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Hyper Jet Fusion Corporation Final Scientific/Technical Report Plasma Guns for Magnetized Fuel Targets for PJMIF DOE Grant Number DE-AR0001236 Award:DE-AR0001236 Sponsoring Agency:USDOE, Advanced Research Projects Agency - Energy (ARPA-E) Lead Recipient:Hyper Jet Fusion Corporation Project Team Members:Hyper Jet Fusion Corporation Project Title:Plasma Guns for Magnetized Fuel Targets for PJMIF Program Director:Dr. Robert Ledoux Principal Investigator:Dr. F. Douglas Witherspoon Contract Administrator:Erin Gilley Date of Report:04/21/2021 Reporting Period:03/27/2020 - 03/26/2021 The information, data, or work presented herein was funded in part by the Advanced Re- search Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Num- ber DE-AR0001236. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ⇥This Report contains no Protected Data. Contents Contents2 1 Table of Figures/Tables2 2Public Executive Summary3 3Acknowledgments4 4Accomplishmentsand Objectives4 5Project Activities5 6Project Outputs5 7Follow-On Funding6 1Tableof Figures/Tables Table 1. Key Milestones and Deliverables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2 2Public Executive Summary In plasma jet driven magneto-inertial fusion (PJMIF) an array of discrete supersonic plasma jets is used to form a spherically imploding plasma liner, which then compresses a magnetized plasma target to fusion conditions. With funding from ARPA-E’s ALPHA program and from Strong Atomics LLC, Hyper Jet Fusion Corp and Hyper V Technologies Corp. previously de- veloped the plasma guns required for an experimental demonstration of the plasma liner formation part of the concept. A 36-gun demonstration of an imploding spherical plasma liner is currently underway on the PLX facility at Los Alamos National Laboratory. This present project addresses the next step required for a complete PJMIF concept, i.e. develop- ing the magnetized plasma target. We proposed to form the target by stagnating a number of magnetized plasma jets in the center of the target chamber. This is accomplished by adding a bias field coil to the plasma liner gun to form a magnetized plasma jet. This experimental development took place at Hyper Jet in a geometry replicating the bias field environment that will be seen on a PLX port so the results are directly transferrable to the PLX exper- iment. The objective of this effort was the technical development and characterization of a new magnetized plasma jet using a high-performance, high momentum flux, contoured-gap coaxial plasma gun appropriate for use on the next stage of the PLX experiment. Electromagnetic modeling of the coil indicated the coil was best placed around the alu- minum tubes of the gun transmission line, rather than around the chamber port as originally proposed. This maximized field strength in the breech and yielded much better flux linkage between gun electrodes. A 30-turn coil was ultimately implemented, allowing long pulses that could diffuse through the metal walls on the timescale of interest. Mach2modeling predicted that less capacitance in the main PFN could potentially improve plasma jet ve- locities due to better matching of the current to the smaller plasma mass and the existing electrode contour designed to suppress blowby. This proved true, as testing showed markedly improved performance when the original 600μFbank was reduced to 400μF.Anumberof diagnostics were built and/or upgraded in order to characterize the plasma jets, inluding laser interferomtery for density, photodiodes for velocity, Bdot probes for magnetic field measurements, a Triple probe for temperature measurements, and spectroscopy for impurity content. A plasma gun with the 30-turn magnet coil installed, a 70% reduction in gas valve plenum volume, and a 33% reduction in main PFN capacitance produced a dense, high velocity, well magnetized plasma jet. We met or exceeded virtually all of the plasma jet parameter goals. Peak velocity of 135km/s exceeded the 100km/s goal by 35%, while the peak density of over 1.0⇥10 15 cm 3 was 3.3 times the goal of>3.0⇥10 14 cm 3 .Shot-to-shotrepeatabilityis excellent, with a jitter of less than 300ns observed on the arrival fronts of the photodiode signals from one shot to the next. Average plasma jet lengths of 33cm (at 120km/s) were slightly longer than the 20cm goal, but jets as short as 11.8cm were observed at 135km/s. Mass is much higher than the targeted goal, averaging 106μgper shot compared to 20μg. The magnetic field is still a bit lower than desired, with a maximum to date of⇠811G, compared to the goal of 1000G. Average values, though, were typically in the 300-450G range, when measured further downstream after some expected in flight decay. Temperature measurements are still a work in progress. Increasing the B field and completing temperature measurements will be continued on into the ongoing BETHE project. 3 3Acknowledgments We would like to thank Dr. Robert Ledoux and Dr. Colleen Nehl for many programmatic and technical discussions that helped to make this project a success. We appreciate their intense interest and support of this project. We would also especially like to acknowledge and thank Dr. Samuel Langendorf, Principal Investigator of the BETHE project at Los Alamos National Laboratory, who has been instrumental in keeping us focused and encouraging us onwards during a difficult year. His technical insights and advice have been extremely helpful during weekly and other Zoom meetings. Finally, we would like to thank ARPA-E for its financial support of this project. 4Accomplishmentsand Objectives The main objective was to design, build, and test a magnetized jet with the parameters listed in Table 1 under Milestone 2. We did that and we met or exceeded all those parameters except the B field, which is lower than desired at present, and we still need to make a temperature measurement to confirm we reached the 5e V goal. Table 1Key Milestones and Deliverables Key Milestones Achieved Milestone 1(go/no-go at 6 months): Demonstrate operational magnetized jet Successfully demonstrated magnetized jet Milestone 2(at end of SEED project): Magnetized jet parameter goals:Magnetized jet parameters achieved: Velocity >100 km/s135 km/s Density >3⇥10 14 cm 3 >1⇥10 15 cm 3 Mass >20μg>100μg Length Scale⇠20 cm11.8 cm (typ 12-30 cm) B jet ⇠1000G811G (typ 300-450G further downstream) Temperature >5e VMeasurement is still in progress 4 5Project Activities In plasma jet driven magneto-inertial fusion (PJMIF) an array of discrete supersonic plasma jets is used to form a spherically imploding plasma liner, which then compresses a magnetized plasma target to fusion conditions. This SEED project focuses on initial development of the magnetized plasma jets needed to form the target. The magnetized plasma jets are generated by adding a magnetizing coil to the exterior of the existing plasma liner guns previously developed on the ALPHA program. The coil position, geometry, and driving circuit parameters were developed using EMS electromagnetic modeling software coupled with Solid Works and bench testing. Three experimental campaigns were performed making diagnostic measurements to confirm the magnetized plasma jets met the desired performance specs. Mach2modeling indicated that a reduced capacitance pfn would match better to the existing electrode contour and the accelerated mass, and this was confirmed by testing. With the better matched pfn, the spec for density was exceeded by a factor of about four to>1.0⇥10 15 cm 3 ,velocityof135km/sexceededthe100km/sgoal,theobservedmass of 100μgexceeded the goal of >20μg,thelengthrangeoftypically12-30cmmatchedthe desired 20cm length (with a best value of 11.8cm), and the maximum observed embedded jet magnetic field of 811G came close to the goal of 1000G, although more typical values were in the range 300-450G. Temperature measurements are still in progress to determine if we met the 5e V goal. A more extensive final technical report has also been prepared which more fully documents project activities and results. 6Project Outputs A. Journal Articles None yet B. Papers None yet C. Status Reports Quarterly Reports were submitted through e Pic. D. Media Reports None yet E. Invention Disclosures None yet F. Patent Applications/Issued Patents None yet G. Licensed Technologies None yet H. Networks/Collaborations Fostered Los Alamos National Laboratory 5 I. Websites Featuring Project Work Results None yet J. Other Products (e.g. Databases, Physical Collections, Audio/Video, Software, Models, Educational Aids or Curricula, Equipment or Instruments) None K. Awards, Prizes, and Recognition None yet 7Follow-On Funding BETHE project “Target Formation and Integrated Experiments for Plasma-Jet Driven Magneto- Inertial Fusion” (award DE-AR0001268) is in collaboration with Los Alamos National Lab- oratory, the University of Alabama - Huntsville, and the University of New Mexico. Table 2Follow-On Funding Received Source Funds Committed or Received ARPA-E DE-AR0001268$2,738,034 6 DOE Grant Number DE-AR0001236 Plasma Guns for Magnetized Fuel Targets for PJMIF An ARPA-E SEED Project Final Technical Report Extended Version 21 April 2021 Prepared for the United States Department of Energy Advanced Research Projects Agency - Energy Edward Cruz, Andrew Case, F. Douglas Witherspoon (PI) Adam Cook, Marco Luna, Robert Becker Hyper Jet Fusion Corporation Chantilly, VA 20151 SBIR/STTR RIGHTS NOTICE The information, data, or work presented herein was funded in part by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0001236. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof. ⇥This Report contains no Protected Data. Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Contents Contents2 1Introduction3 2Background3 2.1 Problem and Proposed Solution . . . . . . . . . . . . . . . . . . . . . . . . .3 2.2 State of the Art and the Technical Approach . . . . . . . . . . . . . . . . . .4 3Modeling(Task1)7 3.1 MHD Modeling of Plasma Gun . . . . . . . . . . . . . . . . . . . . . . . . .7 3.2 Electromagnetic Modeling of Magnet Bias Coil . . . . . . . . . . . . . . . . .9 3.2.1 Coil Location . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 3.2.2 Thermal Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11 3.2.3 Mesh Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12 3.2.4 Effects on the Gas Valve . . . . . . . . . . . . . . . . . . . . . . . . .14 4Biasfieldcoil(Task2)16 4.1 Initial Magnet Circuit with 8-Turn Magnet Coil . . . . . . . . . . . . . . . .16 4.2 Revised Magnet Circuit with 30-Turn Magnet Coil . . . . . . . . . . . . . .24 5Diagnostics(Task3)34 5.1 Electrical Diagnostics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34 5.2 Photodiodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35 5.3 Interferometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36 5.4 Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37 5.5 B-dot probes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39 5.6 Triple Langmuir Probe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46 5.7 Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47 6Experimentalcampaigns(Task4)48 6.1 Initial Magnet Circuit and 8-Turn Magnet Coil . . . . . . . . . . . . . . . .48 6.2 Revised Magnet Circuit and 30-Turn Magnet Coil . . . . . . . . . . . . . . .53 6.3 Modified Gas Valve and Reduced PFN Capacitance . . . . . . . . . . . . . .55 7Summary65 8Acknowledgments66 References67 2 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 1Introduction Figure 1HJ1 gun with added bias coil so flux links both electrodes. Field lines shown are only illustrative. In plasma jet driven magneto-inertial fusion (PJMIF)[1, 2] an array of discrete supersonic plasma jets is used to form a spherically im- ploding plasma liner, which then compresses a magnetized plasma target to fusion conditions. With funding from ARPA-E’s ALPHA program and from Strong Atomics LLC, Hyper Jet Fusion Corp. and Hyper V Technologies Corp. have been developing the plasma guns required for an experimental demonstration of the plasma liner formation part of the concept[3]. A 36-gun demonstration of an imploding spherical plasma liner is currently underway on the PLX facil- ity at Los Alamos National Laboratory (LANL). We proposed to begin addressing the next step required for a complete PJMIF concept, i.e. de- veloping the magnetized plasma target. We proposed to form the target by stagnating a number of magnetized plasma jets in the center of the target chamber, and to accomplish this by adapting the previously developed plasma liner gun to form a magnetized plasma jet by adding a bias field coil to the gun, as illustrated in Figure 1. This experimental develop- ment took place at Hyper Jet in a geometry properly simulating the bias field environment that will be seen on a PLX port so the results are directly transferrable to the PLX experi- ment. The objective of this proposed effort was the technical development and experimental characterization of a new magnetized plasma jet using a high-performance, high momentum flux, compact contoured-gap coaxial plasma gun appropriate for use on the next stage of the PLX experiment. 2Background 2.1 Problem and Proposed Solution For magneto-inertial fusion (MIF) approaches to be successful, both a suitable liner and target are required. We have made substantial progress in understanding and demonstrating liner formation[3, 4] and now the central challenge we face is the formation of a suitable magnetized plasma target. The target must necessarily be a dense magnetized DT plasma, that can be repetitively formed at the center of a large vacuum chamber. A desirable target behavior is obtained when the plasma electron and ion Hall parameters1andthus thermal conduction losses from the target are reduced in the cross-field direction, but the plasma beta is also >1 and thus the thermal pressure of the plasma dominates the global stability of the target rather than the magnetic pressure. In this latter way the risk of magnetohydrodynamic (MHD) instabilities leading to a catastrophic loss of confinement is sidestepped, being replaced instead with the risks of hydrodynamic instabilities, which have 3 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report historically been survivable at modest convergence ratios[5]. These target criteria are met in the recent successful high-performance Mag LIF implosions performed at Sandia National Laboratories, where an axial magnetic field is used owing to the long aspect ratio (axial length over diameter) of the target and achieved multi-ke V temperatures, high neutron yields, and thermonuclear burn conditions[6]. For a spherical implosion which has unity aspect ratio by definition, another topology is likely needed to provide similar reduction of thermal losses. Multiple options have been considered, including configurations with closed field lines such as spheromak or field-reversed-configuration (FRC) topologies, or open-field-line configurations in which the field lines become tangled enough to impede electron heat conduction[6]. The requirement for the target to exist at high beta in the center of a large target chamber leads to the idea that the target plasma must be transiently formed, e.g. injected into the chamber ahead of the heavy liner which will do the compression and primary heating. This leads to the idea of using a subset of the guns / injectors to dynamically form the target “just in time” ahead of the liner. To achieve the magnetization of the target, a “stuffing” magnetic flux can be established in the gun via external solenoid coils, enabling them to operate in a similar manner to traditional spheromak guns[8, 9]. The amount of initial magnetic flux present in the gun determines degree of magnetization of the resulting jet, with the limit of high initial stuffing flux leading to the formation of a detached low-beta spheromak. Operation at this low beta may not be advantageous for the ultimate compression as it may increase the chance for MHD instabilities to disrupt the global confinement in the target. Ultimately, simulations and experiments are required to explore this area of transient magnetized target formation to identify if and how a suitable target can be formed, and what its confinement characteristics may be when compressed by the heavy liner. 2.2 State of the Art and the Technical Approach In this effort, we planned to develop and experimentally characterize magnetized plasma jets using modified versions of our existing plasma guns. The existing HJ1 gun[1] designed and produced by Hyper Jet/Hyper V are the state of the art for high mass, high momentum flux plasma guns. These guns have been used to form small sections of a liner and perform jet merging studies on the PLX facility at LANL[3]. This work was performed under the ARPA-E ALPHA program with additional funding from Strong Atomics LLC. With the installation of 36 plasma guns on PLX, work is currently underway by Dr. Sam Langendorf and his team at Los Alamos National Laboratory performing the first ever demonstration of an imploding fully formed spherical plasma liner with parameters ultimately applicable to a fusion experiment. The LANL/Hyper V/Hyper Jet ALPHA collaboration is expected to generate a spherical liner and obtain experimental data on that liner. This present project will provide the magnetized plasma target for that envisioned liner-on-target integrated experiment which is the goal of the BETHE project. The basic physics for a magnetized target via magnetized jets have been described in detail in the paper by Hsu and Langendorf[7]. Table 1 details the required jet parameters which are our target. This program is a first step towards experimentally developing such jets. The basic idea is to add a solenoidal bias field[11, 12] to the already high-performance plasma liner guns to produce a magnetized plasma jet with high velocity and mass (momen- 4 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Table 1Plasma parameters for proposed near-term target formation experiments and simulations. Magnetized jets in flight Merged magnetized jets Electron Temperature T e (e V)525 Ion Temperature T i (e V)525 Number Densityn(cm 3 )3.00E+143.00E+16 Velocityv(cm/μs)101 Length Scale Lm (cm)2040 Ion Charge State Z( )11 Ion Mass Ratioμ()11 Magnetic Field B(G)10005000 tum). This is illustrated conceptually in Figure 1. The plasma gun is mounted re-entrantly inside a port on the PLX vacuum chamber. For the magnetized jet, we envisioned winding a coil around the port as shown to form the solenoidal magnetic field. The pulse width for the coil current needed to be sufficiently long so that the field “soaks” through the metal and is essentially static during the time of the much shorter plasma jet duration. Since the port structure at Hyper Jet is slightly different from that on PLX, we duplicated the PLX port by mounting the gun inside an extended port tube with the same thickness and geometry as the PLX port. The innovation in this project is in demonstrating formation of a magnetized plasma jet in a high performance (high momentum flux) compact contoured-gap coaxial gun and determining the optimal operating configuration to achieve the parameters listed above. This had not been achieved before and is critical for the successful formation of a magnetized target plasma needed for the PJMIF concept. There are currently two main approaches under consideration for forming a magnetized plasma target appropriate for PJMIF. One is to produce an imploding target plasma with an embedded magnetic field produced directly within the plasma guns themselves. The second is to generate a magnetic field in a target plasma by the use of laser beat wave driven currents in the target plasma. Here we propose to address only the first case as being the simpler and more direct. We leverage the use of an existing spare HJ1 gun for this developmental effort. We also have seven additional operational Alpha2guns (a previous gun design of comparable parameters) which could be pressed into service if the need arises. The electrode profile contours are identical. The Alpha2guns would only need the gas valve upgraded to the latest drop-in version and the original capillary ignitor array replace by the simpler GPI circuit. Reusing existing hardware will save a great deal of time and funds. Following the discussion in Bellan’s book Spheromaks [13], compact toroids (CT) are a class of magnetized plasma with toroidal configurations that are self-stable on time-scales relevant to fusion. The spheromak and field-reversed configuration (FRC) are the most well understood CT configurations and have the greatest potential as fusion targets. The main difference is that the spheromak contains poloidal and toroidal magnetic fields, while the FRC has only the poloidal fields. The FRC has naturally higher,butrequiresan external magnet for confinement. The spheromak has shown greater confinement times and temperatures, and thus greater overall potential as a viable target for PJMIF. Spheromaks 5 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 2Figure is from Chapter 7 of Bellan’s book “Spheromaks” [13]. are commonly produced by coaxial plasma guns, which Hyper Jet has experience developing and manufacturing. FRC’s can be formed and accelerated entirely inductively, eliminating any potential contamination from electrodes, which could be advantageous later. Common among all spheromak formation schemes is that field-aligned plasma current is driven by a field-aligned EMF. As plasma current increases, it remains in a force-free equilibria with the magnetic field according tor⇥B=B. The net field, B, is then twisted, and it’s twist is an increasing function of, which scales with the plasma current. If the rate-of-rise ofis very slow compared to the characteristic Alfven time, then the system evolves through a sequence of relaxed states. All that is essential for spheromak formation is sufficient build-up of, which is referred to as helicity injection. For a coaxial plasma gun configuration, helicity is injected into the plasma at a rate of 2V, where V is the voltage applied across the inner and outer electrodes andis the flux linking these electrodes. Hyper Jet’s HJ1 plasma gun may present a distinct advantage by inherently having a voltage applied across the inner and outer electrodes during plasma formation from its glow-like pre-ionization system (GPI). The GPI voltage can be varied from 0 to 5k V, and the GPI circuit inductance can easily be adjusted to control the rate-of-rise of .Allthatismissingisamagnettoprovidethelinkedflux,. The work plan is therefore centered around designing and installing a bias field coil along with its power supply, and then performing a detailed experimental campaign to study and elucidate the performance of the magnetized jet formed, with diagnostics being key to understanding the observed results. 6 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 3Modeling(Task1) 3.1 MHD Modeling of Plasma Gun Direct computational modeling of the plasma gun performance using He, H, and Ar was done in-house using Mach2[14]. This was needed to fine tune the gun operational envelope and determine the modified PFN for the lower mass required for the magnetized jets. Mach2is only a 2D axisymmetric MHD code and therefore cannot directly model the formation of a magnetized jet which is a fully 3D configuration. However, Mach2is capable of modeling the basic underlying jet, which has much larger magnetic fields (>10 times) in the driving armature than that of the externally applied bias field. This allows Mach2to be a useful tool to provide guidance on setting up the basic jet parameters and determining the optimal capacitance using the existing hardware. Future fully 3D modeling of the magnetized jet is planned to be accomplished during the BETHE program using thr SPHMax code developed by Prof. Jason Cassibry at University of Alabama - Huntsville. Additional 3D modeling of the jet collisions will be performed by Li at Los Alamos National Laboratory under an INFUSE project. Previous experience with the liner guns showed operation at⇠1mgat50-60km/sat best. Since these present experiments were designed for an order of magnitude less mass, it was necessary to determine what set of current current profile and mass parameters would be required to match to the existing electrode contour. Too much current and or the wrong time profile would likely lead to blowby on either the inside or outside. Initial Mach2modeling confirmed that solutions in the parameter range needed actually existed as shown in Figures 3 and 4. The modeling showed that deuterium plasma jets with velocities in the range 100-150 km/s,1⇥10 14 cm 3 density, and 100μgare possible using only 200-300μFof capacitance in the PFN. This would represent a significant reduction in capacitance compared to the liner guns, which use 600μF. This is mainly due to the lower current required to drive the mass to high velocity. A shorter pulse leads to faster current rise and better matching to the electrode contour, thus suppressing blowby. Figure 3Representative 200μFcase at 4k V.Figure 4Representative 200μFcase at 5k V. 7 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report As will be described in further detail in Section 6 below, initial experimental testing of the magnetized gun was performed with the already existing 600μFpfn, which was sufficient for the preliminary stages of testing the magnet coil and diagnostics. Modifying the pfn required some effort, and so we wished to defer that as long as possible to allow time to scan through the performance possibilities with the already existing pfn. This testing revealed that we were indeed making magnetized jets, but the velocities were observed to be low, typically around 70 km/s or so, whereas our goal was greater than 100 km/s. Figure 5Representative 300μFcase at 3.8k V.Figure 6Density contours for 300μFcase. Mach2modeling had indicated that 200-300μFwould match the mass and electrode contour better than the 600μFas seen in the plots of Figures 3, 4 and 5, but unfortunately, the required current exceeded the peak current limitations on each capacitor, limiting the voltage/current at which we could operate. Figure 7A400μFcase using Helium. This performance roughly matches the observed behavior of the magnetized jet. The solution (see also Section 6.3) was to increase the total ca- pacitance slightly to 400μF(6 groups of 2 caps), midway be- tween the two extremes of 200μF and 600μF. Figure 7 shows the resulting performance. At achargingvoltageof4.9k V,this meets the current limitation of 50 k A per capacitor. The ex- periments confirmed the simula- tion, showing velocities well over 100 km/s and with densities well above the minimum desired value. 8 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 3.2 Electromagnetic Mod- eling of Magnet Bias Coil Figure 8Possible magnet coil locations. See text for discus- sion of A, B, and C. Magnetic field modeling of the bias field coil was performed us- ing EMS for Solid Works. EMS is an electromagnetic field simu- lation software which calculates fields (electric, magnetic, flux, potential, eddy currents), circuit parameters (inductance, capaci- tance, resistance, impedance, flux linkage), mechanical parameters (force, torque), and losses (eddy, core, hysteresis, ohmic). EMS was developed by EMWorks and is seamlessly integrated within Solid Works CAD. In particular, EMS was used to evaluate mag- net location, size and energy re- quirements. 3.2.1 Coil Location Figure 9Static field result from an applied mag- net current of 14k A for an 8-axial turn by 2-radial turn coil at Location A. The first task using EMS was to deter- mine the optimal magnet location. The HJ1 plasma gun has three convenient locations for placing external magnets, identified in Fig- ure 8 as locations A, B and C. Locations Aand Ballowforacoilwoundfromflexi- ble cable, using either the gun or flange as amandrel. Location Cisbettersuitedfora rigid coil. The primary goal is to maximize flux linkage between the inner and outer elec- trodes in the formation region. The forma- tion region corresponds the volume between the inner and outer electrodes around where the gas flows out from the inner electrode, and is referred to as the breech. Initial steady-state modeling was done with magnets at locations A, B and C to eval- uate the merits of each option. Location A was the originally proposed magnet location, and was evaluated with a magnet modeled as an 8-axial turn by 2-radial turn coil wound from 4/0 cable. Static field results for an applied magnet current of 14k A are shown in Figure 9. Overall field intensity is good at⇠5k G in the breech, or⇠22m G/Amp-turn. Field 9 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report orientation is not ideal though, as the perpendicular component linking the inner and outer electrodes in the breech is relatively small compared to the overall field magnitude. Location B was evaluated with a magnet modeled as a 4-axial turn by 2-radial turn coil wound from 2/0 cable. Static field results for an applied magnet current of 10k A are shown in Figure 10. Overall field intensity is good at⇠2k G in the breech, or⇠25m G/Amp-turn, which is slightly better than location A, which gave⇠22m G/Amp-turn. Field orientation is much improved versus location A, with the field lines more normal to the inner and outer electrodes for significantly improved flux linkage. Figure 10Static field result from an applied magnet current of 10k A for a 4-axial turn by 2-radial turn coil at Location B. Figure 11Static field result from an applied magnet current of 10k A for a two part coil as- sembly, a lower coil with 2-axial turns by 3- radial turns and an upper coil with 3-axial turns by 6-radial turns, at Location C. Location C was evaluated with a magnet modeled as two separate coil assemblies, a lower coil with 2-axial turns by 3-radial turns and an upper coil with 3-axial turns by 6-radial turns. Static field results for an applied magnet current of 10k A are shown in Figure 11. Overall field intensity is good at⇠4k G in the breech, or⇠17m G/Amp-turn, which is somewhat less field in the breech per Amp-turn than locations A or B. Field orientation is as good or better than location B. Figure 12Static field results from an ap- plied magnet current of 10k A for a two-magnet model, with magnets at locations B and C. Having magnet coils at multiple locations was also evaluated. Static field results from a two-magnet model, with magnets at locations B and C, for an applied magnet current of 10k A are shown in Figure 12. The details of the mag- net at each particular location are the same as that of the single magnet models at that loca- tion discussed above. Overall field intensity in the breech is >5k G, which is just ok consider- ing how many more Amp-turns are present in this case versus the individual magnet cases dis- cussed above. The magnet at location B alone was giving⇠2k G and the magnet at location C 10 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report alone was giving⇠4k G. Field orientation is very good though, with field lines being al- most completely normal to the gun electrodes in the breech, providing excellent flux linkage. That being said, field intensity at the center of the combined magnet assembly is very high at⇠40k G, which is undesirable. The result of this analysis is that a magnet at location B provides the best combination of safety, convenience, field strength and field orientation in the breech. Transient analysis was then done on a simplified gun model to observe the evolution of the field in the breech of the gun. A 15k A, 10ms current pulse was applied to a 4-axial turn by 2-radial turn coil wound from 2/0 cable. Results are shown in Figure 13 at t=5ms, the peak of the current pulse. Figure 14 shows the corresponding flux down the bore of the plasma gun at t=5ms. Overall field intensity is good at⇠3k G in the breech. Field orientation is also good, with the field lines almost directly linking the inner and outer electrodes. Figure 13Transient field results at t=5ms from 15k A, 10ms current pulse for a 4-axial turn by 2-radial turn coil at Location B. Figure 14Flux down bore of plasma gun at t=5ms from 15k A, 10ms current pulse for a 4- axial turn by 2-radial turn coil at Location B. 3.2.2 Thermal Analysis Figure 15Temperature rise after 10ms from a 80k A-turn magnet pulse. Transient thermal analysis was then done to check temperature rise and evaluate what de- gree of cooling was necessary. Results of tran- sient analysis are shown in Figure 15 and show anegligibletemperatureriseof0.15Kfromre- sistive losses due to eddy currents. Active cool- ing will therefore not be required. This led into experiments using a single magnet, passively cooled, wound around the ground tube of the plasma gun assembly, which are discussed be- low. 11 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 3.2.3 Mesh Analysis A lot of the preliminary modeling discussed above was done with a coarse mesh with under 100k mesh cells to get results quickly. An evaluation was later done of results from using a coarse mesh (tens of thousands of mesh cells) versus a fine mesh (hundreds of thousands of mesh cells). Coarse versus fine mesh cells for a plasma gun model are shown in Figure 16. Figure 17 shows the dramatic difference of the field orientation and magnitude in the breech early in time. For some reason, the coarse mesh model develops strong field parallel to the gun electrodes in the breech early in time. This is most likely due to an error in the eddy current effects due to the coarse mesh. The orientation and magnitude of the breech field later in time is similar though. A fine mesh will be used in EMS for all modeling data presented in later sections to avoid the nonphysical effects exhibited early in time with a coarse mesh. Figure 16Coarse (left) vs fine (right) plasma gun model mesh in EMS 12 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 17B-field with coarse mesh (left) vs fine mesh (right) at t=2.0ms, 8.5ms, 13.5ms and 16.0ms. 13 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 3.2.4 Effects on the Gas Valve The effects of the magnet coil being developed on the gas valve of the gun were also modeled, as there was some concern that the magnetic forces from the coil could have a negative impact on the performance of the gas valve. An 8-turn magnet coil was used to model the magnetic effects on the gas valve, as shown in Figure 18. The magnet coil was placed in relative position to the gas valve coil and flyer plate, which are the gas valve components most susceptible to magnetic forces. Effects were observed from⇠6.5ms long magnet pulses of 5k A and 15k A, shown in Figure 19 for 15k A. Peak field on axis at 15k A was 5.1k G, as shown in Figure 20(a). Peak repulsive force is 4.3N (0.97lbs) at 5k A, where d B/dt= 38.6 T/s, and peak repulsive force is 15.5N (3.5lbs) at 15k A, where d B/dt = 116 T/s. Magnetic force versus time for a 15k A magnet pulse is shown in Figure 20(a). Ultimately the magnetic forces from the bias coil on the gas valve components are small enough to be considered negligible. Figure 18Model used to study magnetic effects of coil on the gas valve from the gun. 14 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report (a) 15k A magnet pulse used to study magnetic effects of coil on the gas valve. (b) Forces on gas valve components from 15k A magnet pulse. Figure 19Effects on Gas Valve (a) cap1(b) cap2 Figure 20Peak field from 15k A magnet pulse for an 8-turn magnet coil with gas valve flyer plate. 15 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 4Biasfieldcoil(Task2) 4.1 Initial Magnet Circuit with 8-Turn Magnet Coil We have fabricated and mounted a suitable bias coil onto one of our HJ1 plasma guns, as shown earlier in Figure 1. The gun is mounted in a re-entrant manner on our vacuum tank, similar to how it is on the PLX vacuum tank. We chose to wind the bias coil around what is called the ground tube of the HJ1 gun based on electromagnetic modeling. This has the distinct added advantage that all components are external to the vacuum, eliminating costly vacuum power feedthroughs and the non-trivial problem of coil cooling in vacuum, both of which will likely be important for future implementation on PLX. This also provided flexi- bility in the design process to allow various configurations of the coil length, number turns, operating current, polarity etc. We investigated the efficacy of various coil configurations at the start of the program, looking at both low-turn-count/low-inductance configurations and high-turn-count/high-inductance configurations. (a) Initially deployed circuit model for 8-turn mag- net coil, along with current and voltage from 60V capacitor charge. (b) Simulated current and voltage signals from ini- tially deployed circuit model at 60V charge. Figure 21Details of first tested circuit. We had several options for driving the bias coil, but we chose to use commercially avail- able electrolytic capacitors, which provided the ideal combination of current capacity, voltage and pulse width needed. Likewise, we went with an SCR circuit to control the long pulse coil current. The initially deployed magnet circuit model is shown in Figure 21(a). Corresponding simulated current and voltage plots from 60V of charge on the capacitors are shown in Fig- ure 21(b). This particular circuit used 2 elec- trolytic capacitors in parallel, SCR switching, a crowbar diode and positive charging to ener- gize an 8-turn magnet coil. The capacitors were rated for 330m F at 63V, for a total circuit ca- pacitance of 660m F and total circuit energy of 1.3k J. The SCR was rated for 18.7k A at 1.4k V and the diode was rated for 23k A at 1.8k V. To test the circuit, semiconductor clamps were necessary for the SCR and diode. The semiconductor clamps were designed and built in-house for this experiment, shown in Fig- ure 22, which allowed for dramatically reduced lead times for a clamp with specific dimensions and load calibration. Development was based on industry standard designs. These clamps used a spring washer system to set the load, along with sleeved bolts to provide the load. Load calibration was performed using precision electronic load cells from Omega. 16 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report (a) Semiconductor clamp calibration using precision load cell. (b) Semiconductor clamp deployed and in use with SCR and diode installed. Figure 22Semiconductor clamp system. Figure 23Bench testing setup for initial magnet circuit with commercially manufactured coils. Shakedown testing of the magnet circuit was then per- formed using a commercially manufactured coil designed for high current, which allowed for circuit validation without the risk of magnet failure that could oc- cur from a hand-wound coil, as shown in Figure 23. The com- mercial coils used had a total in- ductance of 386μHand resistance of 47.5m⌦.Althoughresistance and inductance are high, they are close enough to the ranges of in- terest to provide a good testo the magnet circuit. Low current test results are shown in Figure 24 and show very good agreement with SPICE simulations. 17 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 24Sample test results from initial circuit with commercially manufactured coils Figure 25Bench testing setup for initial magnet circuit with 8-turn magnet coil. Further bench testing was performed with an 8-turn mag- net coil wound from 2/0 THHN stranded building cable, shown in Figure 25. This particular cable was chosen because it provided the best combination of size and strength, but with limited flexi- bility, as a result of using much heavier gauge strands than stan- dard welding cable. THHN cable is stiffand hard to wind, requir- ing a rigid winding jig mounted to a lathe. The winding jig is shown in Figure 26 along with 4- and 8- turn coils that were wound using it. Figure 26(Left) Winding jig used to fabricate initial test coils. (Center) 4-turn test coil wound from 2/0 THHN cable. (Right) 8-turn test coil would from 2/0 THHN cable. 18 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report For initial testing, plastic and metal plates were bolted together to use as clamps to prevent the coils from unraveling from magnetic forces. In-house designed and built semi- conductor clamps with integrated transmission line were used for the SCR and diode. Total circuit inductance and resistance were measured to be 22.8μHand 5.23m⌦,respectively. Measured magnetic field and magnet current are shown in Figure 27 and Figure 28 for two different capacitor charge voltages. These data show good agreement with EMS and SPICE simulations. A clamp on current meter was initially used to measure the magnet current, but was limited with a peak current rating of 1.5k A. A 75μ⌦shunt was later added to the magnet circuit to better measure higher currents. Figure 27Simulated vs experimental data from 1.0k A magnet pulse for 8-turn magnet coil. (Top) Magnetic field data. (Bottom) Corresponding current and voltage data. 19 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 28Simulated vs experimental data from 3.8k A magnet pulse for 8-turn magnet coil. (Top) Magnetic field data. (Bottom) Corresponding current and voltage data. After validating the magnet coil and circuit on the bench, magnetized gun shakedown tests were performed, shown in Figure 29. Results showed 7.2k A peak magnet current at a capacitor charge voltage of 60V. This is significantly more current than observed during bench testing. This is because aluminum transmission line from the gun runs through the magnet center, reducing its effective inductance, resulting in a shorter pulse with increased current compared to bench testing. For the shot shown, the gun was fired 3.8ms after the magnet was fired, just below the current peak. This turned out not to be the best timing for firing the gun as eddy current effects significantly delayed the arrival of peak magnetic field into the breech. 20 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 29(Left) Magnetized gun shakedown test setup. (Top-right) Experimental magnet current and voltage signals. (Bottom-right) Plasma jet photodiode signals An internal B-dot array and 100-turn pickup coil were both used to measure, map and verify the field in the breech. The internal B-dot array is shown in Figure 30. It consists of 16 individual 25-turn coils used to map the magnetic field down the axis of the gun. Figure 31 shows measurements from the internal B-dot array and modeling results of the magnetic field at and around the gas ports in the breech of the gun. Figure 32 shows data from the 100-turn pickup coil, along with the coil itself, which was used as a simple secondary diagnostic placed in the breech of the gun at the gas valve ports. The internal B-dot array was a complicated diagnostic, so the 100-turn pickup coil allowed for a simple sanity check to confirm that the data from the internal B-dot array was good and being interpreted correctly. The data overall shows good agreement with modeling and indicates that the gun would be better fired with a 14.2ms delay relative to the magnet firing, rather than the originally tested 3.8ms, as a result of magnetic diffusion. 21 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 30Internal B-dot array with a total of 16 individual 25-turn coils. (Top-left) CAD model showing location of each individual B-dot coil. (Top-right) Assembled B-dot array assem- bly. (Bottom-left) B-dot array assembly mounted on gun inner electrode. (Bottom-right) B-dot array assembly fully installed. 22 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 31Internal B-dot array experimental and simulated data from a magnet charge voltage of 60V for the 8-turn magnet coil. Figure 32(Left) 100-turn pickup coil data for a magnet charge voltage of 60V. (Right) 100-turn pickup coil mounted on plastic rod, along with 3D printed components used as a winding jig. 23 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 4.2 Revised Magnet Circuit with 30-Turn Magnet Coil (a) Revised circuit model for 30-turn magnet coil, along with current and voltage from 300V capacitor charge. (b) Simulated current and voltage signals from ini- tially deployed circuit model at 300V charge. Figure 33Revised circuit model for 30-turn magnet coil, along with current and voltage from 300V capacitor charge. The experimental results for the magnetized plasma jet using the initial magnet circuit and 8- turn coil were good, but more field in the breech was desired than the 300G to 400G that this initial configuration could provide. A revised magnet circuit was developed with significantly higher energy, along with a more inductive 30- turn magnet coil. The higher inductance was desired to extend the pulse, reducing magnetic forces and stresses as the result of a lower d B/dt for a given prescribed field value. The revised magnet circuit model is shown in Figure 33, along with current and voltage plots from 350V of charge on the capacitors. The re- vised circuit used 17 electrolytic capacitors in parallel, SCR switching, a crowbar diode and positive charging to energize a 30-turn magnet coil. The capacitors were rated for 24m F at 350V, for a total circuit capacitance of 408m F and total circuit energy of 25k J. This was a 19- fold increase in total circuit energy over the ini- tially magnet circuit. The same SCR and diode as before were used, with the SCR rated for 18.7k A at 1.4k V and the diode rated for 23k A at 1.8k V. Effective coil inductance and resistance values were measured on the gun, which includes mutual inductance from the gun’s transmission line. The simulated pulse shapes were fine- tuned against experimentally measured pulse shapes, requiring an additional 8.4m⌦to be placed in series with the SCR switch for the model to match the experiment. The result is a current peak of 10.6k A at 8.5ms vs 7.4k A at 4ms with the initial circuit. Due to the increased turn count of the magnet, this represents a significant increase in the maximum amp-turns in the coil vs the initial circuit, 318 Amp-turns vs 35.2 Amp-turns, respectively, and thus should provide much a higher magnetic field. A partially manufactured version of this revised circuit is shown in Figure 34. 24 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 34Revised magnet circuit during construction for driving 30-turn magnet coil Figure 35EMS results for magnetic field at and around gas ports in the breech of the gun from a capacitor charge voltage of 350V for the 30-turn magnet coil. The SPICE circuit model results shown above were subsequently fed into EMS for com- parison against the measured magnetic field in the breech. EMS results of the magnetic field at and around the gas ports in the breech of the gun are shown in Figure 35 and show an order of magnitude improvement in achievable breech field compared to the initial circuit and magnet combination, 3.1k G with the revised circuit and magnet vs⇠300G with the initial circuit and magnet. The magnetic field now peaks at 28ms in the breech due to eddy current effects, shown in Figure 36. These eddy currents also result in large magnetic forces against the plasma gun’s aluminum transmission line plates, as shown in Figure 37. At 350V magnet charge volt- age, 86k N of peak force is momentarily present against the plasma gun’s transmission line plates. This is reaching, but still within, the mechanical limits of the transmission line assembly. The magnet coil itself momentarily experiences 78k N of peak axial force, which includes forces from the rest of the gun as well, not just the transmission line plates. The eddy current effects also result in relatively poor coupling of field into the breech, as the eddy currents generated in the gun’s aluminum transmission line that runs through the magnet center are large. Despite the poor coupling, modeling indicates that there is still 25 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 4m Wb of flux linking gun electrodes in the breech, which is ultimately the desired amount of Figure 36EMS vector plot of peak mag- net field att = 28ms, when the breech field reaches its peak for the 30-turn magnet coil. flux linkage. If more breech field is desired in the future, replacing aluminum with stainless steel in select portions of the transmission line to mitigate the effect of these eddy currents would allow a good deal more field to make it into the breech for a given applied external field, as shown in Figure 38 and Figure 39. This would allow for 6.8k G of peak field at 13ms in the breech with stainless steel versus 3.1k G at 28ms with aluminum at a magnet charge voltage of 350V, essentially allowing for more breech field without increasing magnet power. This trans- lates to 9.2m Wb of flux linking gun electrodes in the breech with stainless steel transmission line components versus the 4.0m Wb achieved with aluminum components at the maximum mag- net charge voltage of 350V. Magnetic forces are also significantly reduced for a given breech magnetic field, as shown in Figure 40 and Figure 41. With stainless steel, peak force against the gun’s transmission line plates is reduced to 15k N versus the 86k N seen with aluminum at a magnet charge voltage of 350V. This translates to significantly reduced axial force seen by the magnet coil as well, down to 27k N peak with stainless steel versus the 78k N peak seen with aluminum. 26 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 37Repulsive force between the magnet coil and the aluminum transmission line plates on the plasma gun from a capacitor charge voltage of 350V for the 30-turn magnet coil, which experience the largest forces from the magnet pulse. Figure 38Breech magnetic field with aluminum vs stainless steel plasma gun transmission line tubes and gun-side transmission line plates from a capacitor charge voltage of 350V for the 30-turn magnet coil. 27 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 39Breech magnetic field with aluminum vs stainless steel plasma gun transmission line tubes and gun-side transmission line plates versus magnet charge voltage for the 30-turn magnet coil. Figure 40Force between the magnet coil and stainless steel transmission line plates on the plasma gun from a capacitor charge voltage of 350V for the 30-turn magnet coil. 28 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 41Force between the magnet coil and either aluminum or stainless-steel transmission line plates on the plasma gun versus magnet charge voltage for the 30-turn magnet coil. 29 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 42The 30-turn coil is wound manu- ally at Hyper Jet with the help of a large lathe. The 30-turn coil required considerably more care in its manufacturing than the initial coils due to its potential for so much higher fields. The 30-turn magnet coil was wound from 2/0 THHN cable, using a modified version of the lathe mounted winding jig shown above. The winding process for the 30-turn coil is show in Figure 42. The wound coil was wrapped with fiberglass strapping tape between layers and two layers of 1/16” fiberglass weave around the OD. The fiberglass wrapped coil was then encap- sulated using 20-3001 NC epoxy, which has a 5,700psi tensile strength, and vacuum degassed, as shown in Figure 43. Once encapsulated, the 30-turn coil was mounted on a gun for testing with the revised magnet circuit, as shown in Figure 44. The revised circuit was validated prior to testing using the same commercially manufactured coils used to validate the initial circuit. Figure 43Fiberglass wrapped 30-turn coil epoxy encapsulation and vacuum degassing. 30 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 44Experimental setup for 30-turn coil testing, mounted on gun with revised magnet circuit. 31 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report The 100-turn pickup coil discussed above was used to measure the resultant magnetic field in the breech from the 30-turn magnet coil. The experimental breech magnetic field and magnet current pulses are shown in Figure 45 and Figure 46 and compared against EMS and SPICE modeling results for two different capacitor charge voltages. These data show excellent agreement with the EMS and SPICE simulations and confirm that the gun would be best fired with a 28ms delay relative to the magnet firing with the 30-turn magnet coil. Figure 45Simulated vs experimental data. (Top) Breech B-field. (Bottom) Corresponding current and voltage pulses from 50V capacitor charge for 30-turn magnet coil. 32 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 46Simulated vs experimental data. (Top) Breech B-field. (Bottom) Corresponding current and voltage pulses from 150V capacitor charge for 30-turn magnet coil. 33 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5Diagnostics(Task3) 5.1 Electrical Diagnostics The key electrical parameters of the gun during operation are measured using either Ro- gowskis integrated using 100 microsecond time constant RC integrators, or Pearson current transformers. The voltages are measured by putting 2k⌦high current resistors in parallel with the load, and measuring the current using 1V/A Pearson current transformers. The Gas Valve current is measured using a 0.01 V/A Pearson, and the current from each of the six PFN switches is individually measured using Rogowskis custom wound using a winding jig. Typical PFN current traces are shown in Figure 47. Figure 47Typical plasma gun PFN current traces from Rogowski coils for magnetized jets of He. 34 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5.2 Photodiodes The photodiode velocimetry system has been upgraded for improved spatial resolution. The two photodiodes collect light from viewing volumes separated by 150 mm. The light is collected by two focusing collimators which convey the light into fibers that bring it to the Thorlabs PDA10A amplified PIN diodes located in the screen room for noise immunity. Velocity is inferred from the difference in arrival times at the two viewing volumes. This velocity is then used in the calculation of mass and width based on the interferometer signal. Typical traces are shown in Figure 48. There is some ambiguity regarding the exact time of arrival at the viewing location since the shapes of the two photodiode signals differ. Accordingly, we are using 4 different methods of finding the time difference between the plasmoid arrival times at the photodiode viewing volume. These are (1) time to rise to 50% of peak value, (2) peak of the signal, (3) centroid of the top 10% of the signal, and (4) centroid of the top 25% of the signal. The first method gives the fastest velocities (shortest time difference) with the least deviation from shot-to- shot, whereas the remaining three tend to give slightly lower velocities with more spread in values from shot-to-shot. Figure 48Typical plasma gun photodiode signals for magnetized jets of He. 35 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5.3 Interferometer Figure 49Motorized mirror mounts devel- oped for the Hyper Jet interferometer system. The interferometer is upgraded from the previ- ous version, adding the capacity for a second channel at the center of the chamber if needed. The key elements for the second channel are in place, awaiting only a few additional mirrors (cost $500 total) to be brought into regular op- eration. The interferometer itself is a quadra- ture heterodyne system using a 635 nm He Ne laser, which has been designed for two chan- nel operation, though only one channel is com- plete, as mentioned above. The beam is modu- lated at 40 MHz by an Acousto-Optic modulator which also serves as the splitter for the reference and sample beams. The sample beam is passed through the chamber offa retroreflector on a custom vibration isolation stand and returned to a polarizing beam splitter for recombination with the reference beam. The reference beam travels a path of the same length (within about 5mm). Aquarterwaveplateisinsertedintothe sample beam so the first pass transforms the lin- early polarized light (laser is polarized) into cir- cularly polarized light, and on the return pass it transforms it into linearly polarized light at 90 degrees to the original polarization. This allows the recombination beam splitter to be placed so the horizontally polarized outgoing sample beam passes through undeflected, but the vertically polarized return beam is reflected by the splitter onto a path collinear with the reference beam. The reference beam is horizontally polarized, so to get an interference pattern we pass the combined beam through a polarizer at 45 degrees, picking offthe rele- vant component of each contributing beam so they interfere and produce good modulation of the interference pattern. Rotating the polarizer slightly either side of 45 degrees changes the relative contribution of each beam, which is helpful since sometimes there is much less power in the sample beam than the reference beam, so tuning by rotating the polarizer allows us to make the two contributions to the interference pattern similar in magnitude so as to maximize depth of modulation. The interference signal is detected by a Thor Labs PDA10A PIN diode with >100 MHz bandwidth. The photodiode signal is sent to the demodula- tion circuit, where it is mixed with the reference output from the Acousto-Optic modulator driver, producing two output signals, one proportional to the cosine of the phase shift and one proportional to the sine. The phase is extracted by taking the arctangent of the ratio of these signals, and then it is a simple proportionality constant multiplier to obtain line integrated density. There is ambiguity in the arctan when the phase change exceeds 2⇡, which must be handled carefully to avoid spurious jumps in the signal. The existing interferometer channel is 355 mm from the muzzle of the gun, halfway 36 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report between the photodiode viewing volumes. The noise on the interferometer signal due to the firing of the PFN switches has been dramatically reduced by moving the detector (a reverse biased PIN photodiode) into the screen room. In addition, a motor-controlled mirror has been added which allows the interferometer to be tuned from within the screen room. This is because over the course of a day of shooting the interferometer, signal quality tends to degrade as small shifts in the position of the optical table or retroreflector stand due to thermal effects or vibration from firing the gun accumulate, requiring tuning up every hour or so. The remote tuning allows this to be done without entering the HV bay, which requires de-arming the HV systems, saving time and reducing opportunities for errors. The motorized mirror mounts (2 channels) can be seen in Figure 49, the screen room control system in Figure 50, and a sample interferometer trace is shown in Figure 51. Figure 50Screen room control system devel- oped for motorized mirror mounts. Figure 51Representative interferometer trace for magnetized jet of He. 5.4 Spectroscopy Figure 52LANL spectrometer The spectrometer sent to Hyper Jet from LANL is operational with 4 fibers viewing the plasma, 2paralleltothedirectionofmotion,and2per- pendicular. The parallel views are in fact 15 de- grees offaxis since the diagnostic fork is in the way, but primarily so that they are not looking directly into the muzzle of the gun, which would contaminate the signal with light from the ring- ing of the bank exciting residual gas in the gun. The spectrometer is shown in Figure 52. The instrument is a 0.25 m Czerny-Turner configuration, with 3 gratings installed, 100 lines/mm, 150 lines/mm, and 300 lines/mm. This gives us 0.46 nm/pixel for the 100 lines/mm, and 0.155 nm/pixel for the 300 lines/mm grating. Currently we are using the 100 lines/mm grating to get survey information, 37 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report but once we have found a good operating regime in hydrogen we will switch to the higher resolution gratings to improve sensitivity. We will look first for the H-alpha line, and if there is enough light we will look at H-beta, which should give us insight into the magnetic field strength due to Zeeman splitting. The image from a shot using Helium is shown in Figure 53(a), with the corresponding lineout shown in Figure 53(b). The top two sets of lines are from the two parallel views. The third set of lines is from the perpendicular view, with the fourth set just offthe bottom of the CCD, so not shown here. (a) Spectrum from magnetized He plasma.(b) Corresponding lineout from spectrum shown above along with mixed He/air Prism Spect fit. Figure 53Spectrum of He plasma 38 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5.5 B-dot probes The modular diagnostic fork system consists of three main assemblies: the head assembly, the rod seal assembly, and the back-end cable breakout assembly, as shown in Figure 54. A great deal of effort has been put into getting the best possible data out of the B-dot probes themselves. They are 3-axis probes, with 10 turn coils for each of the x, y, and z axes. The coils are wound on a custom 3D printed form shaped so that the centers of the coils are at the same position and the total cross-sectional area of each coil is the same. The coils are shielded from the plasma by a 10mm OD, 8mm ID closed-ended quartz tube mounted on an aluminum fitting which contains a plug for getting the signals out. The individual wires for each B-dot coil are twisted together, then each twisted pair is spread apart from one another and shielded with copper tape grounded through a pin in the connector to reduce the noise from outside electrical interference. All this fits inside the quartz tube cemented into the aluminum vacuum adapter subassembly. The orientation of the B-dots to the connectors were critical in maintaining a constant plane orientation of the B-dot coils. This fitting mates to a fork structure giving multiple probes for spatial resolution and the ability to replace B-dot probes with other probes such as a triple probe for electron temperature measurement (discussed in the next section). A picture of a 3-axis B-dot probe assembly is shown in Figure 55. Figure 54Hyper Jet modular diagnostic fork system overview. 39 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 553-axis B-dot probe assembly The diagnostic fork system features a modular head for future diagnostic purposes, as shown in Figure 56. A cross-sectional view of the head assembly is shown in Figure 57. The modular head assembly design also allows the option to perform maintenance through the windows of the vacuum chamber. The individual probes are interchangeable/removable, in addition to the entire head. The modularity of the system should prove useful for making quicker changes to future designs. Figure 56Modular diagnostic head assembly with B-dot probes 40 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 57Cross-sectional view of modular diagnostic head assembly. The rod seal assembly allows linear movement of the diagnostic stalk while under high vacuum, shown in Figure 58. A cross-section of the rod seal assembly is shown in Figure 59. The modular diagnostic fork system allows for differential pumping through the rod seal assembly while maintaining electrical connectivity with the B-dot coils. Differential pumping reduces relative pressure against vacuum seals into the chamber, allowing the main chamber to achieve high vacuum more easily. Component materials were generally chosen to reduce outgassing, although this could not be done for all the wires and connectors required by the electrical system, necessitating the use of differential vacuum pumping, rather than pumping the entire system to high vacuum. Figure 58Diagnostic rod seal assembly 41 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 59Cross-sectional view of diagnostic fork system rod seal assembly. The back-end cable breakout assembly is where all the diagnostic wires within the sys- tem transition to BNC connectors for monitor- ing and recording, shown in Figure 60. The ca- ble breakout assembly is a vacuum cross featur- ing custom flanges with 38 BNC feedthroughs, of which the center conductors are wired to two 25-pin d-sub connectors, as shown in Figure 61. Using only the center conductors diminishes the likelihood of crosstalk between channels. The d- sub connectors then connect to a custom-made wire bundle consisting of coaxial cables, further reducing electromagnetic interference and keeping inductance low. The other ends of the coaxial cables are then attached to a 3d- printed connector which adapts to the two 25-pin connectors in the head assembly. Figure 60Back-end cable breakout assembly of diagnostic fork system. Figure 61Custom made BNC feedthrough flange with d-sub connector for back-end cable breakout assembly. The originally deployed diagnostic fork system is shown in Figure 62. The originally deployed fork structure has 4 locations for probes, as shown in Figure 63. The figure also shows the arrangement of the probes in the chamber, and the movable fork on the sliding rod seal assembly for scanning along the z-axis of the vacuum chamber. The small yellow fixture on the leftmost probe is a 3-axis Helmholtz calibration coils with 10-turns per coil developed for calibration of the probes in-situ. The form is 3D printed and fits precisely over the probe tips so that the center of the calibration coils is coincident with the center of the B-dot pickup coils. 42 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 62Originally deployed diagnostic fork system mounted on Hyper Jet vacuum chamber. Figure 63Originally deployed 4-probe diagnostic head assembly in Hyper Jet vacuum chamber with 3-axis Helmholtz calibration coils attached. 43 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report Figure 643-axis Helmholtz calibration coils with 10-turns per coil solid model for EMS. The calibration coils were modeled in EMS, as shown in Figure 64, to attain flux through X (inner-most coil), Y (mid- dle coil) and Z (outer-most coil) b-dot coils per amp applied to the calibration coils. This was used to interpret the signal being seen on the B-dot probes during calibration. The B-dot coils were modeled as flux surfaces centered between Helmholtz coils. The 10-turn Helmholtz calibration coils were modeled as coils wound with 24awg wire. The X and Y axis calibration coils are separated by 17mm. The Z axis calibration coils are separated by 23mm. Resultant flux through the X, Y and X flux surfaces of the B-dots from the corresponding Helmholtz calibration coils were 1.177⇥10 8 Wb/Amp, 1.375⇥10 8 Wb/Amp and 7.834⇥10 9 Wb/Amp, respectively. EMS plots of the resultant field for each axis are show in Figure 65. Figure 65B-field from 3-axis Helmholtz calibration coils for B-dot probes. (Top) Z-axis B-field. (Middle) Y-axis B-field. (Bottom) Z-axis B-field. 44 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report A new diagnostic head assembly with a 3-probe fork has recently been deployed, along with an improved rod seal assembly and linear motion control. The reason for the change is to provide better high current connections for the triple probe, which can be placed on the center location or swapped out for another 3-axis B-dot probe, as well as allow for easier and more accurate positioning of the probes within the vacuum chamber. The new head is shown in Figure 66, along with two B-dot probes and the triple probe. The revised linear motion feedthrough carriage assembly is shown in Figure 67. Figure 66Revised 3-probe diagnostic head assembly. Figure 67Revised linear motion feedthrough carriage assembly. 45 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5.6 Triple Langmuir Probe Figure 68Hyper Jet’s 4-tip triple Langmuir probe. Experiments have begun in an effort to refine the triple probe measurements for electron tem- perature based on prior results from exploratory experiments. A new probe with 4 tips has been constructed, allowing a free probe tip to be able to double up on the floating potential measure- ment if desired. An image of the 4 tips is shown in Figure 68. One of the modifications going for- ward will be to make the tips shorter to reduce the current. The circuit originally used for the ex- ploratory experiments was biased to 18 Volts between the two non-floating tips by using two 9Voltbatteriesinseries,butthisappearstobe too high of a bias. Accordingly, we will be using arrays of 1.5 Volt D cell batteries, allow- ing us to vary the bias by adding and subtracting cells. The high-speed signal from the plasma requires that we use capacitors in parallel with the batteries so as not to load down the tips with the battery impedance. Current is measured using a 1 V/A Pearson current transformer. Figure 69Tektronix TBS2000B differential probe and shielded box. The potential of the floating tip will be measured using a Tektronix P5200A differential probe with a 50 MHz bandwidth and the abil- ity to measure millivolt signals on top of up to 1.3 k V bias. The need for a differential probe is largely due to the possible coupling of the plasma potential to the PFN current, which is still ringing by the time the plasma jet reaches the probe. It is not yet clear if the plasma is electrically connected to the PFN by the time it reaches the midpoint of the chamber where we take the triple probe measurements, but the possibility cannot be discounted. The differential probe requires a dedicated oscilloscope placed in the HV bay since the probe cable is too short to reach the screen room. We have purchased a TBS2000B Tektronix oscilloscope for this purpose and placed it in a shielded box in the HV bay, with electrical isolation from the power lines to reduce noise. The box and differential probe assembly are shown in Figure 69. 46 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 5.7 Imaging We have used a Nikon SLR camera to obtain time integrated (open shutter) images as shown in Figure 70, somewhat tilted due to mounting constrains on the camera. The camera shutter is open for the entirety of the shot, and the plasma is viewed through 3 layers of 90% neutral density filters to cut down light. As a result, the image contains light captured almost solely from the plasma. Figure 70Nikon open shutter image of magnetized He plasma jet striking B-dot probes. 47 Hyper Jet Fusion Corp.“Plasma Guns for Magnetized Fuel Targets for PJMIF”Final Report 6Experimentalcampaigns(Task4) Several experimental campaigns to explore and fine tune the operational envelope of the magnetized plasma jets were performed with extensive efforts to diagnose and understand the jets formed. During this effort, we investigated various biasing coil configurations, the details of which are discussed above in Task 2. The basic experimental setup for all magnetized jet experiments is shown in Figure 71. The magnetized plasma gun with 30-turn magnet coil is on the left, along with the 2 main photodiodes on the top middle-left, the interferometer beam on the chamber axis between the photodiode projections and the diagnostic stalk with B-dot probes on the right, with the B-dot probe tips at the center of the chamber. A A B B