39IhA IAAIASMWSAWASEE Joint Propulsion Conference & Exhibit AIAA-2001-3671 Huntsville, Alabama July 20-23,2 003 A PLASMOID THRUSTER FOR SPACE PROPULSION Syri J. Koelfgen* and Clark W. Hawkt University of Alabama in Huntsville (UAH) Propulsion Research Center S225 Technology Hall Huntsville, AL 35899 Richard Eskridge,' James W. Smith' and Adam K. Martin' NASA Marshall Space Flight Center (MSFC) Propulsion Research Center, TD40 Huntsville, AL 35812 ABSTRACT probes, high-speed cameras and a laser interferometer. Also of key importance will be There are a number of possible advantages to measurements of the efficiency and mass using accelerated plasmoids for in-space utilization. Simulations of the plasmoid thruster propulsion. A plasmoid is a compact plasma using MOQUI, a time-dependent MHD code, will structure with an integral magnetic field. They be carried out concurrently with experimental have been studied extensively in controlled fusion testing. research and are classified according to the relative strength of the poloidal and toroidal INTRODUCTION magnetic field (B, and Bt, respectively). An object with B , / Bt 1 is classified as a Field Reversed High specific-impulse engines are needed for )' Configuration (FRC); if B , = Bt, it is called a large Av space missions.' Furthermore, relatively Spheromak. The plasmoid thruster operates by high thrust allows for greater mission flexibility. A producing FRC-like plasmoids and subsequently plasmoid thruster has many potential advantages ejecting them from the device at a high velocity. for these missions. Since plasmoids are formed The plasmoid is formed inside of a single-turn inductively in a Field Reversed Configuration conical theta-pinch coil. As this process is (FRC),* they do not suffer from the electrode inductive, there are no electrodes. Similar erosion losses inherent in many advanced experiments have yielded plasmoid velocities of at propulsion concepts. Due to this electrode-less least 50 km/s, and calculations indicate that operation, the device is not limited to specific velocities in excess of 100 km/s should be propellants. As the FRC is a self-contained possible. This concept should be capable of plasmoid, it is not connected to the walls by producing Isp's in the range of 5,000 - lS,OOO s magnetic field lines and thus does not require a with thrust densities on the order of lo5 N/m . The magnetic nozzle. Consequently, the problem of current experiment is designed to produce jet detachment is lessened or possibly eliminated. - powers in the range of 5 10 kW, although the Insulation of the plasma from the walls of the concept should be scalable to several MW's. The device also leads to a higher temperature than plasmoid mass and velocity will be measured with that of wall-supported plasmas. In addition, the a variety of diagnostics, including internal and FRC is relatively free of impurities that would external B-dot probes, flux loops, Langmuir otherwise cool the plasma through line-radiation. * AlAA Student Member, NASA MSFC Graduate Student ' AlAA Member, NASA MSFC Aerospace Systems Engineer Researchers Program Fellow & UAH Graduate Research NASA MSFC Mechanical Engineer Assistant NASA MSFC Physicist AlAA Fellow, Director of UAH Propulsion Research Center and Professor of Mechanical & Aerospace Engineering This paper is declared a work of the US. Government and is not subject to copyright protection in the United States. 1 American Institute of Aeronautics and Astronautics .\ i Moreover, this high plasma temperature positively by sending an optical trigger signal to a high- affects efficiency. Plasma resistivity is proportional voltage pulser that then triggers the spark-gap to Ti3'*, so a high temperature plasma has a low switch. The capacitors and their associated resistivity (q), and therefore a high magnetic circuitry sit inside of a tank filled with mineral oil to Reynolds number, R = p,,vUq, where p,, is the prevent arcing. When triggered, the spark-gap permeability of free space, v is the plasma switch closes, allowing the capacitor bank to velocity, and L is a characteristic scale length. discharge through the coil. Thus it may be possible to accelerate the In the current experiment, only single plasmoids with high efficiency. FRC's are made. If the concept proves out, the There are, on the other hand, several experiment will be upgraded to burst mode technological requirements that must be met for operation. successful operation of a plasmoid thruster. Pulsed plasmoid formation requires rep-rates of 10 FRC Plasma Coils hertz or greater.3 The device would therefore mXkmm/m-- rnm require low-voltage (i.e. less than 20 kV) formation and acceleration technology such as solid-state - switches, or high-voltage (10 40 kV) switches and high energy density capacitors. Much improvement is required over existing technology for the in-space operation of a plasmoid thruster. The potential advantages however make it a worthy candidate for study. FRC's are empirically well understood, as they have been studied for years as a fusion core.4 Open Flux - Cl~~edFlu~ 1( a) Field Reversed Configuration (FRC) PLASMOID THRUSTER EX PER1M E NT (PTX) A plasmoid is an object with an internal magnetic field structure. If the plasmoid has only toroidal current and poloidal magnetic field, it is called a Field Reversed Configuration (FRC); if both the current and magnetic field have poloidal and toroidal components, the lasmoid is classified as a Spheromak (Figure l)! In practice there is a continuum between the two. The Plasmoid Thruster Experiment (PTX) produces FRC plasmoids in a theta-pinch formation chamber (Figure 2). The plasmoids are produced in a quartz tube situated inside of a theta-pinch coil. l(b) Spheromak The solid aluminum coil has a conical half-angle of 17.5" and provides a peak magnetic field of 5 Figure 1. Plasmoid Magnetic Field Structure kGauss. The quartz tube is connected to a rectangular vacuum chamber for exhausting the plasmoids. Four cylinders with internal permanent magnets surrounding the vacuum chamber are used to generate a bias field. Before firing, the chamber is filled with gas through the quartz tube. The gas is preionized by a high-voltage DC power supply that is connected between the gas inlet and the chamber (ground). A current pulse is then sent through the theta-pinch coil to produce a reversed magnetic field, i.e. a field in the opposite direction of the bias field. The coil is driven by a Figure 2. Plasmoid Thruster Experiment (PTX) 640 nF, 40 kV capacitor bank which is switched Schematic with a Perkin-Elmer spark-gap. The bank is fired 2 American Institute of Aeronautics and Astronautics EXPERIMENTAL OPERATION charging voltage was used, resulting in a peak quarter-cycle current of 28 kA (Figure 5), which The PTX plasmoids are formed as FRC's in a was lower than predicted by SPICE. theta-pinch coil as described previously. Typically, separate capacitor banks are used for pre- ionization, bias-field generation, and reversed field generation (compression).2 PTX uses one bank to perform all three stages of formation. This method is referred to as firsthecond half-cycle formation (Figure 3). This technique was used in the earliest days of FRC research and has the virtue of being relatively simple. The capacitor bank and coil constitute an L-C circuit, which rings sinusoidally. To begin operation, the gas is first introduced. A static fill is being used at present, but the system is being upgraded to incorporate gas-puff valves. The circuit is then closed with a high-current, high- voltage spark-gap switch, causing the capacitor to Tifiul..l discharge through the coil. At time to the axial magnetic field changes rapidly. This creates an Figure 4. PTX Circuit Simulation using SPICE electric field that pre-ionizes the plasma. Simultaneously, the plasma is seeded with the B- field produced during the first half-cycle. When the current/field swings through zero, the chamber contains a cold, partially ionized plasma with an imbedded magnetic field. As the field increases in the opposite direction, the plasma is shock heated to full-ionization and is compressed. 1 0 5 lirn&ns) 15 m eni Compression an (W 0 Field Reversal -7 \" Bias F"lu "/ Figure 5. Actual PTX Capacitor Bank Discharge -200 -5 0 5 io 15 20 Time (ms) DIAGNOSTICS Figure 3. Ideal Coil Current for FirstlSecond A variety of diagnostics will be used to study the Half-Cycle Formation formation and acceleration of the FRC plasmoids. Excluded Flux Array SPICE was used to simulate the PTX A singular feature of the FRC is that circuit using assumed circuit component values of simple external magnetic diagnostics provide 640 nF capacitance, 194 nH external inductance, detailed information about the internal conditions 60 mR external resistance, 28 nH coil inductance, of the plasma. In equilibrium, the FRC plasmoid and 35 kV charging voltage. The peak coil current sits inside of an open, solenoidal magnetic field; generated was predicted by SPICE to be 53.4 kA the boundary surface between the open and at the quarter-cycle time tI4l = 0.6 ms (Figure 4). closed field lines is called the separatrix. FRC's When the PTX capacitor bank was actually are typically highly prolate, and this is discharged through the theta-pinch coil, 32 kV of characterized by an elongation E, 3 American Institute of Aeronautics and Astronautics Conical pin\ h Coil where ZSep and rTepa re the length and radius of the separatrix respectively. The shape of the separatrix is typically found to be either elliptical, corresponding to a high pressure on the open field lines, or race-track like, corresponding to a low pressure on the open field lines. For a sufficiently prolate FRC, there is a simple formula, the Barnes p relation, that links the plasma (the ratio of the plasma pressure to magnetic field pressure) and the separatrix radius by the equation: =l--x,v 1 2 2 Figure 6. Excluded Flux Array with so that: (3) where n is the plasma density, k is Boltzmann's constant, Te and Ti are the electron and ion where rwa//i s the radius to the vacuum wall. plasma temperatures respectively, Bwall is the Hence, from the two independent measurements magnetic field at the vacuum wall and rcoili s the of Bwll and CD, rTepc an be measured at every radius of the theta-pinch coil. axial probe/flux-loop location, giving an axial The separatrix radius is measured using profile of r,yep,a nd therefore the shape of the an excluded flux array, which consists of a flux- separatrix, as well as the internal plasma pressure loop5 and associated B-dot probe5s6 located from the Barnes relation. beneath every coil, just outside of the vacuum wall (the quartz tube). (Figure 6) The B-dot probe Internal B-dot Probes measures Bwalla nd the flux loop measures the flux Internal B-dot probes5s6w ill be used to within that loop, where flux is given by measure the magnetic fields in the open flux region, in the exhaust region, and within the FRC itself. Six sets of two B-dot probes (for a total of (4) twelve probes) have been constructed for measuring B, and Bo, the axial and azimuthal The flux within the separatrix is zero by definition. magnetic fields, respectively, of the translating One then defines an 'excluded flux', ACP , from: plasmoid. (Figure 7) The probes were wound with 36 gauge magnet wire around a 3.15 mm square form made from G-10 circuit board material. During construction of the probes, the fragile wire was kept taut by winding it around spools mounted to an optics table. 4 American Institute of Aeronautics and Astronautics uniform field, probably due to the fact that the coils were heating up during calibration and changing the effective current. Figure 10 shows the calibration data for the Helmholtz coil. 14- 12- f 10- QJ oa- m 08- 04- a I I I I Figure 7. Internal 6-dot Probes A Helmholtz coil (Figure 8) was built for Figure 9. Axial Magnetic Field of. Helmholtz the purpose of calibrating the B-dot probes. The Coil Helmholtz coil is a set of two coaxial current loops with equal radii, separated from each other by that radius. The radius of the PTX Helmholtz,coil loops is 2.5 inches. Figure 10. Helmholtz Coil Calibration Curve In order to calibrate the B-dot probes, the Helmholtz coil was pulsed with a 300 VDC trigger, and the resulting voltage (translated into current and then magnetic field) responses of the Helmholtz coil and B-dot probes were measured with a digital oscilloscope. The magnetic field B Figure 8. Helmholtz Coil seen by the B-dot probe is obtained through the relation Equal currents flow in the same direction 1 in each of the current loops. The magnetic field B = JBdt = -NA j Vdt (7) produced by the Helmholtz coil is extraordinarily uniform inside the volume of the coils, hence it provides a valuable calibration device. The where B (B-dot) is the change in magnetic field Helmholtz coil magnetic field was calibrated with a aB power supply (current source), multimeter and Hall with time, i.e. -, as measured by the probe, dt at probe. The magnetic field inside the coils along the “z” axis is shown in figure 9. The measured is the time of measurement, N is the number of data is not completely flat in the area of the turns of the probe, A is the cross-sectional area of 5 American Institute of Aeronautics and Astronautics the probe, and V is the voltage measured across where e and me are the charge and mass of the the leads of the probe. electron respectively, 6 is the permittivity of free The calibrated item, NA, was found to be space, c is the speed of light, il is the wavelength within -80% of the predicted value of NA = of the laser light, and n is the plasma number 10*(3.15 mm)' = 9.92E-5 m2 for the probes. The density (electron number density). The phase shift frequency response of the B-dot probes was also is proportional to the line-integral of the plasma assessed by applying a sine wave to the density along the scene beam. Helmholtz coil at frequencies from 1 kHz to 3 MHz. The reference beam is frequency-shifted Responses of the Helmholtz Coil and the B-dot by 40 Mhz using an acousto-optic Bragg cell. The probes were measured with the oscilloscope. scene and reference beams are recombined at a Figure 11 shows a typical plot of the calibration photo-detector and the resulting voltage signal is constant NA. AC-coupled to an analyzer-circuit. There, the signal passes through a filter that selects the difference frequency. The interference signal (phase shift) is encoded as a frequency modulation of the 40 Mhz carrier signal. This 15~10~- 'heterodyning' preserves the sense of the phase - shift when it goes through zero, and makes the - 10- instrument relatively insensitive to electrical noise E I and variations in amplitude. 05- The analyzer-circuit splits the interference signal into two signals. One of these signals is 00-- mixed with the original 40 Mhz carrier signal, and I ' I I I I the other is mixed with the carrier signal shifted by 0 200 400 600 800 90". These are then low-pass filtered, yielding two i= signals proportional to sin(A@) and cos(A@) Figure 11. Typical B-dot Probe Calibration Plot respectively. From these, A@ may be reconstructed unambiguously. The line-averaged density is then calculated from this phase-shift. At Lanamuir Probes least 3, and perhaps as many as 6 chords of Quadruple Langmuir probes' will be used interferometry will be available for PTX. to make local measurements of electron temperature T, and number density ne in the open Hiah-speed PhotoaraDhy flux region and in the exhaust region. Three of the High-speed photography will be used to four probes on the quadruple probe sample the image the FRC end-on, and to measure its Current-Voltage characteristic, and from this yield velocity as it is translated. A variety of cameras measurements of T, and ne. The fourth bent probe with nanosecond resolution are available for the provides an estimate plasma velocity. high-speed photography. PTX has access to a Cordin 2208 camera that can take 8 pictures at lnterferometrv 100 million frames/second, a Hamamatsu streak- A heterodyne, quadrature He-Ne laser- camera and two Princeton Instruments PI-Max interferometer will be used to measure the plasma cameras, which have an intensified CCD array number density in the FRC. This instrument is (512 x 512 pixels) and shutter-times down to 2 ns. described briefly in Reference 8. In laser interferometry, the laser beam is Bolomet split into two beams of approximately equal ?silicon XUV photo-diode bolometer6 will intensity. One, called the scene-beam passes be used to measure the integrated radiation flux through the plasma; the other, called the reference from the plasma, which will be instrumental in beam, passes outside of the vacuum chamber. determining the overall energy balance. The scene beam acquires a phase-shift due to the plasma given by 6 American Institute of Aeronautics and Astronautics EXPERIMENTAL OBJECTIVES plasma density. The volume of the plasmoid will be determined by using the internal B-dot probe In order to have a high efficiency in a working array as well as the high-speed photography. device, it will be necessary to efficiently ionize the Thrust-related measurements to be plasma and form the FRC. As with all electric obtained include plasmoid mass and velocity. The propulsion systems, ionization represents a large mass and velocity will be acquired as previously frozen-flow loss in a plasmoid thruster.lP2 It is described. Overall jet power, and therefore also necessary to reduce its overall influence on the thrust, will be limited by the rate at which the system energy budget. With the theta-pinch system can be fired. Currently, the experiment is formation method, the plasma is formed in a being fired in single-shot mode, however upgrades partially ionized state and is fully ionized during to repetitive operation are being developed. Gas- the shock heating caused by the field reversal. It puff valves operable at frequencies of up to 500 hz has been found though that FRC equilibrium is are being installed. For pulsed operation, it is sensitive to the pre-i~nization.~V arious pre- also necessary to design capacitor banks and ionization techniques for PTX will be investigated, switching circuits that can be repetitively fired. including ringing magnetic field ionization, coaxial This will be done after initial experiments are plasma injectors, and perhaps rotating magnetic performed. fields (RMF). One of the advantages of the FRC thruster For a practical system, it will also be is that it should behave almost like a mass-driver, necessary to minimize resistive losses in the coils ejecting a magnetized plasmoid that is not (by, for example, liquid nitrogen cooling), and to magnetically attached to the nozzle. It will be maximize the recapture of inductive energy in the necessary however to actually demonstrate this coils and transmission line. experimentally. Detachment of the plasmoid from To evaluate the performance of the the magnetic field lines will be investigated by plasmoid thruster, measurements used to assess characterizing how much of the plasmoid plasma specific impulse, thrust and efficiency will be stays attached to the field lines with increasing taken. Since propulsion performance is also distance from the theta-pinch coil. Magnetic characterized by how much of the plasma exits diagnostics and high-speed photography will be from the device, detachment of the plasmoid from used to evaluate the shape and magnetic field the magnetic field lines will also be investigated. structure of the translating plasmoid. In addition to Similar experiments have yielded plasmoid the internal B-dot probe array, an excluded flux velocities of at least 50 km/s, and calculations array will also be used to this means. It may also indicate that velocities in excess of 100 kmls be favorable to allow the plasmoid to decay should be possible. The concept should b - e just as it reaches the nozzle (by making ZnUwde cay capable of producing Isp’s in the range of 5,000 15,000 seconds with thrust densities on the order comparable to Tramit), so as to recoup, at least of lo5 N/m2. The current experiment is designed partially, the frozen-in magnetic and plasma - energy, and convert it into directed kinetic energy to produce jet powers in the range of 5 10 kW, in the exhaust. although the concept should be scalable to several Simulations of the plasmoid thruster using MW’s. MOQUI, a time-dependent MHD code, will be Isp-related measurements to be obtained carried out concurrently with experimental testing. include velocity and mass utilization. Velocity will be measured by use of high-speed photography, INITIAL RESULTS the internal B-dot probe array, laser interferometry and the quadruple Langmuir probe. Mass Images of PTX translating plasmoids have been utilization will be evaluated by taking an inventory captured with the Cordin camera. Figure 12 of the propellant mass put into the device shows a plasmoid propagating in air at 43 mTorr compared to the mass of the plasma coming out. backfill pressure. The Cordin camera has 8 CCD The mass of the plasma emerging from the device cameras in a circular configuration that take will be determined by measuring the density and photos at a rate of 100 million frames/second, with volume of the ejected plasmoid. Density will be a gate time of 10 ns per frame. The plasmoid measured using the quadruple Langmuir probe velocity has been estimated from the Cordin and the laser interferometer. The Langmuir probe images by assessing the amount of time for the will provide a local measurement of electron plasmoid to traverse the vacuum chamber number density, while the interferometer will make viewport over the 8 frames. The plasmoid shown a non-intrusive measurement of line-averaged 7 American Institute of Aeronautics and Astronautics in Figure 12 (which was frame 6 of the 8) was a camera shutter speed of 20 ns) correlated with estimated to have a velocity of 5.3 km/s. the coil current trace was made (Figure 13). From this plot it is proposed that two separate plasmoids are formed during a test firing. CONCLUSION The plasmoid thruster is a promising concept for in-space propulsion with a number of possible advantages. The Plasmoid Thruster Experiment seeks to evaluate the concept. The plasmoid thruster operates by producing FRC-like plasmoids in a theta-pinch coil and subsequently ejecting them from the device at a high velocity. To evaluate the performance of PTX, the plasmoid Figure 12. Translating Plasmoid mass, mass utilization, velocity and thruster efficiency will be measured with a variety of diagnostics, including internal and external B-dot In order to begin evaluating PTX probes, flux loops, Langmuir probes, high-speed ionization, the Cordin camera has also been cameras and a laser interferometer. placed at the end of the vacuum chamber so that it is looking into the throat of the theta-pinch coil. For the 6% H2 in He gas, a 128 mTorr static fill and preionization, a plot of plasmoid images (with High Speed Plasmoid Images (20 ns Shutter Speed) Correlated with Theta-Pinch Coil Current Data 1.5 2.0 2.5 3.0 3.5 4.0 Tine (pa) Figure 13. Plasmoid Images with Coil Current 8 American Institute of Aeronautics and Astronautics REFERENCES 1. ) “Physics of Electric Propulsion,” Robert G. Jahn, McGraw-Hill, 1968. 2.) “Field Reversed Configurations - Review Paper,” M. Tuszewski, Nuclear Fusion 28(1 l), 1988. 3.) “Pulsed High-Density Fusion Rocket,” J. Slough, NASA Fusion Propulsion Workshop, Nov. 8-10, 2000, NASA Marshall Space Flight Center, Huntsville, Alabama. 4.) “Inductive Field-Reversed Configuration Accelerator for Tokamak Fueling,” A.L. Hoffman et. ai., Fusion Technology, 36, Sep. 1999. 5.) “Principles of Plasma Diagnostics,” I. H. Hutchinson, Cambridge University Press, 1987. 6.) “Plasma Diagnostic Techniques,” Richard H. Huddlestone and Stanley L. Leonard, Academic Press, 1965. 7.) “Application of a Quadruple Probe Technique to MPD Thruster Plume Measurements,” R. L. Burton and S. Del Medico, Journal of Propulsion and Power - 9(5), pp. 771-777,S ep. Oct. 1995. 8.) “Photographic, magnetic, and interferometric measurements of current sheet canting in a pulsed plasma accelerator” T.E. Markusic, E.Y. Choueiri, 37‘h AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, July 8- 77, 2007, Salt Lake City, Utah, AlAA Paper 2001 -3896. 9 American Institute of Aeronautics and Astronautics Syri J. Koelfgen, NASA MSFC GSRP Fellow University of Alabama in Huntsville Propulsion Research Center 3gthA IAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit July 20-23,2003 Huntsville, Alabama w"M-+ --?%+* Experimental Team NASA Marshall Space Flight Center / TD40: Dr. Adam Martin, Richard Eskridge, Mike Lee, James Smith, Tommy Reid, Jeff Richeson University of Alabama in Huntsville: Syri Koelfgen, Dr. Clark Hawk 1 Presentation Out Ii n e 1. Background II. Pl asmoid Pr o puI si on 111. Plasmoid Thruster Experiment (PTX) IV. Diagnostics V. Imaging Results VI. Future Work Propulsion System Comparison I 10-5 1W 1oJ 10 -2 10 -1 1 10 102 Vehicle Acceleration or T/W Ratio (g’s) 2 I . Compact Toroids (Plasmoids) A Compact Toroid is a plasma w/ a self-contained magnetic field structure (i.e., a magnetized plasmoid) a) If it has only toroidal current and poloidal magnetic field, it is called a Field Reversed Configuration (FRC) b) If both the current and the magnetic field have both toroidal and poloidal components, it is called a Spheromak In practice, there is a continuum between the two 4 I I I (a) Field Reversed Configuration (b) Spheromak I Theta-Pinch (FRC) Plasmoid Formation FRC plasmoids formed with a theta-pinch coil Propelled to high velocity in acceleration stage by compressing the B field behind it -v=20-100km/s; 3.) 4.) - ISP= 5,000 - 15,000s Thrust determined from plasma density and rep- 5.) rate (subject to power constraints) initially single plasmoids made; later upgrade to burst mode operation 3 Plasmoid Propulsion . FRC-produced plasmoids formed . repetitively & magnetically VaaM accelerated Maonebc Feld No electrodes (inductive [FonmtiorJAccderabiar coid . formation) \ A . Potential performance: F . - Jet Power: 10 kW MW’s . Specific Impulse: 5,000-1 5,000 s . Efficiency: 50-80 % . Potential applications: Low power (Pj,T< 100 kW), high -spec ific impulse (Isp > 5,000 s) . missions - High power (PjET MW’s), high specific impulse missions I Plasmoid Propulsion Advantages & Requirements Advantaqes Technoloaical Reauirements . High Isp (5,000-15,000 s) Pulsed plasmoid formation requires: High thrust density Rep-rates > 10 hz pcham=be iro 5 - 106 pa Recapture of inductive energy No electrodes (inductive formation) Either.: Plasma exhaust contained by Low-voltage (< 20 kV) internal magnetic field (aiding in formation and acceleration detachment) technology: solid-state FRC‘s are empirically well switches understood -the technology draws or on results routinely obtained in FRC High-voltage (10-40 kV) 9 fusion research switches, and high energy- FRC’s have been accelerated to 250 density capacitors km/s - yielding an “Isp”e quivalent of 25,000 s Much improvement reauired over Potentially scalable to high powers existina technoloqy (up to MW’s) 4 .. .a 1 Plasmoid Thruster Experiment (PTX) II PTX Side View PTX Oblique View PTX Schematic: n u.4 - I Plasmoid Thruster Experiment (PIX) I I 7- &Pinch Coil (Above), Coil & Pyrex Tube (Below) 5 .. p h a m b e r ,Th eta-Pinch Coil, and Bus (Top) Capacitors, Switch and Bus ~ Capacitors etc. in Oil Tank (Bottom) I Experiment (PTX) SPICE Simulation of PTX CoiVsank Firing of PTX Capacitor Bank C= 640nF t,,4 = 0.6 ms PTX Capacitor bank discharged thru Lefi = 194 nH I,,,, = 53.5 kA 0-pinch coil L,,,, = 35 nH Charge voltage = 32 kV, Peak current = Flea = 60 mi2 28 kA - lower than predicted by SPICE Vcbw = 28 kV 6 .. PTX Diagnostic Tools . . E.x cluded Flux Array In.t erferometry Axial array of flux loops and Heterodyne, quadrature B-dot probes that measures He-Ne laser interferometer . . plasmoid separatrix radius measures line-averaged ne In.t ernal B-dot Probes High-speed Photography for imaging Measure shape and internal magnetic field structure of plasmoids in exhaust . translating plasmoids c.ha mber Quadruple Langmuir . PI-Max gated CCD cameras -P.ro be . Hamamatsu Streak Camera - Takes local measurements of Cordin Camera takes 100 T, and ne as well as plasma Million frames per second flow velocity I 1 Excluded Flux Array Axial array of flux loops and Bdot probes measures rs (separatrix radius) along compact toroid: - F = Bp(r,2 r,2) For a prolate, cyllndrlcally svmmetric compact toroid (FRC), the volume averaged B may be found from: <p> = 1 - r,2 / (2 rc2) I, /@, ’ B-dot pmbes Loops .. I 1 Internal B-dot Probes Internal B-dot Probes: 6 Sets of 2 B-dot Probes (12 total) for measuring B, and B, of translating plasmoid Probes wound with 36 gauge magnet wire - 10 turns, Form Cross-Sectional Area (3.15 mm)* - Internal B-dot Probes holtz Coil I 1 - Helmholtz coil 18 aauae manet wire. Radius = 2.5 In.. Windina W AM^ Left). ComDleted COII (Above Rlahtl I4 12 I4 0.8 4.0 0.4 .. - Internal B-dot Probes Calibration Calibration of Internal B-dot Probe NA's (number of windings area) with Helmholtz Coil - Pulsed Helmholtz Coil with 300 VDC; Measured Helmholtz Coil and 6-dot Probe response with oscilloscope - NAs within -80% of predicted NA of lO'(3.15m m)2=9 .92E-5 m2 Frequency response calibration - Sine wave input to Helmholtz Coil for 1 kHz - 3 MHz n I 1 High-speed Photography Hamamatsu C77W Optical Streak Cordin model 2208 Framing Camera: Camera 8 CCD cameras In circular configuration take up to 100 million Captures high veioclty plasma front frames per second, 8 frames total motion Gate time: 10 ns per frame Camera placed at side of chamber (shown above) and at end of chamber 9 ^. Translating Plasmoid 21 us 22 us 23 us 24 us 25 us 26 us 27 us 26 us Propagating piasmoid in Air at 43 mTorr backfill pressure (view from p side of vacuum chamber); Velocity for this test was 5.3 kds I I 10.0 us 10.5 us 11.0 us 11.5 us 12.5 us 13.0 us Side view of piasmoid propagating into backfilliiken with Cordin 2208 framing camera. Center of image is 20.4 cm downstream of theta-pinch coil. Gas conditions: 6% H, in He, at 95 mTorr. Times indicated are from start of discharge. images are consistent with a piasmoid traveling at hypersonic velocity. n 10 1 .@ Plasmoid Images I I- 2 40 ua 2 45 "* 2 60 w 2 66 "8 3.00 w 3.06 un 3.10~ 3.16~8 3.26 UB II 335- 3 40 urn 345w LIU 3 65 UB 3.60 us 3.66 UB 3.70 UB 3.75 us 3.80 W 3.86 u Time development of plasmoid (looking into throat of &pinch coil). Coil current 53.5 kA. Exposure ti ne 20 ns, time between frames 50 ns. Pressure = 125 mTorr. Gas: 6% H2 in He 1 Plasmoid Images Along Current Trace 40 7.0 0 -20 11 Future Work . . Take additional diagnostic measurements from: . Excluded Flux Array Internal B-dot Probes . Quadruple Langmuir Probe 1 He-Ne Laser Interferometry Measure initial performance characteristics: . Thrust-related measurements: plasmoid mass and velocity 1 . Isp-related measurements: velocity and mass utilization . Detachment of plasmoid from magnetic field lines Radiation losses . Try a variety of propellant gases: He, H, D, Ar, Ne, N Modify PTX: . 0-pinch coil with smaller cone-angle 1 . Quartz tube Crowbar switch Perform detailed modeling using MOQUl n 12