Portable Diagnostic Package for Thomson scattering and Optical Emission Spectroscopy on Princeton Field-reversed Configuration 2 (PFRC 2) N. Kafle, D. Elliott, B. Berlinger, Z. He, S. Cohen, Z. Zhang, and T. M. Biewer 1) Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 2) Princeton Plasma Physics Laboratory, Princeton, NJ 08543, USA 3) University of Tennessee-Knoxville, Knoxville, TN 37996, USA (*Author to whom correspondence should be addressed: [email protected]) (Dated: 26 September 2022) An Advanced Research Projects Agency-Energy (ARPA-E) funded diagnostic system has been deployed to the Princeton field-reversed configuration 2 (PFRC-2) device, located at Princeton Plasma Physics Laboratory. The Portable Diagnostic Package (PDP), designed at Oak Ridge National Laboratory, allows measurement of Thomson Scattering (TS) for electron density and temperature and Optical Emission Spectroscopy (OES) for ion temperature, impurity density, and ion velocity. A tunable spectrometer on the PDP with three gratings provides the flexibility to measure low (1 eV) and high (1000 eV) electron temperature ranges from TS. Additionally, using a second spectrometer, the OES diagnostic can survey light emission from various ion excitation levels for wide wavelength ranges. The electron density (< 2 × 10^19 m^-3) of plasmas generated in PFRC-2 has been below the PDP TS discrimination threshold, which has made TS signal detection challenging against a high-background of laser stray light. The laser stray light was iteratively reduced by making modifications to the entrance and exit geometry on PFRC-2. Rayleigh scattering experiments on PFRC have yielded the TS discrimination sensitivity to be > 1 × 10^20 m^-3 for the PDP. A recently implemented narrow-band notch spectral filter that masks the second harmonic 532 nm Nd:YAG laser wavelength has increased the system’s TS light discrimination sensitivity 65 times compared to the instance when the notch-filter was not implemented. The hardware implementation including design changes to the flight tubes and Brewster windows will be discussed, along with results from the Rayleigh and Rotational Raman scattering sensitivity analyses, which were used to establish a quantitative figure of merit on the system performance. The Raman scattering calibration with the notch-filter has improved the PDP electron density threshold to 1 ± 0.5 × 10^18 m^-3. Thomson scattering (TS) as a plasma diagnostic tool was developed in the late 1960s and has been a benchmark diagnostic technique measurement of electron temperature (Te), electron density (ne), and in some cases the entire electron velocity distribution.1–8 The Portable Diagnostic Package (PDP)9 has been designed as a traveling diagnostic set consisting of TS along with optical emission spectroscopy (OES). The PDP is designed to measure Te, ne, ion temperature (Ti), and ion density (ni). The PDP’s performance was tested on the Electrothermal-Arc (ET-Arc) source at the Oak Ridge National Laboratory (ORNL) before its deployment.10 Helium emissions spectra from OES provided the He I line ratios for a collisional radiative model (CRM) and He II emission for Ti measurement. Rotational Raman Scattering (RRS) and Rayleigh Scattering (RS) calibrations for TS provided Te and ne. After commissioning on the ET-Arc, the PDP was deployed to the Princeton Field Reverse Configuration 2 (PFRC-2) device located at the Princeton Plasma Physics Laboratory (PPPL). PFRC 211,12 is a second generation of the FRC device designed at PPPL. FRC devices are quasi-linear in geometry and have high β (β ≡ plasma pressure/magnetic field pressure). High β will allow use of aneutronic fuel such as D−3He rather than conventional D−T fuel.12–14 The PFRC-2 vacuum chamber is made out of LEXAN poly-carbonate and has 22.71 cm inner diameter and 83.82 cm axial length. A helicon antenna produces a seed plasma before a rotating magnetic field (RMF) is applied. The helicon plasma can be pulsed or operate at a steady state, but the pulsed RMF nominally produces 4 ms long discharges between 30- and 40-kW power. Most of the commercial-off-the-shelf (COTS) components of the PDP are modular and portable which are easy to procure and quick to deliver. The components can also be setup promptly. The PDP largely consists of a Lumibird Q-SMART 1500 (10 Hz, 1500 J at 1064 nm and 850 J at 532 nm) laser, two Teledyne Princeton Instrument Isoplane SCT320 tunable spectrometers, each with three gratings, two PI-MAX 4-1024f iCCD cameras, Nikon 50 mm f/1.8 lens, 3×11 800 µm fiber optics bundle, and a Gentec pyroelectric energy detector. Using these components (that are modular, have smaller footprint, and are quick to setup) has greatly enabled the PDP’s portability. After the commissioning phase, the PDP was setup at PFRC-2 at Princeton Plasma Physics Laboratory (PPPL) within a week which included loading (at ORNL), transportation, unloading (at PPPL), and reassembly of the system. The Q-SMART 1500 laser and its power supply are physically smaller compared to other TS legacy laser systems. Moreover, with most of the PDP’s components in the diagnostic cart9, it can be placed closer to the plasma device without taking significant lab space. The following sections of the paper will discuss the PDP’s integration on PFRC-2, the RS calibration measurement for threshold electron density estimation without discriminating the central laser wavelength, the use of Volume Bragg gratings (VBG) notch filter to reject the stray light, and the electron density threshold estimation using RRS calibration with the VBG notch filter. Finally, the paper will summarize the findings, provide potential optimization options and discuss the future work. The schematic of the PDP on PFRC-2 is shown in Fig. 1. The 532 nm Nd:YAG is routed upward using 532 nm laser graded coated mirrors from the optical table. The laser enters the vacuum chamber at a 45° angle to the collection optics. The laser traverses across the diameter of PFRC-2. A half wave-plate is placed past mirror 3 (M3) to rotate the laser polarization 90° from vertical to horizontal. Right before steering the beam into the vacuum vessel, a 750 mm plano-convex, focusing lens is used to focus the laser beam at the geometric center of the chamber. Two UV Fused Silica Brewster-angled windows at the entrance and exit of the laser beam create the air-vacuum interface. After the exit, another mirror directs the laser beam at 90° towards the laser beam dump. The laser energy meter was located within the beam dump. The information from the laser energy measurements are used for both signal and stray light normalization during analysis. The total length of the laser path from the laser to the machine axis is ~2.25 m and ~1.95 m from the machine axis to the laser beam dump. The BraggGate VBG notch filter is placed in front of only one spectrometer on the top-shelf. Of the 33 available optics fibers from the collection bundle, 11 intersect the laser beam path, and the remaining 22 can be used for the OES diagnostics. Eight out of these eleven intersecting views lie above the machine axis, one intersects the machine axis, and the remaining two views lie below the machine axis. The spatial resolution for the TS and OES diagnostics differ because of the 45° angle between the laser path and the collection optics. The total measurement length for TS along the laser path is ~7.9 cm, and the tangency radii for OES spans ~5.6 cm for OES, as shown in Fig. 1. Synchronization of the PDP laser pulses with the PFRC-2 discharges was difficult due to the differences of scale in all of the timing. A direct 1 Hz trigger from PFRC-2 to the 10 Hz laser was problematic as it would mean that 90% of the data collected would be stray light and noise. The Q-SMART 1500 Laser used as part of the PDP has a repetition rate of between 9.9−10.1 Hz, yet the laser pulse is only 10 ns long. Moreover, the laser must operate for 10−20 min in order to stabilize in energy and it was not possible to trigger the PFRC-2 with the laser’s internal timing. To ensure that the laser was able to perform optimally and synchronize both systems, a function generator was used to send 10 triggers to the laser at 10.01 Hz. The burst of 10 triggers beginning with every 1 Hz trigger from the PFRC-2 experiment maintained the laser operations in its optimal performance range, as well as kept it synchronized with the PFRC-2 discharge pulse. Additionally, an “AND circuit” was created so that the intensified camera could be triggered based on the Q-switch of the laser only when the plasma discharge was underway. Figure 2a shows the trigger synchronization logic between the TS camera, laser, and PFRC-2 discharge including the use of the function generator for the laser flash lamp input. This allowed for < 50 ns of jitter between the laser timing and the intensified camera timing. Ultimately this led to an intensified camera integration of 35 − 75 ns. Ideally the intensifier on the camera would be perfectly synchronized with the laser and have its width match that of the laser pulse, so the highest fraction of the collected light would then be from the scattered laser light. Figure 2b represents the trigger sequence during plasma discharges. A. Threshold from RS in initial PDP configuration Rayleigh scattering calibration15,16 is one of the most common electron density calibration techniques used for TS diagnostics. RS calibration experiments were conducted to identify the electron density discrimination threshold of the PDP for TS measurement on PFRC-2. Since the RS photons are collected close to the laser wavelength, presence of larger stray light counts influence the instrument’s sensitivity calibration. In the initial installation the entrance and exit apertures on PFRC-2 were ~12.4 mm. Assuming the singular Gaussian mode, the beam diameter for the laser was about 9±2 mm, which was roughly the size of the aperture. The Gaussian tail which potentially carried 0.4% of laser beam might have been interacting with the entrance aperture and scattering into the vessel. A vacuum vent was conducted, where the aperture size was increased to 25 mm (3× beam diameter), as well as an in-vessel view dump was added opposite to the collection optics. With all the above stray light mitigation techniques, the RS calibration measured the ne discrimination threshold of > 1×10^20 m^-3, which was above the PFRC-2 expected electron density (< 2×10^19 m^-3) for any discernible TS signal from the FRC plasmas. One of the limiting factors in the stray light reduction at PFRC-2 has been the clear vacuum chamber which allowed the light to exit the vessel from all surfaces and potentially scatter back into the collection optics. A 3D printed mask was custom-made and designed to replace the PI-MAX 4 camera mounting plate. A 1mm wide mask on the mounting plate at the central pixels would block the laser wavelength. However, the mask did not block the central wavelength as intended. The mask was designed to be normal to the entrance aperture of the camera, but the dispersed light from the spectrometer was likely entering the camera at a non-normal angle. It was empirically estimated to have an offset of ~15−20° to the left of the detector. Hence, this technique was rejected as a solution. B. VBG notch-filter implementation Based on success elsewhere, a narrow-band BraggGate VBG notch-filter has been implemented to attenuate the laser stray light intensity.1 The transmission efficiency of the filter is specified to be at 95% and the attenuation at 99.99%. Two 40 mm focusing lenses are used on either side of the notch-filter to collimate the light from the fiber bundle. Figure 3 shows the optical layout of the notch-filter, which is arranged on the top-shelf of the PDP cart. Figure 4 shows an image of the white-light intensity source on the detector with the notch-filter, where alternate fibers (6 out of 11) were illuminated. The VBG notch-filter nominally blocks 532 nm wavelength light which produces the dark band of pixels with reduced intensity. However, the attenuated intensity region is not vertically aligned across the detector but tends to bow inwards away from the center fiber channel. The VBG notch filter blocks light according to a specific physical geometry. Light of a specific wavelength, which is incident on the filter at a specific angle, satisfies the Bragg condition for destructive interference, and is not transmitted. In our geometry, imperfectly collimated light originating from fibers that are off-optical-axis, encounter the filter at a slight angle, consequently the Bragg condition is satisfied for light that is at a slightly different wavelength. This manifests itself as a blocking wavelength that is curved in wavelength space for light rays that are increasing with distance from the optical axis. Consequently, in the current setup only one out of eleven fiber channel, near the center of the detector, that efficiently blocks the 532 nm laser stray light. This reduces the PDP’s spatial resolution for the TS measurements from eleven channels to one channel for a single plasma discharge. A single channel TS measurement is still quite significant for PFRC-2 experiment. A radial profile will be conducted by changing the fibers between plasma discharges. Figure 5 shows the effectiveness of the spectral notch filter in the stray light attenuation on one fiber channel. Figure 5 (top and bottom respectively) compares the stray light intensity without the notch-filter and with the notch-filter in place. Adding the VBG notch filter has reduced the stray light intensity ~65×, as indicated by the counts in Fig. 5. The attenuation is much less than the vendor specified OD 4 reduction, which gives opportunity for further improvement of the system. Replacing 40 mm L1 and L2 lenses (shown on Fig. 3) with a longer focal length (~100 mm) lens will improve the collimation, which should reduce the stray light intensity to the vendor specified values. C. Threshold from RRS in VBG notch-filter PDP configuration The VBG notch-filter affects the RS calibration, as it blocks the laser wavelength, where the majority of the RS scattered light gets collected. The Rotational Raman Scattering calibration is another commonly used method to calibrate the density threshold for Thomson scattering1,17,18. RRS spectral peaks represent the transition from one rotational state to another where Stokes (higher energy state) and anti-Stokes (lower energy state) peaks are obtained on the right and the left respectively of the excitation wavelength. RRS spectra (IRRS) were measured around the 532 nm laser line for nitrogen gas at six different pressure points (350, 240, 180, 20, 10 and 2.4×10^-6 Torr), as shown in Fig. 6. The gray line is the background signal (Ibackground), which is equivalent to the laser stray light near vacuum. The data shows the total counts obtained from 6000 laser pulses at an average energy of 440±10 mJ at each N2 pressure. The plotted data shows the RRS signal after subtracting the laser background (i.e. IRRS = Iraw_signal − Ibackground). The increase in the intensity at the 532 nm wavelength at higher N2 pressures can be attributed to the RS photons. This is due to imperfect attenuation by the VBG notch-filter at the laser wavelength. Although not shown in Fig. 6, RRS signal was obtained until 3.3 Torr. Using this pressure, the discrimination ne threshold for the TS measurement is calculated to be 1±0.5×10^18 m^-3. The VBG notch-filter has improved the PDP’s electron density threshold below the estimated electron density in PFRC 2 (< 2×10^19 m^-3), thus making the TS measurement likely during RMF discharges. This paper presented the deployment and installation of the PDP on PFRC-2. Lessons learned from the project will enable progress in the second iteration of the system. The project encountered several issues associated with high laser stray light, but several solutions have been implemented to improve the PDP’s performance. The implementation of VBG spectral notch-filter that blocks the 532 nm light has been the most effective solution in reducing the laser stray light by 65×; hence improving the TS sensitivity threshold of the PDP. However, the performance of VBG can be further improved by using longer focal length lenses to improve the collimation. Longer image distance may also improve the fiber optics pointing into the notch-filter, which will increase the spatial resolution. Additionally, the effect of a close-packed fiber bundle with a smaller diameter for a signal spatial point can be studied to improve the throughput of the scattered photons. These optimizations should increase the performance of PDP. The PFRC-2 system upgrade has left the device inoperative for the time being. Future work may include an attempt at Thomson scattering measurements in RMF plasmas. While the focus so far has been on achieving the TS measurement on PFRC-2, using the OES diagnostic hydrogen lines emission (Hα, Hβ, and Hγ) spectra has also been collected. The hydrogen Balmer series can be used to calculate Te and ne from the CRM model for hydrogen plasmas. Additionally, Kr II lines collected from the high-resolution OES during the RMF discharge can be used to interpret plasma characteristics. Further analyses of these data is required in the future. ACKNOWLEDGEMENT This paper has been authored in part by UT-Battelle, LLC, under contract with the U.S. DOE. The U.S. government retains and the publisher, by accepting the article for publication, acknowledges that the U.S. government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for U.S. government purposes. DOE will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). This project was supported by the U.S. Department of Energy, Contract No. D.E.-AC05-00OR22725. The authors thank the Program in Plasma Science and Technology for support under DOE Contract No. D.E.-AC02-09CH11466. The authors also greatly appreciate the funding support provided by the Advanced Research Projects Agency-Energy (ARPA-E). DATA AVAILABILITY The data that support the findings of this study are available from the corresponding author upon reasonable request. REFERENCES 1 B. Vincent, S. Tsikata, S. Mazouffre, T. Minea, and J. Fils, “A compact new incoherent Thomson scattering diagnostic for low-temperature plasma studies,” Plasma Sources Science and Technology 27, 055002 (2018). 2 B. L. Klarenaar, O. Guaitella, R. Engeln, and A. Sobota, “How dielectric, metallic and liquid targets influence the evolution of electron properties in a pulsed He jet measured by Thomson and Raman scattering,” Plasma Sources Science and Technology 27, 85004 (2018). 3 R. Agnello, Y. Andrebe, H. Arnichand, P. Blanchard, T. De Kerchove, I. Furno, A. Howling, R. Jacquier, and A. Sublet, “Application of thomson scattering to helicon plasma sources,” Journal of Plasma Physics 86 (2020). 4 T. Carlstrom, F. Glass, D. Du, A. McLean, D. Taussig, and R. Boivin, “Thomson scattering measurements on DIII-D using in-vessel laser mirrors and lenses to diagnose a new divertor location,” Review of Scientific Instruments 89, 10C111 (2018). 5 P. Shi, P. Srivastav, C. Beatty, R. S. Nirwan, and E. E. Scime, “Incoherent thomson scattering system for phase space mapping (phasma) experiment,” Review of Scientific Instruments 92, 033102 (2021). 6 A. Milder, J. Katz, R. Boni, J. Palastro, M. Sherlock, W. Rozmus, and D. Froula, “Statistical analysis of non-maxwellian electron distribution functions measured with angularly resolved thomson scattering,” Physics of Plasmas 28, 082102 (2021). 7 B. Vincent, S. Tsikata, and S. Mazouffre, “Incoherent Thomson scattering measurements of electron properties in a conventional and magnetically-shielded Hall thruster,” Plasma Sources Science and Technology 29, 35015 (2020). 8 H. Tojo, R. Pasqualotto, A. Fassina, L. Giudicotti, H. Sasao, H. Homma, and N. Oyama, “Design of JT-60SA core Thomson scattering diagnostic system,” Review of Scientific Instruments 92, 043556 (2021). 9 N. Kafle, D. Elliott, E. W. Garren, Z. He, T. E. Gebhart, Z. Zhang, and T. M. Biewer, “Design and implementation of a portable diagnostic system for Thomson scattering and optical emission spectroscopy measurements,” Review of Scientific Instruments 92, 063002 (2021). 10 Z. He, N. Kafle, T. E. Gebhart, D. Elliott, H. B. Flynn, Z. Zhang, and T. M. Biewer, “Multi-channel Thomson Scattering Measurements on an Electrothermal Arc Source,” Review of Scientific Instruments, submitted (2022). 11 S. Cohen, B. Berlinger, A. Brooks, C. Brunkhorst, M. Edwards, J. Gumbas, and C. Myers, “Construction of the PFRC-2 device,” in American Physical Society, 53rd Annual Meeting of the APS Division of Plasma Physics (2011). 12 S. Cohen, C. Brunkhorst, A. Glasser, A. Landsman, and D. Welch, “RF plasma heating in the PFRC-2 device: Motivation, goals and methods,” AIP Conference Proceedings 1406, 273–276 (2011). 13 S. A. Cohen and A. H. Glasser, “Ion heating in the field-reversed configuration by rotating magnetic fields near the ion-cyclotron resonance,” Physical Review Letters 85, 5114–5117 (2000). 14 S. A. Cohen, B. Berlinger, C. Brunkhorst, A. Brooks, N. Ferraro, D. P. Lundberg, A. Roach, and A. H. Glasser, “Formation of collisionless high-β plasmas by odd-parity rotating magnetic fields,” Physical Review Letters 98 (2007), 10.1103/PhysRevLett.98.145002. 15 K. Muraoka, K. Uchino, and M. Bowden, “Diagnostics of low-density glow discharge plasmas using thomson scattering,” Plasma physics and controlled fusion 40, 1221 (1998). 16 K. Warner and G. M. Hieftje, “Thomson scattering from analytical plasmas,” Spectrochimica Acta Part B: Atomic Spectroscopy 57, 201–241 (2002). 17 J. Barrett, “Generation of coherent anti-stokes rotational raman radiation in hydrogen gas,” Applied Physics Letters 29, 722–724 (1976). 18 B. LeBlanc, “Thomson scattering density calibration by rayleigh and rotational raman scattering on NSTX,” Review of Scientific Instruments 79, 10E737 (2008).