Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma

In research on magnetic fusion energy (MFE) devices, x-ray spectroscopy was developed early on as a powerful diagnostic for measuring fundamental plasma parameters and remains an indispensable tool for determining concentrations of multiple impurity ion species in the plasma core. This capability of x-ray spectroscopy is especially important now that plasma-facing components made of high-Z metals are receiving increased consideration for use in burning plasma environments. Our work aims to apply detector technology developed by the astrophysics community to transform MFE x-ray diagnostics by providing a paradigm shift in spectroscopic capabilities. We have begun work toward adapting state-of-the-art x-ray microcalorimeter detector systems for MFE applications by installing and testing an existing microcalorimeter spectrometer, originally developed for astrophysics, under fusion-relevant plasma conditions at the Madison Symmetric Torus (MST).

Beiersdorfer et al. (2010)¹ introduced the anticipated performance of an x-ray microcalorimeter instrument for ITER. Such an instrument was originally suggested as a complement to the ITER core imaging x-ray crystal spectrometer to mitigate risks, such as those associated with monitoring impurities in only a narrow spectral range as allowed by the crystal spectrometer design.² Here, we expand upon that initial work, and in Sec. II, we present a more detailed introduction to microcalorimeters and their basic operational principles and to the strengths, both scientific and technical, of using microcalorimeters as diagnostics for MFE devices. In Secs. III and IV, we describe the first phase of our diagnostic development work toward proving microcalorimeters for MFE, including preliminary results from our initial measurement campaign at MST. In Sec. V, we present an overview of the second phase of our diagnostic development work that will take place over the next three years, and Sec. VI provides a summary.

X-ray microcalorimeter spectrometers combine the best characteristics of the instrumentation currently available on fusion devices: the high spectral resolution of crystal spectrometers and broadband coverage provided by pulse-height analysis (PHA) systems. These instruments offer multi-keV spectral coverage: 0.1–12 keV is a typical operating range, which can easily be extended to much higher x-ray energies. Microcalorimeters are non-dispersive spectrometers, as opposed to crystal and grating spectrometers, and they have several advantages. For example, they are relatively easy to absolutely calibrate, and they only require a small access port to measure broadband line emission from impurities ranging from ions of Be to W. See Sec. II D for more details.

Originally driven by the needs of x-ray astrophysics missions, several research groups have developed these revolutionary x-ray detectors to enable imaging x-ray spectroscopy with relatively high spectral resolution. Although developed initially for observations from space, we have operated microcalorimeter spectrometers since 2000 on the EBIT-I electron beam ion trap device at Lawrence Livermore National Laboratory (LLNL).^3,4 This work is enabled by an ongoing collaboration between LLNL and the x-ray calorimeter group at NASA’s Goddard Space Flight Center (GSFC). Our collaboration focuses on laboratory astrophysics and instrument testing and calibration. During the past two decades, we have employed GSFC-built calorimeters at the LLNL EBIT to study the x-ray emission from a variety of highly charged ions ranging from B³⁺ to W⁷¹⁺. Our collaboration’s extensive use of calorimeters has in turn provided valuable feedback to the community on how to improve the functionality of these devices. Groups in Japan have also been active in both space- and ground-based calorimeter development and used an early instrument at a MFE device during two test campaigns.^5,6 More recently, a wider variety of x-ray microcalorimeter spectrometers and concepts have been developed for ground-based experiments, including at synchrotron light sources and other large facilities (see Refs. 7 and 8 and references therein).

A microcalorimeter is a type of x-ray detector designed for high energy resolution, consisting of an x-ray absorber, a sensitive thermometer, and a weak link to a thermal bath (Fig. 1, left). The absorber converts the energy of an incoming x-ray to heat, thus raising the temperature of the absorber. Desired properties of the absorber are low heat capacity (to magnify the temperature rise) and high quantum efficiency. After photon absorption, the device cools through the weak link to reset for the next photon. Thermodynamic noise associated with the weak link means microcalorimeters work best at cryogenic temperatures. In addition to photons, microcalorimeter pixels are also sensitive to heat deposited by particles, including neutrons.

Several thermometer technologies currently under development rely on various semiconducting or superconducting material properties, including ion-implanted silicon thermistors and transition-edge sensors (TESs).

Instruments based on silicon thermistors are very mature, stable, near-turn-key systems that are robust against many environmental factors. The silicon thermistor instruments in use at LLNL use micro-machined ion-implanted silicon thermometers and HgTe x-ray absorbers. When a detector pixel absorbs an x-ray, a change in resistance causes a change in the voltage drop across the sensor, and the height of the corresponding voltage pulse is proportional to the energy of the photon. The equation for variable range hopping⁹ describes the resistance in a doped semiconductor at low temperatures, albeit with some deviations reported.¹⁰ The devices have high resistance, and a junction gate field-effect transistor (JFET) reads out each pixel individually. Arrays like those in Fig. 1 (right) use 10 μm-thick HgTe absorber tiles engineered to have low heat capacity, and devices routinely achieve better than 5 eV FWHM (full width at half maximum) energy resolution at 6 keV.¹¹ 

In contrast, instruments based on TESs enable better energy resolution, larger array size, and higher count-rate capability. A TES exploits the narrow superconducting-to-normal-state transition of a thin metal film as an exceedingly sensitive thermometer. A small change in temperature in a TES creates a large change in resistance. TES microcalorimeters typically operate under voltage bias. An absorbed x-ray causes a brief increase in the TES resistance and thus a pulsed decrease in the device current. The height of the current pulse, measured by an inductively coupled SQUID ammeter, is proportional to the energy of the photon. The array in Fig. 2 uses a microns-thick Au/Bi absorber, designed to thermalize the absorbed energy quickly, with a TES made from a Mo/Au proximity-effect bilayer. The bilayers are tuned to a transition temperature of ≈100 mK, which is compatible with standard cryogenic platforms [e.g., adiabatic demagnetization refrigerators (ADRs) or dilution refrigerators].

These high filling-fraction, high quantum-efficiency TES microcalorimeter devices consistently achieve energy resolutions better than 2 eV FWHM at 6 keV.¹² These state-of-the-art results follow years of research in device physics and fabrication techniques by groups throughout the world. For the European-led Athena x-ray observatory (launch in the early 2030s),^13,14 NASA is currently designing arrays with identical pixel performance but higher pixel count (∼3000 pixels) and a chip-size designed to accommodate bond pads for all pixels (3000 pad pairs). These large two-dimensional detector arrays enable larger fields of view for astrophysics applications, whereas, for applications that do not require imaging, such as many ground-based applications including fusion research, the increased pixel count translates to an increase in the total count-rate capability.

For a more detailed introduction to x-ray microcalorimeters, see Refs. 15 and 16.

Because the energy resolution of TES microcalorimeters now rivals that of high-resolution x-ray crystal spectrometers, present-day TES calorimeters can, in principle, already measure the Doppler width of the lines emitted by impurity ions in burning plasmas. For example, the core ion and electron temperatures in ITER are expected to exceed 20 keV. In this case, the linewidth of the K-shell line of helium-like krypton, which had been proposed as an ITER ion temperature diagnostic,¹⁷ is in excess of 10 eV, i.e., more than five times larger than the resolution of current TES detectors. The widths of L-shell tungsten lines also proposed as an ITER ion temperature diagnostic² are also multiple times that of the TES detector resolution.

In fact, calorimeters can, in principle, already be used to infer the core ion temperature of present-day plasmas heated with neutral beams or radio frequency power. Even at a temperature of only 4 keV (routinely achieved with auxiliary heating), the linewidths of the K-shell lines of helium-like iron are still twice as large as the TES detector resolution, and the contribution from Doppler broadening is readily inferred. Note also that this high spectral resolution would be useful during the start-up phase of ITER, when high-power auxiliary heating will not be available and the electron temperature will be below 10 keV. For electron temperatures in the range of 2–10 keV, the line broadening for elements such as Ar and Fe will need to be recorded to provide the ion temperature. Furthermore, even a single line of sight through the plasma by an x-ray calorimeter can provide some information on the radial profile of the ion temperature, stemming from the capability of the calorimeter to simultaneously record emissions from multiple ion species and from the fact that the emissivity for each ion species peaks at a certain electron temperature. Using a separately measured electron temperature profile (e.g., from Thomson scattering), we can determine the approximate location where line radiation from each ion species was emitted. Correlating the ion temperature inferred from each linewidth with the plasma minor radius enables production of an approximate ion temperature profile (see Fig. 6 of Ref. 1).

In addition to the ion temperature and impurity species, a fusion plasma x-ray spectrum contains information about the ion flow velocity (from the Doppler shift of ion emission lines) and electron temperature and density. One of the more successful techniques, exploited in both astrophysical and fusion contexts, has been measurement of line intensity ratios involving forbidden or intercombination transitions.^18,19 In relatively low-density plasmas, such as the solar corona and MFE experiments, the population of excited levels that decay only via forbidden transitions can be high. For each forbidden line, a range of electron densities exists where electron collisions are comparable to the radiative decay rate.²⁰ In this range, changes in the electron density strongly affect the intensity of the forbidden lines. It is this feature that can be exploited as a plasma electron density diagnostic.

In MFE experiments, forbidden transitions are sometimes among the strongest x-ray emission lines, especially for high-Z ions, such as tungsten, because of the scaling properties: forbidden transition probabilities scale as Z⁸ or higher, whereas allowed electric dipole transition probabilities scale as Z⁴.²¹ An example of this situation is the bright unresolved Ni-like W feature observed at 7.93 $Å$ in the ASDEX-U tokamak²² that was resolved into two lines at the LLNL EBIT facility, one from an electric quadrupole transition and one from a magnetic octupole transition.²³ Modeling shows a large change in line ratio over the range of electron densities typical for a tokamak, making this pair of lines a candidate for robust n_e measurements in present-day MFE experiments and in a substantial portion of ITER operational space.

As we have already noted, MFE research has shown renewed interest in x-ray measurements due to the use of molybdenum and tungsten as plasma-facing components and the associated issues with transport and confinement of these and other metals in all regions of the plasma, from the divertor region to the plasma core. For example, a crucial area of fusion research is understanding energy and particle losses from the core plasma due to radiation, radial transport, and various instabilities. Core impurity accumulation studies are of special importance in this context, as even small amounts of a high-Z impurity in the core can dramatically degrade plasma confinement. X-ray measurements are needed to quantify the influx and accumulation of impurities in the core plasma and, thus, to understand the impact of plasma–surface interactions with tungsten, molybdenum, or other plasma-facing components on the core plasma. Because calorimeters simultaneously record x rays from all impurities with Z ≥ 4 and with high resolution limited mainly by the plasma Doppler motion, they offer an unprecedented opportunity to absolutely quantify species-resolved impurity concentrations in the plasma as a function of time and provide much-needed data for transport simulations. Bringing x-ray diagnostics into the x-ray calorimeter age is timely to properly support the utilization of high-Z plasma-facing materials in the progression toward producing burning plasmas.

Microcalorimeters are non-dispersive spectrometers, as opposed to crystal and grating spectrometers, a characteristic that provides several advantages: the energy resolution does not degrade with the source size and the instruments are easy to align, have high quantum efficiency, and are straightforward to calibrate. Another key advantage for future MFE applications is their small port-access requirement. Such spectrometers will only require an ∼25 mm diameter radial sightline through the core of a high-temperature fusion plasma, and the instrument itself can be placed tens of meters away.¹ The small plasma viewing window means microcalorimeters offer orders of magnitude lower neutron hazard compared to crystal spectrometer designs (e.g., see Fig. 1 of Ref. 1). The microcalorimeter dewar and electronics rack can be placed far from the plasma vessel and behind significant shielding, and the instrument itself would be serviceable. For example, Fig. 3 shows a simple model of our calorimeter dewar installed at MST. In this case, the dewar is only 4 m from the plasma vessel, but that distance could be significantly increased in larger fusion devices, and would likely only be limited by space constraints (e.g., the size of experiment hall or building). This configuration also lends to easily adding a microcalorimeter to a small viewport auxiliary to a crystal spectrometer (or other) port on a large fusion experiment.

Finally, the x-ray microcalorimeter spectrometer represents a type of passive measurement technique that may be critical for the next generation of fusion devices, such as ITER’s successor, the DEMOnstration power plant (DEMO). Port access for diagnostics will be extremely limited in these devices, and techniques that rely on the injection of particle or photon beams into the plasma (such as interferometry, Thomson scattering, and charge-exchange recombination spectroscopy) will be extremely challenging or impossible.^24,25 Thus, the diagnostic challenge will shift to a need to implement passive diagnostics that measure plasma parameters by recording emission from the plasma, such as the x rays produced by impurities. Developing microcalorimeter spectrometers for MFE devices directly addresses this diagnostic challenge.

In 2017, we began our program to implement and test x-ray microcalorimeter spectrometry in an MFE experiment, to address challenges in adapting such instruments to the MFE pulsed-plasma environment. Our original intent was to install an instrument on NSTX-U or DIII-D; however, construction and upgrade efforts at these facilities reduced the availability of these devices for development of x-ray calorimetry. Instead, we established a strong collaboration with University of Wisconsin–Madison researchers and aimed to perform tests at MST. In this section, we describe our work that resulted in deploying and operating a microcalorimeter spectrometer at MST in 2019.

MST is a toroidal magnetic plasma confinement device typically operated as a reversed-field pinch (RFP).²⁶ MST is a flexible experiment facility able to produce fusion-relevant plasma conditions with electron temperatures up to 2 keV at an electron density of 10¹⁹ m⁻³. The major and minor radii of the device are 1.5 and 0.5 m. The typical fuel gas is deuterium, and a variety of noble gases can be introduced as impurity dopants. Graphite and aluminum are the main plasma-facing surfaces, and thus, C and Al are the dominant impurities. Large amounts of data can be collected over a variety of plasma conditions, as the equilibrium flat-top period of each plasma discharge is 10–30 ms long and ∼100 discharges are produced in a typical run day. The daily run schedule can be very flexible, and, for example, researchers can gain access to the experiment floor between shots.

The microcalorimeter instrument we chose to deploy from LLNL to MST for our initial measurement campaign has a 32-pixel detector array based on silicon thermistors. We used this instrument, known as the XRS/EBITv2 (XRS for short), as a facility instrument at the Livermore EBIT-I until 2007,²⁷ and in 2011 and 2012 for campaigns at the Linac Coherent Light Source (LCLS) x-ray-free electron laser at SLAC.

The XRS has 625 × 625 × 8 μm³ HgTe absorbers, each mounted on a doped silicon thermistor. Each pixel is independently wired. We typically operate the detector with a heatsink temperature of ≈60 mK, achieved using an adiabatic demagnetization refrigerator (ADR) installed in an IR Labs dewar. The system requires a pumped liquid helium bath, as well as a liquid nitrogen shield. The detector views the source out of the bottom (long end) of the dewar through four aluminized polyimide infrared blocking filters, which have been photometrically calibrated. The quantum efficiency of the HgTe absorbers was also calibrated prior to instrument assembly.

The XRS is operated using the same software and duplicates of the hardware developed for space flight instruments. The main components are the calorimeter analog processor (CAP), computers used for digital processing of the photon pulses, and computer-controlled housekeeping electronics to read out and control thermometers and the ADR. The system is fully automated and capable of real-time processing of x-ray spectra.

In spring 2019, pre-ship tasks were completed at LLNL, including re-assembly of the electronics, testing of the assembled instrument, and calibration using an ⁵⁵Fe radioactive source and the LLNL EBIT-I. The x-ray emission from mainly He-like and H-like ions created using EBIT-I is ideal for calibrating the energy gain scale and line-spread function of the instrument. Figure 4 illustrates the high spectral resolution of the XRS, showing a best-fit combined resolution of 5.35 eV FWHM at 5.9 keV, with per-pixel resolutions ranging from 4.7 to 6.3 eV.

Other pre-ship work included selection of a port at MST and design of an instrument stand, external filter stack, and beam–tube assembly needed to interface with MST. Accelerometer data taken during MST discharges have confirmed substantial vibration at acoustic frequencies produced by the transformer components and partially transferred to the vacuum vessel and mounting structure. Therefore, vibration isolation was a priority in the mechanical design of the pillar assembly upon which the XRS is mounted. The beam–tube assembly incorporated a series of filters to allow definition of the spectral waveband and apertures to provide an energy-independent reduction in x-ray flux.

Installation of the XRS at MST took place in summer 2019. The beam–tube assembly and instrument stand shipped to MST in June 2019 and were installed and attached to a facility vibration isolation pillar. In July, the calorimeter instrument shipped to MST, and following installation underwent a post-shipment checkout using a radioactive calibration source. The heatsink temperature stability in this post-shipment checkout was better than that at LLNL, indicative of the good vibration isolation provided by the pillar assembly. Figures 5 and 6 illustrate this implementation of the XRS at MST. The measured instrument spectral resolution was slightly degraded compared to the tests at LLNL (Fig. 4), with a combined spectral resolution of 6.5 eV FWHM at 5.9 keV and per-pixel resolutions ranging from 4.9 to 8.1 eV. This change is due to two factors: increased 60 Hz noise and a low calibration-source flux that limited our ability to track and correct small temporal changes in the gain. Both of these factors are straightforward to address, and we expect this small degradation will be eliminated in the next experimental campaign.

First-light experiments during MST discharges took place in August 2019, during which the XRS recorded x-ray spectra of photons emitted by Al impurity ions. This first-light campaign employed a range of MST plasma conditions, combined with various beam tube filters and apertures, to achieve specific count rates. For most of the acquisition sequences, the combination of plasma conditions and optical path was targeted to achieve ∼1 ct pixel⁻¹ shot⁻¹ to ensure the data had the highest achievable spectral resolution. The tests incorporated the MST trigger signal into the microcalorimeter data acquisition system and analysis confirmed that each photon arrival was time-tagged relative to the MST discharge trigger with an uncertainty of <1 ms and that the x-ray signal appeared when expected relative to the discharge profile (see Fig. 7). Most microcalorimeter systems in existence do not use an external timing trigger (e.g., for astronomy or many beamline applications); however, the experience using spectrometers equipped with an external trigger at the LLNL EBIT proved invaluable in quickly achieving these results.

These data are informative regarding how the electromagnetic and acoustic noise of an MFE experiment affects the instrument performance. The raw data showed evidence of interference (likely electrical, possibly magnetic) consistent with the “sawtooth” features routinely observed during MST discharges due to magnetic reconnection events; however, the interference signal was small compared to the x-ray pulse signal and had minimal overlap with our signal bandpass and thus has limited effect on the spectral resolution. In addition to electrical or magnetic pickup, resolution can be degraded by fluctuations in the detector heatsink temperature, which can be thought of roughly as causing time-dependent fluctuations in each pixel’s gain. The heatsink temperature of the XRS, which we controlled at 58.50 mK, showed only small (<0.05 mK) disruptions during the discharge owing to the good vibration isolation provided by the pillar assembly.

The number of photons in the example spectra (Fig. 8) is limited because of the short initial data run. However, even given the low number of counts and degraded resolution due to small gain uncertainties, the calorimeter spectra allow initial probes of physical mechanisms. For example, comparison to calculations shows that the spectra have much lower charge balances than would naively be expected. The spectrum of the 400 eV plasma (blue curve) likely has significant contributions from lower charge states of Al than the model (black curve)—Be-like and B-like Al features have peaks at 1560 and 1545 eV, and the pulsed parallel current drive (PPCD) spectrum (green curve) lacks the strong hydrogen-like Al emission lines present in similar models of 600 eV plasma. These differences are related to charge exchange with neutrals, which is a significant effect at MST: Fig. 9 shows an example of the difference in the modeled ionization-fraction balance at MST when incorporating charge exchange. MST does not have a divertor so the effect is heightened, but even a small neutral population can affect plasma and can be relevant at many MFE devices. Microcalorimeter spectra can quickly assess charge balance for impurity ions across the waveband of interest.

Considering continued construction and upgrade efforts at both DIII-D and NSTX-U and advantages we have found in the flexibility afforded by deploying our instruments to a mid-size physics experiment, we plan to continue developing x-ray calorimetry for application as a fusion diagnostic at MST over three upcoming campaigns, one per year. The first two campaigns will use the XRS that is currently deployed at MST. For the final campaign, once we have fully assessed the operating environment of the calorimeter and implemented required changes, we will deploy a TES-based spectrometer, as such an instrument is more suitable for the high-count rates produced at MFE facilities. This work includes verifying our neutron mitigation scheme that will be needed for use on larger fusion experiments. Completion of our future work will resolve critical issues and establish a path for implementation on fusion facilities, including ITER.

Similar to x rays, incident neutrons can cause a rise in the temperature of the microcalorimeter absorbers, which may result in contamination of the x-ray signals. In addition, neutrons impinging on the detector substrate could cause unwanted noise or low-level signals in one or more pixels. To prevent undue heating of the array by neutrons, the x-ray flux from the plasma must be deflected by a grazing incidence mirror. For installation of a microcalorimeter on a burning plasma device, we plan to arrange the geometry of the mirror such that the deflected x-ray flux proceeds to the detector, while shielding material absorbs the undeflected neutron flux (Fig. 10). The mirror will deflect the incoming x rays by about 0.5°. To test this method of neutron protection on MST, we will install a grazing incidence mirror in the x-ray calorimeter flight tube. This deflection means that, over a 4 m distance, the incoming neutron beam (defined by the initial collimating aperture before being bent by the grazing-incidence mirror) will be separated from the x-ray beam by about 3.5 cm. This separation should be sufficient to keep unscattered neutrons from hitting the microcalorimeter array.

Operated as an RFP, MST is an extremely flexible source of D–D neutrons (2.5 MeV). At low plasma currents, the neutron flux is below the scintillator detector limit. At high plasma currents, the neutron production reaches 10¹¹ s⁻¹ immediately following magnetic reconnection events. The reconnection events produce a suprathermal deuterium population.³² The D–D fusion cross section is extremely sensitive to ion energy, so a small number of fast ions can contribute more to the total neutron flux than the entire thermal population, and, in fact, the fast ions completely dominate neutron production in standard MST RFP plasmas. In addition to standard RFP operation, we can operate MST in a high-confinement mode into which we can inject a 1 MW deuterium neutral beam. This mode of operation can produce neutrons at a rate of up to 10¹² s⁻¹ (integrated over the entire plasma and into all directions). At this neutron rate, approximately ten neutrons would impact each pixel of the unshielded XRS microcalorimeter detector during each MST discharge, which is more than sufficient to test the neutron mitigation method described above. Standard high-current RFP discharges with a peak neutron rate of 10¹¹ s⁻¹ are probably more suitable for these tests, since approximately one neutron will impact each pixel of the XRS during a discharge.

Although our upcoming experimental work will take place at MST, we plan to continue our ongoing discussions with diagnostics teams at DIII-D and NSTX-U and to begin preparing for installation on such facilities in the near future, based on lessons learned from the measurement campaigns on MST. While our research gains tremendously from using existing spectrometers developed primarily by NASA for space missions and often using spare or prototype detector components, several aspects of the detector and system design should be optimized specifically for MFE diagnostic requirements. We plan to lay out a conceptual design for a system appropriate for ITER or for an experiment such as JT-60SA. Building such an instrument for fusion application will leverage the decades of development by NASA and other agencies. However, building this instrument will require (at least) a dedicated design and fabrication run. For ground-based instrumentation, as compared to space-based instruments, many requirements can be relaxed, allowing much greater design flexibility, particularly in the array (fill fraction) and readout (multiplex factor) architecture. A combination of increased array size (number of pixels) and decreased detector pulse time constant allows a higher counting rate but taxes the multiplier electronics. We plan to study such tradeoffs and may choose a modular detector assembly, possibly with separate arrays optimized for different x-ray energy ranges, coupled to a microwave SQUID multiplexer.³³ We could reasonably implement these alterations in a short time, since, compared to space missions, aspects such as instrument volume, mass, and power needs are not constrained.

In this paper we provide an introduction to high-resolution x-ray microcalorimeter spectrometers and scientific and technical advantages of using these instruments as x-ray diagnostics for burning plasmas. We present our experimental diagnostic development work to date, having deployed and operated a microcalorimeter spectrometer at MST, as well as our plans for development over the next few years. This diagnostic development is consistent with the needs of the fusion and plasma science community as highlighted during the recent plasma physics community planning process (CPP). The cross-cut measurement and diagnostics section of the CPP report³⁴ describes the need for high-resolution x-ray spectroscopy of high-Z elements: “High-resolution x-ray imaging spectroscopy of impurities is critical …. The needed instrumentation advancements include improved spectral resolution, broader spectral coverage, and higher time resolution, ideally with small port access requirements and small equipment footprint.”

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract No. DE-AC52-07NA27344. The experiments were conducted at the Wisconsin Plasma Physics Laboratory (WiPPL), a research facility supported by the DOE Office of Fusion Energy Sciences under Contract No. DE-SC0018266 with major facility instrumentation developed with support from the National Science Foundation under Award No. PHY 0923258.

The data that support the findings of this study are available from the corresponding author upon reasonable request.