Work at WiPPL will be featured at 2020 High-Temperature Plasma Diagnostics Conference
Two projects on MST have been selected for invited talks at the High-Temperature Plasma Diagnostics conference in May 2020:
Patrick VanMeter, University of Wisconsin–Madison
Robust analysis of space-, time-, and energy-resolved soft x-ray measurements of magnetically confined fusion plasmas
A new concept for a multi-energy soft x-ray (ME-SXR) diagnostic is being developed by a PPPL/WiPPL collaboration on the MST facility. This diagnostic is based on a custom calibration of a solid-state detector consisting of a 2D array of ∼100,000 pixels. The lower photon absorption cutoff energy can be independently set for each pixel. Results from MST illustrate how the detector can be used to study the evolution of thermal and impurity density profiles. Similar diagnostics will soon be installed on NSTX-U and WEST, including a hard x-ray variant based on a CdTe sensor. This diagnostic may eventually be applied to ITER, where small, non-perturbative diagnostics will be necessary for routine steady state operation.
Megan Eckart, Lawrence Livermore National Laboratory
Microcalorimeter measurement of x-ray spectra from a high-temperature magnetically confined plasma
The LLNL/NASA x-ray microcalorimeter spectrometer installed on the MST facility has recorded x-ray photons emitted by aluminum impurity ions in a deuterium plasma. X-ray microcalorimeter development has been driven by space applications, where they have been used to make detailed measurements of astrophysical x-ray sources. The goal of this project is to adapt microcalorimeters for magnetic fusion energy research, and demonstrate the value of such measurements for fusion science. Microcalorimeter spectrometers combine the best characteristics of the x-ray instrumentation currently available on fusion devices: high spectral resolution similar to an x-ray crystal spectrometer, and the broadband coverage (0.1–12 keV) of an x-ray pulse height analysis system.
High-resolution x-ray microcalorimeter on MST
The effort to develop fusion for energy generation has been described as “bringing a star to earth.” Recently, a type of instrument similar to those being used to study the emission from hot, extraterrestrial plasma, such as from stars, black holes, and supernova remnants, has been installed on the MST experiment at UW-Madison to study hot terrestrial plasma.
This instrument, called an x-ray microcalorimeter spectrometer was commissioned on MST by a team from Lawrence Livermore National Laboratory, Wisconsin Plasma Physics Laboratory, and NASA/Goddard Space Flight Center. The first run campaign with this instrument on MST, which took place during the last two weeks of August 2019, produced highly encouraging results, despite the challenge of observing a pulsed plasma in an electrically noisy environment. Expected lines from He-like aluminum ions (Al11+) were observed with acceptable resolution, and data was recorded for a variety of MST plasma conditions. The next steps are to further hone data analysis techniques, improve in-situ spectral calibration, and increase instrument flexibility to adapt to plasma conditions. The combination of high spectral resolution and broad spectral coverage provided by the spectrometer will provide plasma diagnostic capability important to the development of fusion.
This work was performed, in part, under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344, based upon work supported by the U.S. Department of Energy, Office of Science, Office of Fusion Energy Sciences.
Researchers recreate the sun’s solar wind and plasma “burps” on Earth
A new study by UW-Madison physicists mimicked solar winds in the lab, confirming how they develop and providing an Earth-bound model for the future study of solar physics.
Scilight: Spectroscopic technique sheds insights on how confined plasmas flow
A recent Physics of Plasmas article by Darren Craig (Wheaton College) is highlighted in the AIP’s Scilight: Spectroscopic technique sheds insights on how confined plasmas flow” DOI: 10.1063/1.5119247.
The Scilight reports:
Craig et al. have reported the first direct measurements of plasma flows in a reversed field pinch, a device used to generate and confine plasmas. Their findings which validate many common assumptions about plasma flow behavior and identify eddy currents as the dominant momentum loss mechanism during improved confinement are published in Physics of Plasmas.
Co-author Darren Craig said the team used a technique called charge exchange recombination spectroscopy. The method works by first exciting impurity ion emission with a neutral atom beam and then measuring the Doppler shift to determine the velocity of the ions.To make the measurements, the team had to overcome several technical challenges.
“First, we built a custom high-throughput spectrometer to enable measurements with high time resolution” Craig said.”Second, we had to develop techniques to deal with a high background signal in the MST plasmas we studied.”?
Developing a reliable absolute wavelength calibration was the final step that enabled the authors to measure how the plasma flows varied in space and time.
The article is available here:
Intrinsic flow and tearing mode rotation in the RFP during improved confinement? by D. Craig, E. H. Tan, B. Schott, J. K. Anderson, J. Boguski, D. J. Den Hartog, T. Nishizawa, M. D. Nornberg and Z. A. Xing, Physics of Plasmas (2019). The article can be accessed at https://doi.org/10.1063/1.5095620.
Steph Kubala wins Blewett Fellowship
WiPPL graduate student Steph Kubala has received a Blewett Fellowship from the American Physical Society (APS). After pausing her research to care for an ailing parent, Ms. Kubala has resumed her research, where she specializes on MST’s Thomson scattering diagnostic. Her thesis work focuses on characterizing how magnetic field fluctuations scale with Lundquist number, a parameter that characterizes magnetic fluctuations and the quality of confinement in the RFP plasma. The Blewett Fellowship will enable Ms. Kubala to continue her work toward completing her Ph.D.
Pursuing high thermal pressure plasmas with tangled magnetic fields
In February 2019, the Big Red Ball (BRB) was operated for an initial investigation of compact toroid (CT) collisions as proposed by a Los Alamos National Laboratory (LANL) group. LANL researchers Scott Hsu and Tom Byvank visited WiPPL to work with UW-Madison physics graduate student Doug Endrizzi on these experiments. A chief motivation for the work was a hypothetical plasma state described by Dmitri Ryutov [Fusion Science and Technology 56, 1489 (2009)] wherein the normalized thermal pressure is high (beta>1) yet the plasma is magnetized (wt>1) by a tangled field. Such a state might be of interest as a compression target for magnetized target fusion or as a laboratory setting with connections to astrophysical molecular clouds.
Byvank and Endrizzi operated BRB in order to inject and collide two CT plasmas, diagnosing their speed, magnetic structure, and kinetic properties with moveable arrays of internal probes. Each CT plasma was formed and injected with one of two CT injectors at WiPPL, one on loan from TAE Technologies, Inc., the other designed by Endrizzi based on the TAE design and built by WiPPL. The video below shows an example of the collision process, with the two CT plasmas colliding together in the core region of BRB where the diagnostic probe arrays are visible. The results of these initial experiments are promising, and further collaborations are being planned.
UW-Madison team receives NSF grant for programmable power supply construction
A UW-Madison Physics Department research team led by Dr. Brett Chapman was awarded a two-year Major Research Instrumentation grant from the National Science Foundation (NSF) for construction of programmable power supply (PPS) modules for experimental use on the Big Red Ball (BRB) and the Madison Symmetric Torus (MST) at WiPPL. The PPS modules are digitally programmable and feedback controlled, based on a prototype, pictured above, already in use on MST to drive toroidal plasma current, which generates a poloidal magnetic field confining the high-temperature plasma. The modules, six of which are shown in the picture, can be used individually, or they can be used in groups as large as desired, providing a wide range of maximum current. The modules provide for substantially more flexible and sophisticated plasma experiments at WiPPL. Some experiments will be accomplished much more readily and quickly with the modules, but the modules will also enable other experiments that would otherwise range from difficult to impossible. This capability is illustrated with the MST plasma current signal plotted below, with 100,000 amperes of MST plasma current being modulated with an embedded “UW” waveform. On BRB, the modules will enable an easily scannable toroidal guide field for ongoing magnetic reconnection studies, as well as future experiments on the formation of spherical tokamaks for adiabatic compression and ion heating studies, for example. On MST, they will help enhance the capabilities of the power supplies now in use to provide both poloidal and toroidal magnetic fields, enabling increased operational performance for physics studies of reversed-field pinch and tokamak plasmas. This new power supply capability funded by the NSF will substantially enhance and broaden the plasma science research that can be carried out at WiPPL.
Princeton Users See First light from Multi-Energy Soft-X-ray camera
A collaboration between Dr. Luis? Delgado-Aparicio (Princeton Plasma Physics Laboratory), Dr. Lisa Reusch, and Dr. Daniel Den Hartog (UW-Madison) has successfully installed and operated a new soft-x-ray camera based on the Pilatus3 detector made by DECTRIS Laboratories in Switzerland. The detector has the novel features of using a thick silicon layer bonded to a 100k pixel CMOS detector and per-pixel control of the pulse height comparator providing sharp energy thresholding. A pinhole in front of the detector allows it to be used as an x-ray camera with extremely high spatial resolution.? The combination of a large number of pixels with individual energy settings allows the camera to be operated with both high spatial and spectral? resolution enabling it to function as a high resolution Multi-Energy Soft-X-ray (ME-SXR) detector.
This ability to simultaneously set multiple energy thresholds enables ? simultaneous measurement of both continuum and line emission features in the 2-8 keV range where highly-charged ions contribte to the emission spectrum. The camera was installed in? the spring of 2018, and the first data collected with the system is shown in Figure 10. It shows the number of counts generated from x-ray photons with energy greater than? 4 keV. These x-rays were created by thermal electrons in a 500 kA MST discharge.
To get a sense of the spatial resolution o? f the camera, images were taken when fast electrons from the plasma were striking objects on the wall of the vessel, lighting them up with x-ray emission. In the image on the left, different structures are apparent in the image.
This camera will be used to measure temperature and density, structures in helical plasmas, detect fast electrons generated by fast magnetic reconnection events, and quantify resistive dissipation in? MST plasmas.
Compressible MHD turbulence studied in high-beta plasmas
In March, the Big Red Ball at WiPPL was operated for an investigation of compressible MHD turbulence proposed by a collaboration of Swarthmore College professor Mike Brown, Swarthmore undergraduate student Emma Suen-Lewis, and Bryn Mawr College professor David Schaffner. They proposed to measure the correlations between density and magnetic field fluctuations to distinguish between fast and slow waves in turbulent, high-beta magnetized plasmas. Suen-Lewis and Schaffner visited WiPPL to oversee the initial plasma run days. UW-Madison graduate students Doug Endrizzi and Ethan Peterson operated an array of biased plasma guns with a background axial magnetic field in order to create the experimental flux-rope plasmas, an example of which is seen in the video below. Endrizzi also designed, built, and operated a novel combination Langmuir and magnetic fluctuation probe to carry out the correlation measurements. Suen-Lewis is analyzing the experimental data for an undergraduate thesis at Swarthmore.
Italian visiting scientists commission fusion neutron detector
A group of three scientists from Italy made use of the MST device during a two week stay this spring. ? Dr. Matteo Zuin (Consorzio RFX), Dr. Luigi Cordaro (University of Padua), and Dr. Cristiano Fontana (Istituto Nazionale di Fisica Nucleare Sezione- National Institute of Nuclear Physics) used MST plasmas with a substantial fusion neutron flux to commission a set of neutron and gamma discriminating detectors.
The diagnostic boasts of a very fast time response with on-the-fly pulse shape discrimination to distinguish between neutron and gamma counts on the scintillator. ? This cross-discipline collaboration was especially productive in adapting nuclear physics? counting techniques to a plasma physics short pulse, high flux discharge. Previous pulse counting techniques on plasma physics experiments record the detector output for the full duration of the discharge, then analyze the pulses? between shots. ? The counting specialist, with a nuclear physics background, is accustomed to a detector that free-runs for long periods of time, ? processing the data and registering counts as they come in??” ? a technique well suited for low count rates. ? In discharges with neutral beam injection on MST,? each of the five detectors was routinely subjected to count rates exceeding 50kHz, which challenged the system’s ability to keep up.
In just under a week with the full crew on site, the typical challenges of experimental installations were all solved. and Dr. Fontana? stayed on another week to collect data in several different plasma conditions, including neutral-beam heated discharges with varying deuterium concentration, as well as discharges with a natural fast ion population from both strong ‘sawtooth’ reconnection events and from quiescent, high-confinement plasmas where such events are suppressed. ? It was a very successful visit overall, with the diagnostic operating robustly through all conditions. ? One challenge remains: ? with the ability to measure neutron flux on five independent detectors, a suitable collimation system will allow instantaneous measurement of a fast ion density profile. ? This addition will help answer questions on the details of the reconnection-based ion heating, the dynamics of fast ions during the transition to the RFP’s single helicity state, and the effects of fast ion driven instabilities in the Reversed Field Pinch.