Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session CP11: Poster Session II: In-Person, Hall A (2:00-3:30pm) and Virtual Poster Presentations (3:45-5:00pm)
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Room: Exhibit Hall A and Online |
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CP11.00001: MFE: LOW ASPECT RATIO Session Chairs: |
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CP11.00002: On the role of neutrals in achieving flat temperature profiles and exciting tearing mode activity in LTX-β Santanu Banerjee, Dennis P Boyle, Anurag Maan, Dick Majeski, Nathaniel M Ferraro, Ronald E Bell, Christopher J Hansen, William J Capecchi, Drew Elliott Flat Te(r) - profiles are obtained earlier in LTX-β, with lithium coated first wall, once the external fueling is stopped. Due to strong hydrogen retention and low recycling of lithium coatings, the flat Te profiles are obtained at reduced density. The goal of the present work is to explore the possibility of achieving this unique operation regime at higher and sustained densities with suitably tailored fueling, using the high field side gas puffing (HFS-GP) and/or the supersonic gas injection (SGI). LTX-β is equipped with toroidal and poloidal arrays of Mirnov probes, with one of the toroidal Mirnov probes being simultaneously acquired at a fast digitization rate (3 MHz). It is observed that beyond a critical fueling limit by either HFS-GP or SGI, the Te profile shifts from flat to peaked and a tearing mode is also destabilized. The critical fueling limit is experimentally manifested through a line averaged density threshold (<ne>th) of 1 x 1019 m-3 for the tearing mode to get destabilized. Below <ne>th flat Te profiles are achieved while maintaining <ne>. Mode analysis by singular value decomposition confirms the mode structure to be m/n = 2/1. Our hypothesis is that the destabilization of the tearing mode and peaking of Te profiles beyond <ne>th is due to the edge cooling by the cold neutrals beyond a critical fueling flux. This is supported by neutral density estimation at the edge by DEGAS 2. Linear tearing stability analysis will be reported as a function of edge neutral flux to further investigate the hypothesis. |
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CP11.00003: Extending and sustaining the low-recycling regime with higher performance discharges, liquid lithium walls, and NBI-heating in the Lithium Tokamak Experiment-β Dennis P Boyle, Jay K Anderson, Santanu Banerjee, Ronald E Bell, William J Capecchi, Drew B Elliott, Christopher J Hansen, Paul E Hughes, Robert Kaita, Shigeyuki Kubota, Benoit P LeBlanc, Anurag Maan, Dick Majeski, Filippo Scotti, Vlad Soukhanovskii, George J Wilkie, Leonid Zakharov Recent experiments in LTX-β have extended the duration, performance, and operating conditions of the low-recycling regime first observed in LTX and achieved record values of current Ip, temperature Te and Ti, pressure p, and confinement τE that were 50-200% higher than LTX. The flat Te profile and hot edge unique to the low-recycling regime has now been sustained with steady, moderate density for multiple τE in high performance discharges, and has now been observed in discharges with liquid Li walls. TRANSP analysis assuming neoclassical ion thermal transport is generally consistent with available core Ti measurements. TRANSP estimates of τE are up to 3 times the Linear Ohmic Confinement scaling and also exceed H-mode confinement scalings in a variety of high-performance discharge types with fresh solid, passivated solid, or fresh liquid Li walls, with or without flat temperature profiles. As thermal conduction losses decrease with flat temperature profiles, core density measurements demonstrate that energy is carried primarily by particle convection, consistent with low recycling. Fast-ion confinement was low in initial experiments but recent high density discharges with ~doubled Ip, reduced NBI energy, and adjustments to the beam and plasma position showed distinct heating. Experiments are underway to demonstrate NBI heating in low-recycling, flat-Te discharges. |
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CP11.00004: Neutral beam heating of flat-temperature profile plasmas in LTX-β William J Capecchi, Jay K Anderson, Dennis P Boyle, Anurag Maan, Santanu Banerjee, Richard Majeski, Ronald E Bell, Christopher J Hansen, Drew B Elliott Flat temperature profiles are achieved in LTX-β via a lithium coated first wall. A 20 kV Neutral Beam Injector, installed in 2019, has recently been observed to heat the plasma. This auxiliary heating, essential for studying the energy confinement scaling in the low-recycling environment and the response of flat temperature profiles to auxiliary heating, was achieved only after a concerted modeling and experimental campaign. Utilizing thermocouple data from the calorimeter, the neutralization fraction was optimized, a misalignment of the source was detected and corrected, and a measure of the beam width as a function of perveance was made, consistent with CHERs data. Modeling suggests that the higher current discharges made available as of 2021 after an upgrade to the ohmic power supply have moved LTX-β into a regime where good coupling is now possible but remains challenging. Many factors influence beam coupling, including the beam energy, perveance, and tangency radius, as well as plasma density, current, temperature, and tearing mode activity. Although TRANSP modeling suggests beam heating increases with beam energy up to the max operating voltage of 20 kV, heating was not observed until the beam tangency radius was increased by intentionally misaligning the beam source relative to the neutralizer tank and operating at the mid-range energy of 13 kV. Exploration of the variables controlling beam heating in LTX-β are still underway and the present findings will be presented, alongside future plans to install an NPA and realign the beam during an upcoming vent. |
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CP11.00005: Expansion of the LTX-beta Thomson scattering system, including high-field side measurements with off-normal viewing angles Drew B Elliott, Dennis P Boyle, Benoit P LeBlanc, Anurag Maan, William J Capecchi, Santanu Banerjee, Robert Kaita, Grant M Bodner, Richard Majeski, Theodore M Biewer There has been an expansion of the Thomson scattering(TS) electron temperature and density measurement system on LTX-beta. This expansion extended the range of major radii sampled to the high-field side of the device. The motivation for this increased sampling range is to further support flattened electron temperature profile measurements, to better locate the plasma centroid, and to provide TS data for smaller major radii plasmas. Machine hardware limitations and the requirement to make use of an existing laser beam led us to utilize an unusual scattering geometry where the polarization lies on the scattering plane. By staying away from a normal scattering angle, collected TS light is kept above our detection limits. Despite avoiding viewing parallel to the polarization, the total amount of collected light is still reduced by over 3 times. The detection system consists of 5 polychromators, each with four spectral channels. The spectral filters were originally optimized for normal viewing angles and 500 eV temperatures. Single shot measurements have been achieved but typically the signals are ensembled to reduce uncertainty, improve statistics, and better correlate to the core measurement system. Calibration, stray light mitigation, analysis methods, and initial results will all be discussed, as well as composite profiles from both the core and high-field side TS measurements. |
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CP11.00006: Density Fluctuation Measurements with Reflectometry on LTX-β Shigeyuki Kubota, Richard Majeski, Santanu Banerjee, Dennis P Boyle, Anurag Maan, Terry L Rhodes, Christopher J Hansen The introduction of Li-coatings to plasma facing components has resulted in improved confinement and reduced edge turbulence in many fusion plasma devices. On the LTX and the LTX-β devices, Li evaporated onto a close-fitting shell surrounding the plas- mas has yielded discharges having a flat electron temperature profile with a hot edge, and a peaked density profile. The upgraded reflectometry system on LTX-β is uniquely equipped to measure low-k core and edge turbulence in this regime. This diagnostic consist of two frequency-modulated continuous-wave (FM-CW) channels (13.1−20.2 and 19.5−33.5 GHz) and two fixed-frequency channels (13.1−21 and 20.5−40 GHz) which can be scanned from shot to shot. Electron density profile measurements are possible in the range of 0.2−1.4 × 1013 cm−3 with a time resolution down to 4 μs. The fixed-frequency channels extend the density fluctuation coverage to 2.0 × 1013 cm−3 with a bandwidth ≤5 MHz. Since the FM-CW and fixed-frequency channels share a bistatic antenna array, measurements of the fluctuation level, spectrum, and radial correlation length are possible with a high degree of temporal and spatial localization. Initial measurements and analysis results will be presented at the meeting. |
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CP11.00007: Lithium surface conditioning and its impact on the scrape-off layer and fuel recycling in the Lithium Tokamak eXperiment-β Anurag Maan, Dennis P Boyle, George J Wilkie, Manaure Francisquez, Dick Majeski, Robert Kaita, Santanu Banerjee, Drew B Elliott, William J Capecchi, Christopher J Hansen, Shigeyuki Kubota, Elizabeth Perez, Filippo Scotti, Vlad Soukhanovskii
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CP11.00008: Near term plans for LTX - β Dick Majeski, Santanu Banerjee, Ronald E Bell, Dennis P Boyle, Dylan Corl, Benoit P LeBlanc, Thomas Kozub, Anurag Maan, Javier Morales, William J Capecchi, Drew Elliott, Shigeyuki Kubota, Terry Rhodes, Leonid Zakharov, Enrique Merino, Vlad Soukhanovskii LTX-β began operations in the summer of 2018, and has now operated without a vent or other major modifications for over four years, through the end of FY2022, which may be a record for any tokamak. A vent is planned for early FY23, to service internal components of the tokamak. The vent will also incorporate a number of changes and modest upgrades to improve operations. Modifications will include new porting for the neutral beam (added as part of the upgrade to LTX-β) to increase the tangency radius, which is expected to reduce the first orbit fast ion loss fraction, and improve beam coupling at higher injection energy. A new set of instrumented beam dumps and scrapers will be installed, to relocate and replace the existing beam dumps. The high voltage power supply for the neutral beam will also be upgraded, to increase the pulse length by 3×. Separate collection optics and APD-based polychromators for the Thomson scattering system, which view to the high field side of the plasma axis, were installed in 2021 – 2022, and improvements to this system planned for the vent include a more extensive viewing dump to reduce stray light. We are also investigating the implementation of a second pass for the laser beam to improve the signal level. A new remotely actuated lithium deposition system will be installed. LTX-β is fueled by a combination of high field side gas puffing, with a low field side supersonic gas injector. An additional high field side nozzle will be added during this vent. These modifications, along with near term research plans, will be discussed. |
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CP11.00009: Modeling lithium deposition on plasma facing components of LTX-β Javier J Morales, Anurag Maan, Richard Majeski, Dennis P Boyle The Lithium Tokamak eXperiment β (LTX-β) explores the benefits of using lithium as a plasma-facing component (PFC). The LTX-β has two lithium evaporators used to evaporatively coat the inside of the plasma-facing shells. The evaporators are located 180 degrees apart toroidally, each directly under a Quartz Crystal Microbalance (QCM), which is used to determine the thickness of the deposited lithium. Prior work has modeled lithium deposition on the shells using a point source approximation, but discrepancies remain between the model and observations. Specifically, coverage of the high field side limiting edges is not well diagnosed. This work extends the model to a surface source accounting for the effect of both the emission and deposition angle. This model facilitates an understanding of the lithium coverage of the shell and its interaction with the plasma. A comparison of the model's prediction with data collected on LTX-beta QCMs will be presented. |
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CP11.00010: The EqRV code for sensitivity analysis of equilibrium reconstruction in LTX-β and tokamaks in general. Leonid Zakharov The EqRV code, developed in 2008 for promoting MSE-LP (Line Polarization) and MSE-LS (Line Shift) to ITER, is made compatible with tokamak data on magnetic, MSE (-LP, -LS), Faraday rotation signals, and the use of transport code profiles as additional signals. The technique is based on calculation of perturbations of signals from solutions of linearised Grad-Shafranov equation in which the perturbations (variances) of plasma boundary, and pressure and current density profiles are considered as the source of perturbated signals. The application of SVD to the responce matrix, which relates signals and variances, results in determination of eigen values and eigen vectors of variances, which allows to classify the variances and visible, barely visible, and undetectable given the specific diagnostics. In its turn, this allows to judge on accuracy of reconstracted plasma shape, plasma paramenters, e.g., beta, li, or pressure and q-profiles. The number of visible variances inicates is a measure of quality of the diagnostics used. The EqRV code exposes the most and least valuable signals for reconstructions, thus, allowing to optimize the measurements on the device. The results of application of EqRV to LTX-β will be presented. |
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CP11.00011: SOLPS-ITER Simulation of MAST Opaqueness versus Aspect Ratio Yi-Cheng Chuang, Saskia Mordijck, Richard Fitzpatrick, Richard Reksoatmodjo For continuous tokamak operation, with a high core density, constant fueling through recycling, gas puffing or pellets will be necessary. However, in future large-size devices and at higher density, opaqueness, defined as ne x a, will increase, reducing the ability of neutrals to penetrate past the separatrix [1]. The opaqueness depends on ne approximated as the average of pedestal and separatrix electron densities, and a which is the minor radius. It is unclear how changes in aspect ratio affect opaqueness. In this poster, we will study the relationship between neutral penetration and aspect ratio using SOLPS-ITER simulation. We will start with matching an H-mode from MAST to determine boundary conditions and radial transport coefficients. We will in SOLPS-ITER increase the major radius, while keeping all other aspects identical to increase the aspect ratio to match the one in DIII-D in multiple steps. We will study the changes in neutral density as a function of aspect ratio in these SOLPS-ITER simulations. |
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CP11.00012: Initial active beam diagnosis of beam ions in MAST-U Garrett Prechel, William W Heidbrink, Clive A Michael, Andrew Jackson, Deyong Liu A solid state neutral particle analyzer (SSNPA) [1] diagnostic oriented to measure trapped ions and a tangentially viewing fast-ion D-alpha (FIDA) [2] diagnostic that measures mainly passing fast ions in MAST-U. For initial validation of the measurements, TRANSP NUBEAM calculates the neoclassical distribution function neglecting fast-ion transport by instabilities and then FIDASIM [3] computes synthetic signals for comparison with the data, including SRIM[4] simulation of the energy cutoffs for the SSNPA diagnostic. Unfortunately, suitable MHD-quiescent discharges for diagnostic validation from the first campaign are unavailable. For a discharge with a locked mode, the radial profile measured by the SSNPA is much narrower than predicted, perhaps due to fast-ion transport by the locked mode. Shots to produce MHD-quiescent validation discharges during the second campaign have been allocated. New comparisons using those discharges, including improved subtraction of soft x-ray backgrounds, are planned. |
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CP11.00013: Heated and Lithium-coated Marshall Gun CHI performance on SLiC Spherical Tokamak Stephen J Howard, Wade Zawalski, Don Froese, Kelly Epp, Kristin Skrecky, Joshua Hawke, Philippe Laberge, Alex Mossman, Victoria Suponitsky The SLiC device produces small scale (R=12 cm, a = 7cm) spherical tokamak plasmas using fast Coaxial Helicity Injection (CHI) with external formation current (500 kA peak) applied for a duration of 50 ms followed by Ohmic decay of internal plasma currents (up to 350 kA peak) over a lifetime of 1.5 ms. The small volume/surface-area ratio of SLiC makes plasma lifetime very sensitive to wall conditions and works well as a testbed for studying liquid Li as a plasma facing surface. We have recently added 4 new lithium input ports to the Marshall gun section and increased the heating capacity to enable plasma formation with the gun up to 250 C and with a liquid Li layer on the center and outer electrodes. These conditions are similar to what would occur in the Marshall gun of our CHI-formed Magnetized Target Fusion reactor concepts. Traditionally, most experiments with Marshall guns do not operate at sustained elevated temperatures because of the technical complications that adds, with SLiC being the first to also operate with liquid Li on the high-current electrode surfaces. Preliminary results to date show no decrease in performance when running with gun electrodes that are wetted with liquid Li. Latest results will be shown along with a summary of efforts to control Li surface quality in situ, implementation of plasma diagnostics in a lithium-vapor environment, and simulations of free surface motion of liquid Li in the SLiC system performed with in-house MHD solver |
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CP11.00014: Making the most of measurements: results from the integrated diagnostic analysis of the ST40 tokamak Marco Sertoli, Jonathan Wood, Bart Lomanowski, Ephrem Delabie, Michael Gemmel, Hannah Willett, aleksei dnestrovskii, Stanley M Kaye, Alsu Sladkomedova, Oleksandr Marchuk The characterization of magnetic-confined-fusion plasmas requires a comprehensive set of diagnostic systems measuring a wide range of parameters. New machines, such as the ST40 high field spherical tokamak owned and operated by Tokamak Energy Ltd, typically start with a small subset and gradually increase the diagnostic suite to include more complex and comprehensive systems. To make the most of these first phases of operation of ST40, in parallel to standard diagnostic analysis methods, forward models of the various diagnostic systems have been developed and integrated in a single workflow together with modelling tools such as FIDASIM and TRANSP. This has enabled the detection of unexpected errors in various diagnostic systems, correction of geometric uncertainties of line-of-sight mapping, and, above all, has allowed to constrain the kinetic profile shapes and their central values despite the absence of profile diagnostics. This contribution explains the analysis philosophy, the details of the methodology, and discusses the most recent achievement of attaining ion temperatures in excess of 108 K (> 8.6 keV) on the ST40 spherical tokamak during the 2021-22 campaign. |
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CP11.00015: Linear Gyrokinetic Stability Analysis of ST40 Hot Ion Plasmas Yang Ren, Stanley M Kaye, aleksei dnestrovskii, Michele Romanelli, Marco Sertoli The high field compact spherical tokamak ST40 is an important step toward ST-based fusion reactor [1]. Temperature and density profiles, and their uncertainties in recent hot ion ST40 plasmas with central ion temperature exceeding 8.6 keV have been inferred from experimental and modeling constraints in the absence of full profile measurements. Linear gyrokinetic stability analysis has been carried out to identify most unstable micro-instabilities in these hot ion plasmas. In one of these plasmas, it is found that linear growth rates of both ion- and electron-scale ($k_\theta\rho_s \geqslant 0.2$) modes decrease from the edge toward the core of the plasma (i.e. from $\rho=0.8$ to 0.3 where $\rho$ is the square root of normalized toroidal magnetic flux) while the change of linear growth rates at $k_\theta\rho_s < 0.2$ is non-monotonic. In particular, at $\rho=0.3$ no unstable mode was found at $k_\theta\rho_s> 5$, and about an order of magnitude reduction in the maximum linear growth rate in the ion-scale wavenumber range of $ 0.2\leqslant k_\theta\rho_s < 1$ is seen at $\rho=0.3$ compared with the $\rho=0.8$ location. TEM and/or ubiquitous mode are found to be important in this hot ion plasma. \\ |
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CP11.00016: Transport Analysis of ST40 High Performance Discharges Stanley M Kaye, marco sertoli, Peter Buxton, Alexei Denstrovskii, Michele Romanelli The main mission element of the ST40 tokamak is to attain fusion-relevant ion temperatures (100M Kelvin) in a small, compact spherical tokamak. These temperature levels have been achieved at higher aspect ratio, but only in much larger devices with very high heating power levels. ST40 has a major radius of 40 cm, Ip up to 800 kA, BT up to 2.2 T (the highest in a spherical tokamak to date), and it is heated by up to 2 MW of D neutral beams. Near central impurity ion temperatures in excess of 100M K (~8.6 keV) have recently been achieved in ST40 for both H+ and D+ thermal plasmas. Temperature and density profiles, and their uncertainties in these ST40 plasmas have been inferred from experimental and modeling constraints. TRANSP has been used to analyze the transport properties of these high-performance plasmas and confirm the hydrogenic temperature levels. While lower than the impurity ion temperature level, hydrogenic temperatures do reach and exceed the 100M K level. Both ion and electron transport are large, and the ion temperature exceeds neoclassical levels in the outer region of these plasmas. The TRANSP results are being used as a basis for both gyrokinetic studies of the source of these turbulence levels as well as for studying fast ion driven instability observations. |
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CP11.00017: Physics and engineering drivers for the mission and design of an EXCITE facility Jonathan E Menard, Brian A Grierson, Tom Brown, Chirag Rana, Yuhu Zhai, Francesca M Poli, Rajesh Maingi, Walter Guttenfelder Compact tokamaks have been proposed as a means of potentially reducing the capital cost of a fusion pilot plant (FPP). However, compact tokamak FPPs with higher magnetic field and reduced major radius and surface area face the challenge of integrating high core confinement, plasma pressure, and high divertor parallel heat flux and wall loading. This integration is sufficiently challenging that construction and operation of an EXhaust and Confinement Integration Tokamak Experiment (EXCITE) has been proposed by the U.S. community to close this integration gap. This presentation will review EXCITE in the context of present and planned facilities and describe recent systems studies and design activities for superconducting steady-state EXCITE facilities. Specific core-edge integration challenges including edge density operating windows and choice of impurities for detached divertor operation, the potential to access turbulence-driven broadening of the scrape-off-layer heat fluxes, and H-mode access and sustainment with high core radiation fraction will be described as a function of facility size and aspect ratio [1]. |
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CP11.00018: Time-dependent modeling of the X-I ECH and ECCD for solenoid-free start-up of a compact pilot plant Masayuki Ono, Nicola Bertelli The elimination of OH solenoid maybe the most impactful design driver for an economical compact fusion tokamak reactor system. To address this challenge, an efficient solenoid-free start-up and ramp-up scenario utilizing the X-mode at w = Ωe (X-I) was investigated to drive ~ 10 MA of plasma current for a compact Sustained High Power Density tokamak facility with a modest ECH power of ~ 10 MW utilizing the 170 GHz gyrotrons. The high ECCD efficiency is due to the strong wave-particle interactions at the Doppler broadened w = Ωe resonance, together with the accessibility constrained absorption on uni-directional passing electrons. The efficiency of the well localized current drive remains high for a broad range of launched parallel index of refraction suggesting that a relatively simple waveguide launcher placed outboard could be used. As a next step, a time dependent model of the X-I solenoid-free start-up and ramp-up is been developed. Interestingly, the X-I current ramp-up process appears to possess a positive feed-back of key parameters, ECCD, Ip, L-mode confinement time, and Te, where increase in any one of the parameters results in corresponding increases in other parameters resulting in a virtuous cycle of continuous current ramp-up even with a constant applied power. |
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CP11.00019: Nonlinear simulations of GAEs in NSTX-U Elena Belova A set of nonlinear simulations has been performed to study the nonlinear evolution of unstable global Alfven eigenmodes (GAEs) in the NSTX-U. Results of the single toroidal mode number simulations are compared with a full nonlinear simulation (all toroidal harmonics included). It is shown that single-n results are close to full nonlinear simulation only for the most unstable mode, in which case the saturation amplitudes and changes in the fast ion distribution are comparable. In contrast, peak amplitudes of subdominant modes in all-n simulations are smaller by a factor of 5-10 compared to single-n runs due to flattening of the beam ion distribution by the fastest growing mode. In single-n simulations, the conservation of two integrals of motion of a particle in a cyclotron resonance with the mode is demonstrated, resulting in a one-dimensional evolution of the particle distribution. Nonlinear simulations show a significant redistribution of the resonant fast ions, especially in the pitch parameter. Thus, the changes in the resonant particles parallel and perpendicular energies can be several times larger than the mode energy. This implies that even a relatively small amplitude mode can significantly modify the beam distribution in the resonant region. |
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CP11.00020: Localized 2nd Harmonic Global Alfvén Eigenmodes on NSTX(-U) Eric D Fredrickson When GAE are present in NSTX, modes are also commonly observed at the second frequency harmonic. The modes are weak, as would be expected if they were the result of non-linear terms in the wave equation. In the low field, low n shots, the second harmonic GAE wavelengths and frequencies are consistent with expectations from non-linear modeling. However, at higher fields, high toroidal mode number (shorter wavelengths), the qualitative behavior of the 2nd harmonic GAE becomes more complex. Previous studies have found that the unstable GAE frequencies and toroidal mode numbers scale nearly linearly with toroidal field. The GAE toroidal mode number is < 6 for toroidal field below about 3 kG, and over 10 at the highest toroidal fields reached on NSTX-U of about 6 kG. While the observed frequencies at high field are still consistent with non-linear modeling, the toroidal mode numbers (wavelengths) are not well defined. Rather, the 2nd harmonic modes, with expected toroidal mode numbers of order 20, become toroidally localized. The toroidal array of magnetic sensors has limited coverage, but the toroidal asymmetry in mode amplitude is quite clear. Conversely, the fundamental ICE instability, with somewhat higher frequencies than the 2nd harmonic GAE, but lower toroidal mode numbers, is toroidally symmetric within measurement uncertainties. No strong toroidal asymmetries of CAE at similar frequencies with toroidal mode numbers 10 to 14 have been seen. These observations could have an impact on modeling for even Toroidal Alfvén eigenmodes in ITER, as the toroidal mode numbers are expected to be large on ITER. |
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CP11.00021: NBI+RF synergy and its effect on Alfvén Eigenmode stability in NSTX/NSTX-U Mario L Podesta, Nicola Bertelli, Vinicius N Duarte The synergy between NBI and RF injection can result in a large distortion of the original NBI distribution function by generating a tail extending to very high energies (E>>100 keV) or by causing increased losses of NB ions that are displaced to loss orbits by the RF field. The modification of the NBI distribution by RF can then lead to different stability properties for Alfvénic modes, making RF a candidate actuator to affect the mode stability. The expected modifications of the NBI distribution on NSTX and NSTX-U as a function of the injected RF spectrum will first be assessed via the ORBIT code. The RF fields from the full wave code TORIC are used to evaluate the RF quasi-linear diffusion coefficients, which can then be used to modify the NBI energetic particle distribution. The impact of the modified NBI distribution on Alfvénic instabilities will then be assessed by quantifying the power exchanged between fast ions and the modes. This modeling exercise would provide initial guidance on the expected effects of the NBI+RF synergy for NSTX-U scenarios, and it would serve as a starting point to design dedicated experiments. |
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CP11.00022: Framework for study of neoclassical tearing modes in NSTX James J Yang, Eric D Fredrickson, Mario L Podesta, Francesca M Poli A framework for the study of neoclassical tearing modes in NSTX has been developed. This work is motivated by the need to interpret the experimental data and to provide a predictive modeling prior to the experiments regarding the neoclassical tearing mode activities in NSTX and other tokamaks. The framework focuses on solving the generalized Rutherford equation [1,2] for the time evolution of the magnetic island width. Each term in the generalized Rutherford equation is allowed a degree of freedom, represented by a coefficient, to incorporate the measurement uncertainties in the parameters involved. The coefficients are fitted to match the simulated and measured time evolution of the magnetic island width. The input data for the model is provided by synthetic soft X-ray diagnostics, which provides the measured magnetic island width [3], and TRANSP, which self-consistently calculates both thermal and fast ion transport considering the effect of the measured magnetic island width utilizing the Kick model assumption [4]. The results show that adding a fast ion term [5] to the generalized Rutherford equation is necessary for the fitted coefficients to be within a realistic range in NSTX. It has been found that the stabilization effect of ion polarization current can be exceeded by the destabilization effect of fast ion driven uncompensated cross field current for otherwise marginally stable neoclassical tearing mode. The result of a database study involving NSTX discharges will be presented. |
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CP11.00023: Simulation of Thermal, Particle, Impurity, and Momentum Transport in NSTX Discharges Using the Multi-Mode Anomalous Transport Module Tariq Rafiq, Christopher Wilson, Jan Weiland, Eugenio Schuster The theory-based multi-species multi-fluid multi-mode anomalous transport module (MMM) [T. Rafiq et al. Phys Plasmas 20, 032506 (2013)] is utilized to calculate electron/ion thermal, electron particle, impurity, toroidal, and poloidal momentum transport in low, medium, and high collisionality NSTX discharges. The effects of isotope mass on transport are also investigated. The ion thermal transport is found to be close to zero in the core of NSTX plasmas with equilibrium flow shear. The results agree with NSTX experiments and gyrokinetic ion-scale simulation on the fact that ion thermal transport is neoclassical. The electron thermal transport is found to be anomalous due to unstable electron temperature gradients and microtearing modes. The latest version of MMM includes ion and electron temperature gradient modes, trapped electron modes, peeling and kinetic ballooning modes, drift Alfven modes, microtearing modes, ideal MHD, and drift resistive inertial ballooning modes. The goal is to implement MMM in the integrated modeling code TRANSP in order to not only compute the temperature, density, current, rotation, and other profiles that are measured in existing experiments, but to also extrapolate these predictions to future planned devices. |
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CP11.00024: Linear and Nonlinear Gyrokinetic Electron Temperature Gradient Mode Simulations for NSTX Discharges and NSTX-U Projections Cesar F Clauser, Tariq Rafiq, Walter Guttenfelder, Eugenio Schuster Experiments on NSTX have demonstrated that electron thermal transport dominates ion thermal transport. One of the modes that is responsible for electron thermal transport is the electron temperature gradient (ETG) mode. A series of NSTX discharges and NSTX-U projections were analyzed to investigate the anomalous electron thermal transport caused by ETG modes. The CGYRO code was employed for this purpose and simulations were carried out in both electrostatic and electromagnetic limits. The results were compared to earlier studies and extended to NSTX-U projections to investigate low collisionality regimes. The gyrokinetic ETGM thermal flux has been utilized to verify and calibrate a recently developed fluid model [1] for electromagnetic toroidal ETG driven drift mode. The fluid ETG model will be used as a component of the Multi-Mode anomalous transport module [2] in the predictive integrated modeling code TRANSP to predict time-dependent electron temperature profiles in NSTX-U and conventional tokamak plasmas. |
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CP11.00025: Transport modeling of NSTX plasmas with TGLF reduced transport model Galina Avdeeva, Kathreen E Thome, Sterling P Smith, Orso-Maria O Meneghini, Joseph T McClenaghan, Tomas Odstrcil, Gary M Staebler, Devon J Battaglia, Walter Guttenfelder, Stanley M Kaye The TGLF reduced transport model is being validated against core profiles in L/H-mode plasmas from NSTX for use as a predict-first tool for future scenario development on NSTX-U. In a low beta L-mode discharge, where ETG/ITG instabilities are expected to be dominant, heat-flux-matching TGYRO predictions of plasma temperature profiles are in a good agreement with experimental data. TGLF simulations at multiple radii indicate that the contribution of low-ky ion scale modes to electron heat transport can reach up to 80% of the total flux in the ITG unstable region. Ion heat flux in this shot is predicted to have a comparable fraction of neoclassical and turbulent contributions. This is different compared to H-mode discharges with higher plasma density and beta, where simulations show fully neoclassical ion heat flux. In H-mode shots a discrepancy between predicted electron temperature and measurements is observed inside the region ρ ∼ 0.5−0.7 and potentially can be caused by MTM instabilities unresolved by TGLF model. To deepen the understanding of what drives the heat transport in the core region of NSTX, we will verify the critical inverse temperature gradients predicted by TGLF against gyro-kinetic simulations. |
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CP11.00026: Effects of MHD island on bootstrap current and turbulence in spherical tokamak plasmas Weixing X Wang, Min-Gu Yoo, Edward A Startsev, Stanley M Kaye, Walter Guttenfelder, S. Ethier, Jin Chen Magnetic islands, by altering the topological structure of the confining magnetic field, may strongly impact both neoclassical and turbulent dynamics in fusion devices such as spherical tokamaks. Major effects are associated with island-induced three-dimensional ambipolar electric field. First, a magnetic island induces a localized Er-well across island inner boundary; further, strong turbulence-driven Reynolds stress gradient contributes to continued growth of E×B flow beyond the neoclassical level. The resultant E×B shear layer is shown of capable of triggering the formation of an internal transport barrier inside the island rational surface for an NSTX-U L-mode plasma. A magnetic island also drives non-resonant electric potential islands at the island edges, which may introduce a major change in plasma self-driven current through an efficient nonlinear parallel acceleration of electrons, resulting in a significant global reduction of electron current with respect to the neoclassical bootstrap current in large aspect-ratio tokamaks. The reduction of the axisymmetric current scales with the square of island width. Remarkably, this effect of bootstrap current reduction by MHD islands is significantly weaker in STs, in particular, in the reactor-relevant high-βp regime where NTM islands are more likely to develop. |
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CP11.00027: Conceptual design of NSTX-U Radial Polarimeter-Interferometer diagnostic Jie Chen, David L Brower, Paul Li, Brentley C Stratton Conceptual design of a 935 GHz Radial Polarimeter-Interferometer Diagnostic (RAPID) for the NSTX-U spherical tokamak is presented. The diagnostic will utilize 3-wave technique to accomplish simultaneous Faraday-effect and line-integrated density measurements with ~0.1 µs time response and ~0.01 degree phase noise, to provide internal magnetic and density fluctuation measurements. The primary application of this diagnostic is to investigate MHD instabilities, energetic particle driven modes and turbulence for purposes of physics validation and plasma control. High-level diagnostic requirements are determined, such as implementation of two chords with 4.5 degrees toroidal separation, which will allow toroidal mode number measurement up to n=40 and is critical for physics validation. Engineering challenges are identified and possible solutions are developed, including fabrication of low-profile retro-reflectors with 1 inch diameter, modification of high-field-side graphite tiles and rearrangement of window positions on the low-field-side port flange. Optical design with realistic configuration compatible with the NSTX-U test cell is developed. |
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CP11.00028: Investigating emissivity evolution of NSTX-U graphite and Li-coated stainless steel for IR thermography Promise O Adebayo-Ige, Shota Abe, Anurag Maan, Kaifu Gan, Evan T Ostrowski, Jhovanna Garcia, Richard Majeski, Bruce E Koel, Rajesh Maingi, Brian D Wirth
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CP11.00029: Internal rotation of ELM filaments on NSTX Mate Lampert, Ahmed Diallo, Stewart J Zweben, James R Myra Edge localized modes (ELMs) are a threat to tokamaks due to their high heat and particle loads on the plasma facing components (PFCs). A significant portion of this energy is carried and deposited by the emerging ELM filaments, whose dynamics are directly connected to their impact. Therefore, understanding their underlying physics is important for the operation of future fusion reactors. We extend our knowledge of ELM filaments by investigating their internal rotation around their own axis. Our analysis of gas-puff imaging (GPI) data on NSTX show that ELM filaments are characterized by internal rotation in the direction of the ion-gyro motion with 15.2krad/s median angular velocity. The characteristic size of the ELM filament was also assessed and found to be similar to the blobs emerging in intermittent SOL turbulence. A nearly linear trend was found between the angular velocity of the ELM filament and both its radial velocity and its distance from the separatrix, as well. An analytical model called the shear-induced rotation model was identified as a candidate for explaining the physics of the observations. Our results show that the modelled mechanism could significantly influence the rotation of the ELM filament, however, it cannot be a sole contributor. |
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CP11.00030: MHD bifurcations in NSTX discharges Stefano Munaretto, Nathaniel M Ferraro, Eric D Fredrickson, Galina Avdeeva, Kathreen E Thome, James J Yang The observed bifurcations of the low frequency and low toroidal periodicity (n) MHD activity often present in the initial part of the NSTX discharges can be explained by the evolution of the radial profile of the safety factor (q) crossing multiple rational surfaces in the core. Important performance limiting instability mechanisms in the NSTX spherical tokamak are often linked to low frequency and low-n MHD activity. They are quite common in long-pulse NSTX plasmas. They can be present at the beginning of the plasma current flat-top, at the end of the discharge or during the whole duration, and they have been observed to deleteriously impact performance over a wide range of q95. An interesting feature observed in some NSTX discharges is the presence of a bifurcation in the frequency of the low n modes, as low as n=1, that have frequencies comparable to the plasma core rotation divided by n. Equilibrium reconstructions constrained by magnetic diagnostics data and MSE pitch angle radial profiles suggest that the observed bifurcations are linked to a fast evolving minimum value of q. 3D non-linear resistive MHD simulations are presented, to understand the physics of the observed bifurcations. |
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CP11.00031: Temperature and Current Density Profile Prediction in NSTX Discharges Christopher Wilson, Tariq Rafiq, Jan Weiland, Eugenio Schuster The new physics-based Multi-Mode anomalous transport module (MMM) is implemented as a component in the integrated modeling code TRANSP. The MMM module consists of a combination of theory-based transport models that are used to predict the evolution of electron and ion temperatures, electron density, and toroidal and poloidal rotation profiles in tokamak plasmas. A combination of models is required to account for the various physical phenomena that contribute to transport in various radial regions of a plasma discharge. This study predicts the time evolution of current density, electron, and ion temperature profiles in discharges with low and high collisionality. The role of electron-scale turbulence in these predictions is studied by varying electron density and temperature profiles. The equilibrium is computed using the TEQ module. Neoclassical transport is calculated using the Chang-Hinton module. NBI heating current drive is obtained using the NUBEAM module. The latest version of MMM includes a new electron temperature gradient mode model as well as updated component models, which include the physics of ion temperature gradient modes, trapped electron modes, peeling and kinetic ballooning modes, drift Alfven modes, microtearing modes, ideal MHD, and drift resistive inertial ballooning modes. |
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CP11.00032: Filament Shape Characterization on NSTX Jaela C Whitfield, Mate Lampert Filamentary transport accounts for a significant fraction of the unwanted heat and |
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CP11.00033: Model-Based Optimal Control of Core Kinetic+Magnetic Profiles and Scalar Plasma Properties in NSTX-U Hassan R Al Khawaldeh, Brian Leard, Sai Tej Paruchuri, Tariq Rafiq, Eugenio Schuster Active control of key plasma properties will be necessary to achieve high-performance scenarios in NSTX-U. These scenarios, which are characterized by magnetohydrodynamic stability, improved confinement, and possible steady-state operation, rely on active shaping of the kinetic and magnetic profiles and/or active regulation of important scalar (integrated over space) properties. A response model based on the one-dimensional magnetic diffusion equation in combination with a zero-dimensional energy balance equation has been exploited in this work to synthesize some of the needed active control algorithms. The infinite dimensionality of the magnetic diffusion equation is reduced by spatial discretization and both equations are then linearized to obtain a response model for control synthesis. Model-based optimal control techniques are later used to synthesize controllers with a variety of control objectives, such as the simultaneous regulation of the safety factor profile and the plasma stored energy or the simultaneous regulation of the plasma internal inductance and the normalized beta. The controllers are tested in higher-fidelity (simulation models are more complex than those used for control synthesis) nonlinear simulations using the Control Oriented Transport SIMulator (COTSIM). |
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CP11.00034: Development of COTSIM+NUBEAMNet Predictive Transport Simulation Capability for Scenario Optimization in NSTX-U Brian R Leard, Cesar F Clauser, Hassan R Al Khawaldeh, Tariq Rafiq, Eugenio Schuster, Mark D Boyer Plasma transport predictive models require a sufficiently small computational effort to enable their use in model-based scenario-control design. The Control Oriented Transport SIMulator (COTSIM), which captures the control-relevant physics involved in a tokamak discharge, has been developed to fulfill these computational-time constraints and to serve the specific purpose of control design. In this work, the predictive capabilities in COTSIM for NSTX-U scenarios have been significantly improved. The dynamics of the plasma is modeled by the magnetic diffusion equation, the heat transport equation for electrons and ions, and the angular momentum equation. The current, torque, and heating depositions by the neutral beam injectors are modeled using a neural network, NUBEAMNet [1], which reproduces the results of NUBEAM in a fraction of the computation time demanded by the original Monte Carlo code. Semi-empirical transport models, Coppi-Tang and Bohm/gyro-Bohm, and fixed boundary equilibrium solvers are also integrated with the transport equations. Predictions by this fast-simulation capability are validated against those by TRANSP. This new version of COTSIM for NSTX-U will enable a myriad of plasma-control applications, including model-based scenario optimization. |
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CP11.00035: Fast-Ion D-Alpha spectroscopy studies at NSTX-U Aidan J Edmondson, Benedikt Geiger, David Smith, Colin Swee, Ryan Albosta Knowledge on fast-ion confinement properties is important since fast-ions heat the background plasma and, if lost before thermalization, can damage the vessel walls. One method to study the fast-ion distribution function is Fast-Ion D-Alpha (FIDA) spectroscopy which measures the Doppler shifted Balmer-Alpha emission emitted by neutralized fast-ions following charge-exchange reactions. Here we present the design and first calibration results of an upgraded FIDA spectrometer system at NSTX-U. Moreover, we introduce a novel python version of the FIDASIM code that models synthetic FIDA spectra based on an input 4D fast-ion distribution function. A detailed benchmark between the new code, using a less memory intensive simulation scheme, and the FORTRAN version of FIDASIM will be presented. |
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CP11.00036: Real-Time Density Feedback Control Using FIReTIP Calvin W Domier, Yilun Zhu, Jon Dannenberg, Yang Ren, Brentley C Stratton, Neville C Luhmann The Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) is under development to provide time-resolved, chord-integrated electron density and tangential B-field data on NSTX-U. The FIReTIP beam will enter on Bay G, reflect from an internal retroreflector placed within the Bay B port cover, and exit back through the Bay G entrance window. A heterodyne HeNe interferometer will provide real-time vibration compensation, with a field programmable gate array (FPGA) digitizing the voltage outputs of both FIReTIP and HeNe interferometer systems. The FPGA will analyze the data, correct the FIReTIP data for the mechanical motions observed on the HeNe interferometer, and then output a corrected signal in real time (except for a short time delay due to the amount of time required to perform the analysis) with a throughput of at least 5 kHz. The FIReTIP FPGA outputs will provide real-time density feedback control signals for the Real-time Control and Protection (RTCP) system of NSTX-U, and can serve as real-time constraints for EFIT equilibrium reconstructions. Details of the FIReTIP system, including ongoing FPGA development activities, will be presented. |
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CP11.00037: Remote Control Operation of High-k Scattering and FIReTIP on NSTX-U Ke Yao, Calvin W Domier, Jon Dannenberg, Yilun Zhu, Xianzi Liu, Brentley C Stratton, Yang Ren, Neville C Luhmann Two laser-based plasma diagnostic systems, the high-k poloidal scattering system [1] and the Far Infrared Tangential Interferometer/Polarimeter (FIReTIP) system [2], are under development for NSTX-U. The high-k system will collect density fluctuation data at 8 simultaneous scattering angles corresponding to poloidal wavenumbers from 7 cm-1 to >30 cm-1, while FIReTIP will provide time-resolved, chord-integrated electron density and tangential B-field data. Both diagnostics use CO2 and optically pumped FIR lasers to generate THz source beams, and are to be remotely controlled from the NSTX-U control room using a dedicated LabVIEW program. Key features include: (1) Real-time monitoring of the CO2/FIR laser status, (2) remote cavity control of each laser to enhance both power and frequency stability, and (3) remote control the high-k scattering launch mirror and 4-axis receiver optics to optimize the placement of the high-k interaction region to maximize physics output. Details of each diagnostic will be presented, detailing both the hardware and software elements of the remote controls. |
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CP11.00038: Divertor peak heat flux reduction induced by ELM filaments in NSTX Kaifu Gan, Rajesh Maingi, Travis Gray, Eric D Fredrickson, Kevin L Tritz, Promise O Adebayo-Ige, Elijah Stafford, Brian D Wirth, Adam McLean The occurrence of small ELMs with high (> 4 MW) neutral beam injection is observed to decrease the divertor peak heat flux in NSTX. Small ELMs are routinely observed in many discharges, resulting in < 50% transient increase in deposited power on the divertor. ELM filaments are observed with these small ELMs. The ELM filaments observed from divertor imaging are consistent with magnetic perturbations and filament rotation from the Mirnov diagnostic. The ELM peak heat flux decreases with the number of ELM filaments, and the divertor heat flux width increases with the number of ELM filaments. The divertor peak heat flux decreases during the ELM rise time with two or more ELM filaments. With one or no ELM filament, the divertor heat flux increases during the ELM rise time. Compared with the ELM-free phases in discharges, the integral power decay width (λint) at the ELM peak time can reach ~ 300% larger than in the ELM-free phase. Broadening of the heat flux footprint makes the divertor peak heat flux at the ELMs peak <40% of the divertor peak heat flux in ELM-free periods. The divertor heat flux width during these small ELMs and inter-ELM periods does not decrease with the plasma current. |
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CP11.00039: ELM divertor heat flux with edge harmonics oscillation in NSTX Elijah D Stafford Previously, strike point splitting induced by an edge harmonic oscillation (EHO) was observed in NSTX (K.F. Gan et al., 2017 Nucl. Fusion 57 126053). The EHO also appears during inter-ELM. The rotation of the EHO was accelerated when the ELM appeared, and the EHO disappeared quickly during the ELM decay time. During the ELM rise time, the ELM divertor heat flux distribution is consistent to the divertor heat flux with EHO, which indicates that the ELM heat flux deposited on the divertor has an n=1 strike point splitting structure. During the ELM decay, the disappearance of the EHO caused the strike point splitting to disappear along with a narrowing of the heat flux distribution. The integral heat flux (λint) within the ELM decay time is decreased by 50% compared to that at the ELM peak time; while the deposited power during the ELM decay time decreased slowly, such that the peak heat flux at the ELM decay time could be >100% larger than the peak heat flux at the ELM peak time. |
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CP11.00040: Solenoid-Free Startup in the Pegasus-III Experiment Michael W Bongard, Molly W Aslin, Stephanie J Diem, Abigail L Ferris, Raymond J Fonck, John A Goetz, Armand K Keyhani, Mark D Nornberg, Jilliann K Peery, Christopher Pierren, Joshua A Reusch, Alexander T Rhodes, Cuauhtemoc Rodriguez Sanchez, Rachel K Sassella, Carolyn E Schaefer, Aaron C Sontag, Timothy N Tierney, Justin D Weberski Solenoid-free startup techniques offer the potential to simplify the cost and complexity of tokamak fusion power plants by reducing the technical requirements of, or need for, a central solenoid. Robust solutions are critical to the spherical tokamak (ST) concept as well. The Pegasus-III Experiment provides a dedicated US platform for comparative solenoid-free startup studies. Presently completing construction and beginning commissioning, this new solenoid-free ST (A ≥ 1.22, Ip ≤ 0.3 MA, BT ≤ 0.6 T, tpulse ~ 0.1 s) will be equipped with local helicity injection (LHI) and coaxial helicity injection (CHI) startup systems; a 28 GHz radiofrequency (RF) system for heating and current drive; and enhanced diagnostics. The research mission is centered on developing the underlying non-solenoidal startup physics basis via validated, predictive models and the needed technology to effectively implement these techniques. Its long-term objective is to facilitate the deployment of MA-class startup concepts on larger-scale STs and beyond. Initial experiments are planned to extend LHI scenarios to BT ~ 0.3 T and Ip = 0.3 MA; develop transient CHI scenarios; and characterize electron Bernstein wave emission to optimize the RF system. |
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CP11.00041: The Pegasus-III Spherical Tokamak John A Goetz, Molly W Aslin, Michael W Bongard, Michael T Borchardt, Stephanie J Diem, Abigail L Ferris, Raymond J Fonck, Armand K Keyhani, Benjamin A Kujak-Ford, Benjamin T Lewicki, Mark D Nornberg, Jilliann K Peery, Christopher Pierren, Joshua A Reusch, Alexander T Rhodes, Cuauhtemoc Rodriguez Sanchez, Rachel K Sassella, Carolyn E Schaefer, Aaron C Sontag, Timothy N Tierney, Justin D Weberski, Gregory R Winz The Pegasus-III experiment is a solenoid-free spherical tokamak dedicated to comparative non-solenoidal tokamak startup studies. Major upgrades to the magnets and power systems will increase BT four-fold to 0.6 T for up to 100 ms with improved shape control while maintaining low A~1.2. This increased field directly supports the mission of researching non-solenoidal power plant-relevant startup techniques such as local and coaxial helicity injection and RF wave injection. These techniques will be contrasted and possibly combined. The Ohmic solenoid has been removed and the toroidal field system now is comprised of a new 24-turn center rod and outer coil system, torque assemblies with crossover finger joints, and integrated divertor coils. Approximately 175 MVA of programmable power systems and 7 MJ of stored energy are available to support the four-fold toroidal field increase, new divertor and poloidal field coils, local and coaxial helicity injectors, RF systems, and a diagnostic neutral beam. A new digital control system uses FPGA technology and real-time computing to control the electromagnets (IGBT buck converters) and helicity injectors (multi-level buck) while providing fault detection and protection. |
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CP11.00042: Emission Measurements of EBW for Development of a Microwave Heating System on the PEGASUS-III Experiment Jilliann K Peery, Steffi J Diem, Tim S Bigelow, Michael W Bongard, Raymond J Fonck, John A Goetz, Robert W Harvey, Cornwall H Lau, Mark D Nornberg, Yuri V Petrov, R Sassella, Joshua A Reusch, Aaron C Sontag Development of non-solenoidal heating and current drive (CD) techniques are advantageous to advanced tokamak concepts and critical for spherical tokamak (ST) concepts for compact fusion power plants. PEGASUS-III is an ultra-low A solenoid free ST focused on non-solenoidal plasma startup and sustainment techniques using local helicity injection (LHI), coaxial helicity injection (CHI) and microwave injection. PEGASUS-III will implement a microwave heating system employing a 28 GHz gyrotron for both electron Bernstein wave (EBW) and electron cyclotron (EC) heating. To aid the development of this system, measurement of mode-converted EBW emission will be carried out using a synthetic aperture microwave imaging diagnostic (SAMI), allowing the first exploration of EBW emission measurements from HI generated plasmas. The emission measurements will be used to validate the ongoing work of modeling EC and EBW injection into HI plasmas, optimize the antenna launch angle, and guide initial experiments. The 28 GHz system on PEGASUS-III will allow direct tests and investigation into synergistic improvement of proposed startup and ramp-up scenarios, providing results relevant for larger scale ST and reactor technologies.
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CP11.00043: Suppression of microtearing transport in diamagnetic well regimes for high-beta spherical torus configurations David R Smith, Michael W Bongard A diamagnetic well and local minimum |B| region was readily accessed in high-β plasmas driven by local helicity injection in the A ~ 1 Pegasus ST. ▽B reversal on the low-field-side is stabilizing for drift waves, reduces the trapped particle fraction, and expands the parameter space for fast ion trapping. The high-β plasma, however, remains net-paramagnetic with near omnigeneity (|B| ≈ |B|(ψ)) in the bad curvature region. Here, we report on the gyrokinetic stability of microtearing modes (MTM) in the Pegasus minimum |B| regime. Multiple classes of MTM at kyρs ~ 0.1-1 arise in the region ψN ~ 0.3-0.9. Collisionless high-k modes (kyρs ≈ 1) with narrow parallel mode structures are destabilized at βcrit ≈ 3%, and collisional low-k modes (kyρs ≈ 0.3) with extended parallel mode structures are destabilized at βcrit ≈ 12%. Nonlinear gyrokinetic simulations for a conventional monotonic |B| equilibrium show that the low-k MTM produce electromagnetic electron thermal transport, but the transport and low-k modes are suppressed in the diamagnetic well configuration. With finite-beta suppression of ITG and TEM, the results point to an attractive confinement regime for ST devices. |
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CP11.00044: Engineering Design of a Coaxial Helicity Injection System for Non-Solenoidal Startup Studies on Pegasus-III Joshua A Reusch, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, Benjamin T Lewicki, Aaron C Sontag, Timothy N Tierney, Justin D Weberski, Gregory R Winz, Roger Raman DC helicity injection is a promising way to address a critical challenge facing the spherical tokamak: non-solenoidal startup. Coaxial helicity injection (CHI) is one such technique studied on several devices. An engineering design is in process for a CHI system for Pegasus-III that will address outstanding issues for this startup scheme, including: eliminating the need for an axisymmetric vacuum vessel break; the scaling of Ip with injector and/or flux footprint shape and separation; minimizing impurity sourcing from the electrodes; the degree of axisymmetry required to achieve high Ip; and understanding the roles of reconnection and dynamo drive mechanisms. This design features two coaxial, segmented, floating electrodes in the upper divertor region. Their size and position enables Ip scaling studies with projected maximum Ip > 0.3 MA. Segmented electrodes enable rapid changes to shape and plasma facing material for impurity control. Current is sourced by six power feeds, enabling diagnosis and control of the drive symmetry. These features enable experiments to test the reconnection and dynamo current drive scalings that set the achievable peak Ip with CHI. |
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CP11.00045: Plasma Initiation by Local Helicity Injection (LHI) on Pegasus-III Aaron C Sontag, Molly W Aslin, Michael W Bongard, Michael T Borchardt, Steffi J Diem, Abigail L Ferris, Raymond J Fonck, Armand K Keyhani, Benjamin A Kujak-Ford, Benjamin T Lewicki, Mark D Nornberg, Jill K Peery, Christopher Pierren, Joshua A Reusch, Alexander T Rhodes, Cuauhtemoc Rodriguez Sanchez, Carolyn E Schaefer, Timothy N Tierney, Justin D Weberski, Gregory R Winz The Pegasus-III program is focused on developing the physics basis and predictive models for non-solenoidal tokamak startup. Local helicity injection (LHI) is a non-solenoidal tokamak startup technique utilizing localized current sources near the plasma edge. Electron beams injected into the vacuum magnetic field relax into a tokamak-like state, and drive toroidal current up to limits determined by either the helicity injection capability of the hardware or power balance. Several issues must be addressed before this technique is ready to deploy on a large facility including reliable operation with high LHI drive voltage, injector geometry optimization, gas control and minimization of impurity production. The LHI system on Pegasus-III features 4 discrete electron injectors near the outboard midplane capable of a total Iinj ≤ 16 kA and Vinj ≤ 1 kV, and a prototype non-circular injector capable of Iinj ≤ 8 kA and Vinj ≤ 500 V. A new Stark broadening diagnostic for arc fueling optimization and a new cathode spot detection system to minimize impurity injection and improve injector reliability are being deployed. These systems will facilitate investigation of the outstanding issues of LHI physics during the initial campaign of Pegasus-III. |
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CP11.00046: Diagnostic Overview for Non-solenoidal Startup Experiments on Pegasus-III Mark D Nornberg, M W Aslin, Michael W Bongard, Michael T Borchardt, Steffi J Diem, A. Louise Ferris, Raymond J Fonck, John A Goetz, Armand K Keyhani, Benjamin A Kujak-Ford, Benjamin T Lewicki, Jilliann K Peery, Christopher Pierren, Joshua A Reusch, Alexander T Rhodes, Cuauhtemoc Rodriguez Sanchez, R Sassella, Carolyn E Schaefer, Aaron C Sontag, Timothy N Tierney, Justin D Weberski, Gregory R Winz The diagnostic set for initial experiments on Pegasus-III is designed to quantify the efficacy of DC helicity injection and microwave injection as approaches to non-solenoidal tokamak startup. The diagnostic set must constrain kinetic equilibrium reconstructions, quantify magnetic helicity dissipation, characterize the MHD and kinetic activity associated with current drive and magnetic relaxation, quantify impurity content and radiative losses arising from plasma-material interaction, and characterize the current drive sources. The magnetics measurements utilize cable shielding and differential balancing to suppress volt level EMI from switching power supplies driving the electromagnets. Active charge exchange spectroscopy, microwave interferometry, and a Thomson scattering system provide constraints for the kinetic profiles. Radiation measurements from an AXUV diode array, a visible bremsstrahlung system, and SPRED are used with STRAHL simulations to quantify impurity dynamics. Visible imaging and Stark broadening measurements are used to characterize the dense injector plasmas. Microwave imaging is used for optimizing the launch angle and quantifying mode conversion efficiency for electron Bernstein wave injection. |
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CP11.00047: Characterization of Current Stream Structure in Local Helicity Injection on the Pegasus ST Carolyn E Schaefer, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, Mark D Nornberg, Aaron C Sontag, Joshua A Reusch, Justin D Weberski Local Helicity Injection (LHI) uses small, high-power electron current injectors at the plasma edge to provide non-solenoidal tokamak startup. LHI dynamics on Pegasus are consistent with a discrete injected current stream structure that persists in the plasma edge region following relaxation. Scaling LHI to larger devices requires an understanding of how this current stream structure evolves in time and its potential effect on equilibrium properties. Strong (~bZ/Bf ~ 10-2), low-frequency (~20–50 kHz) n = 1 activity is observed on the low field side (LFS) during LHI on Pegasus and is well-characterized by a singly line-tied kink instability of the injected current streams. A simple model of an oscillating helical current stream just outside the plasma edge closely reproduces LFS dBZ/dt and B(R,t) measurements. Accounting for the effects of this 3D current stream structure is important because prior work on other tokamaks has shown that even small, nonaxisymmetric perturbations (dB/B0 ~10-4) can greatly modify plasma performance. The helical current stream model can be used to inform future 3D equilibrium studies of LHI using codes like the Generalized Perturbed Equilibrium Code (GPEC). |
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CP11.00048: Identifying Beam-Driven Instabilities Responsible for Current Drive on Pegasus-III R Sassella, Stephanie J Diem, Mark D Nornberg, Joshua A Reusch, Alexander T Rhodes, Carolyn E Schaefer Local Helicity Injection (LHI) is a non-solenoidal startup technique that requires an Alfvénic instability to generate turbulence that drives toroidal current through magnetic relaxation. Significant broadband magnetic turbulence has been measured in LHI discharges on Pegasus and is responsible for current amplification of injected current streams through dynamo-like effects. The turbulence arises from instabilities driven by super-Alfvénic electrons. Several candidates are kinetic Alfvén wave, electron beam ion cyclotron, and electron beam Alfvén wave instabilities. Dependence on relative electron beam density, Alfvén Mach number, and guide field can distinguish these instabilities. Identifying the instability responsible for initiating the turbulence is important to predicting the requirements for a helicity injection system for larger tokamaks and ultimately a fusion power plant. Upcoming experiments leverage insertable probe arrays, the improved Pegasus-III diagnostic set, the expanded operating space of Pegasus-III, and multi-dimensional correlation analysis to identify the instability or instabilities driving the turbulence and explain the observed current drive scaling. |
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CP11.00049: Confinement Scaling Projections for Local Helicity Injection Plasma Startup on Pegasus-III Justin D Weberski, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, John A Goetz, Mark D Nornberg, Joshua A Reusch, Alexander T Rhodes, Aaron C Sontag Ip(t) in plasmas produced by Local Helicity Injection (LHI) is described by global helicity conservation and a Taylor relaxation limit that is less restrictive as the device size and BT is increased. In the helicity-limited regime, the ability to project Ip(t) depends on understanding the dissipation of injected helicity. As no first-principles model presently exists for confinement during LHI (during which stochastic and/or reconnection-driven transport may also occur), modeling to date has assumed resistive dissipation (η∼Te-3/2) and utilized tokamak τE scalings. Observations of LHI plasmas on Pegasus at BT < 0.15 T in transport equilibrium were consistent with estimates from neo-Alcator and collisional stochastic models, assuming modest auxiliary heating from reconnection. Modeling considering the expanded helicity drive, increased BT (< 0.60 T), and ne in Pegasus-III suggests favorable, distinguishable trends in Ip and Te amongst the assumed confinement regimes. Recent extensions to an LHI 0D power-balance model incorporate a dynamic estimate of τE and self-consistently find Ip(t) and Te(t) to treat scenarios outside transport equilibrium and assist in design and execution of initial Pegasus-III LHI studies. |
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CP11.00050: Investigating The Role of Impurities in Plasma Startup Via LHI on Pegasus-III Cuauhtemoc Rodriguez Sanchez, Stephanie J Diem, Raymond J Fonck, Mark D Nornberg, Gregory R Winz The influence of impurities in plasma start-up is investigated for local helicity injection on Pegasus-III. Impurity studies are critical to understanding helicity dissipation during the initial ramp-up and the power balance after the handoff. Previous Pegasus experiments at low BT show that transport plays an important role in the charge-state balance of impurity distribution and radiative power losses. The new Pegasus-III facility will operate up to BT of 0.6 T enabling a wider range of transport levels as confinement in LHI is predicted to improve with BT. In preparation for these experiments, the impurity diagnostics have been upgraded. A high-resolution grating was installed on SPRED to help identify previously unresolved lines. New coupling optics on a visible spectrometer allow the use of the whole CCD to improve the measurement of bremsstrahlung to determine Zeff. A new 32-channel AXUV radiometer was designed and calibrated for Prad measurements. This set of diagnostics will be used in conjunction with STRAHL to assess the impurity dynamics. The information obtained from these experiments is used to extrapolate the performance of LHI in larger scale devices and potentially future fusion power plants. |
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CP11.00051: Characterizing the Thomson Scattering System on the Pegasus-III Experiment Timothy N Tierney, Stephanie J Diem, Roger Raman, Joshua A Reusch, Cuauhtemoc Rodriguez Sanchez The Pegasus-III experiment uses a multi-point Thomson scattering system to observe the electron temperature and density evolution of non-solenoidal startup plasmas. As part of the upgrade to Pegasus-III, detailed characterization and calibration of the Thomson system was performed to improve the measurement quality and ease-of-use of the diagnostic. Specifically, a series of photometric calibrations were used to quantify the gain and noise enhancement as a function of voltage for the intensifiers used on the CCD cameras. Additionally, improvements were made to the spectrometer's fast shutter design to reduce the shutters' open time from 10 ms to near 2 ms, depending on the components used. One near-term use for the system is to examine the temperature evolution of the coaxial helicity injection (CHI) plasmas. CHI is a form of non-solenoid startup using current driven on open field lines connecting two co-axial, electrically isolated divertor plates to create the plasma. CHI modeling indicates that the electron temperature plays a key role in plasma current evolution and closed flux current generated. Thus, Thomson measurements are critical to understanding CHI dynamics and comparing to these models. |
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CP11.00052: Diagnostic Neutral Beam and Charge Exchange Recombination Spectroscopy Diagnostic for Studying Non-Solenoidal Tokamak Plasma Startup in Pegasus-III Armand K Keyhani, Michael W Bongard, Stephanie J Diem, Raymond J Fonck, Benjamin T Lewicki, Mark D Nornberg, Gregory R Winz Localized measurements facilitated by a diagnostic neutral beam are essential for characterization of tokamak plasmas produced by non-solenoidal startup methods. Beam-based diagnostics provide constraints on kinetic equilibrium reconstructions, enable the measurement of impurity densities, and allow for indirect measurement of current density profiles. A 60–80 kV, ≤ 4A, H0 beam is being developed for evaluating plasmas in the Pegasus-III spherical tokamak. Ion heating from magnetic reconnection during helicity injection (HI) contributes significant pressure to the equilibrium, and edge biasing induces flows that can affect transport. Understanding the impact of reconnection heating and flows on HI plasmas is needed to inform the performance scaling and optimization of the technique. A new charge-exchange recombination spectroscopy diagnostic will measure ion temperature and flow velocity profiles to explore the impact of anomalous ion heating on power balance, impurity transport, and equilibrium parameters in HI plasmas. Initial tests of the uninsulated high-voltage beam power supply have produced 5 kV pulses with less than 0.2% RMS ripple. |
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CP11.00053: Design considerations for a transient CHI gas injection system for Pegasus-III Roger Raman, John Rogers, Joshua A Reusch, Steffi J Diem Transient Coaxial Helicity Injection, a method first developed on the HIT-II experiment and then validated on the NSTX device is a method to initiate an inductive-like tokamak plasma discharge without reliance on the central solenoid. In both these devices, toroidal ceramic insulators were used to electrically separate the inner and outer vessel components. Magnetic flux that initially connects the inner and outer vessel components could be grown into the vessel using J x B forces to generate a closed magnetic field line configuration. In reactors, the installation of large toroidal insulators as part of the vacuum vessel boundary may not be possible. To address this design requirement, a first of its kind, floating double biased reactor-relevant CHI configuration is being developed for Pegasus-III. An equally important requirement for successful transient CHI discharge initiation is the need for injecting a relatively small amount of gas while simultaneously satisfying the requirements for gas breakdown in the injector region and avoiding breakdown in other parts of the vessel. This stringent requirement generally requires that a small gas plenum be located close to a toroidal gas manifold in the injector region in a high gas conductance configuration. Design aspects for a transient CHI gas injection system for the Pegasus-III geometry will be described. |
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CP11.00054: Vertical control modeling and simulation for the STEP prototype reactor Zichuan A Xing, Himank Anand, Jayson L Barr, Anders Welander, David A Humphreys, Oliver Bardsley, Morten Lennholm Vertical stability control systems that are robust in radiative environments are a critical need for the design of the Spherical Tokamak for Energy Production (STEP) prototype reactor. Diagnostic models and control schemes are developed and simulated using the TokSys[1] suite of tools in order to evaluate feasibility and controllability of the proposed diagnostic and actuator combination. Modeling shows that having dz/dt calculated from the induced coil current in passive stabilizing hoops, combined with z position estimation using double-null diverter heat flux imbalance measurements, can achieve adequate vertical control of STEP’s flat-top scenario, as long as the heat flux-based z estimation can achieve an effective response time shorter than 0.1s. Simulations are used to find the optimal controller gain settings for a range of effective time constants, with and without simulated diagnostic noise. TokSys simulations are further used to scan elongation to evaluate the controllability of alternative plasma shapes and establish a range of plausible flattop plasma shapes. The maximum controllable displacement control metric is evaluated for the range of parameters simulated. |
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CP11.00055: MFE: SUPERCONDUCTING Session Chairs: |
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CP11.00056: Self-consistent integrated modeling for the Q≥1 steady-state mission of the Burning Experimental Superconducting Tokamak (BEST) Siye Ding, Joseph T McClenaghan, Andrea M. Garofalo, Lang L Lao, Tim Slendebroek Following two different approaches, fully non-inductive scenarios have been developed aiming at a Q≥1 steady-state goal for the Burning Experimental Superconducting Tokamak (BEST), a new superconducting tokamak under design in ASIPP, China. One striking feature of BEST is the long-pulse D-T operation capability with metal wall and tungsten divertor. To meet the Q≥1 steady-state goal for BEST, fully non-inductive scenarios are designed using the self-consistent STEP physics module running under the OMFIT integrated modeling framework. Two different approaches were pursued: high density, bootstrap current dominated (high-βP approach) and low density, external driven current dominated (reversed shear approach). In the high-βP regime, several operational points are explored at Ip~4 MA, βN~2.4 and fGr (Greenwald fraction) ranging from 0.8 to 1.2. ITBs at large radius (ρ=0.75) are predicted in these cases. Pellet-like external particle source at ρ~0.6 is important for establishing such ITB in density. The reversed shear approach is more challenging, due to limited NBCD capability and LHW accessibility at high pedestal temperature. An operational point close to the goal is found at Ip~5 MA, βN~2.0 and fGr~0.3. Work supported by GA ASIPP BEST Project under 22KH000044US. |
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CP11.00057: Interpretative modelling of boron transport in the WEST boundary plasma in experiments with impurity powder dropper Kirill Afonin, Grant M Bodner, Hugo Bufferand, Guido Ciraolo, Corinne Desgranges, Pascal Devynck, Ahmed Diallo, Alberto Gallo, Jonathan Gaspar, Christophe Guillemaut, Remy Guirlet, James P Gunn, Nicolas Fedorczak, Thierry Loarer, Robert A Lunsford, Yannick Marandet, Philippe Moreau, Federico Nespoli, Nicolas Rivals, Patrick Tamain One of the goals of the WEST tokamak is to test the viability of W plasma-facing components (PFCs) in a tokamak environment in order to prepare operation for future devices like ITER and DEMO. Usage of W PFCs leads to the contamination of the core plasma by W, which can cause significant radiative energy loss. A common technique to prevent W contamination of tokamak plasma is conditioning of PFCs with low-Z coating layers. Active wall conditioning is being studied in WEST through the injection of B powder with a PPPL Impurity Powder Dropper (IPD). An open question in this process is the nature of the transport and deposition of the B atoms on PFCs. For this reason, a modelling workflow is developed to analyze the impact of B injection on the plasma and to interpret experimental data. The workflow consists of a) deuterium-oxygen (D+O) plasma modelling to simulate plasma prior to B injection; b) B powder ablation modelling using the Dust Injection Simulator (DIS) code; and c) D+O+B plasma modelling with B neutral source provided by dust ablation modelling. Synthetic diagnostic tools are applied to measure the impact of B injection on plasma parameters in the simulation and the results are compared with the measurements by various diagnostics during IPD experiments in WEST. |
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CP11.00058: Integration of OEGDE Modelling into synthetic diagnostics for assessment of W Transport in the Scrape Off Layer of WEST Sean R Kosslow, David C Donovan, Alex GROSJEAN, Jake B Maeker, Nicolas Fedorczak, Christophe Guillemaut, Jamie P Gunn, Tennessee Radenac, C.Christopher Klepper, E.A. Unterberg, Curtis Johnson The WEST tokamak provides a unique capability to study the performance of a fully-tungsten, long pulsed plasma similar to the ITER operating scenario. Understanding tungsten (W) transport in the scrape off layer (SOL) is crucial for preventing contamination of the core plasma. To improve the understanding of SOL models, synthetic diagnosis techniques are needed to make direct comparisons with the experimental diagnostic data. A synthetic spectroscopy diagnosis technique has been developed using wavelengths and photon emissivity coefficients (PECs) for the full neutral W spectrum calculated by ColRadPy to compare with visible spectroscopy brightness (ph.m-2.s-1.sr-1). The PECs are used to create emissivity profiles based on the OEDGE background plasma and W profiles. The CHERAB framework is used to simulate the expected intensities of the emission lines in the visible wavelength range by integrating the emissions over the lines of sight at the lower divertor and baffle and accounting for reflections on the all metal walls of WEST. This workflow is employed to compare the expected W excitation line of sight intensities and W-I spatial impurity profiles produced from modelling of a WEST L-mode power scan 1.5 – 2.35 MW reaching the SOL to the corresponding experimental data. The level of agreement of the synthetic spectra impurity profiles with measured spectra and impurity brightness profiles is evaluated to benchmark the workflow. |
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CP11.00059: Developing an iterative synthetic diagnosis workflow for light impurities on WEST Curtis A Johnson, Ezekial A Unterberg, Chris Klepper, Yannick Marandet, Madhusudan Raghunathan, Sean R Kosslow, Christophe Guillemaut, Nicolas Fedorczak, Alex GROSJEAN, David C Donovan
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CP11.00060: Experimental quantification of lower hybrid current drive power on EAST Seung Gyou Baek, Miaohui Li, Paul T Bonoli, Bo Jiang Ding, Jin Ping Qian, Gregory M Wallace Quantitative evaluation of the lower hybrid current drive power absorption on EAST is critical for characterizing the confinement quality and for wave-model validation. While the high-density operation toward the high-performance regime may exacerbate parasitic edge losses, estimations of the effective power absorbed are not readily available. To evaluate the power absorption coefficient, α, a temporal modulation of the LH power of 1 MW is applied to the high-density H-mode plasmas (nebar = 4x1019 m-3) with on-axis electron cyclotron heating. The rise and fall of the stored energy allows experimental evaluation of α by taking the approach in [V. Pericoli et al, PPCF 39, 1115 (1997)]. One feature observed is that the rise in magnetic energy (internal inductance) precedes the increase in thermal energy, implying fast electrons are produced in the on-axis region. Initial analyses show that α ≈ 0.5 (0.35) for the fLH=4.6 GHz (2.45 GHz) cases. A phasing scan at 4.6 GHz yields a slight increase in α when the launched n|| is increased 2.04 → 2.48, consistent with the improved wave accessibility, but within the error bars. Further experimental data analysis and GENRAY/CQL3D modeling to examine wave propagation and absorption will be presented. |
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CP11.00061: Model-based Feedback Control Design Towards Robust Realization and Sustainment of Advanced Scenarios in EAST Zibo Wang, Eugenio Schuster, Xiao Song, Tariq Rafiq, Yao Huang, Qiping Yuan, Zhengping Luo, Jayson L Barr, Wilkie Choi, Christopher T Holcomb, David A Humphreys, Alan W Hyatt, Will Wehner The safety factor profile plays a critical role in the achievement of advanced tokamak scenarios, which are characterized by high confinement and the non-inductive sustainment of the plasma current necessary for steady-state operation. In order to satisfy magnetohydrodynamic stability constraints and performance requirements, the shaping of the safety factor profile usually needs to be complemented by tight control of other plasma kinetic scalars such as the plasma internal energy or the normalized β. Thus, model-based off-line (Liner-Quadratic-Integral (LQI) Control) and on-line (Model Predictive Control (MPC)) optimal feedback controllers have been developed to regulate the safety factor profile at different spatial locations, or a function of the spatial integral of this profile such as the plasma internal inductance, in combination with a set of plasma kinetic scalars. These control algorithms, which actuate the plasma current, the plasma density, the low-frequency (2.45 GHz) and high-frequency (4.60 GHz) lower-hybrid-wave powers, and the individual neutral-beam-injection powers, have been recently implemented in the EAST Plasma Control System (PCS) and tested experimentally. The to-be-reported experimental results show significant progress towards robust scenario control in EAST. |
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CP11.00062: Non-Inductive Plasma Vertical Position and Current Profile Measurements on EAST David L Brower, Hui Lian, H.Q. Liu, W.X. Ding Plasma vertical position is measured non-inductively using a Faraday rotation based POlarimeter-INTerferometer (POINT) diagnostic on EAST. The non-magnetic Faraday-effect approach is fusion reactor relevant as it has no integrator issues like those encountered using flux loop techniques. In addition, the Faraday measurement offers fast time response like magnetic pickup coils and can be used to track changes in position on the time scale of MHD instability growth rate. During the most recent run campaign, the POINT diagnostic has been successfully used to measure the plasma vertical position for the record 1056 second high-temperature discharge. In addition, the time response of the position measurement has been tested in plasmas where the vertical position changed by greater than 1 cm on a sub-millisecond timescale. Furthermore, current profile broadening with injection of ECRH at different radii is measured directly by the POINT system on EAST. The measured current change is consistent with decreasing plasma inductance. |
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CP11.00063: Gyrokinteic simulations of microturbulent transport in KSTAR in the presence of magnetic islands Javier H Nicolau, Zhihong Lin, Gyungjin Choi, Tyler B Cote, Dmitriy M Orlov, Jong-Kyu Park, Eric Howell, SeongMoo Yang, SangKyeun Kim Gyrokinetic simulations of microturbulence in the presence of island in KSTAR are presented. Magnetic islands generated by RMP coils are included in the equilibirum plasma which is obtained from M3DC1 MHD simulations of discharge 19118. Global gyrokinetic simulations using the GTC code are perfomed including kinetic electrons. ITG turbuelnce is observed during the simulated time. Linear analysis indicates a simlar growth rate, frequency and spectrum of the ITG instability when the islands are artificially removed. Nonlinear simulations are indiciating as well similar overall transport levels. However, larger transport levels are observed near the X points of the island compared to the O points. The generation of a vortex mode with the same structure of the magnetic island is observed. |
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CP11.00064: Pedestal optimization right after L-H transition by machine learning-based preemptive RMP application algorithm for avoiding ELM-crash event in KSTAR Giwook Shin, Minwoo Kim, Hyunsun Han, Sanghee Hahn, Wonha Ko, Seong-moo Yang, Sang-Kyeun Kim, Jae-wook Kim, Gunyoung Park, June-woo Juhn, Ju-hyueok Jang, Jongha Lee The unmitigated large ELM crashes can cause severe damage to plasma-facing components (PFCs) in ITER tokamak. Especially the first large ELM crash right after L- to H-mode transition is a concern in that even a single ELM crash can also reduce the lifetime of the PFCs. In order to solve the issue, the RMP is preemptively applied to the plasma right after the H-mode transition by a machine learning-based RMP control algorithm in KSTAR [1]. In the previous study, we applied the preemptive RMP after the moment that the plasma is less affected by the shaping effect for focusing on the first ELM crash suppression. However, since we want to apply the preemptive RMP at a typical onset of H-mode transition where the plasma shape rapidly changes from circular to diverted, the ML-based method encounters a harsher condition to suppress the first ELM crash. Through the optimization process to overcome the condition, we found the optimized slew time and amplitude of the preemptive RMP. As a result, the plasma obtained after the optimization shows different plasma profile patterns from those shown in conventional RMP-ELM crash suppression experiments. In addition, after the pedestal in which the first ELM crash is suppressed is formed, the ELM suppression is maintained over 37 seconds. |
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CP11.00065: FUND: NON-NEUTRAL, ANTIMATTER, STRONGLY COUPLED PLASMAS Session Chairs: |
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CP11.00066: Fully Self-Consistent Calculation of Neoclassical Transport in Nonneutral Plasma Columns Thomas M O'Neil, Andrey Kabantsev, Daniel H Dubin Slow radial expansion of a nonneutral plasma column of length L, caused by θ-asymmetric applied static E or B fields, has for decades posed challenges to theory. One issue is that the self-consistent plasma potential in the presence of the field error must be calculated accurately, including finite-length effects. For large B, a simplified bounce-averaged plasma response is sufficient to obtain transport predictions2 that match experiments. However at lower magnetic fields (< roughly 1 T for 1 eV electrons at 107 cm-3) the plasma response is dominated by bounce-rotation resonances, and a fully self-consistent calculation of the plasma potential must be carried out. This involves a numerical iteration technique whereby the plasma distribution function is calculated using the Fokker-Plank equation, in the presence of a given field error potential; then this distribution is used to calculate an improved potential and the process is repeated until convergence in the potential is achieved. For known applied field errors consisting of either a tilt of the magnetic field or an applied electrostatic asymmetry, this numerical method accurately matches experimental transport rates. Results will be presented for both L and B transport scaling from such errors. |
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CP11.00067: Global thermal equilibrium for non-neutral plasma in non-uniform field traps with cylindrical symmetry Patrick Steinbrunner, Matthew R Stoneking, Thomas M O'Neil, Daniel H Dubin The confinement of a non-neutral plasma in a global thermal equilibrium state is known to |
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CP11.00068: Evaluating emission characteristics of field emission cathodes over long pulse durations Madison R Howard, Joshua E Coleman, Steven Lidia The cathode test stand (CTS) at LANL is designed for voltages up to 500kV and utilizes a PFN which can deliver pulse durations up to 2.6 μs. We are able to measure the emission characteristics of various field emitters on the CTS over a large range of long pulse lengths. Diagnostics include cathode emission imaging, current density (J(x,y)) and emittance measurements with a scintillator coupled with a pepperpot mask and temporal characterization with B-dots and E-dots. In addition, a multi-frame camera has been employed, providing the ability to track J(x,y) on the scintillator throughout time. Here, we will show the extracted characteristics of a given cathode material for several pulse durations. In addition, we describe the correlation between the beam brightness and cathode material. |
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CP11.00069: Inviscid damping of a perturbed elliptical vortex in an ExB strain flow in nonneutral electron plasmas Pakorn Wongwaitayakornkul, James R Danielson, Noah C Hurst, Daniel H Dubin, Clifford M Surko The inviscid spatial Landau damping of the oscillation of a perturbed two-dimensional (2D) vortex under an applied external strain flow is studied using an electron plasma confined in a Penning-Malmberg trap. The experiments exploit the unique properties of magnetized, single-component plasmas to study 2D vortex dynamics through the isomorphism between the 2D Drift-Poisson equations that describe plasma E x B drift dynamics and the Euler equations that describe inviscid incompressible fluid flows. An imposed external strain field is produced by fixing the electrical potential at the boundary. An elliptical vortex patch under a constant strain flow undergoes nutation. With a finite peripheral vorticity gradient, the oscillatory motion of the vortex damps towards a stationary elliptical equilibrium state. The damping rate is found to be independent of strain rate for quasi-flat vortices and is determined by the initial state of the system. Vortex-in-cell simulation results for non-linear effects such as separatrix crossing of peripheral vorticity and interactions with the harmonics of the fundamental resonance will also be discussed. |
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CP11.00070: Gabor lens for use within a Laser-hybrid Accelerator for Radiobiological Applications (LhARA) Christopher Baker Most radiotherapy treatments are currently undertaken using photons, although the use of particle beams are becoming more widespread as they can more precisely target tumours. However, even though particle beams typically operate at low dose rates (<10 Gy/min) with tailored beam characteristics, damage to healthy tissue limits the deliverable dose and hence the clinical efficacy. Recently, the identification of the so-called ‘FLASH’ radiotherapy (> 40 Gy/min) regime presents an exciting development to overcome these limitations, although further understanding of the radiobiological effects is required. |
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CP11.00071: Mode coupling of Trivelpiece-Gould (TG) and diocotron modes in a nonneutral plasma column Daniel H Dubin, Nicola Panzeri, Andrey Kabantsev When an l = 2 diocotron mode is launched to amplitude A, a single l = 0 TG mode is observed to branch out into three waves: an upper and lower frequency TG wave, and a weaker central TG wave, whose frequencies all approach one-another as A decreases with time (due to weak diocotron mode damping). The frequency splitting of the TG waves is proportional to A at larger A values. For infinite magnetic field B this splitting is analogous to the behavior of the eigenmodes of an elliptical drumhead. At finite B it may be analyzed using four-wave mode-coupling theory with the diocotron mode acting as a low-frequency pump wave, and the upper and lower TG waves acting as daughter waves that are admixtures of nearly degenerate l = ±2 and l = 0 TG waves, mode-coupled by the pump. The central TG wave is a third more weakly-coupled daughter wave with θ symmetry that differs from the other branches; hence the weaker coupling. The mode-coupling theory of this effect compares well with the experiments. |
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CP11.00072: Density Mapping of Negative Hydrogen Ions in Cold Electron Plasma Andrey Kabantsev, Charles F Driscoll In a room temperature electron plasma, a small fraction of negative hydrogen ions (H− ions) gradually accumulates due to dissociative attachment in collisions of cold electrons with excited hydrogen molecules, and it is continuously carried away radially by faster ion transport. This fraction of H− ions changes many properties of the plasma column; including a straightforward decrease in the frequency of plasma waves, and it makes a large contribution to the waves (collisional) damping rates. In our work, the radial density distribution of H− ions is mapped in an electron plasma column confined in a Penning-Malmberg trap. The trap is equipped with an electron-sensitive Phosphor Screen/CCD (PS/CCD) only, so a direct imaging of H− ions is not possible. Rather, we find nH(r) by comparing images of ne(r) with and without H− ions stripped of their extra electrons. Here, the H− ion stripping can be done by intense (laser) light (hν ≥1eV) or by electron detachment in collisions of accelerated H− ions with heavy (background) molecules. Fortunately, the electron detachment cross section in collisions of 10÷20eV H− ions with heavy molecules is quite big (∼20Å2). At a residual pressure of 10-9 Torr, the H− stripping rate is about 0.5/sec. At a selected moment of plasma evolution, the H− ions are quickly (<0.1sec) energized in axial velocity by bounce-resonant sloshing of the plasma column. Then, during a few seconds of confinement, the accelerated H− ions lose their extra electrons, which now contribute to the electron density ne(r) at subsequent dump to PS/CCD. By subtracting a similar image taken without the H− acceleration/stripping cycle, we obtain the distribution nH(r). |
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CP11.00073: Dipole traps for non-neutral plasma and pair plasma studies Matthew R Stoneking, Alexander Card, Patrick Steinbrunner, Thomas Sunn Pedersen Dipole traps have demonstrated good confinement properties for non-neutral [1] and quasi-neutral plasmas [2]. We describe two such traps, one under construction and the other in the design phase that will be used to study electron-positron pair plasma and pure electron plasma confinement, respectively. The construction of the APEX (A Positron-Electron eXperiment) – levitated dipole is nearing completion. It makes use of a 15 cm diameter high-temperature superconducting (HTS) coil (I < 60 kAt) that is inductively charged using a second HTS coil (I < 164 kAt). Stable levitation is achieved with active feedback on the lifting coil current and we anticipate levitation times of order one hour with the use of a cooled radiation shield surrounding the trapping region. We present progress on construction and commissioning of system components as well as results of experiments in a prototype supported dipole trap – results that include efficient injection of positrons into a dense electron cloud [3]. A new supported dipole trap is in the design stage at Lawrence University. The design of this device is guided by emerging theory and calculations regarding the requirements for well-confined thermal equilibrium states for non-neutral plasma [4]. These states reside exclusively on the outboard midplane and are therefore likely accessible in a supported dipole trap. A primary aim is to identify a suitable target electron plasma into which to inject positrons in the APEX levitated dipole. |
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CP11.00074: Buffer-Gas Trap for the NEPOMUC High-Intensity Positron Beam. J. R. Danielson, A. Deller, E. V. Stenson, M. Singer, S. Nissl, C. W. Rogge, M. R. Stoneking, C. Hugenschmidt, T. Sunn Pedersen, C. M. Surko Buffer-gas positron traps have dramatically extended the scope for atomic and non-neutral plasma physics experiments involving positrons. In these devices, a continuous beam of positrons is magnetically guided into a Penning-Malmberg trap, wherein inelastic collisions with low-density molecular gases allow for efficient capture in a single pass. The APEX collaboration aims to produce a neutral pair plasma, comprised of equal quantities of electrons and positrons, confined by the magnetic field of a levitated dipole. More than $10^10$ positrons are needed to achieve a short-Debye-length plasma with a volume of 10 litres and a temperature $< 1$~eV, which necessitates new advances in positron accumulation. We present our plans for the installation of a buffer-gas trap at the NEPOMUC neutron-induced positron source in Munich. Beyond the pair plasma experiments, an intense trap-based positron beam will also facilitate new applications in positron materials interactions. |
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CP11.00075: Preliminary design of charged particle injection for EPOS (Electrons and Positrons in an Optimized Stellarator) Jason Smoniewski, Eve V Stenson, Thomas S Pedersen, Matthew T Beidler, Matt Landreman, Michael Drevlak Pair plasmas, with both species of identical mass, have attracted theoretical and computational attention for years as a fundamental plasma scenario. As part of the APEX (A Positron Electron eXperiment) collaboration, the EPOS device, a quasi-symmetric tabletop-sized stellarator, is being designed to confine such a pair plasma. Numerical optimization provides a number of two-field-period quasi-axisymmetric candidate equilibria, and coil sets are evaluated for the possibility of positron injection. Experiments at APEX have previously demonstrated lossless transport of positrons across the magnetic field by ExB drift injection. For EPOS, drift injection requires stray magnetic field connecting to the positron beam line, and sufficient space for tailored electrostatic potentials. At the low energies, high fields, low rotational transform, and extremely low collisionality in EPOS, simulations of individual particle trajectories can address questions of injection and confinement. These simulations are underway, and are starting to address questions of the required rotational transform and degree of quasi-symmetry. |
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CP11.00076: Moving into higher fields and collective behavior: recent advancements in the direction of matter-antimatter pair plasmas Eve V Stenson, Alexander Card, James R Danielson, Adam Deller, Juliane Horn-Stanja, Christoph Hugenschmidt, Paul Huslage, Stefan Nissl, Thomas Sunn Pedersen, Carl Wilhelm Rogge, Lutz Schweikhard, Markus Singer, Martin Singer, Jason Smoniewski, Patrick Steinbrunner, Clifford M Surko, Matthew R Stoneking, Jens Von Der Linden The ultimate goals of the APEX (A Positron Electron eXperiment) Collaboration are the generation and investigation of confined, strongly magnetized, electron-positron plasmas in the laboratory. Our road map to achieving this requires unifying and advancing state-of-the-art physics and engineering in several areas, including: extended accumulation and high-capacity storage of large numbers of positrons, originating from a world-class source; two superconducting, tabletop-sized toroidal confinement devices with complementary magnetic topologies (a dipole and a stellarator), in which the positrons will be combined with electrons and their plasma properties studied; and the development and verification of a number of essential enabling techniques --- e.g., efficient transport of positrons across magnetic flux surfaces and subsequent trapping (previously demonstrated in the single-particle regime [1, 2]). This poster will provide a broad overview of recent key progress along that road map, such as non-neutral plasma trap development [3]; the lossless injection of positrons into an electron cloud dense enough to generate a substantial space charge [4]; the development of the "primary" positron beam down to lower energies [5]; the extension of injection techniques to higher fields, as well as much broader range of velocity distributions for the incoming positrons; and the development status of the two toroidal traps. |
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CP11.00077: A 48 BGO detector array to measure volumetric and localized annihilation from a magnetically confined electron-positron pair plasma Jens Von Der Linden, Stefan Nissl, Adam Deller, Juliane Horn-Stanja, James R Danielson, Matthew R Stoneking, Alexander Card, Eve V Stenson, Thomas S Pedersen The APEX collaboration aims to magnetically confine electron-positron pair plasma in a levitated dipole and diagnose the plasma with FPGA processing of annihilation detections from an array of 48 Bismuth-Germanate (BGO) scintillators. Direct annihilation and decay positronium formed through radiative and three-body recombination produces a volumetric source of gammas. Two-gammas from direct or para-positronium annihilation can be detected in coincidence, allowing for tomographic reconstruction of the volumetric source. Positronium drift and subsequent ionization and pair collisions with neutrals and plasma drive cross-field transport which results in localized sources of two-gamma annihilation at the wall and magnet. The rates of various mechanisms depend on plasma temperature and density and the partial pressures of background gases. Triangulation, distance attenuated single-photon counting, and the ratio between localized and volumetric decays will provide diagnostics for the evolution of the plasma. We are developing techniques to differentiate between volumetric and localized sources and have conducted measurements with β+ emitters placed on rotating turntables to emulate pair plasma distributions. |
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CP11.00078: A Kinetic Model for Pure Electron Plasma Compression via the Rotating Wall Technique Malcolm Lazarow, Eugene Kur, Andrey Zhmoginov, Joel Fajans, Thomas M O'Neil, Jonathan S Wurtele We present a model for pure electron plasma compression in which a rotating wall (RW) electric field couples to the ExB rotation and axial motion of the electrons. The model assumes that the plasma is in a slowly evolving thermal equilibrium. Kinetic theory is employed to find an expression for the coupling of the rotating wall field to resonant plasma particles, and averaging allows for the derviation of equations for the angular momentum and temperature of the plasma. Numerical solutions of the model are consistent with previous experimental results [Danielson and Surko, Phys. Plasmas 13 (2006)]. The model predicts, depending on system parameters and initial conditions, three compression regimes: rapid compression, exponentially long wait time to compression, and no compression. We outline a method that, within the approximations of the model, enables compression to high densities. The scheme envisions a sequence of jumps to ever higher rotating wall frequencies, with the jumps occuring just as the plasma ExB rotation frequency approaches the rotating wall frequency. The numerical method for solving the equations, which is based on solving for the phase portraits of the flow, will be presented. |
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CP11.00079: Patch Potential Measurements in a Penning-Malmberg Trap Andrew J Christensen, Joel Fajans In the ALPHA-2 spectroscopy trap, positron plasmas heat more quickly after the trap is illuminated by laser light. In the new vertical ALPHA-g gravity measurement trap, positron plasmas expand to the trap wall in seconds, preventing antihydrogen formation. By measuring the displacement of electron plasmas from the center of the trap, we verify that these issues are due to variations in the electric potential along the trap surface (patch potentials). We also find that patch potentials can be worsened by laser light, that directing an electron beam toward the trap wall changes patch potentials, and that warming the trap can restore patch potentials to a nominal persistent magnitude. |
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CP11.00080: Adiabatic Cooling of Antihydrogen Danielle L Hodgkinson, William A Bertsche, Joel Fajans Antihydrogen is now routinely formed by merging antiproton and positron plasmas in the ALPHA experiment. Neutral anti-atoms with energy less than around 0.5 K are trapped in a modified Ioffe-Pritchard magnetic trap. Reducing trapped antihydrogen energy is expected to increase precision in experiments that measure fundamental properties of antihydrogen for precise comparison to hydrogen. |
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CP11.00081: Optimizing Adiabatic Expansion Cooling to Improve Precision Measurements on Antihydrogen Nicolas Kalem, Danielle L Hodgkinson, Joel Fajans, Jonathan S Wurtele As a test of the fundamental asymmetries between matter and antimatter, one can measure the electric charge of antihydrogen. In 2016, the ALPHA (Antihydrogen Laser PHysics Apparatus) experiment at CERN used stochastic acceleration (random electric kicks applied to putatively charged antiatoms) to place a limit on the charge of antihydrogen to about one part per billion [Ahmadi, M., Baquero-Ruiz, M., Bertsche, W. et al. An improved limit on the charge of antihydrogen from stochastic acceleration. Nature 529, 373–376 (2016)]. This measurement also set the best-known bounds on the positron charge. Here we use orbit simulations to investigate incorporating adiabatic expansion cooling into the measurement. We find that we can improve the limit on the charge of antihydrogen by a factor of one thousand and, to a lesser extent, improve the positron charge bound. |
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CP11.00082: An Optical Emission Spectroscopic Analysis of Vibrational Excitations in Electron Beam Driven N2 Plasmas Eric R Kaiser, Stuart L Jackson, A. Stephen Richardson, David D Hinshelwood, Stephen B Swanekamp A time-resolved characterization of the vibrational excitations in an electron beam generated N2 plasma is conducted via optical emission spectroscopy. The electron beam is initiated by the Naval Research Laboratory’s Febetron pulsed-power generator which is configured to produce a 100 kV and 4.5 kA peak voltage and current pulse with a 100 ns pulse time width. The beam is directed into an N2 gas fill chamber where beam-impact and thermal heating causes excitation and partial ionization of the gas into a low-temperature plasma. Spectral measurements of the light emitted from this plasma are conducted by means of a 0.5 m focal length Czerny-Turner spectrometer in the near-UV to visible wavelength range. Spectra are imaged with a 2D time-gated intensified CCD that allows tracking of the temporal evolution of vibrational states over the lifetime of the beam pulsed plasma. Analysis of these spectra is focused on measurements of the second positive (SPS) and first negative systems (FNS) of N2. Low pressure (100 mTorr) and high pressure (1 Torr) regimes are explored and comparisons made between vibrational population distributions of each system. Modeling of the vibrational spectra is conducted which allows spectral fitting and measurements of the vibrational temperatures of the plasma at various times during the pulse. |
Author not Attending |
CP11.00083: Revisit of Ion Acoustic wave in MPD Amitkumar D Patel, Zubin Shaikh Shaikh, Rajaraman Ganesh, N. Ramasubramanian, Meenakshee Sharma A new multi-line cusp configured plasma device (MPD) consisting of electromagnets with core material has been constructed with a capability to experimentally control the relative volume fractions of magnetized to unmagnetized plasma volume as well as accurate control on the gradient length scales of mean density and temperature profiles. The electrostatic fluctuations measured using the Langmuir probe radially along the non-cusp region show less than 1% (δn/n< 1%). A controlled experiment on Ion Acoustic waves in the quiescent Argon plasma has been performed in a quiescent Argon plasma of MPD. After this, the strength of amplitude of potential perturbations was increased and given to the grid immersed in the plasma to observe the nonlinearity in the wave. These large potentials increased the amplitude of the IA wave. The properties of ion-acoustic are measured by changing the equilibrium condition of plasma. The equilibrium properties of plasma are changed by increasing the pole magnetic field of the cusp magnetic field. The measured wave amplitude decreased with an increasing magnetic field and after a certain pole magnetic field, the wave disappear. In this article, the probable mechanism of decreasing the amplitude of the ion-acoustic wave has been discussed. |
Author not Attending |
CP11.00084: Ionic Heating of N2 and O2 Gas Discharges Brett Scheiner, Mark C Zammit, Matthew M Hopkins, Christopher H Moore, Eddy M Timmermans
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CP11.00085: Rydberg atom formation rates as a function of magnetic field in ultracold neutral plasmas Ryan Baker, Bridget O'Mara, Jacob L Roberts The heating associated with the formation of Rydberg atoms via electron-ion recombination is a critical limitation to the degree of electron strong coupling that can be obtained in ultracold neutral plasmas (UNPs). In addition, this recombination is relevant to other experimental systems such as those associated with antihydrogen formation. The recombination rate is predicted to decrease as a function of magnetic field, and UNPs are excellent systems in which to carry out relatively precise measurements of the recombination rate as a function of magnetization. We describe our techniques for carrying out these measurements as well as report on initial data that we have obtained. |
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CP11.00086: Low-frequency radiofrequency heating of magnetized ultracold neutral plasmas Bridget O'Mara, Ryan Baker, Jacob L Roberts Applying radiofrequency (RF) pulses to ultracold neutral plasmas (UNPs) heats the electron component in a way that is sensitive to electron-ion collision rates. By measuring the RF heating rate as a function of plasma parameters, the electron-ion collision rate as a function of those parameters can be investigated. For instance, these collision rates can be measured as a function of electron magnetization from weak to extremely magnetized regimes. UNPs are excellent systems for these studies as their low electron temperatures mean that large degrees of magnetization can be obtained at moderate laboratory magnetic field strengths. In contrast to recent work we have done measuring high-frequency RF heating rates, low-frequency measurements should be more easily amenable to theoretical modeling. The larger experimental challenges measuring low-frequency RF heating will be described along with preliminary measurements of the low-frequency heating rates. |
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CP11.00087: Thermodynamic Effects in Multiple-Temperature Plasma Henry Fetsch, Thomas E Foster, Nathaniel Fisch In the infinite mass ratio limit, plasmas can reach a near steady-state with separate temperatures for electrons and ions. The thermodynamics of plasma under these quasi-equilibrium conditions is studied and the responses of the system to heating and compression on various timescales are discussed. Even in the collisionless limit, interaction terms arising from two-particle correlations are seen to mediate between electron and ion temperatures, affecting how input energy is partitioned between species. |
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CP11.00088: Heating Ions by Heating Electrons Without Collisions Thomas E Foster, Henry L Fetsch, Nathaniel Fisch In the limit of massless electrons, no energy can be exchanged via Coulomb collisions between electrons and finite mass ions in a plasma. However, ion heating is still possible due to the change in Debye shielding of the ions when the electron temperature varies, which redistributes energy between the particles and the electrostatic field. To calculate this heating, the BBGKY hierarchy is formulated in the massless-electron limit to describe the evolution of the ion distribution function. Using this formalism, a compact expression is obtained for the curious increase in ion temperature when heat is suddenly supplied to the electrons, rigorously valid for times short compared to the energy exchange timescale. |
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CP11.00089: Dense, cold, nonequilibrium plasma states in nanosecond-scale pulsed laser microdischarges Taemin Yong, Mark A Cappelli We describe studies of the generation of dense non-equilibrium plasma states in atmospheric pressure air driven by pulsed, nanosecond laser discharges. In our most recent studies, we generate initial dense, near-Loschmidt level, plasmas using a Nd:YAG laser (15 ns, 15 mJ, 532 nm) and these are followed by second-stage electron heating to further elevate the electron/ion density using a relatively low energy picosecond laser (20 ps, 1.2 mJ, 532 nm). A third, continuous-wave HeNe laser (10 mW, 632.8 nm), is used to record time-resolved (but line-of-sight averaged) inverse Bremsstrahlung absorption from which electron number density is inferred. The analysis of the data relies weakly on estimates of electron temperature obtained from the continuous background emission in the visible range of the spectrum. Comparisons are made to electron density inferred from Stark broadening the OI 777 nm line. We find that second-stage picosecond laser heating elevates the average electron density by approximately 20 %. A simple model analysis suggests that the level of ionization in air extends into doubly and possibly triply-ionized states. |
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CP11.00090: Improved Milli-Kelvin Laser Cooling of 2D Ion Crystals in Full-Dynamics Molecular Simulations Wes Johnson, John J Bollinger, Athreya Shankar, John Zaris, Scott E Parker Single-Component Penning Trap Plasmas can be laser cooled to milli-kelvin temperatures allowing the formation of crystalline structures [J J Bollinger et al J. Phys. B: At. Mol. Opt. Phys. 36 499 (2003)]. With the implementation of a rotating wall potential the conformation of these crystals can be precisely controlled to give 2D planar ion crystals [J. J. Bollinger et al Phys. Rev. A 71, 023406 (2005)]. Such 2D ion crystals have been an attractive platform for quantum sensing and quantum simulation experiments [G. Bohnet, et al.,Sci. 352, 1297 (2016), K. Gilmore, et al. Phys. Rev. Lett. 118, 263602 (2017)], however recent work has determined that the planar ExB motions of the 2D ion crystals can possess large potential energies that complicate these quantum information experiments [Athreya Shankar et al. Phys. Rev. A, 10 102 (2020)]. In this work we demonstrate a reduction of these large potential energies in full n-body simulations implementing a realistic laser cooling model. We also discuss theoretical results for planar cooling rates and limits of a single ion and its connection to the laser cooling of many-ion 2D ion plasmas. These results will determine desirable experimental configurations for the rotating wall and cooling laser, unlocking the door to improved quantum simulation and quantum sensing experiments. |
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CP11.00091: FUND: WAVES Session Chairs: |
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CP11.00092: Preliminary antenna-driven EM wave measurements in the ALEXIS device Jared C Powell, Saikat Chakraborty Thakur, Edward Thomas The Auburn Linear Experiment for Instability Studies (ALEXIS) is capable of generating magnetized plasmas that can support a variety of plasma instabilities and waves. ALEXIS is currently configured such that strong density gradients can appear in the plasma. Recent experiments are focused on both the production and detection of both electrostatic and electromagnetic waves in the ion cyclotron to low hybrid frequency regimes in ALEXIS. More specifically, an antenna has been constructed to drive waves inside of the plasma, and B-dot probes have been built to measure the associated magnetic field fluctuations. Initial tests have been done in which an RF plasma was produced inside ALEXIS, and EM waves were driven into the plasma via the antenna and measured with the B-dot probes. Results from the initial tests are discussed. |
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CP11.00093: Density depletion and double layer formation in a current-carrying plasma jet Yi Zhou, Paul M Bellan The Caltech plasma jet experiment produces an MHD-driven, current-carrying laboratory plasma jet with an initial radius of a few centimeters, a length increasing to roughly 40 cm, and a nominal electron temperature of 2eV. >6 keV X-ray and 20-60 eV extreme ultraviolet radiation have been observed simultaneously when the cross-section of the plasma jet is choked by Rayleigh-Taylor instability. Visible light images of the plasma jet show that the choked region is likely of low density. Based on these observations, an evacuation instability that can lead to density depletion and the formation of a double layer is proposed. This instability becomes active when the electron drift velocity surpasses the electron thermal velocity. A double layer is formed when a current-aligned, resistive electric field is enhanced due to a reduction in the plasma jet’s cross-sectional area. Charged particles can be accelerated by the enhanced electric field and produce the observed high-energy radiation. This evacuation instability will be compared to the two-stream and ion-acoustic instabilities. Potential applications of this model to particle acceleration in some space plasmas will be discussed. |
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CP11.00094: Experimental studies of inertial Alfvén-wave propagation through an Alfvén-speed gradient Garima Joshi, Troy Carter, Daniel W Savin, Shreekrishna Tripathi, Steve Vincena, Michael Hahn Alfvén waves are ubiquitous in space, laboratory, and astrophysical plasmas. Understanding the reflection and transmission of these waves as they travel through plasma inhomogeneities is important for understanding wave propagation, turbulence, and heating of plasmas in general. Alfvén waves are predicted to reflect when encountering a gradient in the Alfvén speed. However, our recent studies using the Large Area Plasma Device (LAPD) at the University of California, Los Angeles, observed no reflection for kinetic Alfvén waves propagating through an Alfvén-speed gradient. In the present work, we have conducted LAPD experiments to study inertial Alfvén-wave propagation through an Alfvén-speed gradient. The gradients were created by varying the magnetic field in the direction of the propagation of the Alfvén wave. Here, we will present our results for the transmission and reflection of Alfvén waves along the Alfvén-speed gradient. |
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CP11.00095: A hybrid simulation tool for studying laboratory Alfvén waves Feiyu Li, Xiangrong Fu, Seth Dorfman As a fundamental mode in magnetic plasmas, Alfvén waves are ubiquitous in space, astrophysical and fusion plasmas, and play a pivotal role in plasma turbulence, particle acceleration and energy transport at large scales. Detailed space measurements of Alfvén wave dynamics require overcoming several limitations, stimulating considerable interest in laboratory study and associated numerical modeling. Here, we present a new hybrid simulation tool (modified from the 3D hybrid code H3D) for studying laboratory Alfvén waves based on the Large Plasma Device (LAPD) at University of California, Los Angeles. Several novel features including an absorption boundary [1] and 1D Alfvén wave injection [2] are developed to accommodate the LAPD conditions. Such simulations adopting realistic wave-plasma parameters (such as Alfvén wave amplitude, temporal profile, plasma beta and system size) are used to demonstrate a few nonlinear Alfvénic processes with ion kinetics retained, including the parametric instabilities and beat wave excitation, in either single or multi-ion species [3]. Extension to 3D wave injection is underway which may open new investigations such as nonlinear interaction of kinetic Alfvén waves. This tool together with experiment will help guide future Alfvén wave studies on LAPD and gain a detailed understanding of relevant physical processes. |
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CP11.00096: Measuring the Alfvén wave Parametric Decay Instability Growth Rate in the Laboratory Seth Dorfman, Feiyu Li, Xiangrong Fu, Stephen T Vincena, Troy Carter, Patrick Pribyl Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in lab and space. The non-linear behavior of these modes is thought to play a key role in important problems such as the heating of the solar corona, solar wind turbulence, and Alfvén eigenmodes in tokamaks. In particular, theoretical predictions show that these Alfvén waves may be unstable to various parametric instabilities, but observational measurements of these processes are limited. We present an experiment on the Large Plasma Device at UCLA aimed at measuring the Parametric Decay Instability (PDI) growth rate in the laboratory. In these experiments, a high amplitude δB/B0~0.7% pump Alfvén wave is launched from one end of the device and a smaller seed Alfvén wave in launched from the other side. When the frequency of the seed wave is chosen to match the backward wave expected from PDI, damping of the seed wave is reduced. This reduction in damping is the same order as the theoretically expected PDI growth rate and scales with the pump wave amplitude. Analysis is underway to more precisely compare the experimental results with PDI theory and to connect with related numerical simulations. |
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CP11.00097: Full-wave simulations of helicon wave coupling optimization and possible parasitic excitation of slow waves near the edge plasma Eun-Hwa Kim, Masayuki Ono, Syun'ichi Shiraiwa, Matthew J Poulos, Bart Van Compernolle, Nicola Bertelli Helicon waves are thought to be promising since they can penetrate reactor-grade high-density core and drive off-axis current. In the frequency regime ~ 476 MHz, both slow electrostatic and fast electromagnetic helicon waves can coexist in the scrape-off layer (SOL). This work uses the Petra-M simulation code to perform 2D and 3D full-wave simulations of helicon and slow waves. We launch the helicon waves for 2D simulations by assuming an antenna with a finite wave number in radial and vertical directions in a realistic vacuum vessel boundary and discuss SOL power losses of the helicon waves. Slow mode excitation is examined using 3D full-wave Petra-M simulations, accounting for the 3D antenna geometry and SOL plasma. We use a realistic 8-module helicon antenna in Petra-M and limit the simulation domain near the antenna to examine slow and fast wave mode characteristics in detail. The 3D simulations confirm slow mode excitation and indicate that the slow-wave branch can be excited even for zero antenna misalignment angle (Φ). SOL power losses and antenna loading at various values of Φ and SOL density in a 3D simulation are also discussed. |
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CP11.00098: Analytical evolution of coupled, driven waves near their instability threshold Nicholas M Pham, Vinicius N Duarte The nonlinear collisional dynamics of two coupled driven waves in the presence of dissipation is studied analytically within kinetic theory. Sufficiently near marginal stability, time delays become unimportant and the system dynamics are shown to be governed by two first-order coupled autonomous differential equations of cubic order for the wave amplitudes and two complementary first-order equations for the evolution of the phases. It is found that the system of equations can be decoupled and further simplified to a single second-order differential equation of Liénard's type for each amplitude. Numerical solutions for this equation are obtained in the general case while analytic solutions are obtained for special cases in terms of parameters related to the spacing of the resonances of the two waves in frequency space, e.g., wave lengths and the oscillation frequencies. These parameters are further analyzed to find limiting cases of pulsating and quasi-steady saturation. Similarly, to classify equilibrium points, local stability analysis is applied, and bifurcation conditions are determined. The results can be generalized to an n-wave system, where sufficiently near marginal stability, the evolution of the underlying distribution function is shown to naturally reduce to a quasilinear transport regime. |
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CP11.00099: Topological Langmuir-Cyclotron Wave Yichen Fu, Hong Qin In the past decade, topological phases of electronic and photonic systems have become a rapidly emerging field of research, which deepened understanding of the state of matters. Topological Langmuir-Cyclotron Wave (TLCW) is a recently identified topological surface excitation in magnetized plasmas. We show that TLCW is mathematically a spectral flow of a Hamiltonian pseudo-differential operator and originates from the nontrivial topology at the Langmuir wave-cyclotron wave resonance. By isofrequency surface analysis and time-dependent simulations, we demonstrate that the TLCW can propagate robustly along the complex transition interfaces unidirectionally and without scattering. Because of these desirable features, the TLCW could be explored as an effective mechanism to drive current and flow in magnetized plasmas. The analysis also establishes a close connection between plasmas' newly instituted topological phase classification and the classical CMA diagram of plasma waves. |
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CP11.00100: Energizing charged particles by an instability in a slowly rotating magnetic field Eric Palmerduca, Hong Qin, Samuel A Cohen The stability of charged particle motion in a uniform magnetic field with an added spatially uniform transverse rotating magnetic field (RMF) is studied analytically. It is found that the stability diagram depends critically on the chosen boundary conditions. We show that for many boundary conditions and wide regions in the parameter space, RMFs oscillating far below the cyclotron frequency can cause linear instabilities in the motion which break μ-invariance and energize particles. Such energization may appear at odds with the adiabatic invariance of μ; however, adiabatic invariance is an asymptotic result, and does not preclude such heating by magnetic fields oscillating at slow frequencies. This mechanism may contribute to heating in the edge plasma of field-reversed configurations (FRCs) in rotamak-FRC experiments. Furthermore, these RMF-driven instabilities may significantly enhance azimuthal current drive during the formation of FRCs in such devices. |
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CP11.00101: Full-f 6D particle-in-cell simulations of quasi-kinetic equilibrium with spatial inhomogeneity and ITG mode Zhenyu Wang, Hong Qin, CS Chang We are studying the quasi-kinetic equilibrium with spatial inhomogeneity and ion temperature gradient (ITG) modes by six-dimensional (6D) full-f particle-in-cell (PIC) simulations. For the ITG mode, fluid equilibrium with spatial inhomogeneity can be established straightforwardly, but there is no known suitable analytical equilibrium that satisfies the time-independent Vlasov-Maxwell system exactly. Full-f PIC simulations with 6D ions and adiabatic electrons are carried out to search for a quasi-kinetic equilibrium with spatial inhomogeneity. The time evolution of density, ion-temperature and ion-velocity profiles are obtained, and the time-dependent ion distribution function is also numerically constructed. The frequency spectrum of the density, ion-temperature, ion-velocity, ion distribution function from the PIC simulation is compared to the timescale analysis of the dynamics of the Vlasov equation. The threshold of ITG instabilities is estimated using a fluid model for the system parameters adopted in the simulation. We plan to carry out 6D full-f PIC simulations in the unstable region predicted by the fluid model to observe the ITG instabilities. |
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CP11.00102: Nonlinear evolution of the interface of the sausage instability Daniel E Ruiz In this work, I study the nonlinear evolution of the $m=0$ sausage instability, which is a well-known magnetohydrodynamic (MHD) instability occurring in Bennett-type pinches, that is, axially uniform, axisymmetric, cylindrical plasmas. In the proposed model, the plasma column is assumed incompressible and perfectly conducting. To describe the nonlinear dynamics of the plasma surface, I introduce a contour-dynamics formulation, where the interface of the plasma column is modeled as a series of interacting co-axial vortex rings. The radius, axial location, and vortex strength of each ring are allowed to dynamically vary, and the corresponding equations of motion are derived. For small initial sinusoidal perturbations, the calculated linear growth rate agrees with the well-known growth rate determined using simple potential-flow theory. In the nonlinear regime, initial sinusoidal perturbations grow into a "spindle" structure with broad minima in plasma radius and sharp disk-like maxima. Using this model, I numerically investigate the long-time behavior of the sausage instability and quantify its temporal evolution for various initial conditions. For code-validation purposes, I compare the results of this model to numerical simulations using the 2D radiation-MHD code HYDRA. |
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CP11.00103: Laboratory Study of Residual Energy Generation in Strong Alfvén Wave Interactions Mel Abler, Seth Dorfman, Christopher Chen The solar wind is a classic example of a turbulent plasma. At moderate scales (larger than ion-kinetic scales) turbulent fluctuations in the solar wind are often Alfvénic in character, meaning that their magnetic and flow velocity fluctuations are proportional to each other. However, observations of the solar wind have shown that there is a significant difference in the energy in the velocity fluctuations and the normalized magnetic field fluctuations. This difference, called the residual energy, should be zero for linear Alfvén waves, but is consistently observed to be negative in the solar wind, with magnetic fluctuations dominating. This work investigates the energy partition of strong three-wave interactions as a building block of interactions in the turbulent cascade. Preliminary results from an experimental campaign on LAPD studying three-wave interactions of Alfvén waves in an MHD-like regime relevant to the solar wind will be presented. |
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CP11.00104: Inverse Design of Reconfigurable Plasma Metamaterials for Optical Computing Jesse A Rodriguez, Mark A Cappelli Inverse design (or equivalently, machine learning) methods are commonly used to create high-efficiency optical devices that perform exotic functions that otherwise would not be possible to create using conventional ‘human design’ methods. In this study, we apply inverse design methods to produce fully reconfigurable, multi-function, two-dimensional plasma metamaterial (PMM) devices composed of low-temperature plasma discharge tubes. Autograd-compliant finite difference frequency domain simulations are used to design waveguides, demultiplexers, and all-optical logic gates for use in optical computing. Demultiplexing and waveguiding are demonstrated for PMM devices composed of realistic plasma elements with non-uniform plasma density profiles, collisional damping, and resistance to experimental error, allowing for future in-situtraining and experimental realization of these designs. |
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CP11.00105: Space-time quasicrystals in plasma Vadim R Munirov, Lazar Friedland, Jonathan S Wurtele We demonstrate that warm fluid equations of plasma support multiphase solutions for ion acoustic and electron plasma waves. These highly nonlinear waves are excited by using autoresonance -- a phenomenon in which the waves automatically adjust their amplitudes to stay phase-locked with chirped-frequency ponderomotive external drives. The multiphase waves form spatiotemporal quasicrystalline structures that persist even after we stop driving. Due to the threshold nature of autoresonance, such quasicrystals can suddenly "melt" or "crystallize" when the critical condition is being passed. The possible utility of these coherent structures for particle acceleration and plasma photonics experiments is being investigated. We conduct fully nonlinear numerical simulations as well as develop an analytical weakly nonlinear theory based on application of Whitham's averaged Lagrangian method. The analytical theory allows one to estimate and choose the laser parameters required for autoresonant excitation of multiphase waves in plasma. |
Author not Attending |
CP11.00106: Observing Stochastic Ion Heating In a Linear Plasma Device Jesus J Serrano, Troy Carter Ions in a plasma usually pertain into two velocity regimes: thermal and fast ions. Thermal ions go through classical diffusion and have a Maxwellian velocity distribution whereas fast ions are non-Maxwellian and have greater velocities. When EM waves that resonate with the compressional Alfven Eigenmodes (CAE) are introduced into a plasma, thermal ions can gain energy from the waves through stochastic ion heating. This nonlinear heating mechanism has been observed within Tokomak experiments and in plasmas contained in solar winds. With an antenna on the Large Plasma Device (LAPD) at UCLA, we may observe stochastic ion heating with a Fast Ion Energy analyzer. The expected distribution model and diagnostics will be simulated first using a Boris-particle pusher algorithm. By observing SIH in LAPD we hope to better understand the wave instability for the purpose of fusion research. |
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CP11.00107: Interaction of Energetic Ions and Fast Waves in the Large Plasma Device (LAPD) Yhoshua Wug, Gurleen Bal, Shreekrishna Tripathi, Nicola Bertelli, Troy Carter In fusion devices, energetic ions are produced by ionization and charge exchange of injected neutral beams, ion cyclotron heating and current-drive, and byproducts of fusion reactions. The study of energetic ions-wave interactions on magnetic-fusion devices (e.g. dominant fast ion power absorption in some NSTX-U scenarios)1 can be challenging due to harsh plasma environments. Doppler-shifted cyclotron resonance through a proton beam was observed to destabilize Alfvén waves on LAPD2, an 18 m long and 1 m diameter cylindrical device, that produces a magnetized plasma (n = 1011-1013 cm-3, Te= 0.1-15 eV)3. The high-repetition rate (0.2-1.0 Hz), good reproducibility, and various 3D diagnostic tools facilitate detailed exploration of energetic ions and fast wave interactions in LAPD. Interaction between a proton beam (2-15 keV, 15 A) and fast waves launched by a single strap RF antenna was studied. Measurements of 3D wave magnetic-field and beam profiles under varieties of conditions are presented, and energetic-ion diffusion in real and velocity space due to resonant interactions is discussed. |
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CP11.00108: Overstable Alfvén waves in periodic shear flows at low magnetic Prandtl number Adrian E Fraser, Imogen G Cresswell, Pascale Garaud Periodic shear flows are known to exhibit negative eddy viscosity effects (Dubrulle & Frisch 1991). Here, we show that this effect can lead to a destabilization of shear Alfvén waves. In particular, we investigate the linear stability of a sinusoidal shear flow with an initially uniform streamwise magnetic field in the framework of incompressible magnetohydrodynamics (MHD) with finite resistivity and viscosity. This system is known to be unstable to the Kelvin-Helmholtz instability in the hydrodynamic case, and in ideal MHD, provided the magnetic field strength does not exceed a critical threshold beyond which magnetic tension stabilizes the flow. We demonstrate that including viscosity and resistivity introduces two new modes of instability. One of these modes, which we call the Alfvénic Dubrulle-Frisch mode due to its connection to shear Alfvén waves, exists for any nonzero magnetic field strength as long as the magnetic Prandtl number Pm<1. We present a reduced model for this instability that reveals its connection to the negative eddy viscosity of periodic shear flows described by Dubrulle & Frisch (1991). We also demonstrate numerically that this mode saturates in a quasi-stationary state dominated by counter-propagating solitons. |
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CP11.00109: Ion acoustic scattering from a magnetic dipole. Fred N Skiff, Jacob McLaughlin, Daniel Pette, Patrick Langer We present a fluid theory for the scattering of ion acoustic waves from a magnetic dipole immersed in a weakly-collisional neutral plasma. The theory of plasma waves involves the self-consistent interaction of particles and wave modes through collective interactiion. In the vicinity of a magnetic diplole, the dynamical behavior of charged particles is very rich. In the absence of collective interaction there are significant measures of quasi-periodic, chaotic and hyperchaotic (multiple positive Lyapunov exponents) orbits. No computable approach exists to compute the wave modes in such a context. However, the fluid theory can be taken as a starting point and can be expected to correctly describe the mode spectrum in various parts of the spectrum depending on plasma conditions. The approach involves expressing the electrostatic wave equation in dipolar coordinates, and coupling to magnetic field-free solutions in the far field.. We will report progress on the construction of a scattering theory which eventually will be generalized to a kinetic description. Specific conditions will be considered which are relevant to a well diagnosed experimental configuration that is currently being assembled for the validation of the theory. |
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CP11.00110: Measurement of the Ponderomotive Filamentation Instability Growth Rate in Short-Pulse Laser Beams Kyle R McMillen, Jessica L Shaw, Daniel J Haberberger, Dustin Froula In ponderomotive filamentation instability, the ponderomotive ejection of electrons from the high-intensity regions of a laser beam causes modulations to the plasma density and refractive index, which lead to self-focusing and filamentation. We present an experiment that utilizes the joint operation of the OMEGA 60 and OMEGA EP Laser Systems at the University of Rochester’s Laboratory for Laser Energetics to investigate the growth rate of the ponderomotive filamentation instability. In our experiment, a 1w short-pulse (100-ps) laser beam from OMEGA EP is coupled into a preheated plasma on the OMEGA 60 laser–plasma interaction platform. The beam spray of the filamented short-pulse beam is recorded as a time-integrated 2-D image and a time-resolved 1-D image utilizing a specially made infrared transmitted beam diagnostic and coupled streak camera. The measured radial-growth rate is then compared with theoretical predictions and scaling with incident beam intensity and plasma temperature. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0003856 and the Office of Fusion Energy under Award Numbers DE-SC0016253 and DE-SC00215057. |
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CP11.00111: Inductive and helicon plasma for non-linear microwave coupling experiments Kevin Ronald, Kieran J Wilson, Liam Selman, Bengt Eliasson, Colin G Whyte, Mark E Koepke, Alan R Phelps, Robert Bingham, Robert Alan Cairns, Ruth Bamford, Craig W Robertson, Philip MacInnes, Adrian W Cross Non-linear wave coupling and other instabilities can develop in plasma perturbed by powerful EM waves. These effects can arise in microwave interactions in fusion plasma, cool plasma, radio propagation in ionospheric and magnetospheric plasma and laser-plasma interactions. Understanding these dynamics can provide useful mechanisms to introduce energy into plasma, to manipulate plasma conditions or to mitigate undesirable consequences induced by such parametric instabilities. Cool plasmas with critical frequencies in the low microwave range are relatively stable and relatively easier to diagnose. We are preparing experiments to study dynamics of microwave beams propagating in such plasma. A recently commissioned plasma source operating in the inductive/helicon mode has a transverse diameter of 1m and length of 3m. In inductive mode it operates in He and Ar gas with pressures in the range of 10-3 mbar – 10-1 mbar with plasma frequency in the range a few hundred MHz and bulk temperatures of <1eV (a tenuous hotter population is also present) when driven by a 10-200W RF source at 14MHz with appropriate matching. This plasma environment is well suited to studying non-linear wave coupling with readily available 10GHz microwave sources. Progress on this experiment will be reported. |
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CP11.00112: ETG Instability in the Mega Ampere Spherical Tokamak Pedestal Ping-Yu Li, David R Hatch, Benjamin Chapman, Samuli Saarelma, Colin M Roach, Swadesh M Mahajan We present a gyrokinetic analysis of turbulent transport from electron temperature gradient (ETG) driven turbulence in the MAST pedestal. During ELM cycles, the electron density profile builds up faster than the electron temperature profile, suggesting that an electron thermal transport mechanism must be active. ETG transport is considered a plausible candidate because of its large electron heat diffusivity and low electron particle diffusivity. ETG has been investigated recently in NSTX and shown can be important in spherical tokamaks' (ST) pedestals [1]. Local linear and nonlinear results derived from the gyrokinetic code GENE also show that heat flux produced by ETG modes is 10-30% of the neutral beam injection in the MAST upper pedestal and pedestal top during the pre-ELM (80-99% inter-ELM period) and post-ELM (0-20%) periods. A reduced ETG model for conventional tokamak heat flux prediction is tested on MAST's pedestal [2]. Another reduced gyrokinetic model developed for instability identification for the core of conventional tokamaks will also be tested and improved for STs' pedestals [3]. |
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CP11.00113: Kinetic and Two-Fluid Simulations of the Buneman Instability Daniel Alex, James Juno, Petr Cagas, Bhuvana Srinivasan The Buneman instability develops in current carrying plasmas and is observed in laboratory and space plasmas. Relative drifts between electrons and ions result in mode growth where the growth rate varies with wavenumber and phase-space turbulence is produced. This work studies instability growth rates for currents streaming across a magnetic field. We compare growth rates produced from kinetic Vlasov-Poisson simulations, two-fluid simulations, and the respective fluid linear dispersion relation. Additional cases analyzed include the variation of growth rate with mass ratio and wavenumber. |
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CP11.00114: Investigating Magnetospheric Whistler-Mode Chorus Features using SPSC Laboratory Experiments Jim Schroeder, Erik M Tejero, Fred N Skiff, Vijay Harid Wave-particle interactions between electrons and whistler-mode waves play an important role in the dynamics of the outer radiation belt. This work uses NRL's Space Physics Simulation Chamber (SPSC) to explore the kinetic processes responsible for the amplitude growth, chirped frequency, and Ωce/2 power gap common to whistler-mode chorus. Recent SPSC experiments with whistler-mode waves and a counterpropagating electron beam produced nonlinear wave growth and triggered chirped emission of whistler-mode waves. WIth these experimnts, we aim to add measurements of the electron velocity distribution using an energy analyzer and a wave absorption diagnostic. Additional features of the SPSC include the ability to create uniform or flared background magnetic field to study the effect of inhomogeneities on whistler-mode chorus and an electron beam with steerable pitch angle. We will present results exploring optimal conditions leading to wave growth and triggered emissions. |
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CP11.00115: Nonlocal transport witnessed in Vlasov-Fokker-Planck simulations of the return current instability Jeffery Zielinski, Mark Sherlock, Avram Milder, Colin J Bruulsema, George F Swadling, Wojciech Rozmus Electron thermal transport in laser produced plasmas is decreased from classical estimates |
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CP11.00116: Development of a ExB Magnetized Plasma Device Jenny R Smith, David l Cooke, Charles T Hooper A plasma chamber device has been developed to provide an ExB drifting plasma. The ExB Magnetized Plasma Device, EMPD, is a 1m diameter x 2 m long cylindrical chamber with a modified Helmholtz axial magnetic field and up to 330G central field. An independently powered set of pinch coils can provide up to 500G at the ends enabling high mirror ratios at lower entral field. The plasma is generated from a hollow cathode emitter forming a central virtual cathode light saber, and a newly designed hollow cathode based ion source. Biasing the two sources creates a spinning plasma about the central axis. A non-rotating interaction can also be formed by injecting plasma from a side-mounted tube. This device has been previously reported, but more recently upgraded with increased magnetic field, and improved pumping, plasma sources, and diagnostics. One of the goals for EMPD is to study momentum coupling in plasma, which is a mechanism that is central to a wide range of interesting and important phenomena; magnetosphere-ionosphere coupling, solar eruptions, the interaction of an electro-dynamic tether system in the Earth's ionosphere, and the Critical Ionization Velocity (CIV) mechanism are a few examples, with CIV being an easy and persistently observed interaction in EMPD. Progress and results will be presented. |
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CP11.00117: Kinetic Modeling of the Electron Cyclotron Resonance Plasma Cathode with Orifice Anil Bansal, John E Foster, Igor D Kaganovich, Willca Villafana Electron cyclotron resonance (ECR) plasma cathodes utilize wave heating to produce a dense plasma from which electrons are extracted for a variety of applications, such as plasma thrusters. Here, we present the kinetic modeling of a plasma cathode with an orifice, where a region of enhanced ionization forms, so-called fireball. Key questions to be addressed include 1) what role does the anode fireball formation play in enhancing the extractable current, 2) what is the formation mechanism of the observed electron beam at the exit of the orifice, and 3) can this mechanism be exploited to increase the extractable current. We also explore streaming instabilities known to be present in the orifice region. The modeling was carried out using the EDIPIC code [Ref. github], and the modeling results will be validated against experimental data obtained from ECR device operation. |
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