Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Plasma Physics
Monday–Friday, October 30–November 3 2023; Denver, Colorado
Session GP11: Poster Session III:
Poster

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Room: Plaza ABC 

GP11.00001: MEASUREMENT, ANALYSIS, AND CONTROL OF LOW TEMPERATURE PLASMAS


GP11.00002: Emissive probe current in response to timevarying probe bias voltage Emma Devin, Yevgeny Raitses, Michael Campanell Applications of biased emissive probes for measurements of the plasma potential [1] require an understanding of the dynamic response of the probe current to timevariations of the sweeping probe bias voltage. In this work, we explore fundamental and technical limitations of emissive probe measurements with a changing bias voltage at timescales both slower and faster than the inverse of the ion plasma frequency. The focus is on measurements of the timeevolution of the probe current as the sheath between the plasma and a strongly emitting probe restructures in response to the bias change. A comparison of experimental results with simulations [2] will be discussed. 

GP11.00003: Single shot, nonresonant, fourwave mixing laser diagnostics for low and high temperature plasmas Alexandros Gerakis We present our recent experimental and theoretical work in utilizing intense optical lattices for the thermodynamic characterization of different species (neutrals, ions, electrons and nanoparticles) in low and high temperature plasmas. At the core of our study is the use of single shot coherent RayleighBrillouin scattering (CRBS), a fourwave mixing technique, for the measurement of the velocity distribution function (VDF) of these species. The use of single shot CRBS for the characterization of neutral species in a glow discharge has already been experimentally demonstrated^{1} using a custom, inhouse designed and built laser system^{2}. From the experimentally obtained CRBS spectra, macroscopic quantities such as the flow velocity, density and translational temperature can be extracted. CRBS has already been demonstrated to be the coherent analogue of spontaneous RayleighBrillouin scattering and has been used for the insitu measurement of nanoparticles in an arc discharge^{3}. 

GP11.00004: Understanding the propagation of an atmospheric pressure plasma jet inside a helical tube Bhagirath Ghimire, Nageshwar Nagarajan, Gabe Xu Atmospheric pressure plasma jets (APPJ) have become increasingly popular in the last decades due to their practical applications in several areas including biomedicine, agriculture and food safety. During application, the APPJ may need to be confined in different designs such as straight, helix, wave, etc. In this work, we have investigated the characteristics of an APPJ inside a helical tube and compared its propagation within a straight tube of identical dimension. Plasma sources are designed by inserting a stainless steel needle electrode(s) inside linear and helical tube(s) of same inner & outer diameter(s) and operated with argon gas. For the same peakpeak length and total length of the tube, we have investigated the propagation of plasma bullets and emissions from reactive species at the outlet of the tube. Our observations show that the helical design operates at a lower gas temperature, and also produces less hydrogen peroxide than the linear plasma jet of identical configuration. This may be due to the difference in gas flow pattern (swirling vs linear) within the two types of tubes and will be explained through numerical simulations. 

GP11.00005: Improving pulsed laser induced fluorescence signaltonoise through matched filter signal processing Tyler J Gilbert, Katey J Stevenson, Thomas Steinberger, Earl E Scime The PHAse Space MApping (PHASMA) facility was constructed to facilitate laboratory electrononly magnetic reconnection studies at kinetic scale lengths using dual plasma gun discharges. Electron velocity distribution functions (VDF) have been measured during electrononly magnetic reconnection with a Thompson scattering diagnostic. No effect is expected on ions or neutrals in the reconnection event in PHASMA, but this has yet to be successfully measured. A discrete pulsed laser induced fluorescence (LIF) diagnostic is being developed for PHASMA to measure both ion and neutral VDFs during magnetic reconnection. It has also been shown that LIF measurements of Zeeman split spectra offer a method to measure magnetic fields in laboratory plasmas. Using neutral LIF schemes that exhibit strong Zeeman splitting, we can nonperturbatively measure the magnetic field throughout the reconnection event without the use of probes. Performing LIF during a plasma gun discharge presents unique challenges. A pulsed dye laser is necessary to produce sufficient fluorescent signal, but to avoid laser saturation, measurements must be made at relatively low laser energies at which reliable signal is only recovered by averaging over many plasma discharge events. We have implemented a matched filter signal processing technique to greatly improve the signaltonoise ratio of our measurements. This allows us to reduce the number of discharges needed to measure a VDF or to achieve signal at lower laser energies. 

GP11.00006: Towards a RF/MW Instrumentation Suite Design for an Inductively Coupled Plasma Wind Tunnel Steven Latimer, Ethan Leong, Hisham Ali Hypersonic flight is of significant relevance to the atmospheric entry stage of interplanetary missions and national defense, but the formation of a plasma around hypersonic vehicles presents numerous engineering challenges. One such challenge is communication blackout caused by the sheath of plasma attenuating radio signals. The Magnetoaerodynamics and Aerospace Plasma Laboratory at the University of Colorado Boulder (CUMAPLAB) will house an inductively coupled plasma (ICP) wind tunnel facility that will allow for simulation of high enthalpy, continuous plasma flows. Measurement of radio signals emanating from and propagating through plasma is a standard noninvasive technique to measure signal propagation and plasma properties, but is underexplored for ICP torches. This research expands upon previous experimental work by utilizing a numerical simulation in COMSOL Multiphysics. Wave propagation physics are coupled with a simulated ICP torch for a comprehensive characterization of radio signal propagation through an ICP torch across multiple frequency ranges. Results from the numerical simulation will be used to inform radio instrumentation selection to enable experimental studies of signal propagation in the CUMAPLAB plasma wind tunnel. 

GP11.00007: Development of Langmuir and spatially resolved OES diagnostics for arc jet shock and boundary layer measurements relevant to atmospheric reentry environments Ethan Leong, Magnus A Haw, Hisham Ali The secondgeneration miniature Arc Jet Research Chamber at NASA Ames requires physical characterization of its supersonic plasma flow to verify numerical models and material sample testing. Two types of plasma diagnostics will be used: Langmuir probes and optical emission spectroscopy (OES). Several single and triple Langmuir probes have been constructed and will be inserted parallel to the flow via a sweep arm to measure postshock electron temperature and number density along radial profiles at various axial distances. Due to high heat flux (2003000 W/cm^{2}), sweep durations are necessarily short (< 0.1 s). This will be an important metric for validating coupled CFDmaterial codes and providing insight into longstanding thermocouple anomalies attributed to charging of heatshield surfaces. An OES system has also been assembled to obtain spatially resolved density and temperature for various flow species. The system consists of a convex lens that forms an image of the plasma onto a 16channel linear fiber that simultaneously captures the spectra from sixteen different locations. The light emission is then focused by a spherical mirror into a spectrometer and recorded with a camera. These measurements nonintrusively provide a new spatially resolved flow characterization capability, which will initially be used to resolve issues with shock radiation interference in pyrometer measurements of material surface temperature. The design of each diagnostic and any preliminary results will be discussed. 

GP11.00008: The Sheath Exploration Experimental Device (SEED) at UW Madison – a new device to investigate generalized sheath boundary plasmas Peixuan Li, Oliver Schmitz, Gregory Severn The construction of the Sheath Experiment and Exploration Device (SEED) at UW Madison has been finished. It will be used to investigate sheath structure under various plasma conditions. The device consists of a source chamber and a target chamber. Plasmas can be produced within the liner of each chamber by thermionic electrons which are emitted from the biased filaments. A 15cmdiameter stainless steel plate is placed in the target chamber to create various boundary conditions. Diagnostic tools such as Langmuir probes, emissive probes, and laserinduced fluorescence (LIF) are implemented to measure plasma parameters. The latest status of the device and preliminary data will be presented here. 

GP11.00009: Towards timeresolved measurements of solvated electrons at the plasma/liquid interface Adam D Light Contamination of groundwater by polyfluoroalkyl substances (PFAS) is increasingly recognized as a major environmental issue. These compounds bioaccumulate, can cause adverse health outcomes, and are difficult to break down. Low temperature plasma (LTP) offers promising avenues to remediation of contaminated liquids, but many mechanisms remain to be understood. For PFAS, dissolved free electrons ("solvated electrons") may be a key part of effective plasma treatment. To provide insight into relevant processes, we are developing a broadband transient absorption diagnostic to measure the concentration of solvated electrons at a liquid surface. We use light from a subnanosecond, pulsed supercontinuum (FYLA Iceblink) and the total internal reflection geometry of Rumbach, et. al (2015) to sample a broad absorption feature in the thin layer immediately underneath the liquid surface. Lowjitter timing allows us to localize the measurement relative to the start of the discharge, and multichannel lockin detection allows access to the small signal. We will describe the current diagnostic design, results to date, and planned improvements. 

GP11.00010: Ion motion above a biased wafer in a plasma etch tool. Walter N Gekelman, Yuchen Qian, Patrick Pribyl, Alex Paterson, Tugba Piskin, Mark Kushner The behavior of ions in the plasma are essential to the process of industrial etching. We studied the motions and energy distribution of argon ions in a 2 MHz inductively coupled plasma (ICP) etching tool, by the method of laser induced fluorescence (LIF). The silicon wafer posited at the bottom of the machine can be biased with a 1 MHz 1.2 kV peaktopeak sinusoidal voltage. The plasma is formed with a 2 MHz ICP coil which, along with the wafer bias, can be pulsed on and off within 50 microseconds.. The experiment compares the cases of ion motion with and without wafer bias. For both of these cases the two diemsional flow pattern of ions is studied near the center and edge of the wafer. Significant vortex flows are observed near the wafer edge. Experiments in which the wafer is biased in the afterglow, that is when the ICP is off, results in a narrow distribution of ion energy close to the bias voltage close to the wafer. The angles at which ions strike the wafer is smallest under these conditions. The results are compared to simulations using the Hybrid Plasma Equipment Model code. 

GP11.00011: SpaceTimeResolved Measurement of Electronegativity in Inductively Coupled Ar/O_{2} Plasmas Yuchen Qian, Walter N Gekelman, Patrick Pribyl, Alex Paterson, Tugba Piskin, Mark Kushner Pulsed low temperature inductively coupled plasmas (ICPs) are widely used in the microelectronics industry for etching and deposition. The processing discharges frequently use a mixture of argon with various electronegative gases, which adds complexity to the study of these plasmas. A thorough investigation on the local electronegativity is critical to higher etch efficiency and the understanding of the electronegative plasmas properties. We conducted timespaceresolved 3D measurements of the negative ion density in an industrial ICP tool. The stovetop coil plasma source can be run in steady state, or a pulsed mode at 2 MHz and customizable duty cycle, without an impedance matching network. The bottom wafer can be biased up to 1.2 kV peaktopeak and pulsed at 1MHz, separately from the source coil. Plasmas are sustained in tens of mTorr Ar/O_{2} mixtures at any mixing ratio. A 3D probe drive system allows measurement throughout the plasma volume. Langmuir probe (LP) assisted laser photodetachment is used to map out the negative ion density, while a hairpin probe and an RF compensated LP are used for electron measurement. Temporal evolution and spatial distribution of the negative ion density, electron density, electron temperature and induced EM fields are reported and compared for varying ratios of Ar:O_{2} concentrations. The comparisons with 2D calculations will be presented. 

GP11.00012: A novel LIF scheme investigating argon neutral and ion dynamics for dusty plasma diagnostics AKM Mustafizur Rahman, Saikat Chakraborty Thakur, Edward Thomas Understanding the neutral and ion dynamics is crucial for comprehending the fundamental physics behind magnetization and charging of dust microparticles in the Magnetized Dusty Plasma Experiment (MDPX). Application of traditional LaserInduced Fluorescence (LIF) techniques is limited by the relatively high pressures (up to 500 mTorr) necessary to stabilize the dust and the high magnetic fields (up to 4.0 T) achievable in MDPX. The metastable pumping states typically used in traditional LIF schemes can be severely depleted by nonradiative transitions known as collisional quenching, caused by enhanced ionneutral collisions under such highpressure conditions. To overcome this challenge, timeresolved LIF schemes employing a tunable nanosecond pulsed dye laser with response times faster than the collisional quenching process becomes necessary. Here we shall show preliminary results from the development of viable LIF schemes for both argon neutrals and ions in dusty plasma relevant conditions and testing them in a target unmagnetized and weakly magnetized plasma chamber. 

GP11.00013: Overview of the Princeton Collaborative Low Temperature Plasma Research Facility Yevgeny Raitses, Igor D Kaganovich, Mikhail Shneider, Arthur Dogariu, Sophia Gershman, Shurik Yatom, Willca Villafana, Anatoly Morozov The Princeton Collaborative Low Temperature Plasma Research Facility (PCRF) (http://pcrf.pppl.gov) is a collaborative research facility providing expertise and instrumentation for comprehensive characterization of low temperature plasmas (LTPs) with focuses on i) plasmaliquid and plasmasolid interactions, ii) plasma transport, iii) collective phenomena in LTP, iv) use of LTP in modern applications, including nanomaterials processing and synthesis, microelectronics and quantum systems, energy and sustainability, aerospace and plasma medicine. PCRF collaborative users have access to the stateoftheart research capabilities, including advanced plasma diagnostics, a variety of plasma sources, computational codes (e.g. 2D and 3D ParticleinCell codes and fluid codes), and theory support. Since its launch in 2019, 86 users from the plasma and other scientific communities including from academia, national labs and industry have been awarded runtime at the PCRF. In this presentation, we will overview PCRF research, capabilities, and opportunities for collaboration. 

GP11.00014: Ion and Atom Distributions Probing in Plasma via WavelengthModulated LaserInduced Fluorescence Ivan Romadanov, Yevgeny Raitses, Andrei Smolyakov LaserInduced Fluorescence (LIF) spectroscopy is a powerful tool for measuring spectral line profiles of ions or atoms in complex plasma conditions[1], commonly found in laboratory and industrial environments. These spectral line profiles represent Velocity Distribution Functions (VDFs), revealing crucial thermodynamic properties of plasma[2,3]. However, interpreting VDFs can be difficult due to overlapping distributions, various broadening mechanisms, or substantial background emission. We investigates the use of Wavelength Modulation (WM) spectroscopy, [4] a robust derivative spectroscopy technique[5], to address these challenges. Modern diode lasers, with their fast wavelength modulation capability, are ideal for obtaining derivative spectra through WM spectroscopy. The study combines modeling and experimental measurements to explore the use of WM spectroscopy for LIF measurements in plasma. The modeling demonstrated that WM LIF provides more reliable fitting in situations with low signaltonoise ratio and complex VDF profiles, leading to more accurate identification of plasma dynamics. These modeling results were confirmed experimentally by examining weakly collisional argon plasmas with nonequilibrium VDFs using a tunable diode laser. The findings highlight the effectiveness of the WM approach in providing a more rigorous method for VDF analysis. 

GP11.00015: Optical Trapping and Manipulation of Single Particles for Plasma Diagnostics Pubuduni Ekanayaka, Chuji Wang, Saikat Chakraborty Thakur, Edward Thomas Current methods for laser manipulation in dusty plasma mainly focus on manipulating a group of particles within the plasma sheath to study how dust particles interact with the plasma collectively. In our experiment, we have introduced an innovative technique called optical trapping (OT) which allows us to trap individual particles in various plasmas, including plasma jets, DC discharges, dielectric barrier discharges, and rfpowered dusty plasmas. We show that we can not only trap but also transport single particles in both dusty and weakly magnetized rf dusty plasmas in different locations. Our research demonstrates the effectiveness of the OT technology in trapping and transporting particles of different sizes and chemical properties. We utilized imaging systems to monitor the stability and movement of the trapped particles and used optical emission spectroscopy to monitor plasma plumes in real time. This new OTplasma system demonstrates the potential of using a single trapped particle as an insitu microscopic probe for diagnostics of dusty and magnetized dusty plasmas as well as other atmospheric plasmas. 

GP11.00016: Brewster anglecavity ringdown spectroscopy (BACRDS) for measuring plasma species in liquids Chuji Wang, Rongrong W Cohen Cavity ringdown spectroscopy (CRDS) is an ultrasensitive absorption spectral technique, which is widely used for absolute measurement of trace gases in diverse environments including plasmas. In addition to quantification of gas molecules in plasmas, measuring gas molecules in liquids is also important yet remains one of the challenges in diagnostics of low temperature plasma interacting with a liquid. We report on the development of a Brewster anglecavity ringdown spectroscopy (BACRDS) system for low temperature plasma measurements in multiphases. The system can measure gas species in solutions, with a detection limit of 9.1×105, which is equivalent to a detection limit of 0.04 parts per billion for measuring OH radicals in water at 308 nm. With further developments, the detection limit can be potentially up to 106 or lower, which is sufficiently sensitive for measurements of various plasma species in liquids. We present our recent developments in this technology, with a goal to bring a powerful tool to the low temperature plasma community for quantification of reactive plasma species in multiphases or other complex settings. 

GP11.00017: A 10moment multifluid model for partially ionized, partially magnetized plasmas Kentaro Hara, Derek Kuldinow, Yusuke Yamashita Fluid moment models are an attractive option for plasma modeling because macroscopic behavior can be described without needing to track individual particle trajectories. However, the key challenge of fluid models is the closure problem. Particularly in the presence of a magnetic field, anisotropic transport can lead to deviation from local equilibrium, i.e., a Maxwellian velocity distribution function. In this study, a 10moment multifluid model is developed to capture nonequilibrium, nonMaxwellian effects in lowtemperature magnetized plasmas. In addition to mass density and bulk velocity, the full pressure tensor is accounted for, allowing one to capture anisotropy and shear effects without needing explicit viscosity models. Closure is obtained by extension of a Braginskiitype heat flux obtained using ChapmanEnskog expansion. The onedimensional model is developed and applied to the discharge plasma in a Halleffect thruster, and compared to both a 5moment (equilibrium) fluid model [1] and a particleincell MonteCarloCollision model [2]. 

GP11.00018: Numerical thermalization in 2D PIC: Timescales and potential for mitigation in modeling low temperature plasma discharges Sierra Jubin, Willca Villafana, Andrew Tasman T Powis, Dmytro Sydorenko, Alexander V Khrabrov, Igor D Kaganovich Numerical thermalization in particleincell (PIC) simulations has been studied extensively. It is a process that is closely related to the fluctuation spectrum of the simulation; it results in relaxation of the particle velocity distributions as they are driven towards a Maxwellian by the numerical collision operator. This is analogous to Coulomb collisions in a real plasma. For most 2D setups, numerical thermalization rates are inversely proportional to the number of particles per cell. We provide a practical guide for estimating these effects in 2D PIC simulations for a variety of low temperature plasma applications, including example simulations of capacitively coupled plasma (CCP) discharges, inductively coupled plasma (ICP) discharges, beam plasmas, and hollow cathode discharges. In addition to thermalization timescale estimates, we discuss potential methods of mitigating the effects of numerical thermalization. 

GP11.00019: Nonequilibrium air plasma chemistry in boundary layers Tzvetelina B Petrova, George M Petrov Numerical simulations of air plasmas have been performed within boundary layers near the surface of fast moving objects. A twotemperature model is used to cover both equilibrium and non equilibrium conditions, which includes eleven species and twenty six forward and backward hightemperature chemical reactions of ionization, dissociation, and charge exchange [1]. The model is coupled to a conventional boundarylayer model [2] that enables to calculate the temperature and species distribution along the front stagnation line. At these plasma conditions (pressures of 0.0010.1 atm and temperatures of 100010000 K) the plasma does not reach equilibrium due to short interaction times (<ms) and has to be treated as nonequilibrium plasma. 

GP11.00020: ThreeDimensional Kinetic Simulations of NonEquilibrium Partially Magnetized ExB Devices – Anomalous Transport & Coherent Structures Andrew Tasman T Powis, Igor D Kaganovich, Willca Villafana, Jian Chen Nonequilibrium, partially magnetized lowtemperature plasma devices with crossed ExB fields have proven useful in numerous applications. These include for spacecraft propulsion with Hall thrusters and material deposition using magnetrons. This configuration gives rise to a multiscale zoo of waves and instabilities as well as the emergence of largescale structures such as rotating spokes, many of which have been studied in the somewhat simpler Penning discharge device. This complexity has precluded development of analytic models for these systems, necessitating kinetic numerical simulations, often via the particleincell method.


GP11.00021: The Ernst Ratz Analytical Solution in a High Speed Rotating Cylinder Revisited Dr. Sahadev Pradhan In this study the Ernst Ratz differential equation governing the concentration field in a high speed rotating cylinder with and without axial back diffusion was revisited, and the solution of the differential equation was developed to study the effect of product baffle opening on the optimum feed flow rate and on the optimum enrichment for a wide range of normalized countercurrent (L/F in the range 1.2 to 8) with unit cut (P/F) equal to 0.4, 0.5 and 0.6. Here, L is the countercurrent circulation rate, F is the feed flow rate, and P is the product flow rate. The analysis shows that at a given unit cut and normalized countercurrent, the optimum feed flow rate can be reduced by lowering the product baffle opening, and the effect is significant for normalized countercurrent L/F up to 5, and beyond that point the influence is not as much of important, whereas in the case of optimum enrichment (N_{P} – N_{W})_{opt} , as the product baffle opening is lowered, the optimum enrichment increases monotonically with normalized countercurrent. Here, N_{P} and N_{W} are the concentration of the product and waste stream respectively. Next, the flow profile efficiency (E_{F}) and mass flow efficiency (E_{M}) have been studied for wall pressure in the range 20 to 100 mbar based on the mass flow rate in the inner stream (m_{i}), and the analysis indicates that the product of flow profile and mass flow efficiency (E_{F} x E_{M} ) has an optimum for each wall pressure, and the separative power (δu) attains its maximum value at that m_{i} value. The comparison between axial back diffusion and without axial back diffusion reveals that at a given unit cut, the rectifier has an additional length due to axial back diffusion effect, and the influence can be reduced by increasing the aspect ratio (Z/R_{w}) of the cylinder ((Pradhan & Kumaran, J. Fluid Mech., vol. 686, 2011, pp. 109159); (Kumaran & Pradhan, J. Fluid Mech., vol. 753, 2014, pp. 307359)). Here, Z is the effective length of axial countercurrent, and R_{w} is the radius of the cylinder. An important finding is that there is a cross over point for normalized countercurrent beyond which the optimum feed flow rate is higher with axial back diffusion compared to without axial back diffusion, and the cross over point shifted to smaller values with the lowering of product baffle opening. 

GP11.00022: Emissive sheath dynamics in the aidandcompete twoplasma mode of a hotfilament discharge Meghraj Sengupta, Michael Campanell A cylindrical coaxial discharge with an inner radially emitting cathode is a common configuration for many plasma devices, such as the cylindrical magnetron or the hot filament discharge, which find useful applications in the plasma material processing industry and electric space propulsion. The physics of emissive sheaths around biased hot filaments also has implications for emissive probe measurements in plasma diagnostics. 

GP11.00023: Electron energy distribution and instabilities in electron beam generated ExB plasma Nirbhav S Chopra, Yevgeny Raitses Electron beam (ebeam) generated plasmas are promising for low pressure, low damage threshold material processing applications requiring efficient generation of ions and radical species [1,2]. The production of reactive species generated by electron impact is governed by the electron energy distribution function (EEDF). In this work, we investigate the EEDF and plasma parameters of a partially magnetized plasma generated by ebeam in low pressure (0.110 mTorr) argon. The ebeam (energy < 100 eV) is extracted from a negatively biased thermionic filament and injected at one end of a cylindrical vacuum chamber with applied axial magnetic field. The EEDF is measured using Langmuir probes, and plasma potential profiles and ion density oscillations are additionally characterized with emissive and ion probes respectively. Results indicate the presence of beam electrons with energy at roughly the applied cathode potential, as well as a group of warm electrons (1030 eV). Analysis of results of these measurements and their comparison with PIC simulations will be presented. 

GP11.00024: Precision timing controlled pulsed power current distribution solid state switch array for use in plasma and magnetic field generation Mark Moffett, David L Chesny, Kaleb Hatfield Pulsed power systems are required for producing plasma discharges used in propulsion, fusion, and various plasma formation studies. The activation switch that triggers plasma generation requires transmitting > kA in under a microsecond or faster. Spark gaps are low pressure environment switches that use externally sourced "sparks" to initiate the plasma discharge. Precision timing with spark gaps involves constraining variables, such as the incoming voltage to the switch, switching chamber pressure, distance between switch electrodes, and method of initiating the spark. High current silicon controlled rectifiers, known as thyristors, can be used to handle the dV/dT and dA/dT of capacitor bank powered plasma discharges. An array of thyristors in parallel allows for the current to be distributed according to Kirchhoff's junction rule. This current distribution is dependent on simultaneous triggering of the gate on each thyristor in the array using one signal. Differences in timing can be detected using Rogowski coils around each branch of the array. In this study, timing allowance and tolerance of high current discharges through a thyristor array are investigated for the purposes of producing highly coupled plasma and external magnetic field interactions for magnetic reconnection experiments. The timing from two separate pulsed power systems triggered from one signal was observed to be ~10 ns. This provides six orders of magnitude more timing accuracy and control than a spark gap. 

GP11.00025: Demonstration of Ion Energy Distribution Control in a Capacitively Coupled Plasma James R Prager, Joshua Perry, Kevin Muggli, Timothy Ziemba Precision control of the ion energy distribution (IED) in plasma etching is becoming increasingly important to produce highquality highaspectratio (HAR) features for solidstate nonvolatile memory storage. Precision control of the IED is required to minimize bowing and twisting defects in HAR features. Eagle Harbor Technologies (EHT), Inc. has an experimental test bed that consists of a capacitively coupled plasma source driven by a 60 MHz radio frequency generator. The bias tailored voltage waveform is produced by EHT Rapid Capacitor Charger that can charge capacitance to high voltage in tens of nanoseconds and operate at 400 kHz. EHT will present IED measurements made with a retarding field energy analyzer showing control of the IED over different tailored waveform parameters. Based on these measurements, the next steps will be discussed. 

GP11.00026: Confined Filamentation Dynamics in Strongly Magnetized Low Temperature Plasma using the MDPX device Elon Price, Stephen Williams, Saikat Chakraborty Thakur, Edward Thomas The Magnetized Dusty Plasma eXperiment (MDPX) is a unique device that can produce steady state, large magnetic fields up to 4 T in a significant experimental volume, 50 cm in diameter and over 20 cm long. At magnetic fields larger than ~1 T, rf generated, capacitivelycoupled plasmas exhibit the formation of coherent structures that are generally aligned along the magnetic field direction and that can be stable or mobile that are referred to as "filaments". These filaments disrupt the uniform plasma background and cause significant perturbations to dusty plasma experiments. Therefore, understanding the morphology and dynamics of filaments in a dustfree environment has become an integral part of the studies using the MDPX device. This presentation will discuss investigations of confined filaments, achieved by introducing copper rings on the main electrodes, which acts to restrict motion. The localization of the filaments offers more reliable measurements including improved spatial and temporal resolution. A tracking algorithm that has been developed in Python will be used to quantify and classify the translational and rotational dynamics as well as provide insights into transitions between different morphologies. 

GP11.00027: Filamentation Morphology in Capacitively Coupled Highly Magnetized Plasmas Stephen Williams, Saikat Chakraborty Thakur, Mohamad Menati, Edward Thomas Due to the small chargetomass ratio of dust particles, it is often necessary to use large magnetic fields of B ≥ 1 T, in order to observe the influence of magnetic forces in laboratory dusty plasmas. However, when experiments are performed at high magnetic fields in capacitively coupled, radio frequency discharges, the background plasma is often observed to form filamentary structures between the electrodes that are aligned to the external magnetic field which disrupt the uniformity of the plasma and adversely impact our dust experiments. Experiments performed in the Magnetized Dusty Plasma Experiment (MDPX) device have identified and characterized these filamentary structures. This presentation discusses the morphology of several distinct filamentary modes that are formed in low temperature plasmas with different neutral gases. There is strong evidence that each spatial mode has a threshold condition that is dependent on the ion Hall parameter – which is a function of magnetic field, neutral pressure, and ion mass. The criteria for the formation of the filaments are shown to be consistent with predictions of numerical simulations. 

GP11.00028: A Strong Transverse Magnetic Field Effect on Cold Cathode Tube Discharges Harrison Adler, Oleg Batishchev The effects of magnetic fields on glow discharges have been studied extensively in the past [12]. In most cases, however, the Bfield was made predominantly parallel to the applied electric field, or coaxial electrodes were used [3]. We continue investigation of the glow discharges in various gases using Geissler tubes in a strong ~2.4T transverse magnetic field. We detect the growth of ion line emission at certain gas pressures, which indicates enhanced plasma production and/or confinement. Hot dense plasma etches the capillary's inner wall causing erosion of the material and appearance of impurities in the discharge. This opens the possibility to vary the plasma composition in a desired way. We study custom spectral tubes made from different glass types by analyzing broad emission spectra and individual line shapes, including the anomalous Zeeman splitting [45], of the strongest transitions in the expected chemical elements. 

GP11.00029: WAVES, INSTABLITIES, AND TURBULENCE


GP11.00030: Mode Structure in Cylindrical Helicon Plasma Measurements Using a Fast Camera in the WVU PHASMA Experiment Gustavo E Bartolo, Chloelle M Fitz, Earl E Scime We report the experimentally determined dispersion relations for drift waves in a helicon plasma source. The measurements are obtained optically with a Photron fast camera on the West Virginia University PHAse Space Mapping (PHASMA) experiment. Axial measurements of the cylindrical argon plasma allow for radial, angular, and temporal resolution. Assuming cylindrical symmetry we extract mode structures both inside and out of the blue core helicon plasma. Variations in mode structures as a function of forward RF power (550 – 750 W), magnetic field strength (1k – 1.5k G) and direction, fill pressure (33.7 mTorr), and emission wavelength (to separate ion and neutral emission contributions) are presented. Higher m mode structures are more prevalent in larger guide fields with larger forward input power within the blue core region. The helicon plasma becomes increasingly unstable at higher magnetic fields. 

GP11.00031: Gap modes in Arnold tongues: the topological origin of toroidal Alfven eigenmodes Andrew O Brown, Hong Qin The topological origin of the Toroidal Alfven eigenmodes (TAEs) is demonstrated in the limit of high toroidal mode number using the ballooning representation, in which the governing equation reduces to a modified Mathieu equation with a delta function potential perturbation. We prove that the modified Mathieu equation admits isolated gap modes with topological origins in the unstable regions of the Mathieu equation, which are known as Arnold tongues. The modes are identified as TAE modes. A generalization of this argument shows that gap modes can be induced in regimes of instability by localized potential perturbations for a large class of periodic Hamiltonians. 

GP11.00032: Overview of the Basic Plasma Science Facility Troy A Carter, Stephen T Vincena, Shreekrishna Tripathi, Patrick Pribyl, Walter N Gekelman, Christoph Niemann The Basic Plasma Science Facility (BaPSF) at UCLA is a US national collaborative research facility for studies of fundamental processes in magnetized plasmas sponsored by DOE and NSF. The centerpiece of the facility is the Large Plasma Device (LAPD), a 20m long, magnetized linear plasma device. LAPD has been utilized to study a number of fundamental processes, including: collisionless shocks, dispersion and damping of kinetic and inertial Alfvén waves, compressional Alfvén waves for ioncyclotron range of frequencies heating^{,} flux ropes and magnetic reconnection, threewave interactions and parametric instabilities of Alfvén waves, turbulence and transport and interactions of energetic ions and electrons with plasma waves. LAPD is now operating with a new plasma source based on a large area LaB$_6$ cathode along with a new magnet section capable of 

GP11.00033: Laboratory Study of Alfvén Wave Steepening Mel Abler, Seth Dorfman, Alfred W Mallet, Christopher Chen, Stephen T Vincena Alfvénic fluctuations  fluctuations with magneticfield and velocity fluctuations perpendicular to the background magnetic field which are proportional to each other  are thought to be ubiquitous in magnetized astrophysical plasma environments and are observed across scales in our own solar wind. Recent theoretical work by Mallet et al [1] proposes a mechanism by which smallscale, oblique Alfvén waves undergo a onedimensional nonlinear steepening process only at dispersive length scales smaller than the ion inertial length. This work presents the first laboratory tests of this steepening model, comparing predictions for the amplitude of the harmonic of a driven wave to experimental measurements from the Large Plasma Device (LAPD). These tests span highly inertial to highly kinetic conditions at low β to provide insight into turbulence in environments like the solar corona, where the usual counterpropagating Alfvén wave interactions may be suppressed. 

GP11.00034: Laboratory Experiments of Strong Alfvén Wave Interactions Christopher Chen, Seth Dorfman, Stanislav A Boldyrev, Alfred W Mallet, Mel Abler, Stephen T Vincena, Troy A Carter Alfvénic turbulence is pervasive in the solar wind and thought to be common in astrophysical plasmas. The solar wind has allowed us to learn much about this turbulence, however, there are many open questions about how it works and shapes these systems. Lab experiments provide a controlled environment to test the basic physics of such turbulence. Here, we present an experiment on the Large Plasma Device, in which interacting lowfrequency Alfvén waves at small k_{⊥}ρ_{s} are studied. We show that both counterpropagating and copropagating waves result in a spectrum of new modes generated by their nonlinear interaction. Estimated strength parameters, the wave frequencies involved, and their amplitude dependence on the initial waves, indicate the interaction to be in the strong regime – that applicable to space/astrophysical systems. The copropagating waves produce a comparable spectrum to the counterpropagating ones, indicating a nonMHD interaction. We discuss a new nonlinearity, that scales with k_{⊥}d_{i}, which may be responsible, and may also be important for understanding unexplained features of the solar wind. This experiment also allows other aspects of strong Alfvénic turbulence to be studied, such as residual energy, which will also be presented at this meeting. 

GP11.00035: Generation of residual energy by many interacting Alfvén waves Seth Dorfman, Christopher Chen, Stanislav A Boldyrev, Mel Abler Counterpropagating Alfvén wave interactions which transfer energy from large to small spacial scales lie at the heart of magnetohydrodynamic turbulence in the solar wind. An unexpected feature of the turbulence is the generation of residual energy – excess energy in the magnetic fluctuations compared to the velocity fluctuations. By contrast, an MHD Alfvén wave has equal amounts of energy in fluctuations of each type. The current work examines an analytic solution to the reduced MHD equations in the presence of multiple nonlinear interactions. We consider the interaction of two sinusoidal Alfvén modes with arbitrary frequencies and wavenumbers and use the approach of Howes, et. al. 2013 to solve for generalized interaction terms. The result contains both a particular solution at the frequency of the nonlinear drive and a homogeneous solution at the frequency of the associated normal mode. At the resonance where the two frequencies match, secularly growing Alfvén normal modes are produced. Due to the chosen initial conditions, these modes grow in time (not in space), and subsequent interactions involving the secularly growing modes preferentially produce negative residual energy. This result shows up as the condensed region of residual energy near k_{}=0 first derived by Wang, et. al. 2012. Large Plasma Device experiments which have successfully verified residual energy in a driven child mode will be presented in a companion abstract. 

GP11.00036: Finite Ion Temperature Effects on ThreeDimensional Kinetic Instabilities in Partially Magnetized Plasmas Andrew C Denig, Kentaro Hara In partially magnetized plasmas, in which the electrons are magnetized, and the ions are unmagnetized, it is known that the crossfield electron transport does not follow the classical transport theory due to collisional transport. Recent studies suggest that plasma waves driven by instabilities lead to the enhanced transport and diffusion across the magnetic fields. Examples of such kinetic instabilities are the electron cyclotron drift instability (ECDI) and modified twostream instability (MTSI) due to the E<![if gte msEquation 12]>×B drift. In this talk, we extended the capabilities of the previously developed 3D generalized dispersion solver [1] to include the effects of finite ion temperature. The impact of ion and electron Landau damping [2] on the development of ECDI and MTSI is investigated. The finite ion temperature effects contribute to the damping of both modes, depending on the iontoelectron temperature ratio, plasmas density, temperature, and field strengths. The numerical results of the 3D dispersion relation show that the shorter wavelength ECDI mode is more damped than the longer wavelength MTSI mode due to the warm ions. The resulting growth rates and phase velocities are obtained as a function of wavenumber for various plasma conditions and compared with experimental observations. 

GP11.00037: Quasilinear theory for inhomogeneous collisional plasma Ilya Y Dodin The classic formulation of quasilinear theory (QLT) for plasmas interacting with weak turbulence fails to conserve the action of noresonant waves and is inapplicable to inhomogeneous plasmas. We present a reformulation of QLT as a local theory [I. Y. Dodin, J. Plasma Phys. 88, 905880407 (2022)] where these issues are fixed by replacing shortcuts of the classic theory with rigorous calculations. The particle Hamiltonian is kept general; for example, relativistic, electromagnetic, and gravitational effects are subsumed. A FokkerPlanck equation for the dressed "oscillationcenter" distribution is derived from the Klimontovich equation and captures quasilinear diffusion, interaction with the background fields and ponderomotive effects simultaneously. Waves are allowed to be offshell (i.e. not constrained by a dispersion relation), and a collision integral of the BalescuLenard type emerges in a form that is not restricted to any particular Hamiltonian. This operator conserves particles, momentum, and energy, and also satisfies the Htheorem, as usual. As a spinoff, a general expression for the spectrum of microscopic fluctuations is derived. For onshell waves, which satisfy a quasilinear wavekinetic equation, the theory conserves the momentum and energy of the wave–plasma system. The action of nonresonant waves is also conserved, unlike in the standard version of QLT. Dewar's oscillationcenter QLT of electrostatic turbulence [Phys. Fluids 16, 1102 (1973)] is reproduced as a particular case. 

GP11.00038: Stability of the transonic plasma flow in the magnetic nozzle Hunt Feng, Andrei Smolyakov Transonic acceleration of plasmas driven by thermal pressure occurs in convergingdiverging magnetic field configurations such as magnetic nozzles in plasma propulsion devices and magnetic mirror systems for fusion applications. A similar mechanism is responsible for the acceleration of the solar wind where the nozzle effect is created by a combination of the diverging magnetic field and the gravity effect. Wave propagation along the stationary transonic nonuniform flow is of interest for some applications. Alternatively, this problem can be viewed as a problem of the linear stability of the transonic stationary flow. The analysis of the linear stability of plasma flow in the magnetic nozzle using the spectral method suffers from spectral pollution. For purely subsonic and supersonic velocity profiles the spurious solutions can be filtered out using the convergence test. However, the eigenvalue problem for the transonic velocity profile has an additional difficulty due to the presence of the singularity in the eigenvalue problem which occurs at the sonic point. Using the analytical expansion near the singular point, regular solutions to the polynomial eigenvalue problem can be constructed and the regular eigenfunctions can be built in the whole domain by using the shooting method. We show that regular eigenfunctions are spectrally stable. However, the spatial profiles of the eigenfunction can be interpreted as convective amplification of the perturbation entering the transonic acceleration region. 

GP11.00039: Wave topology and topological waves in Hall MHD plasmas Yichen Fu, Hong Qin The topological property of waves in fluids and plasmas has recently been an emerging research topic. It connects the existence of some surface waves in a nonuniform region to the topology of plane waves in a uniform region. Cold plasma waves are shown to have nontrivial topology [12], thereby supporting the socalled topological Langmuircyclotron waves [34]. Here we present the study of topological properties in Hallmagnetohydrodynamics (HMHD) models, where the electron and ion flows are not bounded to each other, and the Hall term is included in Ohm's law. The bulk waves are studied analytically in a uniform system; degeneracy points are shown to exist in the dispersion relation and support nontrivial topology. Topological surface waves are calculated numerically in a 1D nonuniform HMHD equilibrium, which is accurately predicted by the bulkedge correspondence principle. Furthermore, we also show that in a 1D equilibrium, the HMHD system is PTsymmetric and exhibits a purely real spectrum when the PTsymmetry is unbroken, despite its nonHermicity. The result of HMHD wave topology can be generalized to generic weakly nonuniform PTsymmetric systems. 

GP11.00040: Ion hole onedimensional equilibrium and stability Ian Hutchinson Ion holes are electrostatic solitary waves with negative potential, 

GP11.00041: Energy oscillations observed in the MD simulations of plasmas Jawon Jo, Min Uk Lee, JeongYoung Ji The molecular dynamics (MD) simulation is a specialized tool which can capture the effects of particleparticle interactions in a manybody system. A GPUaccelerated MD code has been developed to deal with the Coulomb interactions of particleparticle pairs. This research reports the oscillatory behavior of the temperature and potential energy observed in the MD simulations of plasmas. The numerical experiments are performed to investigate the frequency dependence on the plasma coupling parameter and particle mass. 

GP11.00042: Electrostatic Wave Models of Reflection in a Dipole Magnetic Field Patrick A Langer, Frederick Skiff It is known that plasma experiments can measure reflected waves. Therefore, we utilize onedimensional numerical models as a means of calculating the reflection coefficient of electrostatic plasma waves in a dipole magnetic field. We use two models for both cold and hot plasma effects (Ä1Ñ). As a first approach, we analyze about the midplane of the dipole field and equate the parallel wavelength to be inversely proportional to the field line length. Our model equations result from the taking the dispersion relation for both cold and hot plasma effects, and, under the premise of energy conservation, invert with the equation with k_{x} > i d/dx. These equations are numerically solved and analyzed for to solve for a reflection coefficient. The coldplasma equation yields 100 percent reflection with an incident Ion Acoustic Wave that mode transforms into an electrostatic Ion Cyclotron Wave. This occurs due to a cutoff at $omega = omega_{ci}$, and the phase shift of reflection depends on the frequency of the incident wave. The hotplasma equation yields more complex results, because of mode conversion into the Ion Bernstein Wave. We then discuss the limits of the model, and improvements to the models as a next step. 

GP11.00043: Sensitivity of radiation to the superthermal electron population in mildly relativistic plasma Mikhail Mlodik, Vadim R Munirov, Tal Rubin, Ian E Ochs, Elijah J Kolmes, Nathaniel J Fisch Synchrotron radiation has markedly different behavior in nonrelativistic (T_{e }~ 10 keV) and in mildly relativistic (T_{e }~ 100 keV) plasma. Superthermal electrons which occupy the tail of velocity distribution function have outsized impact on power loss of ~ 100 keV plasma. If electrons with energy more than a cutoff energy are redistributed while keeping the Maxwellian distribution function below cutoff energy intact, both emission and absorption of synchrotron radiation act to decrease the lost power. [1] Similarly, bremsstrahlung radiation is decreased if superthermal electrons are redistributed. [2] These novel radiation transport effects in nonequilibrium plasma suggest large utility in the deconfinement of highenergy electrons to reduce synchrotron radiation in applications where the radiation is deleterious. In particular, these effects can be significant in pB11 burning plasma. 

GP11.00044: Relativistic and Wave Effects on Confinement in Rotating Magnetic Mirrors Ian E Ochs, Vadim R Munirov, Nathaniel J Fisch Rotating magnetic mirror confinement systems are experiencing a resurgence of interest, partly as a result of the great success of shear stabilization. At the same time, new calculations of the protonBoron 11 (pB11) power balance have shown that breakeven aneutronic fusion is less prohibitive than once thought. Here, we examine features of rotating mirror confinement unique to the high temperatures (150300 keV) of the pB11 reaction. Here, relativistic effects are important for electron collisions, reducing the electron confinement [1]. At the same time, electron cyclotron radiation becomes important. These effects can be incorporated in the electron FokkerPlanck equation, leading to new confinement time scalings unique to relativistic plasmas, which are validated with a novel DolfinXbased finiteelement FokkerPlanck code. 

GP11.00045: Topological Langmuircyclotron wave Hong Qin, Yichen Fu It can be proved that the electron cyclotron wave must vanish somewhere on a 2D sphere enclosing the Weyl point of Langmuircyclotron resonance in the parameter space. Consequently, there must exist a topological surface excitation called Topological LangmuirCyclotron Wave (TLCW) in magnetized plasmas [1], which propagates unidirectionally along complex phase transition interfaces without scattering [2]. Due to this topologically protected robustness, the TLCW, whose spectrum span covers those of high harmonic ion cyclotron waves and whistler waves, could be explored for various practical applications. The topological methods recently developed for plasma waves are similar but different to those used in condensed matter physics, where periodic lattices lead to nontrivial topology in momentum space. But in plasmas and other continuous media, nontrivial topology only exists in phase space because of the contractibility of momentum space [1]. Using the algebraic topological concepts and tools developed, such as the boundary isomorphism theorem [1], and an AtiyahPatodiSinger type of index theorem formulated by Faure [3], we prove the existence of TLCW as a spectral flow across the band gap. We show that the TLCW can be represented by a tilted Dirac cone in phase space, the entire spectrum of which, including its spectral flow, is found analytically. [1] H. Qin & Y. Fu, Sci. Adv. 9, eadd8041(2023). [2] Y. Fu & H. Qin, Nat. Commun. 12, 3924 (2021). [3] Faure, arXiv:1901.10592 (2019). 

GP11.00046: Experiments investigating microwave beatwave Raman scattering in inductively coupled plasma Kevin Ronald, Kieran J Wilson, Liam Selman, Bengt Eliasson, Colin G Whyte, Mark E Koepke, David C Speirs, Alan R Phelps, Robert Bingham, Robert Alan Cairns, Ruth Bamford, Craig W Robertson, Philip MacInnes, Adrian W Cross Nonlinear wave coupling 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 plasma and laserplasma interactions. Understanding these dynamics can inform new methods to introduce energy into plasma, manipulate plasma conditions or to mitigate undesirable consequences induced by such instabilities. Cool plasmas with critical frequencies in the low microwave range are relatively easier to diagnose. Experiments are therefore underway investigating the dynamics of microwave beams propagating in an inductively coupled plasma formed in a recently commissioned source with a diameter of 1m and length of 3m. Operating in He gas with pressures in the range of <![if gte msEquation 12]>≈ 10^{3 }mbar – 10^{1 }mbar it achieves plasma frequencies in the range a few hundred MHz and bulk temperatures of <1eV (a tenuous hotter population is also present) when driven by a 10200W RF source at 14MHz, ideal for microwave parametric scattering experiments. The plasma is being modulated by counter propagating microwave beams in the range of 10GHz, with power between 7 and 15kW injected as Gaussian beams using a pair of Satoh horn antenna. Progress on this experiment will be reported. 

GP11.00047: Using Whistlers Waves from LAPD as a Plasma Diagnostic Nicolas A Rongione, Joshua J Larson, Thomas Look, Phil Travis, Stephen T Vincena, Frederick Skiff, Troy A Carter, Richard E Wirz The presence of whistler waves emanating from the main discharge of the LArge Plasma Device (LAPD) at UCLA has been confirmed in recent (unpublished) work by Prof. Frederick Skiff. This work also describes the remote detection of signals resembling these whistlers which propagate beyond the cylindrical plasma's radial boundary. A diagnostic based on the cold plasma dispersion relationship for whistler waves may offer the potential to remotely estimate the magnetic field strength, the electron cyclotron frequency, the plasma density, and plasma temperature. This experiment seeks to indirectly measure plasma properties by detecting electromagnetic waves that emanate from the plasma. Initial observations using a collection of two 6 mm diameter Bprobes separated by 1 cm along the chamber centerline running helium in the LAPD at 1000 G show strong, broadband cross spectral power density from 700 MHz – 2.5 GHz, below the electron cyclotron frequency. The power spectral density collected by a double ridge horn antenna outside the LAPD device looking radially through a nylon viewport contains similar spectral content as that which is detected by either B probe. Ongoing work intends to utilize the COMSOL RF module to study the propagation of the whistler waves from within the plasma to the boundary to develop a new passive, remote diagnostic of the LAPD plasma. 

GP11.00048: Magnetostatic ponderomotive barrier for open field line magnetic confinement configurations Tal Rubin, Jean M Rax, Nathaniel J Fisch Open field line configurations, in particular rotating magnetic mirror machines, are receiving renewed interest. In these devices, field line curvature and centrifugal force add a confining potential on top of the magnetic mirror effect. In this work we propose to add a third type of confining potential for motion along field lines, a ponderomotive potential generated by interaction of the rotating plasma with a static azimuthal magnetic ripple. This ponderomotive effect requires little modification to the rotating mirror concept, and utilizes the existing rotation in order to supply an additional confining potential along the machine axis. 

GP11.00049: Wave dispersion in a plasma with complex orbits Fred N Skiff, Daniel Pette Using the technique of integration along particle orbits, we construct the plasma response function for electrostatic waves in the vicinity of a strong dipole magnet. A large number of particle orbits are integrated numerically using a secondorder symplectic integrator with adjustable timestep. A significant measure of the particle orbits are chaotic, many are also regular and either trapped or untrapped. Some orbits undergo chaotic scattering from the dipole field. Along each orbit a weighted average of E*v is compiled keeping track of the contribution a field of basis modes (spherical Bessel functions) to the eventual phasespace perturbation over a set of discritized spatial points. These point are the final locations of sets of particle orbits. The conditilon of selfconsistency between the perturbed orbits and the phase space perturbations is used to construct either a dispersion relation or a solution to the scattering of an ion acoustic plane wave incident in the farfield. The goal is to provide a point of comparison for experiments of ion acoustic wave scattering from a magnetic dipole. Further extension of the work is planned to address the scattering of the CVK spectrum instead of just the acoustic wave. 

GP11.00050: Laboratory Excitation of Chirped EMIC Waves Stephen T Vincena, Shreekrishna Tripathi The Electromagnetic Ion Cyclotron (EMIC) wave is a ubiquitous and consequential plasma wave mode in the Earth's magnetosphere. It possesses electric field components both perpendicular and parallel to the ambient magnetic field. As such, it interacts effectively with both ambient plasma ions and electrons. Importantly, it can scatter such particles trapped in the radiation belts into the loss cone. 

GP11.00051: The impact of stable modes on the merging characteristics of KelvinHelmholtz vortices Braden Buck, Bindesh Tripathi, Paul W Terry, Ellen Zweibel The dynamics of vortices are ubiquitous in both hydrodynamics and MHD. The merging of vortices is especially relevant to shearflow instabilities and the dynamics of magnetic islands in fusion devices. This study focuses on investigating the dynamics of vortex mergers in shearflow (KelvinHelmholtz) instabilities. We examine the effect of the inclusion and exclusion of the conjugate stable mode to the unstable mode and the effect of varying the Reynolds number and the phase differences between the Fourier modes on the onset time for merger events. An emphasis is placed upon the relative difference between the phases of a given Fourier wavenumber and the wavenumber with half of that value (the first subharmonic) as this has been demonstrated by Guha and Rahmani (2019) to affect the merging time of vortices. The effect of the stable mode on the progression of vortex mergers has not been studied before. This work examines the importance of the stable mode and whether or not its contribution would have a nonnegligible effect on vortex mergers in future studies of shearflow driven instabilities. Furthermore, it is anticipated that the information gathered from this investigation will aid future endeavors that focus on the dynamics of magnetic islands or shearflow instabilities. 

GP11.00052: Studying the Intermittency of Broadband Magnetic Fluctuations Carlos A CartagenaSanchez, David A Schaffner The Bryn Mawr Experiment generates broadband magnetic fluctuations which exhibit intermittent behavior. We report on the progress of the intermittency study using highorder statistical analysis of broadband magnetic fluctuations. The Bryn Mawr Experiment injects magnetic helicity with a magnetized coaxial plasma gun into a fluxconserving cylindrical vacuum chamber. The vacuum chamber has no background magnetic which allow flows and magnetic fields to evolve dynamically. Magnetic field measurements (bdotprobes) show intermittent broadband fluctuations. Understanding this intermittent behavior would provide insight on the dissipation of magnetic energy and the nature of the broadband magnetic fluctuations. 

GP11.00053: On the Existence of MHD Critically Balanced Time Scales Erik Hansen, Akash Shukla, David R Hatch, Prerana Sharma, Justin Walker, Philip J Morrison The Sridhar Goldreich theory (SG95) of strong magnetohydrodynamic (MHD) turbulence in a perturbation of a static, mean magnetic field equilibrium anticipates that the nonlinear energy cascade time will evolve to balance the linear wave period.[1] Several arguments underpinning this conjecture of critical balance are evaluated in a pseudospectral Hall MHD simulation to determine whether the premises of SG95 could be relevant for extended MHD or kinetic models of turbulence. The nonlinearity parameter of SG95 is calculated for each mode to check whether the predicted increase to an asymptotic value of one has occurred. Various metrics of the energy cascade time are constructed from time profiles of energy spectra and spectrograms of the Hall MHD induction and momentum equation nonlinearities, which are in turn compared to the linear wave time. 

GP11.00054: On modulational stability of incompressible magnetohydrodynamic turbulence Suying Jin, Ilya Y Dodin Structure formation in magnetohydrodynamic (MHD) turbulence can be modeled as a modulational instability (MI) of the Alfven waves comprising the background turbulence. We focus on the early stages of structure formation (MI's linear stage) and simple backgrounds, such as monochromatic primary waves, in order to develop a tractable model. In particular, we examine the validity of the popular quasilinear approach to the MI, where only two sidebands of a primary wave are retained when analyzing the MI. We find that such truncated models can be fundamentally inadequate in application to ideal MHD. Even at MI's linear stage, the modulational wave energy can propagate up and down the spectrum in the form of dispersive spectral waves which tend to suppress the growth of coherent structures. These waves also introduce unavoidable dissipation of MHD structures, even in the limit of zero viscosity. That said, deviations from ideal MHD that make Alfven waves dispersive tend to reinstate the quasilinear approximation by restricting the spectralwave propagation and suppressing higher modulational harmonics. 

GP11.00055: MultiScale Interactions between Global Tearing Modes and Trapped Electron Modes T. Jitsuk, A. Di Siena, M.J. Pueschel, P.W. Terry, F. Widmer, E. Poli Interactions between tearing modes (TMs) and smallscale instabilities and turbulence, particularly in reversedfield pinches (RFPs) where these instabilities coexist, can alter the saturation mechanism and increase transport. Understanding their interactions has been hindered by limitations in previous modeling of global TMs in the gyrokinetic code GENE. Correct modeling is accomplished by implementing a shifted Maxwellian distribution into the global version of GENE. The modified code is benchmarked against ORB5, GKW, a fluid model, and theory through parameter scans, demonstrating good agreement. Linear simulations of an MST RFP nonreversed discharge provide insights into TM structure, growth rate, and frequency. Collisionality scans confirm destabilization consistent with theoretical predictions. Linear analysis reveals that TMs predominantly occur in the plasma core, with toroidal mode number n<35, while TEMs reside near the plasma edge, with 35<n<250. TM saturation is achieved by modes in the core coupling to smallerscale stable TMs close to the edge, allowing interactions with microinstabilities. The multiscale interactions of TMs with TEMs are explored, including the degradation of zonal flows by TMs and the resulting increase in TEM transport. This study informs the understanding of the influence of largescale magnetic perturbations on smallscale instabilities, and vice versa, in tokamak plasmas. 

GP11.00056: Investigating ETG in NSTXU Pedestal: Local and Global Simulations with Profile Curvature Correction PingYu Li, David R Hatch, Jason F Parisi, Mate Lampert, Emily A Belli, Swadesh M Mahajan, Michael T Kotschenreuther This study investigates the behavior of electron temperature gradient (ETG) modes in the pedestal region of the National Spherical Torus Experiment Upgrade (NSTXU) using a comprehensive approach of local and global simulations with profile curvature correction. It is observed that global effects can stabilize ETG in the pedestal top for Mega Ampere Spherical Tokamak (MAST). The nonlithiated NSTXU pedestal shows a similar pedestal profile, which might also demonstrate similar global effects on ETG. Local linear and nonlinear simulations are performed at radial locations from the steep gradient region to the pedestal top to gain insight into ETG dynamics and their influence on the pedestal structure. Key parameters affecting ETG growth and stability are identified through the analysis of local simulations. To account for global effects, profile curvature correction terms are incorporated into the local simulations. This allows for the capture of the impact of profile shape on ETG behavior and potential modifications in the pedestal structure. A comparison is made between local simulations with profile curvature correction and dedicated global simulations to assess the adequacy of the profile curvature correction approach in capturing global effects accurately. A combination of local and global simulations, along with local simulations including a profile curvature correction, will inform strategies for modeling the electron temperature pedestal in spherical tokamaks. 

GP11.00057: The Behavior of Edge Instabilities and Turbulence in GasPuffFueled LAPD Discharges Thomas Look, Troy A Carter The recent upgrade of the main plasma source of the Large Plasma Device (LAPD) [1] from Barium Oxide to Lanthanum Hexaboride has motivated a new fueling procedure to support higherdensity operation. Piezoelectric valves near the anode puff gas during the discharge, and the chamber is evacuated between shots. This mode of operation can achieve higher densities and better axial density uniformity than the previous fueling procedure but also has revealed interesting new plasma behaviors. A transition from broadband edge turbulence to a single coherent mode is observed with increased neutral fueling via gas puffing. We will present experimental data demonstrating this behavior and discuss future experiments to understand the nature of this transition. 

GP11.00058: Energy Spectrum of Lost Alpha Particles in Centrifugal Mirror Confinement Alejandro Mesa Dame, Ian E Ochs, Nathaniel J Fisch Due to their good stability and differential confinement features, centrifugal magnetic mirrors have experienced a recent resurgence as a basis for magnetic fusion energy designs. Alpha particles produced in fusion reactions carry anywhere from 20% to 100% of the fusion power, depending on the reaction, and are typically born at temperatures much higher than the confining centrifugal potential, raising the possibility of immediate loss. Knowing their outgoing energy spectrum may be helpful for optimizing direct conversion schemes and is vital to understanding the power balance of the reactor. We derive analytical expressions for the energy spectrum and confinement times of lost alpha particles in centrifugal mirror fusion reactors, and validate them via MonteCarlo simulations. Although we focus on alpha particles in a centrifugal potential, our results hold for any fast ion species subject to any applied potential, providing a wide range of applicability to present day mirror devices. 

GP11.00059: Turbulent plasma wind tunnel studies on the Bryn Mawr Experiment (BMX): Spectra, Magnetic Nozzle, and Insulating Block Experiments David A Schaffner An overview and recent progress of activities at the Bryn Mawr Plasma Laboratory (BMPL) is presented. The main experiment at the facility, the Bryn Mawr Experiment (BMX), consists of a 4mF pulseforming network that generates ~180us of stationary broadband fluctuations of magnetic field and plasma using a magnetized coaxial plasma gun source. A measurement of a dissipation scale, the Taylor microscale, has recently been made with values found on the order of 1020% of the chamber radius. The spectra and intermittency of magnetic field fluctuations are also presented. Results from two recent experiments are also presented: 1) Increasing flow velocities using a magnetic nozzle configuration and 2) Exploring the wake due to an insulating block. 

GP11.00060: Energy Transfer and Scale Dynamics in 2D and 3D Laser Driven Jets Hao Yin, Jessica K Shang, Eric Blackman, Gilbert W Collins, Hussein Aluie We demonstrate a methodology for diagnosing the multiscale dynamics and energy transfer in complex HED flows. The approach separates incompressible, compressible, and baropycnal contributions to energy scaletransfer and quantifies the direction of these transfers in (generalized) wavenumber space. By comparing the kinetic energy (KE) transfer across scales in simulations of 2D axisymmetric versus fully 3D laser driven plasma jets, we find the 2D modeling suffers from significant spurious energization of the bulk flow by a turbulent upscale cascade. A coherent circulation that arises near the jet's leading edge in 2D but absent in 3D helps propel the axisymmetric jet farther (approximately 25% by 3.5 ns) and helps keep it collimated. The methodology may help with intermodel comparison and validation, including future modeling efforts to alleviate some of the 2D hydrodynamic artefacts in HED simulations like astrophysics and inertial confinement fusion (ICF), which are costly to model in 3D. 

GP11.00061: STELLARATORS: W7X, LHD, HSX, CTH, OTHERS


GP11.00062: Auburn CTH and W7X Research Progress and Plans David A Maurer, Nicholas R Allen, Roger Dorris, David A Ennis, Tomas G Gonda, Gregory J Hartwell, David M Kriete, Eleanor Williamson, Noah Bessard The Compact Toroidal Hybrid (CTH) is a torsatron/tokamak hybrid. The main goals of the CTH experiment are to study disruptive behavior as a function of the applied 3D magnetic shaping, and to test and advance computational tools able to describe 3D MHD physics such as the V3FIT reconstruction code and NIMROD modeling of CTH. Recent nonresonant divertor experiments, vertical stability, and hard xray generation studies will be overviewed and their relevance to tokamaks and quasiaxisymmetric stellarators will be discussed. Ongoing diagnostic development for the experiment includes development of new Hall probe array measurements, new spectroscopic studies of W and other high Z materials, and development of a neutral Argon density spectroscopic diagnostic. CTH also serves as a test bed for diagnostic development for our collaborations on the larger facilities like DIIID and W7X. Auburn research on W7X will be summarized also. 

GP11.00063: Edge Plasma Flux Measurements for Characterization of NonResonant Divertor Behavior on the Compact Toroidal Hybrid Experiment Nicholas R Allen, Kelly A Garcia, Aaron Bader, John C Schmitt, Gregory J Hartwell, David A Ennis, David A Maurer Modeling studies investigating the nonresonant divertor concept have demonstrated resiliency of the plasma strike line locations to large changes in the equilibrium flux surface structure of CTH plasmas. To validate these predictions and characterize strike point locations for specific nonaxisymmetric 3D magnetic equilibria, two Langmuir probe (LP) arrays have been developed for ion flux measurements at the edge of CTH plasmas. These LP arrays consist of a radially movable array with 25 probes positioned at the halffield period, and a fixed array with 12 probes at a limiting surface. Each array covers ~1 radian of poloidal extent and measures ion flux at a sampling rate of 500 kHz. These locations are where the measured ion flux is expected to undergo the greatest change when the edge flux surface structure transitions from a limited last closed flux surface to a chaotic edge boundary. This transition, produced by internal plasma current, accompanies field line chaos, loss of edge flux surfaces, and a change in magnetic topology near the halffield period appearing as a helical band that splits into two distinct strike lines. Preliminary measurements of the edge ion fluxes will be presented alongside the anticipated strike line positions derived from previous modeling efforts. 

GP11.00064: Design and Optimization of a Gamma Ray Imager for Runaway Electron Studies in the Compact Toroidal Hybrid Experiment Roger Dorris, David A Maurer, David A Ennis, Gregory J Hartwell The Compact Toroidal Hybrid (CTH) experiment at Auburn University is a currentcarrying stellarator capable of generating up to 10 kA of runaway electron current. A previous study characterized the runaway beam via its relativistic bremsstrahlung emissions using a system of scintillators, observing gamma ray energies in the range of 500 KeV to 10 MeV. It was determined that increases in the magnetic field strength and rotational transform would decrease the runaway energy by a factor of 4 and alter the spatial emission distribution. 

GP11.00065: Dependence of Vertical Stability on Current Profile Peakedness and 3D Shaping in the Compact Toroidal Hybrid Noah Bessard, David A Maurer, David A Ennis, Gregory J Hartwell Poloidally shaped, or elongated plasmas are desirable for tokamak operation in highbeta regimes but are susceptible to Vertical Displacement Events (VDE) resulting from n = 0 vertical motion, leading to disruptions. In the Compact Toroidal Hybrid (CTH) vertical drifts are minimized by the addition of external stellarator transform and the use of a radial field coil (RFC). A feedback controlled power supply has recently been added to the RFC to be used in conjunction with a set of pickup coils mounted above and below the plasma volume for additional vertical control. Vertical drifts in CTH have been detected at varying elongations and fractional transforms using a set of poloidal pickup coils, an interferometer, and a SXR camera array. Additionally, a 3D Hall array consisting of 8 sensors spanning 45 mm has been installed and calibrated for internal field measurements to characterize the edge current gradient. Experimentally reconstructed magnetic field profiles are in good agreement with profiles from a BiotSavart model. The effects of preprogrammed and feedback controlled RFC on vertical drifts are investigated as a function of the current profile peakedness and 3D shaping in CTH. 

GP11.00066: Implementation of a spectroscopic neutral density diagnostic in a low temperature argon plasma Eleanor N Williamson, David A Ennis, Gregory J Hartwell, Curtis A Johnson, Stuart D Loch, David A Maurer, Jared C Powell, Saikat Chakraborty Thakur, Edward Thomas Understanding the transition region between fully ionized and neutrally dominated plasmas is important to the study of the magnetosphere of the earth, the corona/chromosphere transition regions of the sun, and detached divertors in fusion devices. Determining the fractional ionization of a plasma requires accurately measuring the neutral density. An absolute intensity calibrated spectrometer coupled with results from a collisional radiative model solver is used to measure neutral density in the toroidal CTH and ALEXIS linear plasma device. Results will be shown for plasmas with electron temperatures ranging from 1 to 10 eV and electron densities ranging from 1 x 10^{8} to 1 x 10^{12} cm^{3}. A wide variety of plasmas with fractional ionizations ranging from 0.01% to 99.9% have been produced on CTH. The spectroscopic neutral density diagnostic on CTH shows a good match to pressure gauge measurements below 1 mTorr. Theoretical spectra demonstrate a good match to experimental spectra on both devices. Neutral density shows a linear decrease with an increase in power on CTH. Neutral density shows a linear increase with an increase in gas on ALEXIS. Both results support the validity of the neutral density diagnostic. 

GP11.00067: HighResolution Ultraviolet Spectroscopy for Erosion Measurements of PlasmaFacing Components Dane Van Tol, David A Ennis, Curtis A Johnson, Tomas G Gonda, Gregory J Hartwell, Ulises Losada, David A Maurer, Stuart D Loch To investigate highZ impurity erosion and redepostion, an upgraded UV spectrometer and probe have been installed on the Compact Toroidal Hybrid (CTH) experiment. Using spectroscopic measurements of impurity emission, erosion and redeposition rates of plasma facing components can be quantified by using atomic physics coefficients (S/XBs) which relate spectral line intensities to material influx. Challenges arise from insufficiently resolved spectral lines due to impurity blending, pressure broadening, and Zeeman and hyperfine splitting, leading to erroneous inference of erosion rates. As such, a 1.33meter focal length spectrometer with a highresolution diffraction grating and UV optimized sCMOS camera has been developed, providing ~4 pm resolution for a ~3 nm spectroscopic window down to 200 nm. Impurity emission is produced by plasma interaction with a highZ sample at end of a translatable probe. The upgraded probe system allows for faster sample changes while maintaining vacuum cleanliness and can extend the highZ sample to the CTH magnetic axis. A single tip Langmuir probe measures the plasma electron temperature and density at the highZ sample surface. First results of highresolution tungsten spectra using the new probe and UV spectrometer in CTH will be presented. 

GP11.00068: Commissioning and Modeling of the HighResolution XRay Imaging Spectrometer for Tungsten Transport Measurements on the W7X Stellarator Tomas G Gonda, David A Maurer, David A Ennis, Novimir A Pablant, Andreas Langenberg, Felix Reimold, Thomas Wegner, Birger Buttenschön, Naoki Tamura, Daniel Medina Roque We present progress and plans for core tungsten impurity transport studies on the Wendelstein 7X (W7X) stellarator utilizing Xray measurements from the HighResolution XRay Imaging Spectrometer (HRXIS). Tungsten transport into the plasma core can dramatically affect plasma performance and the neoclassically optimized W7X provides important regimes of study for this topic. The HRXIS and XRay Imaging Crystal Spectroscopy (XICS) diagnostics are used to investigate impurity transport in the W7X plasma by measuring emission from highionization states of injected impurities that are introduced to the plasma using the Laser BlowOff (LBO) or TESPEL systems. These Xray diagnostics have a 2 cm spatial and 5 ms temporal resolution and view selected wavelength regions over a 1 – 7.5 Å range with milliangstrom resolution. Modeling of the system's full range of motion has increased the utilizable wavelength range of the HRXIS diagnostic by a factor of 4. During the OP2.1 W7X science campaign, this new capability was used to identify several previously inaccessible W emission lines originating from highly charged states between W43^{+} and W47^{+}. Plans for the upcoming campaign regarding the implementation of impurity emission measurements to study core tungsten transport are discussed. 

GP11.00069: Drift effects and ion temperature measurements in the scrapeoff layer of W7X David M Kriete, Valeria Perseo, Dorothea Gradic, Arun Pandey, David A Ennis, Kenneth C Hammond, Marcin Jakubowski, Ralf König, David A Maurer, Felix Reimold, John C Schmitt, Victoria R Winters 2D ion velocity and temperature measurements provided by coherence imaging spectroscopy (CIS) yield insight into scrapeoff layer (SOL) transport in the W7X island divertor. Experiments investigating drift effects by comparing plasmas with matched core parameters but opposite magnetic field directions, and therefore opposite drift transport directions, find that the poloidal E×B drift alters SOL parallel flows. In lowdensity plasmas (n_{e} ≤ 2×10^{19} m^{3}), the poloidal E×B drift is the dominant particle transport mechanism and induces a large poloidal density asymmetry within the island. This causes the flow stagnation point, which is normally about halfway between targets, to shift toward the Xpoint in the drift direction, leading to nearunidirectional parallel flow throughout the SOL. As density increases, the effects of the poloidal E×B drift decrease substantially, and a counterstreaming flow pattern develops. In magnetic configurations with shorter connection lengths the impact of drifts on parallel flows is weaker. The ion temperature distribution at the divertor is also measured using a new multidelay CIS instrument optimized to have minimal sensitivity to Zeeman splitting while retaining sufficient sensitivity to T_{i}. Temperatures ranging 15–35 eV over the divertor are observed and are in agreement with highresolution spectrometer measurements. 

GP11.00070: Bursty fluctuation regimes in the core and edge plasma of the Wendelstein 7X stellarator Adrian von Stechow, Seung Gyou Baek, JanPeter Bähner, Sean B Ballinger, Neha Chaudhary, Eric Edlund, Olaf Grulke, Matthias Hirsch, Miklos Porkolab, James L Terry Wendelstein 7X (W7X) is a neoclassically optimized stellarator for which turbulence is the dominant contributor to particle and heat transport in most heating and fueling schemes. A major project focus is therefore the characterization of instabilities and turbulence regimes in order to develop turbulenceoptimized scenarios. This contribution explores a parameter window at low ratios of heating power over density (typ. 13 MW / 8e19 m^{2}) which is characterized either by short dropouts (typ. <10 ms) or spikes (typ. <5 ms) of broadband core turbulent density fluctuations measured by the phase contrast imaging (PCI) diagnostic. These phenomena are highly correlated with equally fast changes of scrapeoff layer light and fluctuations measured by the gas puff imaging (GPI) diagnostic, indicating rapid changes in edge profiles, as well as fluctuations of the electron cyclotron emission in the plasma edge. In controlled power stepdown experiments, both regimes are crossed with heating power thresholds that vary with magnetic configuration. Within a narrower range, these events are periodic with a repetition frequency of 40120 Hz, reminiscent but distinctly different from ELMs, especially in their temporal dynamics and lack of strong effect on confinement. 

GP11.00071: The Design of an Ion Beam Probe for Wendelstein 7X and the Study of Optimized Stellarator Physics Diane R Demers, Thomas P Crowley, Peter J Fimognari, Olaf Grulke, Ralph Laube, Humberto Trimino Mora The dynamics of particle and energy transport is particularly important to the physics of optimized stellarators. A critical need exists for experimental data that will complement the models used to estimate power and particle balance. A heavy ion beam probe (HIBP) diagnostic can help fulfill this need with its unique ability to acquire direct measurements of the electric potential, and fluctuations of electron density and potential in the plasma interior. We have developed a full concept design of a HIBP for Wendelstein 7X (W7X) and passed the critical conceptual design review milestone. The system will inject up to a 2 MeV singlycharged ion beam and is designed to access the outboard crosssection of all eight W7X reference magnetic field configurations. The diagnostic will concurrently acquire measurements at three plasma locations that may be displaced: radially, enabling inference of the electric field; or poloidally on a flux surface, permitting measurement of wavenumbers. Fluctuation induced particle flux will be assessed from its measurement of amplitudes, crossphase and coherence among density fluctuations and potential fluctuations. We will present: the results of recent efforts that have emphasized realizing a cohesive HIBP conceptual design, anticipated characteristics of measurements predicted via simulations of beams through W7X plasmas, CAD schematics of the system, and opportunities for collaboration. 

GP11.00072: OPTEMIST: A neutral beam for probing quasiomnigeneous fast ion behavior in Wendelstein 7X Samuel A Lazerson, David Kulla, Christoph Slaby, Paul McNeely, Norbert Rust, Dirk Hartmann A new neutral beam system is ex plored which can directly demonstrate the quasiomnigeneous nature of the Wendelstien 7X stellarator. The Wendelstein 7X (W7X) stellarator high mirror magnetic configuration is predicted to have improved fast ion confinement at finite beta due to the quasiomnigeneous (QO) nature of the device. As the magnetic field becomes more QO in nature, the drift orbits of the toroidally trapped particles drift poloidally which has the effect of averaging out the effects of bad curvature. It is shown that the existing neutral beam system does not directly populate these orbits. Additionally, the energy of the neutral beam fast ions is not high enough to be considered collisionless in W7X discharges. A new 150 kV neutral beam is envisioned which directly populates the deeply trapped orbits with collisionless particles, satisfying the two necessary conditions to demonstrate the QO optimization. Simulations of beam injection and fast ion confinement with the BEAMS3D code demonstrate the ability of such a beam to demonstrate the QO behavior. Synthetic diagnostic signals as modeled by FIDASIM help to gauge the minimum beam line parameters and diagnostic set needed. The use of the ion cyclotron resonance heating system is also considered in this presentation. 

GP11.00073: Direct optimization of ion transport in a W7Xlike reactor case Brandon F Lee, Samuel A Lazerson, Håkan M Smith, Craig D Beidler We directly optimize stellarator neoclassical ion transport while holding electron transport at a moderate level, creating a scenario favorable for impurity expulsion and retaining good ion confinement. Traditional neoclassical stellarator optimization has focused on minimizing ε_{eff}, the geometric factor that characterizes the amount of radial transport due to particles in the 1/ν regime. At reactorrelevant collisionalities, electrons are typically in the 1/ν regime and ions are typically in the √ν regime. Traditional optimizations thus minimize electron transport and rely on the radial electric field (E_{r}) that develops to confine the ions. This often results in an inwardpointing E_{r} that drives highZ impurities into the core, which may be troublesome in future reactors. In our optimizations, we increase the ratio of the thermal transport coefficients L^{e}_{11}/L^{i}_{11}, which previous work has shown can create impurity screening and an outwardpointing E_{r}. Both effects are very beneficial for impurity expulsion. We obtain selfconsistent density, temperature, and E_{r} profiles at reactorrelevant conditions for optimized equilibria. These equilibria are expected to enjoy significantly improved impurity transport properties. 

GP11.00074: Experimental validation of Fast Ion Loss Detector modeling on Wendelstein 7X Alexandra LeViness, Samuel A Lazerson, Anton Jansen Van Vuuren, José RuedaRueda, Sergey Bozhenkov, Manuel GarciaMunoz, Carsten Killer, Kunihiro Ogawa, Novimir A Pablant We present the first validated synthetic diagnostic model for fast ion loss detectors (FILDs) in Wendelstein 7X (W7X), which include the Faraday cup FILD (FCFILD), the FILD provided by the National Institute for Fusion Science (NIFSFILD), and the scintillating FILD (SFILD) currently in development. 

GP11.00075: Direct fastion density measurements with FIDA spectroscopy in Wendelstein 7X Peter Z Poloskei, Oliver P Ford, Thilo Romba, Benedikt Geiger, Samuel A Lazerson, Robert C Wolf Good fastion confinement is one of the main scientific premises of the Wendelstein 7X (W7X) stellarator. In order to gain insight into their distribution, a multiview spectroscopy system is utilized that measures the Dopplershifted Balmeralpha line radiation of chargeexchange neutralized fastions (FIDA). The interpretation of the measured spectra is performed using the FIDASIM code. The fastions originate from three neutral hydrogen beam injectors (NBI) located in two beam boxes, each capable of delivering 1.7 MW of power with an injection energy of 55 keV. The donor neutrals for the charge exchange reactions consist of both recycling neutrals and those provided by the NBIs. In this study, we present the results of fastion experiments and the corresponding observations obtained during the most recent operation campaign of the W7X. 

GP11.00076: Ion Heat Pulse Propagation Experiments at W7X Using High Speed Charge Exchange Spectroscopy Shawn Simko, Colin Swee, Oliver P Ford, Thilo Romba, Samuel A Lazerson, Benedikt Geiger Ion temperature clamping during ECRH in the W7X stellarator experiment is found consistently across magnetic configurations and significantly limits device performance [1]. First results show that the clamping can in part be explained by turbulent transport driven by Ion Temperature Gradient (ITG) modes, which provide a limit on the inversed gradient length a/L_{Ti}. However, detailed measurements of the corresponding temperature profile stiffness via power balance analysis methods were obscured by the strong dependence on the T_{e}/T_{i} ratio, and electronion channel coupling. Here, we report on first ion heat pulse propagation studies, which aim to quantify the T_{i} profile stiffness in W7X similar to electron heat pulse propagation experiments [2]. Successful experiments have been performed using both NBI and ECRH modulation, and clear modulationinduced fluctuations were observed in the electron and ion temperatures. For the iontemperature measurement, a new charge exchange diagnostic has been employed, which allows for measurements with framerates >1kHz. These speeds allow for detailed comparisons of fluctuation amplitude and channel phase delay profiles with physics models for ion heat transport. A new method for forward modelling of heat transport using a numerical scheme similar to that implemented in the STRAHL code will be presented. 

GP11.00077: Development of a Heavy Ion Beam Probe Synthetic Diagnostic for Wendelstein 7X Humberto Trimino Mora, Alejandro Bañón Navarro, Diane R Demers, Peter J Fimognari, Olaf Grulke, Felix Wilms The optimized magnetic field geometry of Wendelstein 7X (W7X) reduces neoclassical transport, with turbulence becoming the governing mechanism for confinement. Challenges characterizing anomalous transport due to turbulence highlight the need for direct measurements of particle transport, instead of relying on particle balance estimates. A Heavy Ion Beam Probe (HIBP) diagnostic can simultaneously measure the plasma electrostatic potential and density fluctuations and thus contribute important information to the characterization of anomalous particle transport and motivates its installation on W7X. 

GP11.00078: Global gyrokinetic simulations with kinetic electrons of microturbulent transport in LHD and W7X stellarators Javier H Nicolau, Zhihong Lin, Tajinder Singh, Animesh Kuley, Xishuo Wei, Pengfei Liu Global gyrokinetic simulations using the GTC code of the ion temperature gradient (ITG) and trapped electron mode (TEM) turbulence in LHD and W7X are reported.This full fluxsurface simulations with kinetic electrons only become feasible thanks to the GTC global fieldaligned mesh in real space, which reduces the number of parallel grid points by a factor of 150. GTC simulations show that electrostatic ITG eigenmode structure is extended in the magnetic field direction but narrow in the perpendicular direction and peaks in bad curvature regions in both the LHD and W7X. The ITG eigenmode structure is localized at the outer midplane in the LHD, similar to that in a tokamak but in the W7X is strongly localized to some magnetic fieldlines at the corners of W7X due to the mirrorlike magnetic fields varying strongly in the toroidal direction. Nonlinear simualtions show that the main saturation mechanism for ITG is the selfgenerated zonal flows. For the TEM eigemode, LHD also exhibits similar characteristic to Tokamaks (localized on the outer midplane side where the curvature is bad). However, GTC code finds a new electrostatic helically trapped electron mode (HTEM) driven by a realistic density gradient in the W7X. The HTEM is excited by helically trapped electrons at the toroidal section with a weak magnetic field. The linear eigenmode is localized to discrete field lines on the inner side of the torus where the curvature becomes unfavorable in the socalled 'straight' section in W7X. Nonlinear simulations indicate that zonal flow play a secondary role in TEM and HTEM saturation. Finally, GTC simualtions of microturublence in LHD are validated using an LHD discharge. 

GP11.00079: Neutronics Modeling and Simulation for Thea Energy Stellarator Design Soha Aslam, Eleanor A Winkler, Alex Koen Thea Energy's first flagship stellarator device will operate with DD plasma, presenting a distinct set of neutronics challenges and solutions compared to the typical DT designs. Neutronics modeling and simulation plays a crucial role in evaluating the performance and safety characteristics of these type of fusion reactors. We developed a neutronics model of our stellarator using the OpenMC Monte Carlo particle transport code. The simulation considered various blanket configurations, materials, and thicknesses, allowing for the investigation and optimization of key neutronics parameters, such as: tritium breeding ratio, neutron flux distribution, and material activation levels. We have also conducted neutronics streaming studies to ensure proper placement of diagnostic equipment and ports. The results highlighted areas for potential optimization and improvements for our stellarator design. This has contributed to a deeper understanding of the neutronics behaviors and performance in a compact and modular DD stellarator. The results obtained through this study will serve as a foundation for further optimization and design refinements, guiding the development of safe, modular, and efficient stellarator designs. 

GP11.00080: Parametric modeling and design of Stellarator systems Andrew Cote, Charles P Swanson, H H Song, Santhosh Kumar, David A Gates Thea Energy is pioneering a novel stellarator architecture that uses a blend of planar toroidal field coils and a unique array of individually controllable planar shaping coils to generate the complex 3D magnetic fields integral to stellarators. This design employs hightemperature superconductors (HTS) REBCO conductors and retains the benefits of the mature physics basis of stellarators. To verify the physics and engineering models of this magnet system, Thea Energy has initiated a prototyping campaign enabled by parametric modeling and simulations. The campaign includes constructing a halfsized halffield version of an HTS planar shaping coil assembly and the design of two cryostats for coil testing at 20 Kelvin. All small shaping coils have undergone design optimization and have achieved accuracy with <0.1% error in ANSYS simulations. Cryostats and thermal management is simulated in COMSOL to verify performance requirements of selected equipment. Prototyping efforts aim to validate the design, comprehend the mutual interactions among the coils, and test scenarios such as fastdischarge quenching, coil manufacturability, coiltocoil variance and performance characteristics. 

GP11.00081: Magnetic Fields with General Omnigenity Daniel W Dudt, Alan Goodman, Rory Conlin, Dario Panici, Egemen Kolemen Omnigenity is a desirable property of toroidal magnetic fields that ensures confinement of trapped particles. All the ideal magnetohydrodynamic equilibria previously found to approximate omnigenity have been either axisymmetric, quasisymmetric, or have poloidally closed contours of magnetic field strength B. However, general omnigenous equilibria are a much larger design space than these subsets with hidden symmetries. A new model is presented and employed in the DESC stellarator optimization suite to represent and discover the full parameter space of omnigenous equilibria. Examples far from quasisymmetry with poloidally, helically, and toroidally closed B contours are shown to have low neoclassical collisional transport and fast particle losses. 

GP11.00082: All Planar Stellarator Coil Optimization Thomas G Kruger, David A Gates Optimized stellarator designs have historically required nonplanar, precisely shaped, highly complex coil systems. Recent advances in stellarator theory have enabled the design of much simpler magnet systems. Our stellarator coil sets are entirely comprised of planar electromagnet coils which are optimized to reconstruct highly quasisymmetric 3D MHD equilibria with an extremely high precision. Planar coil systems greatly reduce the risk of stellarator development. 

GP11.00083: Abstract Withdrawn


GP11.00084: Quasisymmetric stellarator equilibria optimized for good magnetic surfaces Mike F Martin, Thomas G Kruger, Daniel W Dudt, David A Gates Quasisymmetric stellarators with good magnetic surfaces boast the superior confinement properties of tokamaks. However, good magnetic surfaces are not guaranteed in threedimensional toroidal magnetic fields. In general, magnetic islands or regions of chaotic fields will exist, which can degrade particle and energy confinement of the main plasma. Using the Stepped Pressure Equilibrium Code (SPEC) with the Green’s residue method in Simsopt [similar to Physics of Plasmas 28, 092505 (2021)], we show that magnetic islands can be eliminated while preserving good quasisymmetry, in a new equilibrium. Further, we identify an equilibrium with a pressure profile that is selfconsistent with turbulent fluxes, using T3D+GX. 

GP11.00085: Initial engineering design and development of HTS magnet systems for all planar coils stellarator at Thea Energy Honghai Song, A. Cote, D. W Dudt, D. Fort, David A Gates, Thomas G Kruger, M. F Martin, Charles P Swanson, Santhosh Kumar Thea Energy is revolutionizing stellarators by utilizing innovative arrays of small surfaceshaping coils in conjunction with dozens of major toroidal coils, all planar and constructed from hightemperature superconductors (HTS) REBCO conductors. In line with the optimization of stellarator equilibrium, magnetic design and benchmarking of all small shaping coils have been performed and accuracy of <0.1% error has been achieved in Ansys simulations with automation process. The HTS coil design considers the critical current of REBCO as a function of magnetic field and temperature, as well as conductor stabilizer/substrate availability, given the constrained space inside the stellarator system. We are currently engaged in prototype design of single coil and the 3x3 coils panel. Two cryostats are being specially designed for these prototype coil tests at 20 K, including a small directconductioncooled cylindrical cryostat with a pulse tube cryocooler, and a large rectangle cryostat with both cold helium gas and GM cryocoolers. This prototyping campaign aims to validate the design and understand the mutual interactions among the coils, especially during fastdischarge or quenching scenarios. 

GP11.00086: NNBI system based on ITER HNB for beamtarget DD neutron generation in an optimized stellarator Charles P Swanson, David A Gates, Santhosh T Kumar A classical 1D slowing down model is applied to beaminjected deuterons in a deuterium plasma (DD), including the effect of shinethrough, thermal beta limits, and beam beta limits. The model reveals that an economically useful rate of neutrons can be produced by negativeionbased neutral beam (NNBI) at a nearoptimal energy of 1 MeV into an optimized quasisymmetric stellarator with major radius around 2 meters. Parametric dependence of the neutron rate is discussed. This NNBI system is coincidentally at the same energy of the ITER Heating Neutral Beams (HNBs), allowing significant technology reuse. We propose a system which is a trivial scaling from the ITER HNBs, using 2 ion sources instead of 8 and 1segment grids instead of 4. The length of the NNBI system can not be reduced by much, but areas of improvement are discussed. 

GP11.00087: The high field stellarator path to a fusion pilot plant John Canik Type One Energy Group is pursuing the commercialization of fusion energy based on the stellarator toroidal confinement configuration. Type One’s high field, optimized stellarator concept is based on proven science and technology and offers many advantages as a fusion power core owing to its inherently steadystate, disruptionfree operation. Recent advances in confinement theory, simulation, and computational power, paired with the realization of increased magnetic field strength using high temperature superconductors, have opened the opportunity for the rapid deployment of a fusion pilot plant (FPP). These advances enable access to ignited plasma scenarios at significantly lower device size and fusion power than found in previous stellarator reactor studies, improving the overall economics of the system. Further, modern design and manufacturing capabilities can reduce the timeline and costs of deployment. The increased possibilities for an attractive FPP, and Type One’s FusionDirect path to commercial fusion energy, will be presented. 

GP11.00088: Multiobjective stellarator optimization studies Chris C Hegna, Aaron Bader, John M Canik, Antoine Cerfon, Ben J Faber, Walter Guttenfelder, Bharat Medasani, John Schmitt, Luquant Singh The realization of stellarator fusion power plants relies on the use of optimization techniques to provide 3D magnetic field configurations with excellent plasma confinement properties. Optimization activities are reported in targeted areas of plasma physics dedicated to improving the stellarator concept. In this work, both quasiisodynamic (QI) and quasisymmetric (QS) configurations are considered. An emerging area of emphasis is the need to improve the turbulent transport properties of stellarators. Optimization schemes are developed using reduced models to monitor the efficacy of turbulent transport reduction. Configurations produced through optimization using reduced models for turbulent transport are assessed using gyrokinetic based transport modeling. Turbulent improved configurations are sought consistent with excellent neoclassical and energetic particle transport, selfconsistent bootstrap current, robust MHD equilibrium, local and global MHD stability and simplified coil design. Consistency of these designs with scalable divertor solutions are also addressed. 

GP11.00089: Streamlined calculations of the neoclassical bootstrap current and ambipolar radial electric field for stellarator optimization John C Schmitt, Matt Landreman, Aaron Bader Accurate and fast calculations of the neoclassical bootstrap current and ambipolar radial electric fields are essential for finitebeta stellarator optimization. The bootstrap current is a selfgenerated current, parallel to B, which arises due to momentum transfer from bouncing (trapped) particles to passing particles on a flux surface. The bootstrap current and its transient response can alter the rotational transform throughout the plasma which can impact core transport properties and modify the expected edge geometry properties outside of the plasma column which will influence the divertor design. Accurate estimates of the bootstrap current are critical for modeling the plasma response. Typical methods to calculate the bootstrap current can be both timeconsuming and memoryintensive which presents a challenge for an optimization loop. This work presents a set of equations for a streamlined version of a driftkinetic equation solver, sufficient for calculating the bootstrap current and neoclassical transport in many situations. Comparisons will be shown between the streamlined calculations and a more comprehensive model that includes additional physics. 

GP11.00090: Evaluating optimized stellarator performance via gyrokineticbased profile predictions Walter Guttenfelder, Aaron Bader, John M Canik, Antoine Cerfon, Chris C Hegna, John Schmitt, Noah R Mandell, William D Dorland Stellarator configurations that target reduced turbulent transport are now being routinely developed. This is accomplished using optimization approaches that rely on reduced models to capture key qualitative trends of the underlying drift wave stability and turbulence saturation. Here we use firstprinciples gyrokinetic simulations to evaluate the efficacy of these reduced models, and to quantitatively predict and evaluate the transport and energy confinement characteristics of the resulting configurations. Profile predictions are made with Trinity3D [t3d.readthedocs.io], which solves transport equations using fluxes computed from GX [Mandell, 2018; 2022] nonlinear gyrokinetic turbulence simulations. Profile predictions based on previously published configurations [Hegna, 2022] show trends consistent with the reduced models and standalone gyrokinetic analysis. However, the transport characteristics can vary in additional ways not incorporated within the optimization approach, pointing to opportunities for improvement. Similar predictions are being made for optimized configurations being developed by Type One Energy as part of its mission to develop a stellarator fusion pilot plant to address the White House Bold Decadal Vision for Commercial Fusion Energy. 

GP11.00091: Experimental study of the effect of gas puff modifications on the density profile in the HSX stellarator Dionysi Damaskopoulos, Benedikt Geiger, Colin Swee, Wayne Goodman, Brian Putra, Santhosh Kumar, David T Anderson A detailed understanding of the formation and shape of the density profile in the Helically Symmetric eXperiment (HSX) is essential since plasma turbulence has been observed to depend strongly on the normalized density gradient length [1]. Here, we present a comparison of prior data from gaspuff modulation experiments [2] with forward modeling results based on the pySTRAHL [3] and pyFIDASIM [4] codes to model density profile evolution and fueling, respectively. Additionally, new experiments will be conducted where the fueling location will be changed from the tips of the elongated fluxsurface crosssections to the midplane, bringing the fueling source closer to, or further away from the magnetic axis. In preparation for these experiments, significant modifications have been made to the HSX vacuum vessel. These changes include physical cleaning, DC glow discharges, and bakeout. Through these efforts, we expect reduced outgassing from the walls so that gaspuffing primarily fuels and modulates the plasma density. For the new experiments, pyFIDASIM predictions suggest that changes to the fueling location will alter the plasma density profile. Comparisons between forwardmodeled data and experimental results via multichannel microwave interferometry are presented and analyzed. 

GP11.00092: Magnetic FluxSurface mapping using an electron beam diagnostic at the HSX Stellarator Lee Duong, Jarod Delhotal, Benedikt Geiger, Christopher Seyfert In stellarators, small uncertainties of the alignment of the magnetic field coils can cause a significant degradation of confinement properties due to the formation of stochastic regions or magnetic islands. Fieldline mapping is therefore used to check for possible deviations from the expected magnetic field structure. At the Helically Symmetric eXperiment (HSX), initial fieldline mapping has been performed during the commission of the device [1], but no further checks have been performed for more than 20 years. To revisit this important topic, a novel electron gun, i.e., a biased and heated filament, has been designed, built, and commissioned on a tabletop toroidal plasma device. After successful tests, the electron gun will be installed at HSX together with a movable phosphor probe that can be swept vertically to detect the produced electron beam. First experimental results obtained with the new fieldline mapping system will be presented and compared with predictions from a fieldline following code. 

GP11.00093: Mixedtype stellarator coil sets for decreased complexity Todd M Elder, Allen H Boozer, Elizabeth J Paul, Alan A Kaptanoglu Stellarator coil sets are presented which use combinations of different coil types, such as helical and window pane coils, to support the HSX magnetic field. Coil sets of mixed type offer many benefits, from decreased coil complexity to improved access properties of the stellarator. A particular example of helical coils with magnetic dipole coil loops is presented which has great outboard side access properties. To achieve this design, novel improvements of the current potential method were utilized as well as the SIMSOPT suite of codes for spacecurve optimization. 

GP11.00094: Computational Study of Parallel Flows in High Density Plasmas in HSX Celine Lu, Joseph N Talmadge, Santhosh Kumar, Zander N Keith, Benedikt Geiger, Alexander L Thornton, Henrique H Oliveira Miller E × B shear flow plays an important role in reducing turbulence and generating transport barriers. Quasisymmetric stellarators have an advantage over conventional stellarators in that the reduced parallel viscous damping could lead to large plasma flow shear. The Helically Symmetric eXperiment (HSX) is a quasihelically symmetric stellarator with constant B in the helical direction. Because parallel viscous damping is small, neutrals play a significant role in HSX for damping in the symmetry direction as well as damping of the parallel flow. Here, we present a study of the expected parallel flow velocity during high density operation in HSX which will be possible due to the installation of a new 70 GHz gyrotron. The increase in density results in a decrease of the atomic hydrogen neutral density by a factor of 5 in the upgraded plasmas from 1.0 × 10^{10} cm^{−3} to 0.2 × 10^{10} cm^{−3} at r/a = 0.44. Initial results based on a version of PENTA that has been adapted to include neutral damping [1], indicate the parallel flow in HSX increases roughly by a factor of 3 at r/a = 0.44. Results will be presented showing how the profile of the parallel flow in the upgraded plasma compares to that of the 28 GHz gyrotron at 1 T as well as plans to measure the parallel flow using the CHERS system [2]. 

GP11.00095: Design and Prospects of HighSpeed Gas Puff Imaging Diagnostic at the HSX Stellarator Patrick O'Neill, Benedikt Geiger HSX is a quasisymmetric optimized stellarator that has demonstrated excellent neoclassical transport properties [1]. In the outer plasma region, however, confinement is degraded by strong anomalous electron heat fluxes, which are likely explained by density gradientdriven TrappedElectronMode turbulence [2]. To monitor turbulent fluctuations near the plasma edge of HSX, a new gas puff imaging system has been designed. The system is equipped with a highspeed camera (Phantom V12, up to 1 MHz) and will allow for studies of edge turbulence [3]. To provide spatial localization of the measurements a low Z neutral gas is planned to be puffed perpendicular to the B field and within the view of the camera. The viewing system will use a periscope with a single mirror to view the separatrix where the gas will be puffed. The periscope will be adjustable to obtain ideal alignment along the field line of interest and sufficient spatial resolution to view edge turbulence. The system is planned to be installed and commissioned during the upcoming experimental campaign of HSX and the first measurement results will be disseminated. 

GP11.00096: Plans for Experimental Validation of GENEPredicted TurbulenceReduced HSX Configurations Henrique H Oliveira Miller, Michael J Gerard, Xiang Han, Luquant Singh, Konstantin M Likin, Benjamin J Faber, Benedikt Geiger The Helically Symmetric Experiment (HSX) has been shown to be dominated by anomalous transport, most likely density gradientdriven TrappedElectronMode (TEM) turbulence [1]. Recent gyrokinetic studies using the GENE code have found promising coilgenerated magnetic configurations with improved TEM stability [2]. Ongoing computational work also indicates certain configurations may have reduced nonlinear heat flux. Plans for testing these promising configurations are presented here. A set of planned experiments is laid out which include auxiliary coil current scans, profile matching, on and off axis heating, and working gas experiments. An array of diagnostic measurements will be used to discern possible changes in transport physics in these configurations as compared with the standard HSX configuration. The diagnostics and analysis methods discussed include reflectometry, ECE spectroscopy, CXRS, interferometry, and power balance. The goal of these experiments is to ascertain whether the GENEpredicted improvements in heat flux saturation and TEM stability are physically realizable, and validate the method of optimization that provided these configurations in the first place. 

GP11.00097: Correlation electron cyclotron emission measurements in the HSX stellarator Luquant Singh, Konstantin M Likin, Matthijs R Wezeman, M.J. Pueschel, Gavin M Weir, Gavin W Held, Benjamin J Faber, Chris C Hegna Longwavelength k_{⊥}ρ_{s} < 1.5 radiation temperature fluctuations are measured within the midradius of the Helically Symmetric eXperiment (HSX) stellarator using a 16channel CECE radiometer. Despite low singlepass optical depth, the X2 emission may be treated as blackbody under certain conditions present in HSX, and CECE measures turbulent electron temperature fluctuations. Outside the ECRH deposition region (r/a > 0.2), fluctuation amplitudes increase with normalized electron temperature gradient. This increase in amplitude coincides with broadening of the fluctuation frequency spectrum at higher normalized electron temperature gradient. To model the core turbulence in HSX, fluxtube gyrokinetic simulations have been performed with the GENE code. Linear simulations suggest that trapped electron mode is the dominant longwavelength instability. Nonlinear simulations predict that electron temperature fluctuations increase with normalized electron temperature gradient, consistent with CECE measurements. For more detailed comparisons, a synthetic CECE diagnostic has been developed to process the gyrokinetic simulation data and account for diagnostic effects on the temperature fluctuation signal. A comparison of a synthetic and experimental CECE frequency spectrum will be presented. 

GP11.00098: Efficient calculation of the selfinductance, selfforce, and internal magnetic field for thin electromagnetic coils Siena Hurwitz, Matt Landreman, Thomas M Antonsen There exist several quantities of conducting coils that are difficult to numerically evaluate yet are relevant to the design and optimization of magnetic confinement fusion reactors. Lorentz forces are a limiting factor in reactor design due to coil stresses. A conductor's internal magnetic field also describes stress and strain as well as a superconducting coil's proximity to its quench limit. Its selfinductance measures magnetic energy. When computed between coils (e.g., mutual forces), these quantities are simple to evaluate, though when computed on a single coil (e.g., selfforce), evaluation is difficult due to a source point singularity. Importantly, attempts to treat coils as infinitesimally thin fail as the selfinductance, internal magnetic field, and selfforce all diverge. Instead, we present novel models for these quantities using nonsingular integral formulae of reduced dimensions. These formulae were determined rigorously by dividing the domain of integration of the magnetic vector potential into two regions, exploiting the unique assumptions of each region, and expanding in high aspect ratio. Our formulae show good agreement to the full calculations under the high aspect ratio limit, both analytically for a torus and numerically for HSX stellarator coils. 

GP11.00099: A New Baking System for the HSX Stellarator Brian Putra, Jakeb Smiskey, Rex Wagner, Ben Knowles, Chris Seyfert, Zander N Keith The HSX stellarator is an optimized, 4field period stellarator designed to investigate the confinement properties of a quasihelical symmetric field structure. As part of an upgrade of HSX, a 100 °C vacuum bakeout system was designed, installed, and commissioned. Heating is performed by 88 contact heating strips which are proportionalintegralderivative (PID) controlled using a programmable logic controller (PLC). Ceramicbased insulation blankets were installed over the heating strips to even out vessel temperatures and to reduce heat losses. First results during baking up to 50 °C show strong outgassing of water from the walls with an increase of the base pressure by more than two orders of magnitude. After baking, a clear reduction in device base pressure is observed, demonstrating the success of this wall conditioning effort and allowing for stable plasma operation in HSX. First density measurements from the 2023 experimental campaign will be presented and compared with preupgrade data. 

GP11.00100: Magneticfield properties in nonaxisymmetric divertors: Allen H Boozer In toroidal fusion plasmas, the particles that cross the plasma edge must be diverted into pumping chambers. The properties of the magnetic field between the plasma edge and the surrounding chamber walls are the basis for divertor design. Studies of what types of divertors are possible and how they can be controlled is greatly simplified by the use of a magnetic field line Hamiltonian that is a function of the toroidal flux plus a Fourier series in the poloidal and toroidal angles in which each term is somewhere resonant with the rotational transform defined by angleindependent part of the Hamiltonian. A procedure is given in the arXiv version of this abstract for determining this Hamiltonian for any given magnetic configuration. Variations in the magnitudes of the resonant terms determine what divertor features are potentially possible: connection length, width and locations of the interceptions on the chamber walls, and plasma shielding by a broad chaotic region. Variations in the rotational transform explore the robustness of solution to changes in the plasma. Stellarators require nonaxisymmetric divertors, but they could also be used on tokamaks by enforcing quasiaxisymmetry on the core plasma. 

GP11.00101: Flux surface mapping and error field measurement for MUSE table top stellarator Xu Chu, Michael C Zarnstorff, Tony Qian, Bruce Berlinger, Mohammed Haque, Simeon Salia, Eric Zhu, Yousef Nasr, Daniel J Williams MUSE is the world's first quasiaxisymmetric stellarator constructed with planar circular coils and permanent magnets[1]. System assembly has finished[2] and electron beam mapping of flux surfaces[3] has been conducted to confirm its magnetic topology. 

GP11.00102: Stellarator Optimization with Constraints Rory Conlin, Patrick S Kim, Daniel W Dudt, Dario Panici, Kaya E Unalmis, Egemen Kolemen We present new numerical methods, implemented in the DESC code, for optimizing stellarators with nonlinear constraints. We demonstrate these new capabilities with constraints such as MHD equilibrium and stability, bootstrap current selfconsistency, and coil complexity metrics. These methods achieve superior objective values while exactly satisfying specified constraints, in contrast to existing methods which rely on hand tuning of weights and adhoc penalties. 

GP11.00103: Stellarator Turbulence Optimization Based on Flux Surface Triangularity Joey M Duff, Benjamin J Faber, Chris C Hegna, M.J. Pueschel, Paul W Terry A important goal of stellarator optimization is to find configurations that reduce turbulent transport using threedimensional (3D) shaping. Trappedelectronmode (TEM) turbulence can play a significant role in quasisymmetric stellarators [1]. One way to improve the turbulent transport properties of tokamak plasmas is through negative fluxsurface triangularity [2]. Nonlinear gyrokinetic simulations suggests that the heat flux of TEM turbulence correlates with the free energy available in background temperature and density gradients, as quantified by the available energy [3]. In this work, we address the possibility of using negative triangularity as a mechanism to reduce TEM turbulence in stellarator plasmas. Towards this end, a new optimization framework is developed using local 3D MHD equilibrium solutions [4]. This approach has been successfully employed to improve the quasisymmetry properties—a metric for reducing neoclassical transport—while simultaneously reducing the available energy for local 3D MHD equilibria in a stellarator with negative helicallyrotating triangularity and in a stellarator with positive helicallyrotating triangularity. 

GP11.00104: Stabilitybased stellarator optimization with derivatives Benjamin J Faber, Justin Walker, Chris C Hegna The field of stellarator optimization has seen resounding success optimizing for critically important quantities like neoclassical transport and fast particle confinement, making the optimized stellarator a viable magnetic confinement fusion concept.
One of the most important remaining optimization challenges deals with stability on both the macro and microscale, such as MHD ballooning instabilities and kinetic drift waves.
While different reduced models and metrics for stability have been proposed, the availability of computing power makes it now possible to assess stability metric by directly computing the solution to plasma stability eigenproblems.
These solutions alone do not immediately inform optimization efforts, to fully harness modern optimization techniques one needs to have access to derivative information.
A new framework for performing stellarator optimization has been developed in the Julia programming language, enabling seamless integration to automatic differentiation tools. Coupled with Julia defined stability metrics, this framework enables endtoend calculation of eigenvalue and eigenfunction derivatives with respect to magnetic shaping.
Details of the implementation of this framework and a demonstration of robust optimization of stabilitybased metrics will be presented.


GP11.00105: Design and Construction of a Two Coil TableTop Stellarator Thomas Gallenberger, Benedikt Geiger, Michael J Gerard, Ryan Albosta, Christopher Seyfert A new tabletop stellarator experiment called ETOS has been designed with one planar and one helical coil to produce an approximate quasiisodynamic field with fivefold symmetry. The new stellarator configuration has been identified by perturbing the analytic coil parameters and using field line following to identify closed flux surfaces. Moreover, the shape of the employed coils has been optimized for reduced neoclassical transport, a quasiisodynamic magnetic field structure, and feasibility of construction. VMEC equilibrium reconstructions demonstrate a rotational transform of 0.15, and acceptable neoclassical confinement with ε_{eff}^{3/2} values close to 0.1. By considering currents of up to 10 kAturns, the expected magnetic field strength will be 0.09 T on axis, allowing plasma heating by 2.45 GHz ECRH. The helical coil has been designed based on 10 gauge wire and will be supported by a 3D printed structure. The vessel will consist of straight glass tubes, connected by 3D printed stainless steel sections that are shaped to fit the equilibrium flux surfaces. A combination of the size, small coil number and ample space around the coils, the vessel can be easily accessed making it an ideal testbed to explore the feasibility of 3D printed vacuum vessels for plasma experiments. 

GP11.00106: Exploration of NonResonant Divertor Features on the Compact Toroidal Hybrid Kelly A Garcia, Aaron Bader, Heinke G Frerichs, Gregory J Hartwell, John Schmitt, Nicholas R Allen, Oliver Schmitz Nonresonant divertors (NRDs) separate the confined plasma from the surrounding plasma facing components (PFCs). However, a complex scrapeoff layer (SOL), created by chaotic magnetic topology in the plasma edge, connects the core plasma to the PFCs through varying magnetic flux tubes. The Compact Toroidal Hybrid (CTH) is a testbed to study this by scanning across its inductive current. Simulations observe a significant change of the chaotic edge structure and an effective distance between the confined plasma and the instrumented wall targets. The intersection pattern is observed to be a narrow helical band, which we claim is a resilient strike line pattern. However, signatures of fingerlike structures, defined as homoclinic tangles in chaotic domains, within the plasma edge connect the island chains to this resilient pattern. The dominant connection length field lines intersecting the targets are observed via heat flux modelling with EMC3EIRENE. At low inductive current levels, the excursion of the field lines resembles a limited plasma wall scenario. At high currents, a private flux region is created in the area where the helical strike line pattern splits into two bands. These bands are divertor legs with distinct SOL parallel particle flow channels. The results show the NRD strike line resiliency within CTH, but also show the underlying chaotic edge structure determining if the configuration is diverted or limited. This work supports future design efforts for a mechanical structure for the NRD. 

GP11.00107: Application of FAR3d gyroLandau fluid model for analyzing AE instability in stellarators Yashika Ghai, Donald A Spong, Jacobo Varela Rodriguez, Luis Garcia, Dmitry Moseev, Samuel A Lazerson Stellarators are promising fusion devices that aim to achieve controlled nuclear fusion by confining plasma using complex magnetic field configurations. However, the presence of energetic particles (EPs) within the plasma can give rise to instabilities such as Alfvén eigenmodes (AEs) that can degrade plasma confinement. Simulation codes can be extremely useful in understanding the nature of these instabilities and mitigating them. Both global and fluxtube gyrokinetic simulations have been employed for studying plasma instabilities and turbulence in different stellarator devices. These studies highlight the importance of choosing the range of computational domain for the convergence of frequency and growth rate calculations for these modes. In present study, we aim to apply a global gyrofluid code FAR3d to analyze EP driven instabilities in stellarators. Our goal is to identify how the computational domain such as radial resolution, number of toroidal families in simulation, etc., impact the convergence of frequency and growth rate calculations of the AEs, specifically when helical couplings are included. The role of plasma profiles and parameters on AE instability in stellarators shall also be studied. 

GP11.00108: Improved stellarator permanent magnet designs through combined discrete and continuous optimizations Kenneth C Hammond, Alan A Kaptanoglu Designing an array of permanent magnets for stellarator plasma confinement entails solving an optimization problem with tens of thousands of degrees of freedom whose solution, for practical reasons, should be constrained to a discrete space. We perform a direct comparison between two algorithms that have been developed previously for this purpose, and demonstrate that composite procedures that apply both algorithms in sequence can produce substantially improved results. One approach uses a continuous, quasiNewton procedure to optimize the dipole moments of a set of magnets and then projects the solution onto a discrete space. The second uses an inherently discrete greedy optimization procedure. The approaches are both applied to design arrays of cubic rareEarth permanent magnets to confine a quasiaxisymmetric plasma with a magnetic field on axis of 0.5 T. The first approach tends to find solutions with higher field accuracy, whereas the second can find solutions with substantially fewer magnets. When the approaches are combined, they can obtain solutions with magnet quantities comparable with the second approach (up to a 30% reduction from the first approach) while exceeding the field accuracy of what either approach can achieve on its own. 

GP11.00109: FIDA and other observation of improved fastion confinement in the Large Helical Device using shifted magnetic configurations Wataru H Hayashi, William Heidbrink, Masaki Osakabe, Yasuko Kawamoto, Kunihiro Ogawa, Mitsutaka Isobe, Ryosuke Seki, hideo nuga, Hiroyuki Yamaguchi, Tetsutaro Oishi Fastion confinement in the Large Helical Device (LHD) is investigated at various magnetic configurations to validate the behavior of the synthetic diagnostic code FIDASIM when using a nonaxisymmetric distribution from the transport code GNET. Neoclassical confinement in stellarators is improved when the magnetic axis is shifted inward [1]. Improved confinement for fast ions with the inwardshifted configuration has been observed on LHD using neutron measurements [2]. Studies using additional diagnostics have been completed with experiments on MHDquiescent plasmas with magnetic axes R < 3.6m, R = 3.6m, and R > 3.6m. Fast ions in the experiments are populated by tangential negativeion neutral beam injection using 150  170 keV Deuterium sources. Fastion Dalpha, neutral particle analyzer, and vertical neutron camera measurements show improved fastion confinement for R < 3.6 m, this trend is compared to GNETFIDASIM. Successful modeling of neoclassical behavior by GNETFIDASIM will subsequently be applied to validation of a gyrokinetic code distribution function. 

GP11.00110: StellDOS: Alfvéngap density of states (DOS) for stellarators Abdullah S Hyder, Elizabeth J Paul, Donald A Spong StellDOS is a modern opensource highlyextensible objectoriented C++ code that duplicates and extends upon the capabilities of Stellgap. Gaps arise in the shear Alfvén continuum because of poloidal and toroidal mode coupling through the magnetic geometry. Alfvén modes that exist in gaps can be driven by energetic particles via inverse Landau damping, but these modes do not experience continuum damping, potentially leading to instabilities and particle transport. Minimizing the width of Alfvén gaps is an additional criterion that can be optimized for in stellarator design. StellDOS was created to understand the basic features of quasisymmetric configurations that lead to gaps and further study Alfvénic instabilities caused by gap eigenmodes. StellDOS can directly calculate the shear Alfvén continuum of general 3D geometries while accounting for toroidal and poloidal mode coupling. A comparison between an analytical model for the gap eigenmodes and the numerical result from StellDOS is directly made. In the future, StellDOS's extensibility will be further taken advantage of to implement the general Boozer coordinate geometry found in Stellgap and to calculate the density of continuum modes, which can be used to identify gaps. 

GP11.00111: The Magnetic Gradient Scale Length Explains Why Certain Plasmas Require Close External Magnetic Coils Jonathan Kappel, Matt Landreman, Dhairya Malhotra The separation between the last closed flux surface of a plasma and the external coils that magnetically confine it is a limiting factor in the construction of fusioncapable plasma devices. This plasmacoil separation must be large enough so components such as a breeding blanket and neutron shielding can fit between the plasma and the coils. Plasmacoil separation affects reactor scales, coil complexity, and particle loss due to field ripple. For some plasmas it can be difficult to produce the desired flux surface shaping with distant coils, and for other plasmas it is infeasible altogether. Understanding the underlying physics limiting plasmacoil separation can explain why some configurations require close external coils. We explore the hypothesis that the smallest plasmacoil separation is comparable to the shortest scale length of the magnetic field as expressed by the ∇B tensor. We tested this hypothesis on a database of ~ 40 stellarator and tokamak configurations. For this database, the plasmacoil separation compared to the minor radius varies by over a factor of 10. The magnetic scale length is well correlated to the plasmacoil separation of coil designs using the code REGCOIL. There is also a general inverse trend between the number of field periods and plasmacoil separation. 

GP11.00112: Coil topology optimization for stellarators Alan A Kaptanoglu, Matt Landreman, Gabriel Langlois Topology optimization, a technique to determine where material should be placed within a predefined volume in order to minimize a physical objective, is used across a wide range of scientific fields and applications. A general application for topology optimization is inverse magnetostatics; a desired magnetic field is prescribed, and a distribution of steady currents is computed to produce that target field. In the present work, electromagnetic coils are designed by magnetostatic topology optimization, using volume elements (voxels) of electric current, constrained so the current is divergencefree. Compared to standard electromagnet shape optimization, our method has the advantage that the nonlinearity in the BiotSavart law with respect to position is avoided, enabling convex cost functions and a useful reformulation of topology optimization as sparse regression. To demonstrate, we consider the application of designing electromagnetic coils for stellarators. We produce topologicallyexotic coils for several new stellarator designs and show that these solutions can be interpolated into a filamentary representation and then further optimized. 

GP11.00113: Optimization of Nonlinear Turbulence in Stellarators Patrick S Kim, Rory Conlin, William D Dorland, Daniel W Dudt, Rahul Gaur, Rogerio Jorge, Egemen Kolemen, Matt Landreman, Noah R Mandell, Dario Panici Turbulent transport is one of the major limiting factors on performance for stellarators as it determines the rates of heat and particle loss of the plasma. However, directly optimizing for reduced turbulence is usually very challenging due to the large computational cost of typical turbulence simulations. While there have been works minimizing linear/quasilinear metrics of turbulence [1], [2], linear theory may not be able to correctly predict nonlinear turbulence physics [3]. In this work, we directly optimize stellarators for reduced nonlinear turbulent heat fluxes. In order to run nonlinear simulations inside the optimization loop, we use the new GPUnative gyrokinetic code GX [4], which utilizes pseudospectral methods in velocity space. GPU acceleration combined with flexible velocity resolution allows for nonlinear GX simulations that only take minutes to run. The optimization is performed using the stellarator equilibrium and optimization code DESC [5] and employs stochastic optimization methods to robustly handle the noisy heat fluxes from GX. We show different examples of optimized equilibria that have both reduced heat fluxes along with good quasisymmetry to also ensure low neoclassical fluxes. Finally, we study the geometric properties of each equilibria to determine which properties most strongly affect the resulting heat fluxes. 

GP11.00114: NearAxis Constrained Stellarator Equilibria with DESC Dario Panici, Eduardo Rodriguez, Rory Conlin, Patrick S Kim, Daniel W Dudt, Kaya E Unalmis, Egemen Kolemen We present novel results of nearaxis constrained equilibria in DESC, connecting nearaxis expansion theory to global 3D ideal MHD. Nearaxis expansion (NAE) theory can provide analytic insight into stellarator equilibria, but the accuracy of the theory degrades far from axis. Conventional approaches to connect global MHD codes to NAE theory evaluate the NAE at a finite radius and use that as the boundary condition, which uses the NAE information where it is least accurate. In DESC, boundary conditions constraining only the nearaxis behavior to match nearaxis theory have been implemented and shown to result in optimized configurations with nearaxis properties matching the NAE and overall lower force balance. This allows DESC to use NAE theory where it is most reliable (near the axis) and turn to the global MHD equations to satisfy force balance far from the axis. 

GP11.00115: VMEC Convergence Near the Magnetic Axis: Issues, Diagnosis, and Repair James D Hanson, Mark R Cianciosa, Stuart R Hudson, Jonathan Schilling, John Schmitt The VMEC (Variational Moments Equilibrium Code) code is a nonaxisymmetric MHD equilibrium code. It can compute both freeboundary and fixedboundary equilibria, and is widely used for equilibrium reconstruction, stellarator design, and experimental planning and interpretation. Recent comparison of freeboundary zerobeta toroidal currentfree equilibria with BiotSavart fieldline following have shown that some VMECcomputed physics properties near the magnetic axis converge slower than expected (with exponent ~ 1 instead of the expected 2) as the radial resolution is increased. Possible causes of this slow convergence, along with potential algorithm improvements will be explored. 

GP11.00116: Updates on Numerical Implementation and Testing of NIMSTELL Sanket A Patil, Carl R Sovinec NIMSTELL [1] solves viscoresistive MHD equations in 3D geometry. Cylindrical coordinates (R, Z, φ) of the finite element mesh and the perturbed and steady state fields are represented as Fourier series in the generalized toroidal coordinate. Vector potential is expressed using an H(curl) basis of arbitrary polynomial degree to ensure a divergencefree magnetic field. Parallel blockdiagonal preconditioning has been developed for accelerating the large sparse linear solves required for each timeadvance of velocity, temperature, and vector potential. The matrix blocks may span over multiple Fourier components and may have an overlap of several components. This strategy is effective in cases where the Fourier components of dependent fields are coupled due to 3D geometry. An interface between NIMSTELL and DESC [2] has been developed to obtain fluxaligned meshes and the corresponding equilibrium fields. Linear tearing is calculated in helically shaped toroidal configurations with elliptical crosssections of eccentricities up to 0.8, whose equilibria are obtained using the NIMSTELLDESC interface. Progress on calculations of linear interchange in a straight stellarator is also reported. 

GP11.00117: Nonresonant stellarator divertors Alkesh Punjabi, Allen H Boozer Nonresonant stellarator divertor is extremely important for the development of stellarator concept. In our study of the nonresonant stellarator divertor to date, we have found three distinct types of stellarator divertors: the nonresonant stellarator divertor, the hybrid stellarator divertor, and the twomode stellarator divertor. The nonresonant stellarator divertor has no large islands outside the outermost surface; the hybrid divertor has a chain of large islands outside and near the outermost surface and it has features of both the island divertor and the nonresonant divertor; the twomode divertor has chains of many very small islands outside and near the outermost surface. The key property of these stellarator divertors is that the field lines exiting and approaching the region just outside the outermost surface are collimated into magnetic turnstiles. The field lines cross the cantori just outside the outermost surface through the holes in the cantori collimating in the turnstiles. A turnstile always comes in a pair of an outgoing flux tube and incoming tube. Intersection of the tubes with the wall give the magnetic footprints. The two tubes of a turnstile can start at adjacent or separate locations outside the outermost surface. Pseudoturnstiles can also exist. A turnstile is characterized by its probability exponent. The locations of the footprint on the wall are fixed and robust. We will present a comprehensive view of the progress made in our study of nonresonant stellarator divertors. 

GP11.00118: The maximumJ property in quasiisodynamic stellarators Eduardo Rodriguez, Per Helander Quasiisodynamic stellarators, one of the leading concepts of optimised stellarators, tend to exhibit a favourable average magnetic curvature for trappedparticles when the plasma pressure is sufficiently high. This socalled maximumJproperty has several positive implications for plasma performance, such as good fastparticle confinement and suppression of trappedparticle instabilities, which make this concept highly attractive. We show that the maximumJproperty can be achieved to arbitrary accuracy (but not exactly) in quasiisodynamic stellarators, even at zero plasma pressure, unlike in quasisymmetric stellarators (or tokamaks). This requires of a carefully curated field, whose properties we study through a new asymptotic expansion around the magnetic axis. 

GP11.00119: Designing a QuasiSymmetric Superconducting Stellarator eXperiment at Columbia (CSX) Melanie Russo, Carlos A PazSoldan, Naya Nwokorie, Kalen Richardson This research aims to repurpose the Columbia NonNeutral Torus (CNT) into a superconducting, quasisymmetric stellarator called CSX. This work will lay the foundation for a new generation of optimized stellarators, which will play a crucial role in advancing clean energy technologies by testing new stellarator configurations featuring a simple concept with few coils alongside the wide access plasma. The main project focus is building two interlocking, nonplanar hightemperature superconducting coils (~1.2m diameter). Our studies also include exploring lowresistance joint connections between tape segments, lowstrain winding techniques, and designing highcurrent leads to power the tapes. We are investigating the feasibility of 3D printing the bobbin in copper for strong heat transfer, and in interlocking, segmented sections for easy coil assembly. Currently, our configuration design team is optimizing the coil shape to achieve quasisymmetry while our engineering team works on realizing a minimum viable prototype. This prototype aims to validate key aspects of our approach, including 3D printing and assembling a bobbin, soldering HTS segments, maintaining superconductivity during winding and potting of the magnet, testing the cooling system design, and measuring the resulting magnetic field. 

GP11.00120: Filamentary coil optimization for reducing HTS strain Mohammed Haque, Elizabeth J Paul, Melanie Russo, Carlos A PazSoldan


GP11.00121: Characterization of Magnetic Confinement and Plasma Parameters for a Stellarator Using Permanent Magnets Simeon Salia, Eric Zhu, Yousef Nasr, Daniel J Williams, Mike C Zarnstorff, Xu Chu MUSE, a stellarator constructed using permanent magnets, is a cuttingedge fusion device that employs rare earth magnets to confine plasmas. To gain insights into the plasma confinement mechanism, detailed mapping of magnetic flux surfaces was conducted. This mapping technique involved utilizing an oscillating rod coated with fluorescent powder. As the electron beam collided with the rod, it triggered the emission of photons, allowing for the visualization of magnetic surfaces. Additionally, Poincaré maps were generated by capturing images during electron beam shots, enabling a comprehensive analysis of plasma dynamics. 

GP11.00122: Generating magnetic field strength with exact quasisymmetry using soliton potentials Wrick Sengupta, Nikita Nikulsin, Elizabeth J Paul, Stefan Buller, Richard Nies, Stuart R Hudson, Amitava Bhattacharjee Quasisymmetry (QS) is a hidden symmetry of the magnetic field strength, B, that confines charged particles effectively in a nonsymmetric toroidal plasma equilibrium. Recent numerical breakthroughs have shown that excellent QS can be realized in a toroidal volume. Here, we show that the hidden symmetry of QS has a deep connection to the underlying symmetry that makes solitons possible. In particular, we demonstrate that a large class of quasisymmetric B is described by a periodic finitegap soliton potential of the wellknown Kortewegde Vries (KdV) and Gardner equations, drastically reducing the number of independent B parameters on a magnetic flux surface to a few. We show this explicitly for a quasisymmetric vacuum field in slab geometry and extend the results to general geometry, leveraging the connection between LewisErmakov and the parallel adiabatic invariants. We highlight the importance of magnetic shear in QS. Furthermore, we deduce an upper bound on the maximum toroidal volume that can be quasisymmetric. B approaches the form of the 1soliton reflectionless potential in the neighborhood of the outermost surface. The hidden low dimensionality and the analytical insight into the role of magnetic shear could make stellarator optimization schemes significantly more efficient. 

GP11.00123: EPOS: Stellarator design for pair plasmas Jason Smoniewski, Pedro Gil, Paul Huslage, Dylan Schmeling, Matthew T Beidler, Michael Drevlak, Matt Landreman, Eve V Stenson The EPOS device (Electrons and Positrons in an Optimized Stellarator) is being designed to confine a lowdensity, lowtemperature pair plasma. This exotic target requires different considerations compared to the typical fusion goal. Due to the low availability of antimatter, the plasma density is limited by the total number of positrons. In order to achieve plasma densities such that a Debye length is 1/10th of the minor radius, EPOS must be as small as reasonably possible. Typical stellarator coil tolerances become unmanageable at small size but can be improved with stochastic optimization. To avoid positron loss, plasma temperatures must be kept below 15 eV, motivating a 2T magnetic field for effective radiative cooling. This large, steadystate field requires superconducting magnets. Coil design for EPOS includes strain optimization for hightemperature superconducting (HTS) tape, validated with a series of test coils. Finally, positron fueling requires charged particle injection across the magnetic field. The positron beamline will connect to EPOS's stray magnetic field, and electrodes will generate ExB drifts into the confinement region. EPOS requires expanded coils to produce this "weave lane" and allow space for the electrode structures. This contribution discusses the integration and progress on the above tasks towards a HTS tabletopsized quasiaxisymmetric stellarator. 

GP11.00124: Energetic particle instability analysis for high density regime stellarator reactors Donald A Spong, Aaron Bader, Jacobo Varela Rodriguez, Yashika Ghai, Luis Garcia Energetic particle (EP) driven instabilities and the associated EP transport will be an important issue for fusion reactorregime stellarator projects such as the Stellarator Fusion Pilot Plant (FPP). Loss of EP confinement poses a risk both to maintaining selfsustained ignition and damaging plasmafacing components. Since the phase space structure of intense alpha particle populations in 3D systems cannot be created in nonDT experiments, this risk cannot be fully validated prior to operation of reactorscale systems; thus, simulation is critical. We have initiated new simulations of reactor regime DT stellarators using gyrokineticbased models and have studied the effects of the high plasma density regimes that are unique to stellarators. Alfvénic instabilities driven by alpha particles are analyzed along constant fusion power operational contours. Although initial results indicate that high density operation cannot completely suppress these instabilities, their drive can be significantly reduced if the plasma density is above around 3 x 10^{20} m^{3}. Further mitigation methods involving profile and shape optimization are under development that can provide additional control techniques. 

GP11.00125: A Comparison of Ideal MHD Stellarator Equilibria with and without Islands Andrew S Ware, Stuart R Hudson, Mark R Cianciosa, John C Schmitt The numerical calculation of threedimensional MHD equilibria depends crucially on the assumptions used in designing the code that calculates the equilibrium. The VMEC code is an Ideal MHD code that assumes nested toroidal flux surfaces [S. P. Hirshman and H. K. Meier, Phys. Fluids 28, 1387 (1985)] and it has been widely used in stellarator and tokamak physics. The SIESTA code relaxes the assumption of nested toroidal flux surfaces and switches between Ideal and Resistive MHD in the process of searching for an Ideal MHD equilibrium [S. P. Hirshman, R. Sanchez and C.R. Cook, Phys. Plasmas 18, 062504 (2011)], thus allowing for magnetic islands to develop in the equilinrium. The SPEC code is designed with the concept of multiregion, relaxed MHD [S. R. Hudson, et al., Phys. Plasmas 19, 112502 (2012)], where regions of zero pressure gradient (forcefree regions) are separated by interfaces with pressure jumps. In this work, these three codes are used to calculate stellarator equilibria for various configurations. A rotating ellipse equilibrium is used as a test case for comparison. Comparison of rotational transform and magnetic surfaces are presented for both vacuum and finitepressure equilibria. Additional configurations will be examined. 

GP11.00126: Assessing nonlinear MHD stability in optimized quasisymmetric stellarators Adelle M Wright, Nathaniel M Ferraro We report on the application of the M3DC1 extended MHD code to assess the nonlinear stability properties of a recently developed 2fieldperiod quasiaxisymmetric stellarator configuration, optimized at 2.5% plasma beta for good energetic particle confinement and selfconsistent bootstrap current [1]. Overall, the configuration was found to be unstable to fastgrowing magnetohydrodynamic (MHD) modes with moderate to high poloidal and toroidal mode numbers. Strong toroidal mode coupling, arising due to the absence of axisymmetry, leads to rapid destabilization of other modes in the N=2 mode family (modes with even toroidal mode number). Nonlinearly, these modes were found to generate radially elongated perturbations of the pressure profile and led to rapid destruction of nearly all flux surfaces present in the initial equilibrium. 

GP11.00127: Are there disruption forces in stellarators? Robert S Granetz, Thomas S Pedersen, David Anderson, Zander N Keith, Andrew Maris One of the most wellknown advantages of stellarators is the elimination of the problematic issues due to currentdriven disruptions, specifically large induced currents and runaway electron beams, which are problematic in tokamaks with their large current parallel to the magnetic field. In the absence of large J_{‖}, the MHD equilibrium equation, ▽p = J × B, stipulates the necessity for a large perpendicular current density, J_{⊥}= B ×▽p / B^{2}, for a magnetic confinement device with a strong pressure gradient, as expected in a stellarator fusion reactor. If an event occurs for which the plasma pressure is lost rapidly, this diamagnetic current, J_{⊥}, will quench on the same timescale, inducing an electric field per Lenz’s law. Since this electric field is perpendicular to the field lines, it cannot accelerate electrons to form relativistic runaway beams. But it will induce perpendicular current in the vacuum vessel, with resulting I_{⊥}× B force trying to compress the minor radius of the vessel. The magnitude of these effects depends on the timescale in which plasma thermal energy collapses, as well as the magnitude of the drop in plasma pressure. For fast collapses the resulting forces can be substantial and must be taken into account in machine design. This presentation will assess such scenarios in terms of likelihood and avoidance of such events, the relevant time scales, and potential consequences for the design of a stellarator reactor. 

GP11.00128: The impact of rapid thermal loss events on the economics of stellarator and tokamak power plants Andrew Maris, Zander N Keith, Cristina Rea, Robert S Granetz Toroidal magnetic fusion concepts are often considered leading candidates for near term fusion energy, but these devices are susceptible to rapid thermal losses, such as radiative collapses. The threat of “disruptions” in tokamaks, extremely fast thermal collapses followed by toroidal current quenches, have garnered serious attention but have received little treatment in economic studies of magnetic fusion energy. In this talk, we generalize a model presented in [1] to quantify the effect of thermal collapses on the cost of electricity produced by a magnetic fusion power plant and compare the challenges faced by tokamaks and stellarators. We outline the various ways rapid thermal collapses increase costs and decrease revenues, introduce metrics to quantify these effects, and add them to a Levelized Cost of Electricity model. We find that although currentfree stellarator power plants face significantly fewer risks than tokamaks, rapid thermal collapses could still have a significant impact on the cost of electricity, especially for quasiaxisymmetric devices where important uncertainties remain regarding the expected thermal quench impacts and current quench forces. 

GP11.00129: Predictive multiscale gyrokinetic transport simulation capability for stellarator FPPs using Trinity3D+GX+KNOSOS Noah R Mandell, William D Dorland, Felix I Parra, Tony Qian, Jai Sachdev, Jose Luis Velasco Evaluation of a stellarator FPP design requires the capability to predict the equilibrium profiles (and hence fusion power) attainable by the device subject to transport. Trinity3D+GX+KNOSOS is a new framework that leverages multiscale gyrokinetic theory to simulate macroscale profile evolution in stellarators due to turbulent and neoclassical processes. To simulate turbulence we use GX, a GPUnative pseudospectral gyrokinetic code that targets fast reactorrelevant calculations. Neoclassical transport is computed by the KNOSOS code, which uses orbitaveraging to solve the drift kinetic equation very efficiently at low collisionality. Both GX and KNOSOS use a radiallylocal approach, enabling a series of GX and KNOSOS calculations to be embedded in parallel in the Trinity3D transport solver for tractable fusion profile prediction (and evolution) calculations. An implicit Newton method is used to timeevolve the transport equations on the timescale of the energy confinement time. Ion temperature profile predictions subject to ion temperature gradient turbulence can be completed in less than an hour, while multichannel calculations including electron temperature and density profile predictions can be completed in roughly a day. We will demonstrate a validation of the framework via modeling of W7X plasmas and discuss future plans for applying the framework to stellarator FPP design and optimization. 

GP11.00130: DIVERTOR PHYSICS


GP11.00131: Plasma fueling efficiency and fuel cycle implications for a fusion pilot plant Larry R BAYLOR, Trey E Gebhart A fusion pilot plant with high fusion power will require high DT fuel burn efficiency that is dependent on efficient fueling and the ability to efficiently pump the plasma exhaust neutrals including the helium fusion byproduct and any impurities. The DT burn fraction is strongly affected by the global recycling and the fueling efficiency. Recycling is anticipated to be lower in a burning plasma that present day experiments due to the hotter edge and dense scrape off layer limiting the ionization to mostly occurring outside the separatrix. The fueling efficiency can be influenced by the reactor design to use high speed pellets that can penetrate into hot burning plasmas and utilize the gradB and curvature drift to maximize the fuel deposition depth. The pumping of exhaust gas will require a pumping system that has a helium pumping speed comparable to the hydrogenic pumping speed. A continuous cryopump has been developed that demonstrates this capability and is a candidate for future pilot plan deployment. rocessing of the exhaust gas and efficient recirculation must be carefully designed into the fusion pilot plant fuel cycle in order to keep the plant DT inventory at manageable levels. Impurities from the plasma must be effectively separated from the exhaust in order to recirculate the unburned fuel. Makeup of burned DT has to be done in real time to maintain the appropriate isotopic mix. A concept for the entire plasma fuel cycle that meets these requirements is presented. 

GP11.00132: Impurity release and transport from a toroidally symmetric limiters in DIIID Zachary J Bergstrom, Shawn A Zamperini, Aritra De, Jerome Guterl, Tyler W Abrams Dual toroidal limiters will be inserted into the outer midplane of the DIIID tokamak to symmeterize the far scrapeoff layer. The limiters will consist of graphite tiles and may eventually be replaced by silicon carbide (SiC) tiles. Due to the large uncertainty of plasma conditions in the far scrapeoff layer, impurity release and tranport from the limiters remains an outstanding question. Impurity release is modeled with a homogenous mixed material model with particlesurface interactions according to the RustBCA sputtering model. Subsequent impurity transport and redeposition is modeled with the Global Impurity Transport (GITR) 3D Monte Carlo code. The fractional release and deposition of eroded particles are measured as functions of anticipated driving forces in the far scrapeoff layer: temperature, density, flow velocity, and perpendicular transport. It is found that due to the low temperature and plasma densities, impurities undergo substantial longrange transport as opposed to the prompt redeposition observed in divertor regions. The longrange transport of impurities, from both the limiters and the current DIIID configuration, leads to highly localized deposition in locations far from the outer midplane. SiC tiles result in less C erosion than the pure C case, leading to the formation of Sirich interaction regions in the surface layers that may improve the oxygen gettering properties of the main wall. 

GP11.00133: Ejection of Molten Tin Droplets in the Presence of a Hydrogen Plasma James Bramble, Cody Moynihan, Steven Stemmley, Jackson Stermer, Jaime Robertson, David N Ruzic One of the significant obstacles left if the development of a fusion power plant is the development of Plasma Facing Components (PFCs) that can withstand the large heat and particle flux incident on the first wall and divertor region. As solid PFCs struggle with microstructure growth, sputtering and melting, liquid metals have been a popular potential replacement. Liquid metal’s use as a PFC has been increasing due to its ability to selfrepair damage as well as pump lost fuel and waste particles to create a low recycling edge. Currently, tin, lithium and lithium eutectics are the commonly considered liquid metals for use as a fusion PFC. It is therefore important to research the Plasma Material Interaction (PMI) between a hydrogen plasma and the liquid metal PFC candidates. This work, conducted at the Center for Plasma Material Interaction (CPMI), investigated how a molten tin surface reacts when exposed to a hydrogen plasma, building off observations by ASML of particles being ejected from a molten tin surface in the presence of a hydrogen plasma. For this work molten tin was exposed to a hydrogen plasma, of varying electron densities and temperatures, and any ejected particles were collected on a witness plate to determine the particle sizes, angular distribution and mass flux. This work found the ejected tin particles range in size from 10’s of nm<![if gte msEquation 12]> style='msobidifontstyle:normal'> nms to 10’s of microns and that the mass flux of tin from the molten surface is in the 10’s of mg/(m^{2} s)The work done shows macroscopic amounts of molten tin are ejected in the presence of a hydrogen plasma, making tin not suitable as a fusion PFC. 

GP11.00134: Studies of the SOL impurity stagnation point in DIIID using Coherence Imaging Spectroscopy with comparisons to UEDGE Galen G Burke, Filippo Scotti, Steven L Allen, Livia Casali, Max E Fenstermacher, Adam McLean, William H Meyer, Morgan W Shafer, Huiqian Wang, Robert S Wilcox, Menglong Zhao Coherence imaging spectroscopy (CIS) is used to study mainchamber scrapeofflayer (SOL) carbon flows under a variety of divertor configurations in L and Hmode discharges on DIIID. In lowersinglenull (LSN) DIIID Lmode discharges, regardless of the ion BΧ∇B (∇B) drift direction or the degree of detachment, the C^{2+} flow is found to stagnate at a single point near the crown of the main plasma, away from the divertor Xpoint. In contrast, in uppersinglenull and matched discharges, the C^{2+} flow stagnates near the lowfieldside (LFS) Xpoint. These experimental data are compared to UEDGE fluid simulations with drifts. For LSN, UEDGE predictions for the C^{2+} stagnation point roughly agree with experiment in both ∇B drift directions. UEDGE predicts the stagnation point moves from the crown to the outer midplane as the simulation density (n_{e}) increases towards detachment. In separate experimental studies, in LSN Hmode experiments the C^{2+} stagnation point moves from the LFS Xpoint to the plasma crown depending on the direction and magnitude of the plasma current (I_{p}). Impurity gas injection in some highpower Hmodes is found to influence the C^{2+}SOL velocity depending on the ∇B drift direction. Finally, highpower negative triangularity discharges show a dynamic C^{2+} SOL flows stagnation point that depends on n_{e}, injected neutral beam torque, I_{p}, and ∇B direction. 

GP11.00135: Simulation of Silicon Carbide as first wall material using GITR + Surface Model Aritra De Silicon Carbide(SiC) is a promising plasmafacing material for next generation fusion devices because of low hydrogenic diffusion, good mechanical and thermal properties under neutron irradiation and low Z contamination. The goal of this work is to estimate the performance of SiC as a plasma facing component by examining erosion and migration rates of silicon and carbon in the plasma and the evolution of the subsurface concentrations. A semianalytical surface model has been proposed that models surface material interactions (physical sputtering and reflections) with the main plasma ions and impurities. This is coupled with global particle tracking code (GITR [1]) to study erosion and transport of Si and C in the plasma. SiC samples were exposed to Lmode attached plasma using the Divertor Material Evaluation System (DiMES) on the DIIID tokamak [2]. The coupled surface model with a homogeneous mixed material model and GITR simulations have been used to reproduce gross and net erosion rates of silicon carbide coatings in the lower divertor measured during this experiment. Improving upon that, a 1D surface mixing model is proposed to capture the dynamics in the implantation layer. This model is validated with experimental data [3] which measured stoichiometric ratio Si/C as a function of depth in the implantation layer which can be captured using a 1D model. 

GP11.00136: Measuring the Temperature Dependence of the Vapor Pressure and Evaporate Composition of Eutectic LeadLithium Giovanni Diaz, Steven Stemmley, James Bramble, Cody Moynihan, David N Ruzic The interest in using liquid lithium as a plasma facing component (PFC) in fusion reactors has grown in recent years due to the PFC’s selfhealing surface, and low hydrogen recycling. However, the liquid PFC can be hindered by its relatively high vapor pressure (about 1Pa at 800K). This hinderance introduces relatively high Z material into the bulk plasma, thereby decreasing the reactor’s efficiency. However, alloying lithium with lead may substantially lower the vapor pressure (expected to be about: 1mPa at 800K) while also increasing the ability for tritium breeding. 

GP11.00137: Numerical simulations of wall impurity emission from the RF limiters of the Alcator CMod ICRH antennas using the hPIC2 and RustBCA codes Andrea A Gonzalez Galvan, Davide Curreli, Christina Migliore, John C Wright, Paul T Bonoli In this work, we performed numerical simulations of the wall impurity flux produced during ICRH operations in Alcator CMod using the hPIC2 particle in cell code and the RustBCA ion surface interaction code. We considered a deuterium plasma impacting on the molybdenumcovered RF limiters surrounding the ICRH launcher. The electromagnetic fields were calculated using the STIX electromagnetic solver in the frequency domain, which included nonlinear RF sheath boundary conditions. The output from STIX was then utilized to find the ion impact energyangle distributions (IEAD) using the hPIC2 code at different levels of plasma contamination (Mo atomic concentration 05%). The resulting IEADs show the characteristic twopeak energy structure that displayed an increasing energy displacement with larger concentrations of Mo. The RustBCA code was then used to analyze the sputtering behavior of the wall and obtain maps of the Mo impurity fluxes. A parametric study as a function of the antenna strap phasing reveals a minimum on impurity emission also observed in the experiments. 

GP11.00138: Analysis of GITR simulated W erosion and comparison with optical emission spectroscopy in the DIIID SASVW divertor as a step toward validation Alyssa L Hayes, Ane Lasa Esquisabel, Gregory Sinclair, Timothy R Younkin, Tyler W Abrams, Harry Hughes, Robert S Wilcox, Jerome Guterl, Brian Wirth Preliminary impurity transport modeling with a favorable Bt direction (USN with reverse Bt) indicates that the majority of eroded W in the DIIID SASVW divertor is redeposited within the divertor, closer to the slot vertex. Higher net erosion near the outboard slot entrance may contribute to elevated W leakage into the core. Understanding impurity transport is critical for minimizing net erosion of PFCs in a tokamak, and for minimizing core contamination by impurities. Monte Carlo transport analyses using GITR offers insight to the dominance of Lorentz, frictional, and temperature gradient forces and drifts on local transport in the SASVW and their effects on prompt redeposition patterns. 

GP11.00139: Drift effects in SOLPSITER simulations of the COMPASS tokamak SOL Jan Hecko, Michael Komm, Katerina Hromasova, Jakub Seidl, Matej Tomes The final experimental campaign of the COMPASS tokamak encompassed a series of Lmode and Hmode discharges, incorporating both normal and reversed directions of the toroidal magnetic field and plasma current. This presents an excellent opportunity to investigate the influence of charged particle drifts within the scrapeoff layer (SOL) region. 

GP11.00140: Metallic alloys overview for dynamic lithium corrosion of diverter system structural materials based on liquid lithium Aleksandr Khomiakov, Cody Moynihan, Steven Stemmley, David N Ruzic Liquid lithium application in diverter systems for fusion devices requires an understanding of the compatibility of liquid lithium with various structural materials. Taking into account that flowing liquid lithium is more aggressive in comparison with static liquid metal, this work is devoted to the consideration of one of the important aspects of compatibility, namely the corrosion of materials in flowing liquid lithium resulting in the change of mechanical properties of components. The Center for PlasmaMaterials Interactions (CPMI) at the University of Illinois at UrbanaChampaign (UIUC) has built an experimental instrument for testing the dynamic lithium corrosion of materials under lithium conditions, simulating the effect of liquid metal flow on a material by rotating at a controlled frequency of test samples in a volume of liquid lithium. All materials were exposed to lithium at 300° C for 100 hours at a rotational speed of 30 Hz. The level of corrosion of the samples was determined by assessing the changes in the mass of the samples, their surface characteristics (determined using 3D profilometry), as well as strength characteristics, which were measured using tensile tests of the samples. The result of this work is an overview of the mechanical characteristics of a set of metal alloys from the point of view of their application as lithiumfacing materials in fusion devices. As a example, for Grade 5 Titanuim alloy it was found that heating lowered the strength of the material and lithium further lowered the strength while young’s modulus was decreased by heating and further by lithium. 

GP11.00141: An investigation of divertor plasma recombination using a molecular collisionalradiative model Yuzhi Li, Yanzeng Zhang, Xianzhu Tang Divertor detachment provides an optimal way of managing the power and particle exhaust, which is crucial for reducing the target damage and achieving the steadystate operation of a tokamak. The modeling of divertor detachment poses a significant challenge due to the diverse physics involved, especially molecular kinetics. Additionally, the molecular activated recombination (MAR) is suggested to play a crucial role in divertor detachment since the vibrationally excited hydrogen molecules can create effective pathways for the plasmas to recombine at low temperatures (~2 eV). Here, to understand the effects of molecules on plasma recombination at the target, we couple the atomic collisionalradiative (CR) code FLYCHK lite with molecular features that resolves various vibrational levels of the molecules. The molecular chemical reaction rates are evaluated by allorder methods (computationally expensive and complicated methods), which are required in order to achieve good levels of accuracy for nearneutral species (e.g., a recombining plasma). With the newly developed model that resolves the vibrational levels and reactions accurately, we will be able to show the dominant MAR channels where the plasma recombination is enhanced. 

GP11.00142: Radiative dissipation of small ELMs in JETILW highly radiating impurity seeded Hmode scenarios Bartosz Lomanowski, Matthias Bernert, Michael Komm, Dominik Brida, Ephrem Delabie, Stuart Henderson, Juuso Karhunen, Andrew Meigs, Alex Tookey Volumetric dissipation of ELMinduced transient heat fluxes via extrinsic impurity (Ar, Ne, N) radiation in highly radiating JETILW Hmode scenarios can contribute to a significant reduction in the deposited energy on the divertor targets during small ELMs, which occur in pronounced detachment conditions and can coexist with the Xpoint radiator (XPR) regime. Combining ELMresolved measurements of the relative changes to the pedestal pressure, radiated power, heat and particle fluxes on the divertor targets, as well as novel nearUV spatially resolved divertor spectroscopy capabilities, the extent of ‘reattachment’ during fast transient events is analyzed. An upper limit on the ELM size that can be radiatively dissipated (buffered) is derived based on the spatial and temporal dynamics of the Ar^{1+}Ar^{2+}, Ne^{1+}Ne^{3+} and N^{1+}N^{3+} spectral line emission in the nearUV range and the magnitude of the measured heat fluxes as they approach the noise levels of the infrared camera systems. The extent to which the low charge state spectral lines can be used as proxies for the impurity radiation front position in the outer divertor is examined, as are the intraELM plasma conditions near the outer target. The experiment data and its interpretation will be invaluable for validating timedependent SOLPSITER simulations with the aim of developing capability for quantifying the impact of partially and fully buffered small ELMs on plasma facing component lifetime evaluation in nextstep devices. 

GP11.00143: Influences of Porosity on Hydrogen Diffusion and Retention in Lithiated Porous Tungsten Camila Lopez Perez, Martin NietoPérez, Kevin B Woller, Logan Webber Hybrid liquid lithium – porous tungsten materials are being considered as candidate plasma facing components due to their ability to maintain a continuously replenishable lowz plasma interface while tolerating both the high steadystate and transient heat fluxes in the highduty cycle environment of a fusion reactor [1]. 

GP11.00144: Investigation of extruded pebble rods for plasmafacing components in magnetic fusion reactor divertors Erick R Martinez Loran, Eric M Hollmann, Jose A Boedo, Leopoldo Chousal, Christopher Jones, Benjamin W Spencer, Daniel Schwen Design of divertor plasmafacing components which can survive extreme heat loads in future magnetic fusion reactors is extremely challenging. This work presents first manufacture and testing of extruded pebble rods for use as plasmafacing components. The pebble rods consist of 1 mm diameter spheres bound together by a porous carbon matrix. Reactorrelevant (up to 50 MW/m^{2}) steadystate frontsurface heat loads are applied at normal incidence by laser heating. Pebble release at rod recession rates of order 0.5 cm/s and intact pebble recovery are demonstrated. A wide range of pebble materials have been tested, including C, B, BN, SiN, SiC, and W. Recession rate tunability has been demonstrated by varying pebble material, matrix fraction, and pebble size. Comparisons with finiteelement simulations suggest that the pebble rod recession can be understood in terms of pebble expansion followed by matrix cracking. Tests of the extrusion of the pebblebased rods demonstrate that friction between the rods and the stainlesssteel extrusion channel is sufficiently low (<50 N for the expected channel length) for reliable use in a reactor. Front surface outgassing rates below 1000 TorrL/m^{2}/s are achieved, believed to be sufficiently low for use in typical magnetic fusion reactor divertors. 

GP11.00145: Ion orbit modelling of ELMinduced heat load and particle flux on the ITER first wall panels Sunwoo Moon, Tom Wauters, Richard A Pitts, Martin Kocan, Gregor Simic, Matic Brank, James Paul Gunn ELM filaments can carry intense particle fluxes and energies to plasmafacing components (PFC), leading to temperature excursions and potential material erosion. These effects are amplified at the edges of gaps between the castellations required on the actively cooled PFCs which ITER will use. This study focuses on ion orbit modelling of ELM impact on tile edges of the ITER first wall panels (FWP), comparing both Be and W material options. To investigate the timedependent spatial deposition of the ELM filaments and the subsequent penetration of ions with finite ion orbits into the castellation gaps, an idealized model of a filament structure propagating across the scrapeoff layer is first simulated using the SMITER field line tracing environment, in combination with a fluid parallel transport model describing the filament energy loss from a separatrix release point. The model provides scans of the particle energies and fluxes at the FWP impact points, assuming the preELM magnetic equilibrium is preserved during the filament propagation and for varying release parameters (e.g. initial T_{i}, radial filament propagation velocity, total energy of ELM). Deploying the same 3D ion orbit modelling used for simulations of the ELMinduced ITER divertor monoblock edge loading, estimates are made of the ion impact energies and angles on the FWP gap edges for varying gap width and misalignment. These may then be used to assess the enhancement in power loading and material sputtering due to the presence of castellations. 

GP11.00146: MultiFidelity Machine Learning Approach for Fast Determination of Ion Impact EnergyAngle Distributions and Material Sputtering in RadioFrequency Plasma Sheaths Mohammad Mustafa, Logan T Meredith, Mikhail Rezazadeh, Davide Curreli Highfidelity predictions of impurity emission from the plasmafacing components surrounding RF actuators, such as ICRH antennas, often require a large number of computationallyexpensive coupled ParticleinCell and Binary Collision Approximation (PICBCA) simulations. To address this challenge, we developed a Machine Learning (ML) model that integrates a Convolutional Neural Network (CNN) and a MultiLayer Perceptron (MLP) to reconstruct the ion impact energyangle distributions (IEAD) in RF plasma sheaths. In order to train the ML model, we generated large training data sets using the hPIC2 code. The simulations covered up to 14dimensional parameter space, encompassing a wide range of expected plasma conditions. To further enhance the accuracy of the model, we employed a MultiFidelity ML architecture. For computing impurity emission on the ICRH antenna, we then integrated the IEADML model with sputtering data generated by RustBCA. The resulting approach significantly reduces by several orders of magnitude the computational time necessary to estimate impurity emission from RF sheaths. The ML results maintain the same accuracy as the original PICBCA simulations, with bounded error across the whole training interval. 

GP11.00147: Analytic optimization of farSOL protection limiters for reactorscale tokamaks Jacob H Nichols, Ezekial A Unterberg, Peter C Stangeby An analytic framework is presented for the design and optimization of mainchamber farSOL protection limiters, necessary to ensure the survivability of fragile first wall breeding structures in the presence of potentially long farSOL power decay lengths (5+ cm). Reactorscale tokamak devices will face a unique challenge compared to current devices (and ITER), in that much of their first wall must be very thin (~ 5 mm) to allow tritium breeding, limiting the max heat flux density to around 1 MW/m2. This would easily be exceeded if reactor farSOL power decay lengths broaden like electron density decay lengths at high density (the “density shoulder”). Protection limiters, made of thick heatresilient materials, have been put forth as a potential mechanism to remove ionic heat fluxes before they strike the breeding surfaces (at the cost of some fraction of the first wall crosssectional area). The framework is applied to determine the optimal depth, width, spacing, and shaping of protection limiters given a farSOL power decay length, using the plasmawall gap, crosssectional wall area, and deposited heat fluxes as figures of merit. Robustness to uncertainties in the farSOL power decay length is used as an additional optimization factor. It is shown that for FNSFlike plasma parameters, farSOL limiters can reduce the needed plasma wall gap by 50% using less than 1% of the wall area. 

GP11.00148: Plasma Source Development for a Flowing Liquid Lithium Loop Daniel O'Dea, Steven Stemmley, Peter F Buxton, Konstantin Moshkunov, David N Ruzic Flowing liquid lithium Plasma Facing Components (PFCs) offer an attractive solution to the issues faced at the first wall and divertor regions in tokamaks. Lithium is low Z, can getter impurities and reduces hydrogen recycling. However, this uptake of hydrogenic species in the lithium flow, especially tritium, raises operational issues in pilot plants due to stringent limitations on inventory. To address this issue, the Center for Plasma–Material Interactions (CPMI) is developing a recirculating lithium loop containing a free surface PFC. An Electron Cyclotron Resonance source producing a deuterium plasma will be installed allowing for the uptake of deuterium at the lithium PFC and its subsequent transport to be characterized. A 2.45GHz, 3kW magnetron will be used to launch microwaves through a vacuum microwave window into the chamber containing the PFC. To characterize the plasma a custom manufactured diagnostics plate will be installed in the same location as the lithium PFC prior to its operation. This plate consists of an array of 18 Langmuir probes, a retarding field energy analyzer, a radical probe and 4 thermocouples. Supplementing the internal plasma analysis, a wide band spectrometer and 4 filterscopes will be installed with line of sight to the plasma region in front of the plate. This will allow for a range of analysis techniques to be performed such as actinometry, Boltzmann plots and Voigt fitting. This presentation will give an overview of the plasma source, outline the analysis methods and touch on the design of the loop and initial operation. 

GP11.00149: First wall fluxes modelling with 3D PFCs coupled to 3D plasmas in the HEAT toolkit Manuel Scotto d'Abusco, Andreas Wingen, Nathaniel M Ferraro, Stefano Munaretto Leveraging on the Heat flux Engineering Analysis Toolkit (HEAT) [1], re 

GP11.00150: Initial assessment of first wall erosion and retention properties for the General Atomics fusion pilot plant Gregory Sinclair, Tyler W Abrams, Jerome Guterl, Roberto Maurizio, Xinxing Ma, Kurt Zeller, Zachary J Bergstrom, Jonathan H Yu, Anthony W Leonard, David B Weisberg, Brian A Grierson The tungsten first wall of the General Atomics Fusion Pilot Plant (FPP) will exhibit an average surface recession rate on the order of 0.030.05 mm/yr due to erosion by chargeexchange neutrals, corresponding to a total gross erosion rate of 180 kg/yr. First wall panels are made of 2 cm thick monoblocks that consist of a W/SiC functionallygraded composition and an embedded SiC_{f}/SiC coolant tube. The thickness of the pure W layer at the surface above the functionallygraded layer needs to be sufficient to survive one environmental cycle of erosion, thin enough for efficient heat removal, and optimized for neutron multiplication for tritium (T) breeding. The T accumulation rate in the wall also needs to be low enough to keep the total T inventory below any sitelevel regulatory limit. To ensure compliance with these requirements, initial estimates of first wall erosion and retention were made using reduced models based on existing literature at expected high first wall temperatures. The atomic D flux and impact energy for each wall element was determined using Eirene outputs from SOLPSITER simulations on a model equilibrium and wall geometry. These predictions of first wall material performance in FPP provide critical input on componentlevel engineering design that will satisfy system requirements. 
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GP11.00151: Simulations of tungsten and carbon dust transient influx in tokamak divertor plasma. Roman Smirnov, Sergei I Krasheninnikov Dust is a type of impurities in fusion plasmas that can be formed on and ejected from tokamak walls due to various plasmamaterial interaction processes, such as surface layer cracking, peeling, and melting. The intensity of these processes is generally higher for larger particle and power flows to the wall components, particularly, in the divertor region. This leads to intermittent character of dust ejection into fusion plasmas, typically associated with transient highintensity plasma events, such as edge localized modes. Experiments on various tokamaks have demonstrated that the ejected dust can have critical impact on performance of magnetic fusion devices affecting plasma parameters and stability. This work reports on computer simulation studies of transient influx of carbon and tungsten dust particles in divertor of a medium size tokamak. The simulations are performed with coupled plasma and dust transport codes, UEDGEDUSTT, for low and highpower plasma discharge conditions in DIIIDlike geometry. The evolution of edge plasma parameters, scrapeofflayer heat fluxes, and the core impurity accumulation are simulated and compared for different injected quantities of the low and highZ material dust. The results demonstrate complex synergistic phenomena involving evolving plasma conditions and the dust dynamics. The obtained results can clarify mechanisms of transient dust injection effects on the tokamak edge plasmas and help to direct future studies aiming to validate the simulation predictions and mitigate dust impact on fusion plasma discharges. 

GP11.00152: Determining Efficiency of Direct Energy Converters through Simulation Bjorn H Solberg, Ian E Ochs, Elijah J Kolmes, Alexander S Glasser, Nathaniel J Fisch Direct energy converters (DEC) for ions leaving magnetic confinement have been theorized for decades. We examine simulations of some proposed set ups, such as the "venetian blind" arrangement [1,2], to calculate the conversion efficiency for ions. The simulations are run via the Boris particle push algorithm, while the electric and magnetic field set ups are determined through Gmsh and Dolfinx. Efficiency is determined by the change in the kinetic energy once the ion collides with an ion collector. DEC methods for energy capture from fusion reactors theoretically allow for higher energy efficiency than standard heat cycle energy capture. DEC methods lend themselves to open field line configurations. 

GP11.00153: Steadystate and ELM heat flux load predictions on ST40 limiter and divertor PFCs Erin Joy C Tinacba, Travis K Gray, Chris Marsden, Matteo Moscheni, Otto Asunta, Steven McNamara, Adrian Rengle, Andreas Wingen, E.A. Unterberg ST40 is a high field (B_{tor} > 2T) spherical tokamak operated by Tokamak Energy Ltd with an aspect ratio of 1.6 1.8. ST40 can operate up to 0.7 MA of plasma current, I_{p}, with up to 1.8 MW of auxiliary heating, P_{aux}, in various magnetic geometries for ~0.5 sec pulse durations. One of the biggest challenges in highperformance tokamak operations is the management of heat flux on plasmafacing components (PFCs) such as divertors, as these materials are designed to dissipate the plasma heat and particle loads. This makes the PFCs very susceptible to damage and failure. In this research, the Heat Flux Engineering Analysis Toolkit (HEAT) [1] is used to predict the effects of P_{aux}, I_{p}, and magnetic geometry on the heat loads to the divertors and limiters of the ST40 tokamak. MHD equilibrium for diverted (single and double null) and limited plasmas were generated using the FIESTA code. Initial steadystate simulations used P_{aux} of 1.8 MW for a double null plasma and a prescribed Eich radial heat flux profile. This simulation showed that large heat loads (>20 MW/m^{2}) were deposited on the divertors. Initial attempts to predict ELM heat loads on the PFCs will also be shown. The results from these simulations are essential in designing future experiments that fullyprotect the PFCs as well as validating heat flux predictions for future devices. 

GP11.00154: Benchmarking EMC3EIRENE nonaxisymmetric plasma edge simulations based on GPEC and MARSF plasma response models for KSTAR RMP scenarios Jonathan M Van Blarcum, Heinke G Frerichs, Yueqiang Liu, Nikolas C Logan, Oliver Schmitz, SeongMoo Yang For an edge localized mode (ELM) suppressing resonant magnetic perturbation (RMP) KSTAR scenario plasma response GPEC and MARSF produce perturbed magnetic fields with a similar poloidal spectrum and outer divertor magnetic footprint. As EMC3EIRENE simulations of the nonaxisymmetric scrape off layer (SOL) and divertor heat loads are sensitive to uncertainties in the magnetic footprint geometry, the robustness of the footprints suggests that the subsequent simulations (ongoing) will also be robust to the choice between plasma response codes: GPEC, an ideal MHD model including rotational damping effects and MARSF, a resistive MHD model. Progress on the comparison of the EMC3EIRENE simulations to the corresponding experimental data, in support of validating the plasma response and plasma edge models, will also be shown. Currently, the magnetic footprints extend approximately twice as far as the experimental heat flux striation, suggesting a disagreement between the models and experiment, but this is still under investigation. Accurate simulations of the plasma edge for RMP scenarios are critical for ITER to facilitate the choice of an RMP configuration which is not only ELM suppressing but also mediates the divertor heat load. To support that effort, an exploration with respect to RMP phasing of the magnetic footprint width and field line penetration both which affect divertor heat loads, and edge resonant magnitude, Xpt displacement, and kink resonant amplitude, all which correlate with ELM suppression, is being conducted to benchmark these key metrics between the two plasma response codes. 

GP11.00155: FreeSurface Liquid Metal Flows through Magnetic Field Gradients in the Divertor Region Brian R Wynne, Francisco J Saenz, Zhen Sun, Egemen Kolemen Flowing liquid metals (LM) offer an alternative to solid materials for divertors, advantageous for heat removal and the ability to provide a selfhealing surface. Nevertheless, the effect of magnetic field gradients on LM freesurface flows could hinder steady operation, through splashing or flow disruptions. Magnetic field gradients up to 1 T/m were investigated using the LMXU (Liquid Metal eXperiment Upgrade) channel at PPPL to measure the effects on surface velocity, depth, and streamwise currents. LM flow across a magnetic field generally induces perpendicular currents, whereas flowing through an increasing or decreasing field led to induced currents in the streamwise direction. These streamwise (poloidal) currents with a transverse (toroidal) magnetic field cause Lorentz forces in the surfacenormal direction, forcing the LM onto or away from the divertor plate. While the measured effects were relatively small in this experiment, in a larger reactor the interaction of the induced streamwise currents and the toroidal field could generate surfacenormal forces that significantly oppose the flowstabilizing gravitational force. Measurements and simulations using FreeMHD (an OpenFOAMbased custom MHD solver) showed agreement at the LMXU scale. Simulations at the reactor scale then indicated the toroidal magnetic field gradient in the streamwise direction could be an obstacle for divertor freesurface LM flows. 

GP11.00156: Development of a Retarding Field Energy Analyzer for Ion Energy Distribution Measurements in the DIIID Divertor Bingzhe Zhao, Jun Ren, David C Donovan, Mason D Phillips A retarding field energy analyzer (RFEA) system is currently under development for the DIIID divertor. The RFEA is designed to measure main ion temperature (T_{i}) at the divertor target ranging from 10 eV to 200 eV, under typical density of n_{e} ~ e^{19} m^{3} and high parallel heat flux ~ 100 MW/m^{2}, at ~ 1 ms temporal and ~ 1 mm spatial resolutions.The initial testing with a prototype probe will be accomplished at the lower divertor with the Divertor Material Evaluation System (DiMES), where the probe will be facing the “upstream” fieldaligned plasma flow. With refined probe geometry and material choice, some of the critical issues of RFEA were evaluated and addressed by testing with compact plasma source and 3D simulations of fields, particles, and heat flux, including spacecharge, distortion of the ion distribution function, and the heat load on the structure where it is shown by the simulation that surface temperature on the entrance plate rises < 3000 K during 5s operation with local peak heat flux ~ 100 MW/m^{2}. The longterm objective is to develop a permanent RFEA diagnostic array capable of providing reliable T_{i} readings in the SOL, which can be utilized in the studies of heat flux management, divertor detachment, and impurity transport. 

GP11.00157: Predictions of divertor heat load under resonant magnetic perturbations Ben Zhu, Nikolas C Logan, SeongMoo Yang, JongKyu Park Resonant magnetic perturbations (RMPs), the primary technique for edge localized modes (ELMs) mitigation and suppression on current and future tokamaks, break tokamaks' toroidal symmetry. Many of the widely used tokamak transport models based on axisymmetric assumption nowadays are hence not applicable for edge and divertor modeling with RMPs. Recently, BOUT++'s transport model has been extended to incorporate 3D plasma response field from RMPs. Provided by nonaxisymmetric equilibrium code GPEC, this field is often four orders of magnitude smaller than the axisymmetric field, i.e., $ ilde{B}_{rmp}/B_0<10^{4}$. Thus, it is treated as a higher order modification to the equilibrium magnetic field in the plasma transport equations. This small perturbation term has a profound influence on divertor heat loads. Preliminary test shows a distinct striation pattern appears on the divertor targets once the RMPs field is turned on. The new 3D extension of BOUT++ not only enables the 3D divertor plasma transport capability but also opens a path to study edge plasma instability and turbulence dynamics with RMPs. 

GP11.00158: Simultaneous Advance for Extended MHD with Kinetic Closures: Verification and Applications Joseph A Spencer, Eric D Held, Joseph R Jepson We present progress on a simultaneous timeadvance of NIMROD's fluid quantities with consistent kinetic closures obtained from a solution of the ChapmanEnskoglike drift kinetic equation. A semiimplicit formulation is employed to ensure numerical stability and accommodate time steps that are large compared to the compressional Alfven wave propagation time. 

GP11.00159: Saturation of temperaturegradient driven ion acoustic waves in VlasovFokkerPlanck simulations Jeffery Zielinski, Mark W Sherlock, Avram Milder, Colin Bruulsema, George F Swadling, Wojciech Rozmus The generation of Ion Acoustic Waves (IAWs) presents challenges for desirable laser produced plasmas, including laser energy loss via stimulated scattering and limited heat penetration through waveparticle interactions. An important contributor to the latter is the Return Current Instability (RCI), where superthermal electrons rush out from a high temperature region, drawing a current which balanced by a slow, opposing, electron drift. These modifications to the electron distribution function incur their own heat transfer reductions [1], and IAWs are additionally driven when the return current speed exceeds the sound speed. IAWs saturate through a range of processes, and this investigation is the first to be driven by a realistic, reinforced, electron temperature gradient, modelled from a quasistationary period in experiment [1]. 
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