Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Plasma Physics
Volume 65, Number 11
Monday–Friday, November 9–13, 2020; Remote; Time Zone: Central Standard Time, USA
Session BP14: Poster Session: Magnetic Confinement: Stellarators (9:30am  12:30pm)On Demand

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BP14.00001: Small, stable plasmas fully decoupled from the PFCs in W7X. Thomas Sunn Pedersen, Tamas Szepesi, Ralf Koenig, Felix Reimold, Daihong Zhang, Maciej Krychowiak, Andreas Dinklage, Petra Kornejew, Victora Winters, Uwe Hergenhahn, Tullio Barbui This presentation focuses on describing and understanding the physics of some unusual discharges in W7X. One such plasma shrank in minor radius to 0.55 times its original value, triggered by a strong hydrogen gas puff, after having been fullsize for several s. The plasma lasted \textgreater 2 s in a new, stable steadystate with the smaller minor radius  without feedback control  until terminated by the preprogrammed end of ECRH heating. During the phase of reduced size, it had central T$_{\mathrm{e}}$ of \textasciitilde 2.5 keV, central n$_{\mathrm{e}}$ of 46e19 m$^{\mathrm{3}}$ and a confinement time of \textasciitilde 20 ms, in line with expectations when taking into account the smaller minor radius. The plasma clearly had no direct contact with material objects  all the heating power (3 MW) was dissipated in the clearly visible radiating mantle several cm thick defining the edge of the plasma. These plasmas can be thought of as extreme versions of the powerdetached radiatingmantle plasmas seen in W7X before boronization [1], some of which were visibly smaller than attached plasmas [2]. Thoughts on the stability and potential importance and usefulness of these plasmas will also be presented. [1] D. Zhang et al., Phys. Rev. Letters \textbf{123}, 025002 (2019) [2] T. Sunn Pedersen et al., Nuclear Fusion \textbf{59} 096014 (2019) [Preview Abstract] 

BP14.00002: Physics and Engineering of the GasPuff Imaging Diagnostic for W7X J.L. Terry, S.G. Baek, S.B. Ballinger, A. Von Stechow, O. Grulke, C. Von Sehren A Gas Puff Imaging diagnostic is being installed on the Wendelstein 7X Stellarator. It will be commissioned at the start of the next run campaign. The diagnostic will measure the 2d structure and dynamics of plasma fluctuations in a 75x40 mm region near the outboard boundary. In the ``standard'' magnetic configuration, the fieldofview will span (radially) one of the 5 magnetic islands at the boundary, thereby interrogating some of the turbulence dynamics at the island. The fieldofview will also include the last closed flux surface in some magnetic configurations. We expect a spatial resolution of $\approx $5 mm within the fieldofview. The engineering of the system is challenging and features a number of innovative elements. The local gas puff (H$_{\mathrm{2}}$ or He) used to ``illuminate'' the plasma fluctuations is provided by two ``convergingdiverging'' nozzles that collimate the gas cloud to a halfangle of $\approx $12$^{\mathrm{o}}$. The emission from the gas cloud's interaction with the plasma is gathered by a highthroughput reentrant lens system inserted in a port adjacent to the gaspuff port. It features a popup mirror that turns the view through a 115$^{\mathrm{o}}$ angle and acts as a shutter for the mirror and optics when retracted. [Preview Abstract] 

BP14.00003: Integration of the Design of the Heavy Ion Beam Probe Diagnostic with the Wendelstein 7X Stellarator Facility T. P. Crowley, D. R. Demers, P. J. Fimognari, O. Grulke, R. Laube, H. Trimino Mora A heavy ion beam probe (HIBP) diagnostic is being designed for the Wendelstein 7X (W7X) stellarator. The diagnostic will be used to study equilibrium properties and turbulent transport through measurements of electric potential, fluctuations of electric potential and density, fluctuation wavenumbers, the crossphase between fluctuations of density and potential, and electrostatic fluctuation induced particle flux. Recent efforts have emphasized the integration of the design with the W7X facility. Successful integration is dependent on the locations of the 2 MeV accelerator, energy analyzer, and associated beamlines; the magnetic and vacuum properties of the HIBP hardware; and the system's compatibility with radiation safety requirements. Related work involves improvement in beam path and ion optics modeling, advancement of specification for the beam steering systems, deflectors, and focusing elements, and characterization of subsystem hardware. In addition, characterization and restoration of the 340 kV energy analyzer has been initiated. These tasks are key to preparing a cohesive HIBP design for the conceptual design review. [Preview Abstract] 

BP14.00004: Updates and Future Development of The He/Ne Beam Diagnostic for Line Ratio Spectroscopy in the Island Divertor of Wendelstein 7X Erik Flom, O. Schmitz, T. Barbui, M. Jakubowski, F. Henke, C. Killer, M. Krychowiak, R. Koenig, S. Loch, J. Schmitt, J.M. MunozBurgos A lineratio spectroscopy system based on a thermal helium collisional radiative model (CRM) has been implemented to enable measurement of n$_{\mathrm{e}}$ and T$_{\mathrm{e}}$ above the horizontal divertor targets in two modules of the Wendelstein 7X optimized stellarator. Neon has also been implemented in select discharges to investigate the expansion of the measurement envelope of the diagnostic. In this work, modeling results are presented for standard attached and detached conditions in the divertor of Wendelstein 7X to show the helium and neon emissivity as a function of radial position above the divertor target. Also shown is a firsttime comparison between helium beam data and reciprocating Langmuir probe data using a novel flux coordinate system within the standard 5/5 island chain. [Preview Abstract] 

BP14.00005: Overview of Measurements from the Wendelstein 7X Phase Contrast Imaging Diagnostic and Plans for the OP2 Campaign Eric Edlund, Zhouji Huang, Miklos Porkolab, Adrian von Stechow, JanPeter Bahner, Olaf Grulke A phase contrast imaging (PCI) diagnostic was implemented on the W7X stellarator starting in the OP$1.2$ experimental campaign.$^1$ The PCI diagnostic creates an image of plasma density fluctuations using light from a CO$_2$ laser that passes through the plasma which captures the lineintegral of the fluctuations along the optical path of the laser. The typical measurement range spans frequencies of about $2$ kHz to $600$ kHz, and wavenumbers of approximately $1$ cm$^{1}$ to $10$ cm$^{1}$, although the actual range depends on the optical magnifications used. PCI measurements of both coherent Alfv\'enic modes and broadband fluctuations will be presented. Changes to the PCI diagnostic for the OP2 campaign will be presented, with an outlook to possible future additions to the diagnostic including an optical heterodyne system for detection of ioncyclotron resonance heating (ICRH) waves and an alternate optical design that allows for simultaneous measurement of the plasma image and the Fourier spectra of the fluctuations.\\ $[1]$ E.~M. Edlund et al, Rev. Sci. Instr. 89, 10E105 (2018). [Preview Abstract] 

BP14.00006: Peeling stability in quasisymmetric stellarators C. C. Hegna, J. C. Schmitt Quasisymmetric stellarators produce an equilibrium bootstrap current in the presence of finite pressure gradients. As such, current driven MHD stability properties need to be addressed for this class of optimized stellarators. The criterion for peeling stability is derived for general threedimensional magnetohydrodynamic equilibria. For the quasisymmetric class of stellarators, the stability properties of the peeling drive are determined by the sign of the rotational transform gradient and the direction of the equilibrium current. For scenarios with rational surfaces located just outside the last closed flux surface, generally quasihelically symmetric stellarators are destabilizing to peeling modes in the presence of bootstrap current, whereas quasiaxisymmetric configurations are stable against peeling drive. [Preview Abstract] 

BP14.00007: Improving the Physics Basis for Quasihelically Symmetric Stellarators Aaron Bader, B.J. Faber, J.C. Schmitt, D.T. Anderson, M. Drevlak, J.M. Duff, H. Frerichs, C.C. Hegna, T.G. Kruger, M. Landreman, I.J. McKinney, L. Singh, J.M. Schroeder, P.W. Terry, A.S. Ware Quasisymmetric stellarators are an attractive candidate for fusion reactors and pilot plants due to low neoclassical transport, and the promise of transport barriers enabled by low flow damping in the symmetry direction. In particular quasihelical symmetry promises short connection lengths with relatively low bootstrap currents. Expanding the physics basis for reactor relevant configurations requires improving performance with regard to energetic particles confinement, turbulent transport, MHD stability, divertor performance, and feasibility for coils. Each of these targets and new improvements is discussed in turn. For a fusion reactor all of these parameters need to be optimized simultaneously, and some of the tradeoffs will be examined. In addition, the poster will discuss improved optimization algorithms [1]. 1: A. Bader "Advancing the Physics Basis for Quasihelically Symmetric Stellarators" Submitted to JPP [Preview Abstract] 

BP14.00008: Advancing Optimization Algorithms for Reduced Turbulent Transport in Stellarators Benjamin Faber, Aaron Bader, Joseph Duff, Chris Hegna, Ian McKinney, M.J. Pueschel, Paul Terry Driftwavedriven ionscale turbulent transport presents a significant barrier for stellarators as a fusion power plant concept. The manifestly threedimensional nature of the stellarator provides new opportunities for optimizing the magnetic geometry for reduce turbulent transport. An optimization algorithm for reducing turbulence transport is presented that leverages the ability of damped eigenmodes to dissipate turbulent fluctuation energy at the driving scale of the instability. Damped modes play an important role in turbulence saturation in standard tokamak scenarios[1] and in stellarators, the dominant energy transfer channel to stable modes is a function of geometry[2]. A proxy for the nonlinear energy transfer to damped modes is calculated using threewave correlation times and coupling coefficients computed from linear driftwave eigenfunctions in fully threedimensional stellarator geometry. This advanced turbulence transport proxy is coupled with a new package for distributed, multiprocessor optimization to find new stellarator configurations with reduced turbulent transport and optimization calculations will be presented for a quasihelically symmetric geometry. [1] Whelan et al. PRL, 2018 [2] Hegna et al. PoP, 2018 [Preview Abstract] 

BP14.00009: A New Tool for Optimizing Finitebuild Stellarator Coils Luquant Singh, Thomas Kruger, Aaron Bader, Caoxiang Zhu, Stuart Hudson, David Anderson Finding coil sets with desirable physics and engineering properties is a crucial step in the design of modern stellarator devices. Existing stellarator coil optimization codes ultimately produce zerothickness filament coils. However, stellarator coils have finite depth and thickness, which can make the singlefilament model a poor approximation, particularly when coil build dimensions are large compared to the coilplasma distance. We present a new method for designing coils with finite builds using a multifilament model and present a mechanism to optimize the orientation of the winding pack. We show optimization results from the numerical implementation OMIC (Optimization of Multifilament Coils) [1] for the HSX stellarator and a new UWMadison QHS configuration [2]. [1] L. Singh, T. Kruger et al., Optimization of Finitebuild Stellarator Coils, Accepted to JPP June 2020. [2] A. Bader et al., Advancing the Physics Basis for Quasihelically Symmetric Stellarators, Submitted to JPP June 2020. [Preview Abstract] 

BP14.00010: Penalty Functions in FOCUS to Constrain Stellarator Coil Optimization Thomas Kruger, Caoxiang Zhu, Aaron Bader, David Anderson Stellarator coil optimization aims to generate coils that produce a desired magnetic boundary. FOCUS is a coil optimization code that solves for coils in free space. This poster will examine penalty functions, implemented in FOCUS, that optimize coils to better match a magnetic boundary while simultaneously satisfying engineering constraints. Because FOCUS uses analytical derivatives for optimization, all penalty functions need to be differentiable. Penalty functions to optimize for minimum coil radius of curvature and minimum coilcoil separation are presented and their performances are evaluated. It is shown that optimizing for a coilâ€™s average curvature or average curvature squared do not perform as well as optimizations for maximum curvature. Optimizing with penalty functions not only constrains the curvature, but also solves for coils with low normal field values on the desired boundary. [Preview Abstract] 

BP14.00011: Impurity Transport Experiments at the HSX Stellarator J.F. Castillo, B. Geiger, A. Bader, S.T.A. Kumar, K.M Likin, D.T. Anderson The laser blowoff technique is used to inject aluminum atoms into the confined region of HSX. To study the radial propagation and confinement properties of the injected impurities, signals from several arrays of AXUV diodes are evaluated and compared with modeling results from the impurity transport code STRAHL. For plasmas with an ECH power ($P$) of 11.3 kW, the average impurity decay time ($\tau$) is 1.59 ms while an average $\tau$ of 3.16 ms is observed when reducing $P$ to 6.3 kW. A systematic study of the power shows a $\tau \propto P^{1.0}$ dependence, similar to the ISSO4 scaling. In addition, neoclassicallypredicted calculations from the PENTA code show much longer decay times compared to experimental measurements. Thus, neoclassical diffusion alone is insufficient to explain these results and suggest a substantial impact of turbulence on the impurity confinement. This finding is further supported by a sensitivity analysis using STRAHL, showing that uncertainties based on the background neutral density and the scrapeoff layer loss time are low enough to differentiate between neoclassical and anomalous transport. [Preview Abstract] 

BP14.00012: Evolution of the Radial Electric Field and Ion Parallel Flow in HSX with Nonlinear Viscosity Dimitrios Michaelides, Joseph Talmadge, Santhosh Kumar The time dependent momentum balance equations have been solved in two directions on a flux surface with the assumption that the viscosity is linear with the flow. The results show that, due to the quasihelical symmetry in HSX, the parallel flow evolves slower than the radial electric field (Er) in the presence of a driven radial plasma current. At higher flow speeds, the radial current can cause an ion resonance in the plasma at which point the viscosity becomes nonlinear. Due to the lack of toroidal curvature in HSX, the peak in the viscosity occurs at a poloidal Mach number (Mp)$=$3, where in tokamaks the peak occurs at Mp$=$1. Here we model the evolution of the mean ion parallel flow and Er when the viscosity becomes nonlinear. Damping due to neutrals is also included to ascertain when the neutrals might obscure the nonlinearity. A charge exchange recombination spectroscopy (CHERS) system has been used in HSX to determine Er and the mean ion parallel flow from measurements of the inboard/outboard asymmetry of the C$+$6 parallel flow. An experimental program using the CHERS system, a Mach probe and floating probes is detailed that will enable comparisons to the modeling. [Preview Abstract] 

BP14.00013: An Adjoint Method to Calculate Magnetic Island Width Sensitivity Alessandro Geraldini, Matt Landreman, Elizabeth Paul To improve confinement, stellarators are designed to have the smallest possible magnetic islands. However, unavoidable errors in the positioning of the coils during construction cause small perturbations to the intended magnetic field configuration. If the size of an island is too sensitive to such perturbations, the tolerance on the coil positioning may be lower than the expected construction error, and large islands can appear in the device. In NCSX the increase in construction cost, which led to the cancellation of the experiment, was in part due to low coil tolerances associated with island size (Neilson et al, 2010). We describe an efficient method to quantify island width sensitivity that could be used in the early stages of stellarator design. The island width is calculated by following a single field line corresponding to the island centre (Cary & Hanson, 1991). An adjoint method is used to efficiently compute and verify the gradient of island widths with respect to a single parameter for a simple class of magnetic field configurations (Reiman & Greenside, 1986) where the size of an island and its gradient are known analytically. The method is then applied to the calculation of the shape gradient of island widths with respect to coils in configurations such as NCSX. [Preview Abstract] 

BP14.00014: 3D Boundary Identification of Stellarator Plasma Using Cauchy Condition Surface Yasuhiro Suzuki This presentation deals with the new way of the 3D equilibrium reconstruction. For the reconstruction of the stellarator plasma, the identification of the sophisticated 3D plasma boundary shape is a critical issue. Traditionally, the inverse equilibrium calculation such as EFIT was conducted. In this presentation, we apply the "Cauchy Condition Surface (CCS) method" based on the Boundary Element Method (BEM) using the magnetic diagnostics only. The CCS method was applied to a tokamak, JT60U, plasmas, and succeeded for reconstructing the plasma boundary shape accurately and fastly. We extended the CCS method to the 3D configuration. We already tested the 3D CCS method to a stellarator using synthetic magnetic diagnostics and succeeded in the reconstruction of the 3D plasma shape. In this presentation, we discuss the reconstruction based on the experimentally obtained magnetic signal. [Preview Abstract] 

BP14.00015: Nonsymmetric ideal magnetohydrodynamic steady flows Harold Weitzner, Wrick Sengupta Steady ideal magnetohydrodynamic flows which lack symmetry are found by an expansion in a parameter proportional to the amplitudes of the flow and magnetic field. Resonance conditions are identified and similar to problems in the expansion of equilibria , it is shown how to resolve resonances and find the flows. To leading order the flows are parallel, although in higher order the limitation of being parallel is not needed. [Preview Abstract] 

BP14.00016: FiniteBeta Stellarator Optimization and Energetic Particle Confinement Andrew Ware, Jesse Cook, Anna Vonessen, Haley Wilson We present analysis of finitebeta optimization of quasihelically symmetric stellarator configurations, including bootstrap current alignment and improved ballooning stability. This work includes both fixed boundary and free boundary (with coils) equilibria. The transport properties of the optimized configurations are analyzed as well. A summary of recent efforts on stellarator coil optimization for quasihelically symmetric configuration is presented. This includes using the FOCUS code [C. Zhu, et al., Plasma Phys. Contr. Fusion \textbf{60}, 065008 (2018)] to develop and analyze coil configurations for four and fivefield period configurations with quasisymmetry and enhanced energetic particle confinement, including configurations with and without a set of poloidal field coils. Finally, an analysis of energetic particle confinement in these configurations is carried out using the SIMPLE code [A C. Albert, S. Kasilov, and W. Kernbichler, J. Plasma Physics~\textbf{86}, 815860201 (2020)]. [Preview Abstract] 

BP14.00017: Simulation of Hybrid Stellarator Divertor Alkesh Punjabi, Allen Boozer An efficient simulation method for stellarator divertors (A. H. Boozer and A. Punjabi, PoP \textbf{25}, 092520 (2018)) is used to study hybrid stellarator divertor with features of both the nonresonant and the resonant divertors and compare the results with our results for nonresonant stellarator divertor (A. Punjabi and A. H. Boozer, PoP \textbf{27}, 012503 (2020)). The plasma interaction with the fixed magnetic field of a divertor is diffusive but is far more efficiently studied by adding a small radial velocity to an integration of the magnetic field lines. Different magnetic configurations can be created by changing one of the shape parameters in the Hamiltonian for the trajectories of magnetic field lines in nonresonant stellarator divertor. The most novel result of this study of hybrid divertor for stellarator is that the diffusive field lines go into two families of magnetic cantori for all velocities with probability exponents of 2.1 for the primary family and 4.3 for the secondary family. In contrast to this, field lines go into two families of cantori only when the artificial radial velocities are larger than a certain value when there are no islands. The footprints on the wall are stellarator symmetric and have fixed locations on the wall. The hybrid divertor confines larger plasma volume, has higher average shear, larger footprints, lower average density of strike points, lower maximum density of strike points, and longer losstimes than the nonresonant stellarator divertor. The magnetic configuration is robust against small changes in the $\iota $ and large changes in the shape parameter. [Preview Abstract] 

BP14.00018: Towards Measurement of Bootstrap and PfirschSchl\"{u}ter Currents in Stellarators using an Alkali Diagnostic Beam P.J. Fimognari, T.P. Crowley, D.R. Demers The bootstrap and PfirschSchl\"{u}ter currents change the magnetic topology of a stellarator in ways that affect neoclassical transport, stability, and divertor properties. These current profiles are typically inferred using codes that solve for the equilibrium as constrained by magnetic and temperature profile measurements. Our new technique enhances this solution by measuring a component of the magnetic vector potential at select locations in the plasma as an additional constraint. A beam of alkali atoms is injected into a device and undergoes collisions with plasma particles, creating a spray of secondary ions whose Larmor radii are sufficiently large to transport them out of the plasma. In stellarators, the momentum of these particles depends on the local magnetic vector potential both at the point of ionization and along the particles' path. We are studying beam particle simulations through the magnetic equilibrium of the HSX stellarator to determine the relative apportionment of the local and path contributions and thus determine the feasibility, in quasisymmetric magnetic fields, of a diagnostic based on this principle. We have also developed a prototype detector for deployment on the HSX stellarator to study particle and radiative noise signals that may impact diagnostic measurements. [Preview Abstract] 

BP14.00019: DESC: An Efficient Stellarator Equilibrium Code Daniel Dudt, Egemen Kolemen Calculating ideal magnetohydrodynamic equilibria is not trivial in nonaxisymmetric devices, and the existing threedimensional equilibria codes are not adequate for future stellarator demands. This presentation showcases the new code DESC, which is being developed with the unique challenges of stellarator optimization in mind. Instead of the popular energy principle approach, this new technique directly solves the equilibrium force balance equations at discrete points in real space. Through its novel use of pseudospectral methods, DESC properly resolves the magnetic axis and can achieve similar levels of accuracy with fewer independent variables than other codes. The nonlinear system of equations that describe an equilibrium can be solved with routine Newton algorithms that converge quadratically. Furthermore, this formulation provides an easy way to find branches of neighboring equilibria and could be used as a tool to efficiently search for quasisymmetric stellarators. In its current version, DESC solves fixedboundary equilibria and assumes the existence of nested flux surfaces. An overview of the code is presented along with example equilibria calculations and benchmarks against VMEC. Directions for further development are also discussed. [Preview Abstract] 

BP14.00020: Stellarator figures of merit near the magnetic axis Matt Landreman, Rogerio Jorge A new paradigm for rapid stellarator configuration design has been recently demonstrated, in which the shapes of quasisymmetric or omnigenous flux surfaces are computed directly using an expansion in small distance from the magnetic axis [1]. To further develop this approach, here we derive several other quantities of interest that can be rapidly computed from this nearaxis expansion: (1) Magnetic well. (2) Mercier and resistive interchange stability. (3) The $\nabla\vec{B}$ and $\nabla\nabla\vec{B}$ tensors, which can be used for direct derivativebased optimization of electromagnetic coil shapes to achieve the desired magnetic configuration. (4) The minor radius at which the flux surface shapes would become singular, providing a lower bound on the achievable aspect ratio. (5) For configurations that are constructed to achieve a desired magnetic field to first order in the expansion, we compute the error field that arises at second order. [1] Landreman \& Sengupta, J Plasma Phys 85, 905850608 (2019) [Preview Abstract] 

BP14.00021: Opportunities for Reduced Cost Stellarator Pilot Plants. M. Zarnstorff, D. Gates, T. Qian, Y. Zhai, C. Zhu, S. Woodruff High temperature superconductors and permanent magnets offer opportunities to significantly reduce the costs of stellarator pilot plants by simplifying the coil design and construction, and reducing the coil cross section by achieving higher current density. Stellarators provide the advantage of having very low recirculating power and high energy efficiency, reducing system scale at fixed output power. Since stellarator use static magnetic fields, they are particularly suited to superconducting magnets, further simplifying several design aspects. The design space for 100300 MWelectric stellarator pilot plants will be surveyed, considering plasma and configuration constraints to identify attractive approaches to a costeffective pilot plant and modular energy system. [Preview Abstract] 

BP14.00022: Design and Initial Results from a TableTop Stellarator with Permanent Magnets Tony Qian, Arturo Dominguez, Pierre Gourdain, Chris Pagano, Chirag Rana, Michael Zarnstorff, Caoxiang Zhu MUSE is a tabletop stellarator with planar coils and permanent magnets (PM), optimized to be QuasiAxisymmetric (QAS) for good particle confinement. Recent research shows it is possible to produce 3D fields using planar coils and a simple array of permanent magnets [1]. The SAS study at PPPL has applied these principles to design a halftesla QAS stellarator using permanent magnets and the original TF coils and VV from NCSX [2]. MUSE uses the same principles to make a R$=$0.3m B$=$0.15T QAS stellarator with waterjet cut and 3Dprinted materials for lowcost and relatively simple construction. This poster will present methods for permanent magnet optimization using existing codes including REGCOIL and FAMUS, compare multiple QuasiAxisymmetric target configurations, and outline plans for electron beam mapping of flux surfaces and initial plasma experiments. [1] P. Helander, M. Drevlak, M. Zarnstorff, and S. Cowley, Phys. Rev. Lett. 124, 095001 (2020). [2] C. Zhu, M. Zarnstorff, D. Gates, and A. Brooks, 2020 Nucl. Fusion 60 076016 (2020). [Preview Abstract] 

BP14.00023: Fast particle optimization of a quasiaxisymmetric stellarator equilibrium Alexandra LeViness, David Gates, Samuel Lazerson, Neil Pomphrey, Caoxiang Zhu One challenge of stellarator optimization is the reduction of fast particle losses, whether those fast particles come from neutral beam injection (NBI) or, in the case of a reactor, fusion reactions. In this work, the method developed by Sophia Henneberg [1] (single surface quasiaxisymmetric (QA) optimization at midradius) will be used to reoptimize a fixed boundary, 3 period QA stellarator equilibrium for fast particle confinement while maintaining MHD stability and good neoclassical confinement. The original equilibrium used will be a version of the LI383 design scaled to the size and magnetic field strength (0.5 T) of the proposed permanent magnet stellarator SAS [2]. Fastparticle optimization will be performed with the STELLOPT code suite for both NBI and simulated fusion alphas scaled down in energy. 1. S.A. Henneberg et al 2019 Nucl. Fusion 59 026014 2. D. Gates et al 2019, 61st Annual Meeting of the APS Division of Plasma Physics, BP10.00047 [Preview Abstract] 

BP14.00024: Compact Toroidal Hybrid Progress and Plans D A Maurer, N Allen, D A Ennis, G J Hartwell, C Johnson, J Kring, P Traverso, E Williamson The Compact Toroidal Hybrid (CTH) is a torsatron/tokamak hybrid with the ability to vary the confining magnetic field configuration and generate rotational transform profiles that are tokamaklike with ohmically driven plasma current for disruption and MHD studies. 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 the V3FIT reconstruction code and NIMROD modeling of CTH. Past and recent disruption studies will be overviewed and their relevance to tokamaks and quasiaxisymmetric stellarators will be discussed. Ongoing diagnostic development for the experiment includes an upgrade to an interferometer, development of new bolometer arrays, new spectroscopic studies, and coherence imaging of plasma flows. CTH also serves as a test bed for diagnostic development for our collaborations on the larger facilities like DIIID and W7X. These facility collaborations will be briefly summarized along with a new research direction to explore low temperature plasmas on magnetic surfaces. [Preview Abstract] 

BP14.00025: Density Limit Disruption Studies in the Compact Toroidal Hybrid Experiment James Kring, Xingxing Ma, David Maurer, Gregory Hartwell, David Ennis The Compact Toroidal Hybrid (CTH) at Auburn University is being used to investigate the impact of vacuum transform (transform produced only by external coils) on density limit disruptions. While not fully understood, experimentally increasing the vacuum transform allows for plasma densities to surpass the empirical Greenwald Density Limit without using core fueling methods. Utilizing the available diagnostic suite including an interferometer, magnetic diagnostics, and SXR cameras, we present the phenomenology leading to a density limit disruption on CTH for shots taken from 2012 to 2016. Currently, an investigation is under way to study similarly disrupting shots with bolometer arrays. In conjunction with the bolometers, we present the development of a DeConvolutional Neural Network that was trained on synthetic data and used to reconstruct the bolometer emissivity from disruptions on CTH. [Preview Abstract] 

BP14.00026: Spectroscopic neutral density and electron temperature measurements in a microwave generated plasma with varying fractional ionization Eleanor Williamson, David Ennis, Gregory Hartwell, Curtis Johnson, Stuart Loch, David Maurer Understanding the coupling of fully ionized and neutrally dominated plasmas is important to the study of transition regions of the magnetosphere/ionosphere of the earth, corona/chromosphere of the sun, and up stream/detached divertors in fusion devices. Precisely characterizing the fractional ionization of the plasma requires accurately measuring neutral density in addition to the plasma density. We use an absolutely calibrated spectrometer coupled with results from a Collisional Radiative Model (CRM) solver to measure the neutral density in low temperature Compact Toroidal Hybrid plasmas heated by ECRH with up to 2 kW of input power. A triple probe and interferometer measure electron densities in the range of 1 x 10$^{\mathrm{17}}$ m$^{\mathrm{3}}$ to 1 x 10$^{\mathrm{19}}$ m$^{\mathrm{3}}$ and electron temperatures from 1 eV to 10 eV. A synthetic neutral argon spectrum from the CRM shows good agreement with the experimental spectra. Results will be presented from a study of varying the fractional ionization of the plasma for a variety of discharge conditions. [Preview Abstract] 

BP14.00027: Coherence Imaging Spectroscopy Validation on the Compact Toroidal Hybrid N.R. Allen, D.A. Ennis, G.J. Hartwell, C.A. Johnson, D.A. Maurer, C.M. Samuell, S.L. Allen Twodimensional profiles of lineintegrated impurity emissivity and velocity in the Compact Toroidal Hybrid (CTH) experiment are obtained from Coherence Imaging Spectroscopy (CIS), a polarization interferometry technique with fixed delays. CIS impurity flow measurements are beneficial for understanding the role of ion transport in the plasma edge and comparisons to modeling. Characterization of the CIS interferometer on CTH has shown that external environmental factors have less than a 1 km/s influence on the ion velocity measurements. Efforts to validate the CIS absolute ion velocity measurements in CTH have been made by comparisons with traditional dispersion spectroscopy along multiple sightlines yielding preliminary agreement within 3 km/s. CIS measurements of ion flow in differing plasma conditions, such as magnetic field configuration, applied loop voltage and majority ion species, will be presented. . [Preview Abstract] 

BP14.00028: Reconstructions of toroidal current profiles for bootstrap current discharges in Wendelstein 7X. John Schmitt, D. Maurer, N. Pablant, T. Andreeva, J. Geiger, S. Lazerson, U. Neuner, K. Rahbarnia, J. Schilling, H. Thomsen, Y. Turkin, and the W7X Team The reconstruction of the Wendelstein 7X (W7X) plasma equilibrium plays an important role in interpreting diagnostic signals and understanding the plasma. The reconstruction is iterative in nature, involving the repeated calculation of the MHD equilibrium and synthetic diagnostic signals and comparing these signals to measured signals. The parameters that describe the equilibrium (shape and location of the plasma boundary and profile information) are adjusted between iterations to find the bestfit of the measured and synthetic signals. These profiles are then used to interpret diagnostic information and for further physics analysis. The predicted evolution of the current profile is compared to the reconstructions constrained by magnetic diagnostics, Thomson Scattering, interferometry, xray imaging crystal spectroscopy, and boundary conditions based on the proximity to the 5/5island chain. The timeevolved current density profile, based on transport simulations of toroidal current using NTSS, is compared to reconstructions at several times during the bootstrap discharge. The sensitivity of the reconstruction to the current density profile and its parameterization and dependence on diagnostic constraints and initial profiles will be discussed. [Preview Abstract] 

BP14.00029: Coherence Imaging Spectroscopy Measurements of Ion Flows in the W7X ScrapeOff Layer D.M. Kriete, D.A. Ennis, D.A. Maurer, V. Perseo, D. Gradic, R. Koenig To investigate island divertor physics, 2D ion velocity measurements are made in the scrapeoff layer (SOL) of W7X using coherence imaging spectroscopy (CIS). The CIS technique encodes information about lineintegrated ion velocity and temperature into a spatial interference pattern that is overlaid on an image of the plasma emissivity. CIS has high spatial resolution and optical throughput, enabling detailed study of SOL dynamics in the complex magnetic island topology of W7X. Flow measurements are presented in the lowiota magnetic configuration, which exhibits a large 5/6 island chain at the plasma edge and is characterized by the longest average connection lengths in W7X. The structure and direction of the flow changes when the magnetic field direction is reversed, suggesting that particle drifts play an important role in SOL dynamics. A design is also presented for a new CIS system optimized for ion temperature measurements, which will be benchmarked by highresolution dispersive spectroscopy measurements and complement the existing CIS systems optimized for flow measurements. [Preview Abstract] 

BP14.00030: STRAHL modeling of impurity transport experiments with on and offaxis heating during the first divertor campaign on Wendelstein 7X P.J. Traverso, N.A. Pablant, A. Langenberg, R. Burhenn, Th. Wegner, B. Geiger, D. Zhang, B. ButtenschÃ¶n, H.M. Smith, J.D. Kring, J.C. Schmitt, D.A. Maurer In the first divertor campaign of Wendelstein 7X, iron impurity transport experiments were performed via laser blowoff injection during an on to offaxis ECRH scan at constant power. The iron line emission was measured by the xray imaging spectrometer systems, HRXIS and XICS, and the high efficiency XUV overview spectrometer, HEXOS. The spectral measurements show an increase in the global impurity transport time as ECRH power was deposited more offaxis. To understand this observed change, the 1D transport code STRAHL was employed to model the iron line radiation using anomalous diffusion and convection velocity profiles in addition to the neoclassical {\&} classical transport profiles provided by the drift kinetic equation solver, DKES. The observed lineintegrated iron emissivity was matched using a chisquared minimization by varying the anomalous diffusion and convective velocity profiles within STRAHL. Although in all cases the measured data could only be wellmatched with anomalous diffusion at levels 50 times larger than neoclassical predictions, the inferred transport uncertainties were too large to make clear comparisons between the on and offaxis cases. Therefore a sensitivity study using synthetic data was performed to better capture the systematic uncertainties. [Preview Abstract] 

BP14.00031: Optimized finitebuild stellarator coils using automatic differentiation Nick McGreivy, Stuart Hudson, Caoxiang Zhu Existing stellaratorcoildesign codes either solve a convex optimization problem or compute analytic or finitedifference derivatives to perform gradientbased optimization in nonconvex, highdimensional spaces. Historically, such codes have not optimized for finitebuild coils, instead assuming the coils are infinitely thin filaments. An active area of research in this field is to optimize the geometry of coils with nonzero thickness. In this article, optimized finitebuild coils are found using a multifilament approximation of the coilwinding pack. This new code, built on the existing FOCUS code, is called FOCUSADD. The derivatives of the finitebuild coils are computed  with almost no additional effort from the authors  using automatic differentiation. Automatic differentiation (AD) is an automatic method of computing numerical derivatives, which could both simplify the stellarator optimization process and efficiently optimize new targets whose analytic derivatives cannot be found. The coils are parameterized in free space using a Fourier series with a multifilament winding pack that has the freedom to move and twist in space. Optimization results for finitebuild coils are compared with filamentary coils. [Preview Abstract] 
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