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 CP19: Poster Session: Magnetic Confinement: MHD & Heating (2:00pm  5:00pm)On Demand

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CP19.00001: Simple, General, Realistic, Analytic, Tokamak Equilibria Jeffrey Freidberg, Luca Guazzotto The title of the abstract tells it all. We have extended the work of early authors to derive simple, general, realistic, analytic, solutions for tokamak equilibria as described by the GradShafranov equation. What do we mean by all of these adjectives? ``Simple'' refers to the fact that our equilibria contain only a few, intuitively simple terms. Specifically, 7 terms for updown symmetric systems and 12 terms for updown asymmetric systems. These values are not empirical but based on solid mathematical reasoning. ``General'' indicates that our equilibria are valid for a wide range of configurations, including smooth surfaces, double null surfaces, single null surfaces, finite aspect ratio including spherical tokamaks, finite elongation, finite triangularity, and finite beta. ``Realistic'' implies that our profiles are continuous and monotonic with the pressure, pressure gradient, and toroidal current density smoothly vanishing at the plasma edge. Finite edge pedestals in pressure and toroidal current density are also allowed. This is a quantum improvement over the Solovev profiles which always have finite pressure gradient and current density at the edge. Lastly, ``analytic'' indicates that our solutions are exact solutions to the GradShafranov equation, expressed in terms of known functions. Simple analytic expressions for the flux function and importantly its first and second derivatives have been derived. Examples for smooth, double null, and single null configurations will be presented. [Preview Abstract] 

CP19.00002: PIXIE3D simulations of nonlinear saturation of MHD modes Ioannis Keramidas, Luis Chacon, XianZhu Tang The breakup of flux surfaces and the onset of global field line stochasticity provide a natural path for enhanced transport loss thus leading to the thermal quench during a tokamak disruption. In the parallel transport dominated regime, fieldline qualities, such as the magnetic connection length of the open field lines that start in the core but terminate on the first wall and divertor plates, play an essential role in setting the transport level. The nonlinear saturation of MHD modes is the primary route to local and global field line stochasticity. Here, we use the nonlinear 3D MHD code, PIXIE3D, to perform initial value simulations from tokamak equilibria of ITER and existing machines that are unstable to a variety of large scale MHD modes, such as (1,1) kink, double tearing, tearing, and external kink. By using a simulation boundary that closely tracks the first wall, we include the plasma response from both inside and outside the magnetic separatrix. With the NEMATO fieldline tracing code, we further quantify the properties of the disrupted magnetic topology, calculating quantities such as fieldline connection lengths, Lyapunov exponents of neighbouring field lines, and particle diffusion coefficients. [Preview Abstract] 

CP19.00003: Physics Basis for Optimizing 3D Field Coils in Tokamaks Nikolas Logan, Caoxiang Zhu, JongKyu Park, SeongMoo Yang, Qiming Hu The multimodal, nonaxisymmetric plasma response to 3D fields in tokamaks can be predicted and the corresponding 3D fields can be optimized for maximum plasma performance. In this presentation, matrices relating external fields to the resonant response and torque response throughout the plasma are compared and contrasted across a wide variety of plasma scenarios. The fields required to localize the resonant response or torque in the edge or core (minimizing the effects elsewhere) are shown to be distinct from the fields that produce the largest global responses, providing insight for the design of distinct ELM control and EFC coils. The robust features of dominant eigenmodes are used together with stellarator design tools to optimize the geometry of 3D coils, increasing the efficient coupling of these coils to the physics of interest without undesired secondary effects. Importantly, efficient coupling can be maintained even when enforcing large distances between coils and the plasma during the geometric optimization of coil designs. The physicsdriven optimization presented here thus provides a practical path to utilizing exterior coils in future reactors to obtain the powerful 3D field benefits demonstrated on current machines with internal coils. [Preview Abstract] 

CP19.00004: Vessel Forces from a Vertical Displacement Event in ITER Stephen Jardin, Cesar Clauser Disruptions are one of the major concerns in ITER and other future tokamaks. A particularly troublesome type of disruption is a vertical displacement event (VDE) where control of the vertical position of the plasma column is lost. In addition to heat, particle flux, and energetic electrons impacting the first wall, significant electromagnetic loads will arise. For realistic modelling of a VDE disruption, an accurate 3D model of the disrupting plasma and the surrounding conducting structures is required. The structure affects the plasma evolution itself and the plasma acts as a source of currents and fields which produce the electromagnetic loads. Most of the VDE modeling work to date has used the axisymmetric evolving equilibrium codes TSC, DINA, and CarMa0N to describe the disrupting plasma. This paper describes more recent efforts to extend this analysis by using the fully 3D MHD code M3DC1. We have performed several longtime simulations of VDEs in a model of ITER with realistic structure timeconstants. We find that the horizontal (sideways) force depends strongly on the vessel timeconstant, and on the ability of the disrupting plasma to reach a state where the edge safety factor, q(a) \textless 1. This depends on the currentquench time, on the amount of halo current present, and on the rate at which the vertical displacement changes during the current quench. [Preview Abstract] 

CP19.00005: Influence of axisymmetric plasma shape on the plasma response to resonant magnetic perturbations S. Gu, C. PazSoldan, Y. Q. Liu, B. C. Lyons, D. Weisberg, Y. Sun, H. H. Wang, W. Suttrop, M. Willensdorfer, N. C. Logan, J.K. Park, D. A. Ryan, A. Kirk, Q. Ma, M. W. Shafer The nonaxisymmetric plasma response to resonant magnetic perturbations (RMPs) is investigated for DIIID, EAST and ASDEX Upgrade tokamaks using the MARSF code, by systematically varying the axisymmetric plasma shape in triangularity, whilst keeping other equilibrium quantities largely unchanged or deliberately scanning them. The magnetic plasma response decreases with triangularity in both resonant harmonics and sensor measurements. Comparison of experimental measurements and modeling results shows agreement in trends. Multimodal analysis of the simulation results extracts the mode structure and phasing dependence of each mode, and the amplitude of the dominant mode decreases with triangularity like the sensor measurements. Targeted comparisons with the GPEC code also reveal broadly similar trends in the resonant drive with shaping. The plasma response is strongly shielded at high triangularity compared to that at low triangularity, which implies different control effects of edge localized modes (ELMs) in shaping. This result suggests that it is more difficult to control ELMs at high triangularity, thus further coil optimization is required for better control ELMs. [Preview Abstract] 
Not Participating 
CP19.00006: Verification of Hermes2: A 2fluid plasma model including hot ions Nicolas DeStefano, Saskia Mordijck, Ben Dudson Hermes2 extends the Hermes1 to include hot ions. Hermes is built on the BOUT++ framework and is a 5/6field reduced 2fluid plasma model for the study of instabilities and turbulence in magnetized plasmas. It evolves global profiles, electric fields and flows on transport timescales, with fluxdriven crossfield transport determined selfconsistently through plasma turbulence. In order to verify Hermes2 captures the correct physics we will compare its results to analytic driftwave solutions. At first setting the ions to cold, we will reproduce the Hermes1 results, which have been verified. In the second part we will compare the impact of the hot ion component by including an ion temperature gradient and compare the Hermes2 results to analytic predictions for different wave lengths and changes in $\eta_i$, collisionality and ion temperature. [Preview Abstract] 

CP19.00007: Locked Mode stabilization in ITER Richard Nies, Allan H. Reiman, Eduardo Rodriguez, Nicola Bertelli, Nathaniel J. Fisch A strategy of stabilizing locked rather than rotating modes is shown to reduce the EC power requirement in ITER, also being less sensitive to radial misalignment and beam broadening. The short locking time in ITER [1] suggests attention might best be redirected to stabilizing locked modes, which was pioneered experimentally in [2] but received comparatively little attention. The power requirements for rotating and locked mode stabilization scenarios in ITER are obtained by simultaneously evolving the island width and rotation. The results motivate a reevaluation of the optimal EC toroidal launching angle. Finally, we show how current condensation [3] helps to stabilize large locked islands, by selfconsistently modeling the power deposition using raytracing and the island temperature [4]. [1] La Haye et al. Effect of thick blanket modules on neoclassical tearing mode locking in ITER. NF 57, 014004 (2017) [2] Volpe et al. Avoiding Tokamak Disruptions by Applying Static Magnetic Fields That Align Locked Modes with Stabilizing WaveDriven Currents. PRL 115 (2015) [3] Reiman & Fisch. Suppression of Tearing Modes by Radio Frequency Current Condensation. PRL 121, (2018) [4] Nies et al. Calculating RF current condensation with selfconsistent raytracing. Submitted to PoP (2020) [Preview Abstract] 

CP19.00008: Twofluid model of rf current condensation in magnetic islands Suying Jin, Allan Reiman, Nathaniel Fisch RF current condensation can stabilize NTMs in tokamaks with less total power and with less localization of the power [1]. The condensation effect relies on positive feedback between the RF deposition and the resulting island temperature perturbation governed by diffusion. The temperature diffusion has previously been modeled with singlefluid approaches [2,3]. However, in regimes where the ion and electron temperatures are not perfectly coupled, qualitatively new physics is introduced. [1] A. H. Reiman and N. J. Fisch, `` Suppression of tearing modes by radio frequency current condensation,'' Phys. Rev. Lett. 121, 225001 (2018) [2] E. Rodr\'{\i}guez, A. H. Reiman, and N. J. Fisch, `` Rf current condensation in magnetic islands and associated hysteresis phenomena,'' Phys. Plasmas 26, 092511 (2019). [3] S. Jin, N. J. Fisch, and A. H. Reiman, `` Pulsed RF schemes for tearing mode stabilization'' Phys. Plasmas 27, 062508 (2020). [Preview Abstract] 

CP19.00009: Fast sawtooth crash at $q\ge 1$ due to nonlinear interchange Linda Sugiyama Sawteeth at magnetic safety factor $q=1$ often have a fast final stage that expels plasma and energy to well outside $q=1$ and flattens the pressure and current density over the plasma center. It has a sudden onset and accelerates in time. It is nearly independent of resistivity. The final outflow is highly localized poloidally and at least somewhat toroidally. Similar fast crashes occur for double tearing modes (DTM) with two resonant $q=q_s$ surfaces. Analysis of the free energy $\delta W$ identifies the instability as an interchange mode driven by the normal magnetic curvature $\kappa_n$ at the outer $q_s$ surface, triggered when the hotter magnetic island(s) narrows sufficiently that $\kappa_n$ dominates the geodesic curvature and internal kink terms. Unlike the standard toroidal interchange instability, it can be independent of $\nabla p$. In forcefree or incompressible plasmas, it can be driven by the local $J_\perp$, consistent with its existence in 2field reduced MHD simulations of the DTM. If $\nabla p$ exists, it can also contribute to the mode. Fully toroidal MHD simulations with the M3D code illustrate the interchange nature of the fast crash for internal kink and quasiinterchange $q=1$ sawteeth. [Preview Abstract] 

CP19.00010: Nonresonant pressuredriven MHD instabilities in equilibria with low magnetic shear Adelle Wright, Nate Ferraro, Stuart Hudson, Robert Dewar, Matthew Hole We present a new method for investigating nonresonant MHD instabilities that is based on exploiting the rich numbertheoretic structure of the equilibrium qprofile, and use it to investigate pressuredriven instabilities and central pressure gradient accumulation in tokamaklike equilibria with low central shear in which q>1 and no moderational surfaces are present. This method predicts a banded spectrum of growth rate versus toroidal mode number for these modes, which are believed to be resistive infernal modes. Parameter scans using M3DC1 confirm this prediction. It is found that these modes have robust growth and a broad radial mode structure, which may cause central pressure profile flattening in highbeta tokamak equilibria. The banded growth rate spectrum, which has also been observed in previous studies of ideal infernal modes, leads us to suggest that some of the characteristics of infernal modes in lowshear tokamak scenarios may arise due to the nonresonant effects, rather than directly because of toroidicity. Our results also suggest a possible scenario which may enable predictions of the radial extent of temperature and pressure flattening in postsawtoothcrash profiles. [Preview Abstract] 

CP19.00011: Study of MHD spectroscopy by using RMP coils in small tokamak device HYBTOKII Yoshihide Shibata, Masaaki Okamoto, Kiyomasa Watanabe, Noriyasu Ohno, Go Matsunaga, Yusuke Kikuchi Stability analysis of MHD mode is necessary to investigate the stability state of MHD instabilities. The accurate plasma parameter profiles are important for stability analysis. However, it is hard to evaluate these parameters during the plasma discharge, and a realtime evaluation of stability analysis of MHD mode is impossible. MHD spectroscopy, measurement of a frequency response of MHD instability on the external RMP fields, is useful information to investigate the stability of MHD mode in experiment. The stability of MHD mode can be estimated from a damping rate without a stability analysis. In this study, plasma responses and plasma parameters around a resonant surface is investigated during MHD spectroscopy by using the internal plasma measurement in HYBTOKII. Plasma current rampup experiment (610kA) was carried out for MHD spectroscopy. A radial magnetic fluctuation (Br) outside the plasma synchronized with coil current of RMP was measured by a lockin amplifier. Br in rampup exp. was increased with the decrease of the safety factor at the plasma edge. It is considered that increase of the plasma response is generated by the reaction between MHD instability in plasma and RMP. In the presentation, we will show detail analyzed data of MHD spectroscopy. [Preview Abstract] 

CP19.00012: Evolution of plasma density structures during sawtooth events in DIIID D. Liu, W. Fox, S. Bose, Z. Yan, G. McKee, A. Goodman, H. Ji, V. Igochine, Y. Zhu, S. Jardin, N. Ferraro The sawtooth crash in fusion plasmas leads to a fast drop in core electron temperature, and it may be associated with rapid and explosive plasma phenomena in solar and space plasmas. Several models have been proposed for the fast temperature evolution during sawteeth, including fast magnetic reconnection provided by twofluid effects, or the plasmoid instability, or growth of secondary MHD instabilities such as ballooning instabilities. Experiments were conducted at DIIID through the Frontier Science program where the Beam Emission Spectroscopy (BES) system was used for 2D localized density measurements at the sawtooth region during sawtooth events in DIIID to understand the role of these mechanisms. We obtained 2D timedomain movies of the evolution of $\frac{\delta n}{n}$ during sawteeth through 1) cross calibration of the time response and gain for each channel; and 2) careful subtraction of background and plasmaedgelight to isolate the density response in the plasma core. We compare the BES density observations with 3D mode structures determined by external magnetic probes. Techniques of our analysis are presented and results over numerous shots are analyzed and compared to predictions for the competing models. [Preview Abstract] 

CP19.00013: Energetic ion effects on tearing mode stability in tokamaks: magnetic shear and beta are key D.P. Brennan, C. Akcay, C.C. Kim, J.M. Finn Simulations of the effect of a slowing down distribution of energetic ions on the onset of resistive MHD instabilities are presented where the ions can either stabilize or destabilize disruptive tearing modes, depending on the magnetic shear in the core. The strength of the effect depends on the fraction of beta, the ratio of kinetic energy to magnetic energy, that is in the energetic ions. Two cases are compared, one with monotonic shear throughout the profile (q\textunderscore min $=$ 1.1) and one with reversed shear in the core (q\textunderscore min $=$ 1.3). Outside of the reversal surface the equilibrium profiles are nearly identical between the two cases. The drive from energetic ions is stabilizing in monotonic shear and destabilizing in reversed shear, consistent with previous theory. [Preview Abstract] 

CP19.00014: Axisymmetric Stability in Ferromagnetic Tokamaks Oliver Bardsley, Tim Hender In the first generation of fusion power plants, ferritic steels are attractive candidate materials for structural elements of first wall and breeding regions. These materials, however, are ferromagnetic and uncertainties remain as to the magnitude of the negative effects this may have on plasma confinement, particularly with the significant quantities of steel required. Vertical stability and control in tokamaks is an important design concern, particularly so at high plasma elongation. We present an analytical model of vertical displacement events in tokamaks with resistive walls, which assesses the modification to their passively stabilising influence when ferromagnetic effects are included. A dichotomy is found in the dependence of the instability growth rate upon the material’s magnetic permeability. For rapid instabilities, the induced currents penetrate only part way through the wall and its passive stabilising force is depleted. For slower instabilities, the wall is magnetically thin and the ferromagnetic field evolves in a quasisteady manner, providing an additional destabilising influence. In either case, the growth rate is increased by a factor on the order of the effective relative magnetic permeability ($\sim2$). [Preview Abstract] 

CP19.00015: Freeboundary MRxMHD equilibrium calculations using the steppedpressure equilibrium code Stuart Hudson, Joaquim Loizu, Caoxiang Zhu, Zhisong Qu, Carolin Nuehrenberg, Sam Lazerson, Chris Smiet, Matthew Hole The steppedpressure equilibrium code (SPEC) (Hudson et al. 2012 Phys. Plasmas 19, 112 502) is extended to enable freeboundary multiregion relaxed magnetohydrodynamic (MRxMHD) equilibrium calculations. The vacuum field surrounding the plasma inside an arbitrary ‘computational boundary’, D, is computed, and the virtualcasing principle is used iteratively to compute the normal field on D so that the equilibrium is consistent with an externally produced magnetic field. Recent modifications to SPEC are described, such as the use of Chebyshev polynomials to describe the radial dependence of the magnetic vector potential, and a variety of freeboundary verification calculations are presented. [Preview Abstract] 

CP19.00016: The DCON3D Code for the Ideal MHD Stability of SteppedPressure Stellarators Alan Glasser In a recent publication, [Phys. Plasmas 27, 042509 (2020); https://doi.org/10.1063/1.5143455], a procedure was presented to determine the ideal MHD stability of steppedpressure stellarators by the generalized Newcomb method. The EulerLagrange equation (ELE) for making the energy functional $\delta W $stationary is derived as a highorder ordinary differential equation for the complex Fourier components \textbf{U} of the perturbed vector potential $\alpha $ and its derivatives. The related Hermitian Riccati matrix \textbf{P} $=$ \textbf{U}$_{\mathrm{22}}$ \textbf{U}$_{\mathrm{11}}^{\mathrm{1\thinspace }}$is derived. The vanishing of the real scalar $D_{C} \quad =$ det \textbf{P}$^{\mathrm{1}}$ is the condition for the existence of a fixedboundary instability. This procedure has been implemented in a new Fortran 95 code DCON3D. Data are read from a SPEC stellarator equilibrium file. [Phys. Plasmas 19, 112502 (2012); \underline {https://doi.org/10.1063/1.4765691}] In each volume and each interface, components of the EulerLagrange coefficients are computed and the equation is numerically integrated. There are two departures from the paper: native SPEC coordinate (s,$\theta $,$\zeta )$ are used throughout rather than straightfieldline coordinates; and the Riccati equation for \textbf{P}$^{\mathrm{1}}$ rather than the ELE for \textbf{U} is integrated for improved numerical stability. Examples will be presented for an equilibrium with periodicity l $=$ 5, 8 volumes, and finite $\beta $, in which a Newcomb crossing is found in the second interface. The code runs in two minutes of cpu time on a MacBook Pro. [Preview Abstract] 

CP19.00017: GradShafranov equation for nonaxisymmetric MHD equilibria Joshua Burby, Nikos Kallinikos, Robert MacKay The structure of static MHD equilibria that admit continuous families of Euclidean symmetries is well understood. Such field configurations are governed by the classical GradShafranov equation, which is a single elliptic PDE in two space dimensions. By revealing a hidden symmetry, we show that in fact all nondegenerate solutions of the equilibrium equations satisfy a generalization of the GradShafranov equation. In contrast to solutions of the classical GradShafranov equation, solutions of the generalized equation are not automatically equilibria, but instead only satisfy force balance averaged over the oneparameter hidden symmetry. We then explain how the generalized GradShafranov equation can be used to reformulate the problem of finding exact threedimensional smooth solutions of the equilibrium equations as finding an optimal volumepreserving symmetry. [Preview Abstract] 

CP19.00018: Extending M3D$C^1$ to stellarator geometry: preliminary results Yao Zhou, N. M. Ferraro, S. C. Jardin, H. R. Strauss Stellarator plasmas have been observed to be nonlinearly stable even when driven beyond linear MHD stability thresholds. Hence, stellarator designs could employ nonlinear stability considerations to relax linear stability constraints, which can often be too restrictive and costly. However, this possibility has not been systematically investigated due to the lack of a stateoftheart nonlinear initialvalue MHD code for stellarators. We aim to fill this gap by extending the M3D$C^1$ code from tokamak to stellarator geometry. Our approach introduces a set of logical coordinates, in which the computational domain is axisymmetric, to utilize the existing finiteelement framework. The mapping between the logical and the physical $(R, \phi, Z)$ coordinates is then used to calculate derivatives in the latter, in terms of which the existing physics equations are written. This way, no significant changes to the extendedMHD models within M3D$C^1$ are required. So far, we have successfully implemented this approach in 2D and verified its results against the existing code. Preliminary results from 3D implementation will also be presented and discussed. [Preview Abstract] 

CP19.00019: Nonlinear MHD studies of sawtoothing and stationary tokamak states using NIMROD K. J. McCollam, B. E. Chapman, M. D. Pandya, J. S. Sarff, C. R. Sovinec, D. L. Brower, J. Chen, R. Yoneda, W. X. Ding Using the extendedMHD code NIMROD, we simulate tokamak dynamics with the aim of exploring conditional boundaries between sawtoothing and nonsawtoothing states. Our previous singlefluid viscoresistive MHD studies with rectangular tokamak cross sections at Lundquist numbers of $S\approx10^5$ showed characteristically different $n=1$ linear mode structures in magnetic field and flow for zero and finitebeta ($\sim 1\%$) cases. Nonlinear evolution of zerobeta cases exhibited quasiperiodic sawtoothlike events moving the core safety factor $q$ profile above 1 from below. However, spectral pileup prevented successful nonlinear simulations in finitebeta cases, which is ascribed to poor flux surfacespatial grid alignment for the rectangular grids. Thus we are developing elliptically and circularly gridded cases, which will have better fluxgrid alignment. Characterizing linear stability for these cases for zero and finite beta will be followed by nonlinear simulations. [Preview Abstract] 

CP19.00020: Spatial variation of density fluctuation during sawtooth oscillations in DIIID S. Bose, W. Fox, D. Liu, A. Goodman, Z. Yan, G. McKee, H. Ji, S. Jardin, N. Ferraro, V. Igochine, Y. Zhu Sawtooth oscillations are internal relaxation events in a tokamak which lead to a rapid drop of core electron temperature. A significant question for sawtooth oscillations is the short crash time, which, in the traditional Kadomtsev model, is related to how fast reconnection can occur to rearrange the magnetic field. Several possible competing mechanisms have been proposed for the fast crash, including the twofluid effects near the reconnection layer, plasmoid instability, and interchange instability. To understand the role of these processes we have measured the plasma density evolution during sawtooth oscillations using the BeamEmission Spectroscopy diagnostic (BES). Our analysis of BES data obtains 2D images of plasma density near the sawtooth inversion layer. The results show large intensity variations from the 1/1 rotating magnetic structure, suggesting $\delta n/n$ may be as high as 0.2. Comparison of the 2D image of density variation near the inversion layer for a number of sawtooth events having 0.5 and 1.5T toroidal field are presented. We discuss the implication of these results on the existence and nature of magnetic reconnection during sawteeth. [Preview Abstract] 

CP19.00021: Coupled CFD/MHD Simulations of Plasma Compression by Resistive Liquid Metal Paria MakaremiEsfariani, Peter de Vietien In this work we present coupled CFD/MHD simulation results of General Fusion's prototype plasma compression system. The results are obtained from coupling two opensource solvers, both of which have been modified inhouse. The first solver is an OpenFOAM hydrodynamic (compressible) solver that has been modified by adding forces from the poloidal and toroidal magnetic fields. The second solver is the Versatile Advection Code (VAC), an astrophysical magnetohydrodynamics (MHD) code that models the plasma during compression. VAC supplies OpenFOAM with the magnetic fields for the liner evolution, and OpenFOAM supplies VAC with the position of the liner during compression. We use this coupled code to simulate how various liner trajectories, different initial plasma states, and experimentally derived transport coefficients affect plasma and machine performance. In particular, we investigate how magnetic flux diffusing from the plasma to the liquid metal liner affects compression results. [Preview Abstract] 

CP19.00022: Effects on Stable MHD Region of a Magnetized Target Plasma Compression Aaron Froese, Dylan Brennan, Sandra Barsky, Meritt Reynolds, Zhirui Wang, Michel Laberge General Fusion is designing a magnetized target fusion reactor to compress a toroidal plasma inside a liquid metal cavity and heat it to fusion conditions. Plasma compression in realistic geometry is modelled as a series of equilibrium states generated by CORSICA. The resistive and ideal MHD stability of each equilibrium is analyzed using the resistive DCON code. We find plasma conditions that are stable to high compression and show how their range is affected by geometry and current density effects. The results are confirmed for representative cases with a timedependent MHD simulation. Stability is found to be strongly affected by the current density near the plasma edge. Due to the solid central shaft and its effect on plasma elongation, compression reduces the current density near the edge to conserve the q profile. However, this effect is offset by magnetic field diffusion into the liquid metal. At high compression ratios, selfsimilar compression geometries are ideal MHD unstable for intermediate n, but a central shaft and wall resistivity are both found to be stabilizing. [Preview Abstract] 

CP19.00023: Synthetic phase contrast imaging diagnostic for the measurement of helicon waves in the DIIID tokamak S. S. Denk, A. Marinoni, M. Porkolab, J.C. Rost, C. Lau, R. I. Pinsker A synthetic diagnostic is being developed for the phase contrast imaging (PCI) diagnostic on DIIID to aid in measuring the amplitude and structure of highpower helicon waves [R. I. Pinsker et al, Nucl. Fusion 58, 106007 (2018)]. These waves have been predicted to be an efficient source for offaxis current drive. To resolve the width, intensity and wave number spectrum of the helicon beam a novel RF beam modulation system is being integrated into the absolutely calibrated DIIID PCI diagnostic. Integrating this synthetic PCI diagnostic with a full wave code like AORSA [Jaeger et al, Phys. Plasmas 8 (2001) 1573] allows validation studies with DIIID helicon experiments. AORSA incorporates the full hot dielectric tensor allowing it to accurately model the helicon wave propagation in DIIID at around the 50th ion cyclotron harmonic. This approach has high computational costs, however, especially when the full toroidal mode number spectrum is modeled for the synthetic PCI diagnostic. To allow sensitivity studies and predictfirst modeling for experimental planning, a reduced cold plasma full wave model and the raytracing code GENRAY will be benchmarked against AORSA, including for the first time the full spectrum of toroidal mode numbers, for an experimental case of interest. [Preview Abstract] 

CP19.00024: Optimization of the RFperformance of the DIIID high power helicon antenna Bart Van Compernolle, M. W. Brookman, C. Moeller, R. I. Pinsker, R. O'Neill, A. Garofalo, C. C. Petty Helicon current drive, also called fast wave current drive in the lower hybrid range of frequencies, has long been regarded as a promising current drive tool for reactor grade plasmas. A newly installed MWlevel system at DIIID will be the first test of this technology in reactorrelevant plasmas, where full singlepass absorption is expected. A 30module traveling wave antenna has been installed and optimized invessel. The linear electromagnetic characteristics of the unloaded module array have been extensively tested both on the bench and in the vessel at instrumentation power levels. As power is transferred down the antenna through mutual coupling from one module to the next, it was found that adjusting the spacing between modules is crucial for mitigating undesirable effects arising from the lack of toroidal antenna symmetry. Bench testing and modeling of the antenna in different configurations clearly showed the adverse effect of spatial modulation in the mutual coupling between modules. Complete results of the bench testing and optimization procedure will be presented. In the final invessel installation, excellent performance has been achieved, less than 2{\%} reflected power and 1.4{\%} dissipated power per module in air, in a 10 MHz band around the operating frequency of 476 MHz. [Preview Abstract] 

CP19.00025: Diagnosing Helicon and Lower Hybrid Wave Coupling with the Edge Plasma for Current Drive Optimization in DIIID Using Laser Spectroscopy E.H. Martin, C. Lau, A.M. Garofalo, M.W. Brookman, B.Van Compernolle, R.I. Pinsker, S.J. Wukitch, S. Shiraiwa, A.Y. Pankin, D.N. Smithe Over the next several years, the operational space of two novel RF actuators designed for offaxis current drive will be extensively explored in the DIIID tokamak. The goal of these programs is focused on evaluating the potential for efficient current drive in advanced tokamak scenarios. Previous experimental work on CMod and NSTX determined that wave coupling with the scrapeofflayer (SOL) plasma can result in substantial core power loss. However, recent computational studies indicate that the SOL plasma can be optimized to minimize the undesired wave/SOLplasma coupling, A diagnostic based on Dopplerfree saturation spectroscopy (DFSS) has been proposed for direct measurement of the wave's electric field vector (\textbf{E}$_{\mathrm{\mathbf{RF}}})$ in the edge plasma of DIIID. The DFSS diagnostic was designed to provide a local measurement over a 2D region with mmscale spatial resolution and \textless 10 V/cm electric field resolution. The measured 2D \textbf{E}$_{\mathrm{\mathbf{RF}}}$ data will be directly compared with 3D fullwave simulations to quantitatively identify and characterize wave/SOLplasma coupling. Utilizing 3D fullwave simulations the expected \textbf{E}$_{\mathrm{\mathbf{RF}}}$ in DIIID will be discussed and results from mockup performance validation testing of the DFSS diagnostic at ORNL will be presented. [Preview Abstract] 

CP19.00026: Importance of resonant wavefilament interactions for HHFW, helicon, and LH current drive in tokamaks Cornwall Lau, Elijah Martin, Nicola Bertelli, Syun'ichi Shiraiwa, Wouter Tierens, Michael Brookman, Robert Pinsker, Bart Van Compernolle, Abhay Ram, Greg Wallace, Andris Dimits, Jim Myra, Steve Vincena, Xiaokang Yang A significant challenge in tokamaks is to demonstrate efficient current drive in advanced tokamak scenarios for steadystate operation of future reactors. Recent analytical work has hypothesized that the interaction of filaments with waves in the SOL of plasmas can create resonant wavefilament interactions that may lead to large SOL power absorption of waves and reduced current drive efficiency. The potential impact of these resonant wavefilament interactions on current drive schemes such as high harmonic fast waves on NSTXU, helicon waves on DIIID, and lower hybrid waves on Alcator CMod will be presented. Both the cylindrical analytical model and experimentally constrained toroidal fullwave computational models suggests that these resonant wavefilament interactions exist for the abovementioned current drive schemes and may trigger large wave SOL losses in tokamaks. Future work to measure these resonant wave fields on the LaPD linear plasma experiment and current tokamaks will be discussed. [Preview Abstract] 

CP19.00027: Improved heating efficiency of ECH near cut off density with a new injection scheme in the LHD Masaki Nishiura, S. Kubo, Y. Yoshimura, R. Yanai, S. Ito, H. Igami, T. Ii Tsujimura, T. Shimozuma, H. Takahashi, N. Kenmochi We proposed a new electron cyclotron heating (ECH) system from a high field side injection in the Large Helical Device (LHD). High reliable and functional ECH system enhances plasma parameters, and performs the wide variety of physics experiments. The new method is an injection of EC beam perpendicular to the magnetic fields from an outer coil case on the vertically elongated plasma shape of the LHD. The heating efficiency for 77 GHz EC wave with both Omode and Xmode is evaluated by the ray trace calculation of TRAVIS code. We found that the new injection scheme keeps the high heating efficiency up to the cut off density for the Omode by reducing the beam refraction, compared with the conventional ECH. In the case for the Xmode with the same launching position, the oblique injection maintains the high heating efficiency in the electron density from 2x10$^{\mathrm{19\thinspace }}$m$^{\mathrm{3}}$ to 1x10$^{\mathrm{20}}$ m$^{\mathrm{3}}$. The density regime exceeds the cut off density of 77 GHz. The details of the new method will be presented. [Preview Abstract] 

CP19.00028: 3D Full Wave Modeling of ECRH RF Beams in Fusion Devices Vladimir Svidzinski, Liangji Zhao, JinSoo Kim High resolution solution of wave equations in frequency domain in the electron cyclotron frequency range for realistic fusion plasma parameters became feasible with the use of recently formulated hybrid iterative approach [Svidzinski et al, PoP 2018] for solving discretized wave equations. This approach combines time evolution and iterative relaxation techniques into iteration cycles. 3D iterative RF beams simulation tool, implementing this algorithm, is being developed. Dynamic grid adaptation is applied to cover only the part of the volume where the beam is localized. Initial results of full wave 3D RF beam simulations in DIIID in the cold plasma model have demonstrated the capability of this algorithm to model the entire ECRH RF beams in fusion devices on modern supercomputers. Resolution with 100 B degrees of freedom is demonstrated at NERSC. The new tool allows to address RF beams physics not covered by paraxial approximation: diffraction, scattering, mode conversion, interference between modes, propagation in evanescent layers, beam splitting, and evolution of beam section in anisotropic plasma. Details of the algorithm, of the grid adaptation approach, the details of numerical implementation and initial results of modeling of 3D RF beams in DIIID will be presented. [Preview Abstract] 

CP19.00029: Modeling of helicon antenna in DIIID using the VSim code A.Y. Pankin, D.N. Smithe, M.W. Brookman, B. Van Compernolle, A.M. Garofalo, E.H. Martin, R.I. Pinsker, C.M. Roark The helicon antenna recently installed in the DIIID tokamak can become a valuable current drive actuator in future experiments. According to previous modeling results [R. Prater et al. NF {\bf 54} (2014) 083024], the propagation of helicon waves in the plasma core is sensitive to different parameters. In this study, we report the results of the helicon waves propagation computed with VSim [C. Nieter, J.R. Cary, JCP {\bf 196} (2004) 448] both for the prototype and final helicon antenna designs. Analytical fits to the plasma profiles from the DIIID shot 165908 are used in these simulations. Several scans with the pedestal density gradient and outer gap for a simplified antenna geometry are conducted. It is demonstrated that the penetration of fast wave in the plasma core is reduced when the outer gap is increased. In a scan with the poloidal field, an offset between the magnetic field and the antenna polarization is introduced which is equivalent to the introduction of a poloidal field in the SOL to cause the polarization mismatch. A small level of mode competition with the slow wave propagating in SOL before encountering a lowerhybrid resonance near the pedestal base is observed. Development of a surface wave that carries energy along the plasmawall interface is demonstrated. [Preview Abstract] 

CP19.00030: Interaction between High Power RF Waves and Turbulence in LAPD Joshua Larson, Troy Carter, Gurleen Bal, Bart Van Compernolle Experiments have been conducted on the Large Plasma Device (LAPD) to study the effects of high power radio frequency (RF) wave injection from an edge mounted antenna. During these experiments the modulation of core fast waves was observed in the plasma that was not reflected in the antenna current signal. This work investigates the correlation between the core fast wave fluctuation spectrum and the edge density fluctuation spectrum. Strong correlation in the aforementioned fluctuations was observed, particularly at lower frequencies. As RF power was increased the edge density fluctuations also increased. This work explores the mechanisms for power transfer from RF to broadband turbulence and semicoherent features seen at the antenna face. Drift wave instabilities, KelvinHelmholtz instabilities, and threewave interactions are investigated. [Preview Abstract] 

CP19.00031: Validation of Petram fullwave simulation code via RF Wave propagation experiments on LAPD Gurleen Bal, Kunal Sanwalka, Bart Van Compernolle, Troy Carter, Syun'ichi Shiraiwa A 3D fullwave code, Petram, was used to simulate wave propagation from a single strap RF antenna in the Large Plasma Device (LAPD). PetraM simulations allow for importing 3D CAD models of the antenna used in the experiments as well as measured density profiles. The results of the simulations show good agreement with experiments done on the LAPD. The experiments were carried out in a magnetized helium plasma with plasma parameters $n_e$ $\approx$ $10^{18}$ – $10^{19}$ $m^{3}$, $T_e$ ~ 1 – 10 eV and $B_0$ $\approx$ 0.1 to 0.18 T. A standing wave can be observed directly in front of the antenna in both the simulations and experimental results. Simulation results show fastwave propagation in the dense core which is characteristic of a counterclockwise rotating m=1 like mode. This work will also present some preliminary results comparing the short wavelength, slowwave propagation in the plasma edge, lowdensity region. With an improved solver and access to finer mesh elements, we should now be able to resolve the slowwave edge dynamics. Validating the simulated edge interaction with experimental results plays an important role in helping us understand interactions between RF waves and the SOL region of fusion devices. $^1$S. Shiraiwa et al, EPJ Web Conf. 157, 03048 (2017) [Preview Abstract] 

CP19.00032: Improvements to the Folded Waveguide Antenna for ICRF Heating of Fusion Reactors. Tim Bigelow, Chuck Kessel, Greg Hartwell The Folded Waveguide (FWG) has been proposed as an improved high power density Ion Cyclotron Heating antenna that offers the advantages of a waveguide such as ceramicfree all metal construction, low electric fields at the plasma edge, builtin impedance matching, low mutual coupling in arrays. With recent interest in reactor relevant fusion experiments, the further testing of the FWG will offer significant capability particularly for highfield ICRF heated plasma devices. Proposed plasma testing at ORNL and Auburn Univ. will provide nearterm physics results for high plasma density coupling performance that is needed for further development and acceptance of this type of antenna. A travelingwave version of the FWG has been devised for use in certain applications, such as helicon wave launch and fast wave current drive, where a toroidal array of slots can be phased appropriately for optimum coupling to a directional plasma wave. Recent developments in additive manufacturing technologies have the potential to both simplify the fabrication of a complicated FWG face plate with interior helium gas cooling channels and improve its performance by permitting variation in metals between the inside rf and outside plasma facing regions. An additional feature that the FWG could offer is the use of tritium breeding ceramics to fill all or part of the antenna structure to further increase the neutron and energy performance of a fusion reactor. [Preview Abstract] 
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