Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Plasma Physics
Volume 66, Number 13
Monday–Friday, November 8–12, 2021; Pittsburgh, PA
Session GP11: Poster Session III:
Poster
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Room: Hall A |
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GP11.00001: MFE:M-D AND STABILITY; C-2W AND OTHER FRC
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GP11.00002: A hybrid full-wave Markov chain approach to calculating radio-frequency wave scattering from Scrape-off Layer filaments Bodhi Biswas, Syun'ichi Shiraiwa, Seung Gyou Baek, Paul T Bonoli, Abhay K Ram, Anne E White Radio-frequency (RF) wave scattering from turbulence modifies the wave-spectrum, and affects RF heating and current drive (CD) in tokamaks. For Lower Hybrid (LH) launch, scatter from scrape-off layer (SOL) density fluctuations may bridge the spectral gap at low density, and trigger a drop in CD efficiency at high density. Previous scattering models have been limited to weak turbulence treatments [1], or ray-tracing in more realistic, filamentary SOL turbulence [2]. Here, a new model is introduced which retains full-wave scattering effects from filamentary turbulence. A Mie-scattering technique models a single wave-filament scattering event [3]. Next, a Markov chain accounts for several of these events. The resulting wave-spectrum is asymmetrically broadened in wavenumber angle-space; this is not accounted for in previous LH scattering models. The broadened wave-spectrum is modeled in GENRAY/CQL3D to study its impact on CD. For a low density C-Mod discharge, this results in significantly increased on-axis current, and decreased off-axis peaks. This is attributed to a portion of the wave-spectrum that strongly damps on-axis during first-pass. |
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GP11.00003: Finite-Element Solution of a Vorticity Transport Model Including RF Antenna Effects and Application to Scrape-Off-Layer-Turbulence Simulations Andris M Dimits, Thomas D Rognlien, Maxim V Umansky, Mark L Stowell, James R Myra, David N Smithe, Milan Holec, Thomas G Jenkins, Chris J Vogl, Ilon Joseph Radio-frequency (RF) heating and current drive are important for magnetic fusion devices. The associated antenna structures can result in large rectified sheath potentials and ponderomotive forces, which can drive large plasma flows. These flows can drive or suppress turbulence in the scrape-off layer (SOL), which can significantly affect the RF-wave coupling to and propagation through the plasma. To model these plasma flows with the complicated (material) boundary shapes and boundary conditions needed, a drift-reduced fluid-plasma model has been implemented in the the COMSOL Multiphysics finite-element (FEM) package, and in the more scalable Modular Finite-Element (MFEM) package. Steady-state solutions have been obtained with model RF-antenna-structure boundaries and RF-sheath boundary conditions, so far in a simplified two-dimensional, axisymmetric geometry. The values of the potential on a flux surface contained in the FEM simulation domain can be used as boundary conditions in turbulence codes such as the BOUT++-based code SOLT3D. Progress on the generalizing the FEM-based implementations to complicated three-dimensional domains and boundary conditions, model ponderomotive sources, and comparison against and coupling with relevant BOUT++ codes will also be reported. |
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GP11.00004: Modeling RF-induced ponderomotive effects on edge/SOL transport with UEDGE and Vorpal Thomas G Jenkins, David N Smithe, Maxim V Umansky, Thomas D Rognlien, Andris M Dimits EM fields driven by ICRF antennas in the tokamak plasma edge, injected to heat the core plasma, also give rise to transport-timescale ponderomotive effects. Together with the conventional grad(v^2) ponderomotive force, additional terms dependent on density gradients, species charge signs, collisionality, and incident wave polarization may arise; these introduce additional vorticity, energy, and parallel momentum sources to the edge/SOL transport. For large antenna input powers and edge density gradients (e.g. H-mode), RF-induced ponderomotive physics may be large enough to appreciably influence edge plasma dynamics, e.g. by expelling density from, or inducing convective cells in, the region near the antenna. We investigate these effects using Vorpal (FDTD EM+plasma solver) and UEDGE (2D edge plasma transport code). Contributions of various source terms (vorticity, energy, momentum) are assessed with independent UEDGE and Vorpal simulations. RF-induced parallel momentum is estimated to exert greater influence on edge/SOL dynamics than vorticity/energy sources are in representative ICRF heating scenarios. Detailed calculations with the RF parallel momentum source, in which Vorpal and UEDGE are coupled in 2D, explore and quantify the roles its effects might play in edge transport. |
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GP11.00005: Self-consistent reduced model for energetic ion tail formation by ion cyclotron range of frequency (ICRF) power Paul T Bonoli, Jungpyo Lee, Donald B Batchelor, Nicola Bertelli, Samuel Frank, David L Green, Francesca M Poli, John C Wright A high fidelity simulation capability for self-consistent minority ion heating, such as the combined AORSA-CQL3D model [1], has been available for about 15 years. However, these type of models tend to be impractical for implementation in time dependent integrated modeling frameworks because of their computational requirements. Here we revisit previous work [2, 3] where TORIC and CQL3D were coupled iteratively through a quasilinear diffusion coefficient formulated in the ion finite Larmor radius (FLR) limit in TORIC and through the nonthermal ion distribution from CQL3D. The TORIC solver employs a reduced model for the ion conductivity valid in the FLR limit, thus greatly reducing the computational requirements relative to AORSA. The Python-based Integrated Plasma Simulator (IPS) [4] will be used to couple TORIC and CQL3D thus providing the test of a workflow that could be used in an integrated modeling calculation. |
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GP11.00006: Recent Results from the SciDAC Partnership for Simulation of Fusion Relevant RF Actuators David L Green, Paul T Bonoli, Nicola Bertelli, Andris M Dimits, Tzanio Kolev, David N Smithe, Robert W Harvey, James R Myra, Mark S Shephard, Davide Curreli, Lin Mu We present an overview of research related to predicting the behavior of the plasma boundary region in the response to applied radio frequency (RF) power in the ion-cyclotron and lower-hybrid range of frequencies. We focus on the production of impurities via RF sheath enhancement and the non-linear, multi-timescale response of the scrape-off-layer plasma to RF power. Particular results include: (i) a successful multi-implementation benchmarking of the non-linear fluid RF sheath boundary condition, additional verification of that fluid condition with fully-kinetic calculations of the same, and estimates of impurity sputtering as a result for simple geometries; (ii) calculations of the RF ponderomotive force driving terms in full high fidelity 3D geometry in front of an ICRF antenna and initial estimates which show that force as a significant source on density transport; (iii) benchmarking of the combined use of high-order finite-elements with field-aligned meshes towards a new far-scrape-off-layer fluid transport simulation capability; and (iv) demonstrated progress on the effective preconditioning of iterative solvers for the frequency-domain Maxwell equations for larger degree-of-freedom, high geometric-fidelity simulations. |
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GP11.00007: The Effects of Parallel Inhomogeneity in Lower Hybrid Current Drive RF Heating Experiments Samuel Frank, John C Wright, Jungpyo Lee, Paul T Bonoli The effect of parallel inhomogeneity in the magnetic field on cyclotron wave damping in magnetic confinement fusion experiments has long been recognized [1,2]. Introduction of inhomogeneity broadens resonances and induces non-local energy transfer. This effect has been included for cyclotron damping in spectral full-wave codes for many years [3]. However, only recently has parallel inhomogeneity’s influence on Landau damping been calculated [4]. We extend previous analyses of parallel inhomogeneity induced broadening to arbitrary distribution functions and derive an explicit Landau resonance broadening function. We then integrate the function into TORLH full wave tokamak current drive simulations to determine the influence of inhomogeneity in lower hybrid current drive experiments. We also consider the inclusion of the effects of parallel inhomogeneity in the formulation of the quasi-linear diffusion coefficient (DQL) using a modified version of the bounced averaged DQL from [5]. |
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GP11.00008: Development of Impedance Sheath Boundary Conditions in Stix Finite Element RF Code Christina Migliore, John C Wright, Paul T Bonoli, Mark L Stowell Ion cyclotron radio frequency range (ICRF) power plays an important role in heating and current drive in fusion devices. However, experiments show that in the ICRF regime there is a formation of a radio frequency (RF) sheath at the material and antenna boundaries that influences sputtering and power dissipation. Given the size of the sheath relative to the scale of the device, it can be approximated as a boundary condition (BC). Electromagnetic field solvers in the ICRF regime typically treat material boundaries as perfectly conducting, thus ignoring the effect of the RF sheath. Here we describe progress on implementing a model for the RF sheath based on the a finite impedance sheath BC formulated by J. Myra 2015 ~\footnote{J. Myra, et al., Phys. Plasmas 22, 062507 (2015)} which provides a representation of the RF rectified sheath including capacitive and resistive effects. This research will discuss the results from the development of a parallelized cold-plasma wave equation solver Stix that implements this non-linear sheath impedance BC through the method of finite elements in pseudo-1D and pseudo-2D using the MFEM library [http://mfem.org]. With the implementation of the sheath BC, this work proposes investigating the fundamental behavior of the large rectified sheath potentials in the not-magnetically connected far-field case using a cold plasma model approach. |
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GP11.00009: Investigation of a two-dimensional microscale RF sheath model James R Myra, Haruhiko Kohno Radio frequency (RF) sheaths are of fundamental importance in understanding boundary ICRF interactions in fusion devices. In this work the 1D RF sheath microscale model investigated previously [J. R. Myra and D. A. D'Ippolito, Phys. Plasmas 22, 062507 (2015)] is generalized to 2D using a numerical nonlinear fluid model. Two-dimensional effects are important near points where the magnetic field is tangent to the surface, or where the local surface radius of curvature is comparable to the sheath width, as might occur due to surface irregularities. Numerical results from the 2D model and their interpretation will be presented with the end goal of improving the fidelity of the RF sheath impedance boundary condition. |
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GP11.00010: Development of a non-linear rf sheath benchmark suite John C Wright, Christina Migliore, James R Myra, Davide Curreli, David N Smithe, Thomas G Jenkins, Mark L Stowell, David L Green, Clyde J Beers, Tim Younkin, Syun'ichi Shiraiwa, Nicola Bertelli, Matthew J Poulos, Haruhiko Kohno The Bohm sheath on plasma facing components (PFCs) can be energized through rectification by RF waves that reflect off of the PFCs. This can create voltages in excess of 100 V that are sufficient to cause impurities to sputter the surface. The modeling of this effect is a very active area of research. In this poster we present cases that have been benchmarked across several codes using different approaches. Many of these cases were first shown in Kohno Comp. Phys. Comm. (2017). We demonstrate consistency of the solution to high accuracy independent of approach. Cases include non-linear sheaths in 1D and 2D and the sheath plasma wave. These cases should serve as a benchmark suite for developers. |
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GP11.00011: Non-local effect in Petra-M RF simulation Syun'ichi Shiraiwa, Nicola Bertelli Petra-M is an open source FEM platform based on a scalable MFEM finite element library. In RF SciDAC, it has been used for performing a large-scale RF fullwave simulation including realistic 3D antenna models. Here, we investigate the feasibility of expanding the RF wave physics model within FEM discretization. The finite temperature effects make the plasma response from imposed RF electric fields non-local. An accurate treatment of these effects requires extending the FEM assembly process to accumulate the contribution from the neighboring mesh element. Both the first order finite temperature correction term and an all-order term based on a convolution integral [O. Sauter and J. Vaclavik, Nucl. Fusion 32, 1455 (1992)] were implemented. Simulation result of 1D electron Bernstein wave mode-conversion shows that this approach can resolve the thermal EBW. Extension of convolution integral in 2D will be discussed. We also discuss the implementation of various non-linear RF rectified potential models including its asymptotic limit to Petra-M. These models are compared with previous analytic and numerical results [H. Kohno and J. Myra, CPC 220, 129 (2017), W. Tierens et. al., PoP 26, 083501 (2019)] , showing satisfactory agreement. |
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GP11.00012: Adapting Chebyshev Iteration for Solving the AORSA Matrix David N Smithe, Benjamin M Cowan, Shu-Hang Lin The “All Orders” Larmor Radius solve for ICRF in hot plasmas, e.g., the AORSA code[1] and its predecessor, the METS code[2], produce a linear system of equations which is complex-valued, non-symmetric, non-definite, with multiple physical modes of different wavelengths, that makes effective pre-conditioning difficult-to-impossible. Thus, this challenging system is presently solved by direct matrix inversion. However, the N-cubed operation-count of direct matrix inversion restricts the problem size, to iterated 2D, e.g., the axi-symmetric tokamak, but not for the stellarator, or for a non-axi-symmetric perturbation within a tokamak. In order to solve a true non-axi-symmetric 3D problem, an alternate solve technique is required, which we demonstrate. We have adapted Chebyshev accelerated Richardson iteration for use with linear systems having spectra with eigenvalues of both positive and negative real part, and zero or negative imaginary part. We use traditional Richardson iteration, with each iterate providing a different polynomial root, instead of using the Chebyshev recursion formulae. Roundoff is very well controlled with a fractal-based resequencing of the root order.[3] This approach allows us to modify the Chebyshev polynomial by making small changes to the root locations, and also allows us to use a “semi-ellipse” polynomial, where the roots lie on a half ellipse, instead of a line. The combination allows for a half-ellipse spectral hull, which can touch and extend to either side of the origin, thus significantly generalizing the class of non-symmetric problems that can be treated with Chebyshev Iteration, including the AORSA matrix. |
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GP11.00013: Control of the Local Gradient and the Minimum Value of the Safety Factor Profile by Using Moving ECCD Sai Tej Paruchuri, Andres Pajares, Tariq Rafiq, Eugenio Schuster Active control of the safety factor is essential for preventing magnetohydrodynamic instabilities and achieving the needed level of performance (steady-state operation, high pressure, etc.) in advanced modes of operation in tokamaks. The total number of actuators, such as neutral beam injectors and electron cyclotron current drives (ECCDs), and their power saturation limits define the controller's capability to track a given target safety factor. The ECCD deposition location has in general been assumed fixed by safety-factor controllers. A model-based safety-factor controller is proposed in this work by exploiting the capability of changing the ECCD position in real time by mirror steering with the ultimate goal of increasing the tokamak's actuation capabilities. A dynamical model that includes the effect of the varying ECCD position on the plasma dynamics is first developed. Then, controllers considering the ECCD position as a controllable input are designed to tackle two very challenging control problems associated with the safety factor, namely the regulation of its minimum value at a time-evolving location and the regulation of its local gradient around a predefined q value with a time-evolving location. Finally, the performances of these controllers are assessed in closed-loop simulations using the Control Oriented Transport SIMulator (COTSIM) for a DIII-D scenario. |
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GP11.00014: Evaluation of Some Possible Scenarios for Validating the Theory of RF Current Condensation in the DIII-D Tokamak Lanke Fu, Richard Nies, Allan Reiman, Nathaniel J Fisch, Laszlo Bardoczi, Robert J La Haye, Michael W Brookman, Xi Chen, Nikolas C Logan Electron cyclotron current drive can stabilize large islands and is planned for stabilization of tearing modes on ITER. The theory of RF current condensation [1] predicts a nonlinear amplification of power deposition due to the temperature sensitivity of ECCD, which improves stabilization efficiency by self-focusing power deposition at island O-points. Alternatively, premature nonlinear damping can occur at island peripheral, "shadow" island center and reduce stabilization efficiency. We are pursuing numerical simulations to evaluate possible scenarios for validating the theory using the top launch capability of the DIII-D tokamak. Our simulations employ a numerical code OCCAMI (Of Current Condensation Amid Magnetic Islands) [2] which iterates between the GENRAY geometrical optics ray tracer for trajectory and power deposition, and a heat diffusion equation solver for evolving island temperature profile. References [1] Reiman, A. H., & Fisch, N. J. (2018). Suppression of Tearing Modes by Radio Frequency Current Condensation. Physical Review Letters, 121(22), 225001. [2] Nies, R., Reiman, A. H., Rodriguez, E., Bertelli, N., & Fisch, N. J. (2020). Calculating RF Current Condensation with Consistent Ray-tracing and Island Heating. Physics of Plasmas, 27(9), 092503. |
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GP11.00015: Adapting ICRF Transmission Line Components For Use at ~0.5 GHz for Helicon Experiments on DIII-D Robert I Pinsker, M. W Brookman, B. Van Compernolle, C. P Moeller, A. Nagy, E. M Williams A system to test fast wave current drive in the lower hybrid range of frequencies (“helicon”) is operating on the DIII-D tokamak, at rf power levels of up to 1 MW at 476 MHz available from a single klystron. The power is transmitted from the source over a long transmission line to the input of a 30-element traveling-wave antenna of the comb-line type mounted in the DIII-D vacuum vessel. The transmission line is largely constructed of 9”-diameter 50 Ohm coaxial line, with the portion closest to the tokamak built of 6” 25 Ohm coax. The vacuum feedthrough used at the vessel wall is adapted from ICRF feedthroughs originally intended for operation at ~50 MHz. Modifications of the coaxial components proved necessary to obtain acceptably low reflections in the lines at 476 MHz. Characterization of the components as a function of frequency up to 0.5 GHz are shown and the modifications are discussed. The linear (1 mW - 100 W) properties and the properties at up to 0.5 MW of the in-vacuum portion of the transmission lines are discussed, including the important effects of multipactoring at the operating frequency of the helicon system. Operation into plasmas at coupled power levels up to 0.3 MW was achieved in June 2021; initial results are reported. |
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GP11.00016: Radiation force exerted by radio frequency waves on density filaments Abhay K Ram, S.-I. Valvis, K. Hizanidis The scattering of radio frequency (RF) waves by filaments in the edge region of a tokamak plasma has been well studied. In the cold plasma approximation, valid in the scrape-off layer, there exist two distinct modes, either one of which can be a propagating or an evanescent wave. The scattering of an incident wave leads to spatial fragmentation of RF power and coupling of power between the two modes. The RF waves also exert a radiation force on the filament. The full-wave analytical theory formulated for the scattering process [1], in conjunction with the Maxwell stress tensor, is used to determine the radiation pressure. We find that the direction of the force depends on the wave frequency and its polarization, and on the electron densities inside and outside the filament. The force can either pull the filament towards the RF source or push it away. The lower hybrid wave pulls in filaments with higher density (relative to the ambient plasma) and pushes away filaments with lower density. The direction of the radiation force due to ion cyclotron waves varies with the density of the ambient plasma. For a variety of RF frequencies, a detailed analysis of the force will be presented. |
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GP11.00017: Interaction between high power ICRF waves and turbulence in LAPD Joshua J Larson, Troy A Carter, Gurleen Bal, Bart G Van Compernolle Experiments studying high power ion cyclotron range of frequencies (ICRF) wave excitation have been carried out in the Large Plasma Device (LAPD) at UCLA. Fast waves are excited using a single-strap antenna, driven by a high power (100kW) amplifier at ~2.5 MHz (~5 fci). During high power wave excitation, modification of edge turbulence is observed with edge density fluctuations (in the 1-100kHz frequency range) increasing with increasing ICRF power. This work investigates the possible mechanisms for this increase, including profile modification (increased density or flow gradients) and nonlinear three-wave processes. |
Not Participating |
GP11.00018: Kinetic full wave analysis of ion cyclotron waves in tokamak plasmas using integral form of dielectric tensor Atsushi Fukuyama In order to describe the wave structure and power deposition profile in ion cyclotron heating in tokamak plasmas, a considerable progress has been made in developing full wave codes including kinetic effects. Recently kinetic full wave analysis using the integral form of dielectric tensor and the finite element method was extended to two-dimensional configuration including peripheral regions in tokamaks. This approach can describe the inhomogeneous cyclotron resonance and any order of the finite Larmor radius effects, and the code implements efficient parallel processing. For ion cyclotron range of frequencies, the wave structures and parameter dependence in the two-ion hybrid resonance heating are compared with the results of conventional differential operator approach. The description of ion Bernstein waves, kinetic Alfven waves, and ion cyclotron emission in this approach will be also discussed. |
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GP11.00019: Propagation properties of electron cyclotron waves with helical wavefronts in magnetized plasma Toru I Tsujimura, Shin Kubo Propagation properties of an optical vortex with a helical wavefront in cold uniform magnetized plasma are theoretically investigated in an electron cyclotron (EC) range of frequencies. The effects of the helical wavefront of the optical vortex on the wavefields in magnetized plasma are described. These effects are significant when the topological charge of the optical vortex is large or when the distance from the phase singularity is small. The different propagation properties are also confirmed in the propagation of Laguerre–Gaussian (LG) beams by three-dimensional simulations with the finite element method. It is found that a part of the ordinary-mode LG beam excited at the lower electron-density region is converted into the high-wavenumber extraordinary-mode LG beam at the upper hybrid resonance layer. Thus, the vortex EC wave can be a new tool to heat high-density plasma efficiently. |
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GP11.00020: Towards Fast, Accurate Predictions of RF Power Deposition/Current Profile via Data-driven Modeling Gregory M Wallace, John C Wright, E.W. Bethel, Z. Bai, T. Perciano, R. Sadre, Syun'ichi Shiraiwa, Nicola Bertelli Modeling of radio frequency (RF) actuators requires minutes of computation time to simulate a single time slice, however results on the ms timescale are desired for real-time experimental control and integrated modeling applications. This work explores the use of new methods based on machine learning (ML) for the purpose of accelerating these computations through fast surrogate models. Latin hypercube sampling methods ensure that the database of 16,000+ GENRAY/CQL3D simulations covers the range of 9 input parameters (ne0, Te0, Ip, Bφ, R0, n||, Zeff, Vloop, PLHCD) with sufficient density in all regions of parameter space. A comparison of speed and accuracy for several ML regression methods (random forest, multi-layer perceptron neural network, Gaussian process) highlights strengths for each approach in creating a fast surrogate model. Other RF machine learning topics include full-wave preconditioners, the inverse problem for RF current drive (i.e. mapping from a desired current profile to a set of plasma/RF parameters that will produce the desired current profile), and a Hamiltonian physics informed neural network to accelerate ray tracing calculations. |
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GP11.00021: ITER Plasma Physics. ITER Core Plasma Heating by Nonlinearly Coupled Short-wavelength Electron-Bernstein and Ion Bernstein Modes V. Alexander Stefan Electron cyclotron plasma heating is a promising nonlinear method for the ITER plasma environment (X-mode and O-mode launching). In particular, short-wavelength Electron-Bernstein modes (absolute instability, kρe~1; predominantly collisionally dissipated, k-vector normal on B-toroidal field) are efficient in transferring the driver-energy to ions via electron-ion collisions. [1] Nonlinear[2] coupling[3] to short-wavelength Ion-Bernstein modes provides an additional channel for direct deposition of the driver-energy to ion-plasma components. [4] [1] T.H. Stix, 1965 ; W. L. Kruer and J. M. Dawson Phys. Rev. Lett. 25, 1174 (1970). [2] R. K. Dodd et al, Solitons and Nonlinear Wave Equations (Academic Press, New York, 1982; pp. 517—521. [3] V. Stefan and N A Krall, Physics of Fluids 28, 2937 (1985). [4] Evgeniy Pavlovich Velikhov, Kurchatov Center, Moscow, Russia; private communication, May, 2019. |
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GP11.00022: Lower hybrid electric field magnitude and polarization measurement and modeling results on the WEST tokamak Cornwall H Lau, Elijah H Martin, Marc Goniche, Chris Klepper, Gregory M Wallace, Annika Ekedahl, Christophe Guillemaut, Didier Mazon Lower hybrid (LH) current drive is used to drive current and enable long pulse operation of advanced tokamak scenarios. This poster presents recent experimental and modeling results of the LH electric field magnitude and polarization in the scrape-off-layer (SOL) to better understand the coupling of LH waves through the SOL of the WEST tokamak. This is achieved using the DSELF dynamic Stark effect diagnostic to measure the LH electric field over 200+ discharges on WEST, combined with full-wave modeling and a synthetic DSELF diagnostic to validate models against experiments. Recent results show that the spatially averaged measured LH electric field magnitude is consistent with modeling over the experimental uncertainty. At some poloidal locations, the measured LH electric field is significantly different than expected from the model. For most discharges, the measured and modeled LH polarization agrees. However, on average, at higher densities and during MHD events, the measured LH polarization disagrees significantly with the model. These discharges with significant discrepancy in the LH polarization measurement and model appear to be correlated with reduced LHCD efficiency. Hypotheses such as density fluctuations to explain these discrepancies will be discussed. |
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GP11.00023: Relaxation to magnetohydrodynamics equilibria via collision-like metric brackets: toward three-dimensional equilibria Omar Maj, Camilla Bressan, Philip J Morrison, Michael Kraus In our previous work (Bressan et al., J. Phys.: Conf. Series, 2018) we have shown how the idea of metriplectic dynamics (Morrison, Phys. Letters A, 1984) can be employed to design relaxation methods for computing equilibria of Euler's and magnetohydrodynamics (MHD) equations in two-dimensions. Specifically, a metriplectic dynamical system is obtained from a Hamiltonian system by adding a dissipation mechanism in the form of a metric bracket which preserves the Hamiltonian function while dissipating a specific entropy. We observed that the Landau operator for Coulomb collisions can be used as a template for the construction of a specific class of metric brackets that in turn lead to favorable relaxation methods. Here we develop those ideas toward the application to three-dimensional MHD equilibria. We discuss in details a relaxation method for Beltrami fields in three-dimensions. For numerical computations, we show how the key geometric properties of the metric brackets must be preserved at the discrete level and propose a structure-preserving scheme based on finite element exterior calculus. At last a generalization of the method from Beltrami fields to full three-dimensional MHD equilibria is presented. |
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GP11.00024: Minimization of Poloidal Viscosity in Tokamaks Using the FLOW Code Ian F Gustafson, Luca Guazzotto Extensive experimental evidence shows that the inclusion of poloidal flow in tokamaks can dramatically improve transport properties. However, theory shows that these flows are damped by poloidal viscosity. In this work, we utilize the FORTRAN code FLOW [1] to calculate ideal magnetohydrodynamic (MHD) equilibria and estimate the poloidal viscosity with a postprocessor. We then minimize a norm of the viscosity over the input parameters of the calculation, i.e. the input free functions for FLOW, which are associated with intuitive physical quantities. In order to achieve this minimization, the FLOW code has first been functionalized and then wrapped within a Python script for easy use with an open source parallel minimization package. Here, we present and compare results of minimized poloidal viscosities for varying some or all of the FLOW input free functions as well as plasma shapes. We also compare our numerical results with an analytical minimization of the poloidal viscosity. [1] L. Guazzotto et. al., Physics of Plasmas, 11, 604 (2004). |
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GP11.00025: Numerical approach to singular current sheets arising from resonant magnetic perturbations Yi-Min Huang, Stuart R Hudson, Joaquim Loizu, Yao Zhou, Amitava Bhattacharjee General three-dimensional toroidal ideal magnetohydrodynamic equilibria with nested flux surfaces are susceptible to forming singular current sheets at surfaces resonant with externally imposed perturbations. The presence of singular current sheets indicates that magnetic reconnection will ensue, forming magnetic islands or regions of stochastic field lines. Numerically resolving singular current sheets in the ideal MHD limit has been a significant challenge. This work presents numerical solutions of the Hahm-Kulsrud-Taylor (HKT) problem, which is a prototype for resonant singular current sheet formation. The HKT problem is solved by two codes: a Grad-Shafranov (GS) solver and the SPEC code. The GS solver has built-in nested flux surfaces with prescribed magnetic fluxes. The SPEC code implements multi-region relaxed magnetohydrodynamics (MRxMHD), where the solution relaxes to a Taylor state in each region while maintaining force balance across the interfaces between regions. As the number of regions increases, the MRxMHD solution approaches the ideal MHD solution assuming nested flux surfaces. We demonstrate excellent agreement between the numerical solutions obtained from the two codes through a thorough convergence study. |
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GP11.00026: Self-organization and confinement in tokamak plasmas with edge safety factor less than 2 Noah C Hurst, Brett E Chapman, Abdulgader Almagri, Jay K Anderson, Brian S Cornille, Daniel J Den Hartog, Cary B Forest, Steph Z Kubala, Karsten J McCollam, Mihir D Pandya, John S Sarff, Winston Solsrud, Carl R Sovinec In most tokamaks, operation at low edge safety factor q(a) < 2 is challenging or impossible due to a disruptive kink instability. We report measurements and nonlinear MHD modeling of very low safety factor, Ohmic tokamak plasmas in MST (a = 0.5 m, R = 1.5 m, BT = 0.13 T) spanning 0.7 < q(a) < 2.9. The external kink instability is mitigated passively by MST’s close-fitting, thick, conducting shell. The experimental results are compared to nonlinear MHD simulations using the NIMROD code with a similar Lundquist number S ~ 105. Equilibrium reconstructions constrained by local magnetic probe measurements at r/a > 2/3 suggest that q(0) ~ 1 for q(a) ≥ 1. The q(r) profile has a flat region in the core that expands outward systematically with decreasing q(a). Sawtooth-like oscillations are observed in the experiment and in NIMROD simulations which cover q(a) ≥ 1.5, implying self-organization of the current profile. When q(a) approaches 1, the dynamics are dominated by a large-amplitude, saturated m/n = 1/1 helical deformation. As q(a) decreases from 2 to 1.5, the measured electron energy confinement time gradually decreases and the sawtooth cycle becomes more irregular, possibly due to the increasing radius of the q = 1 surface. Work supported by US DOE and the WiPPL team. |
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GP11.00027: Benchmarking NIMROD Continuum Kinetic Formulations through the Steady-State Poloidal Flow Joseph R Jepson, Chris C Hegna, Eric D Held, Joseph A Spencer, Brendan C Lyons
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GP11.00028: Developments of the NIMROD MHD code for next-generation fusion-device modeling and computational hardware Jacob R King, Sina Taheri, Eric Howell, Brian S Cornille, Eric D Held, Andrew Spencer Incorporation of atomic physics associated with multiple species is required to study next-generation fusion device topics such as integration of MHD modeling of resonant magnetic perturbations or edge-harmonic oscillations with advanced edge solutions. We present an operator-splitting formulation of the atomic interactions using a Strang-splitting technique to naturally break equations into constituent ODE and PDE parts and preserve the structure exploited by the semi-implicit leapfrog. By testing on a battery of cases, we show that a second-order-in-time Douglas-Rachford inspired coupling between the ODE and PDE advances is effective in reducing the time-discretization error to be comparable to that of Crank-Nicholson with Newton iteration of the nonlinear terms. Since all of the nonlinear atomic interaction is handled by a local ODE solver using an Adams-Bashforth method, no nonlinear iteration is required and each spatial point can be treated independently and in parallel. This parallelism is advantageous for exploiting GPUs. We use OpenACC with a modern Fortran implementation using a continuous-integration development cycle to port NIMROD algorithms to the GPU architecture. The performance of the ported finite-element and matrix preconditioning kernels is reported. |
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GP11.00029: Nonlinear MHD studies of sawtoothing tokamak states with NIMROD Karsten J McCollam, Brett E Chapman, Mihir D Pandya, John S Sarff, Carl R Sovinec Using the extended-MHD code NIMROD, we simulate sawtoothing dynamics in generic tokamaks with circular, elliptical, and rectangular cross sections. Nonlinear evolution of zero-β cases at Lundquist numbers of ~ 105 exhibits quasiperiodic sawtooth-like events moving the core safety factor q profile above 1 from below. Time-resolved Poincare plots show nonstochastic flux surfaces throughout the sawtooth cycle. Linear mode structures of magnetic field and plasma flow for toroidal mode number n = 1 are characteristically different for zero- and finite-β (~ 1%) cases. Qualitatively, for each equilibrium cross-section shape, the n = 1 magnetic mode structures are more complex in the poloidal plane for zero β than for finite β, and the opposite is the case for the velocity mode structures. At finite β, linear growth rates increase with n, and the high-n (~ 10) mode structure is characteristic of ballooning modes. To allow nonlinear simulations of these finite-β equilibria without spectral pileup, we aim to stabilize the high-n modes by adjusting viscosity and perpendicular heat transport in the NIMROD MHD model or by adding flow shear to the equilibria. |
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GP11.00030: Kinetic Simulations of the Sawtooth Crash Rahul Kumar, Amitava Bhattacharjee, Fatima Ebrahimi The core plasma in tokamaks is known to exhibit sawtooth oscillations when the q-axis drops below unity. In each sawtooth cycle, temperature, density, and current in the core increase gradually, followed by a rapid drop in these quantities. The rapid drop, known as the sawtooth crash, is attributed to magnetic reconnection. We study the sawtooth crash process in three-dimensions using a fully kinetic particle-in-cell method implemented in the PICTOR code. We discuss the differences and similarities between the kinetic simulations and the magnetohydrodynamic simulations with similar physical parameters. Specifically, we find development of significant phase-space anisotropy in the kinetic simulations, which are generally not incorporated in the MHD models. |
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GP11.00031: Mode spectrum characteristics and onset of the low-shear MHD stability regime Adelle Wright, Nathaniel M Ferraro We introduce a new approach for predicting unstable nonresonant MHD modes, which are demonstrated to be destabilised preferentially to any other resonant instability with the same toroidal mode number for n>1, making these modes important for determining overall MHD stability properties in equilibria with weak magnetic shear. |
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GP11.00032: Single Gaussian Process Method for Arbitrary Tokamak Regimes Jarrod Leddy, Sandeep Madireddy, Eric C Howell, Scott E Kruger Gaussian Process Regression (GPR) is a Bayesian method for inferring profiles based on input data. The technique is increasing in popularity in the fusion community due to its many advantages over traditional fitting techniques including intrinsic uncertainty quantification and robustness to overfitting. Most fusion researchers to date have utilized a different GPR kernel for each tokamak regime. This requires a Machine Learning (or simpler) method to first predict the regime, choose the right kernel for that regime, and then use that kernel. The disadvantage of this method is that it requires an additional step, and it is unclear how well it will behave if the plasma enters a new, unexpected regime. We summarize our work developing a general kernel for all regimes (including radially-varying hyperparameters), utilizing heavy-tailed likelihood distributions to automatically handle data outliers, and using GPflow for full Bayesian inference via Markov chain Monte Carlo to sample hyperparameter distributions. We present a single GPR method that is robust across many different tokamak regimes and a wide range of data inputs and quality. |
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GP11.00033: A Mathematical View of the EFIT-AI Project Scott E Kruger, Eric Howell, Jarrod Leddy, Lang L Lao, Cihan Akcay, Torrin A Bechtel, Joseph Mcclenaghan, Sandeep Madireddy, Jaehoon Koo, Samuel W Williams, Matthew Leinhauser, Alexei Pankin The EFIT-AI project will create a modern advanced equilibrium reconstruction code capable of meeting the needs of tokamaks with burning plasmas. Mathematically, equilibrium reconstruction is an inverse problem, where one is seeking to use the data to infer underlying physical properties through the use of mathematical models for the forward problem. Inverse problems have enjoyed a renaissance of mathematical interest in recent years thanks to advances in theories in uncertainty quantification and machine learning. These advances are at the intersection of statistics, functional analysis, computer science, and physics. Here, we review some of these advances in the context of the equilibrium reconstruction problem. |
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GP11.00034: Overview of the SciDAC EFIT-AI Machine-Learning (ML) and Artificial-Intelligence (AI) Enhanced Equilibrium Reconstruction Project Lang L Lao, Cihan Akcay, Torrin A Bechtel, Yueqiang Liu, Joseph Mcclenaghan, David Orozco, David P Schissel, Scott E Kruger, Eric Howell, Jarrod Leddy, Sandeep Madireddy, Prasanna Balaprakash, Jaehoon Koo, Samuel W Williams, Matthew Leinhauser, Alexei Pankin Equilibrium reconstruction is fundamental to tokamak research and operation. The goal of the SciDAC EFIT-AI project is to harness novel ML / AI algorithms to enhance equilibrium reconstruction for modeling and real-time applications. This includes development of a ML-enhanced Bayesian framework to automate and maximize information from measurements and Model-Order-Reduction (MOR)-based ML models to efficiently guide the search of solution vector. A device-independent portable core equilibrium solver has been created to ease adaptation of ML enhanced reconstruction algorithms. An EFIT database comprising of DIII-D magnetic, MSE, and kinetic reconstruction data is being constructed for developments of EFIT-MOR surrogate models to speed up the search of solution vector. Approaches to improve portability between the OpenMP and GPU EFIT versions are being explored on Linux GPU clusters and the new NERSC Perlmutter to create a performance-portable GPU implementation for further optimization. Other progress includes development of a Gaussian-Process Bayesian framework to improve processing of input data, and construction of a 3D perturbed equilibrium database for developments of 3D-MOR surrogate models. |
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GP11.00035: Uncertainty quantification of EFIT reconstructions using the EFIT-AI database Cihan Akcay, Joseph Mcclenaghan, Torrin A Bechtel, David Orozco, Yueqiang Liu, Lang L Lao, Scott E Kruger, Eric C Howell, Jarrod Leddy, Sandeep Madireddy, Jaehoon Koo, Samuel W Williams, Alexei Pankin The EFIT-AI database is a collection of tokamak discharges with multiple equilibrium reconstructions. This database is being developed for the EFIT-AI project because high quality data and open access to it are increasingly becoming a requirement for advancing tokamak science with Data Science methods. Currently, the database features the 2019 DIII-D campaign (approximately 2500 discharges) with 4 different EFIT reconstructions: 1. Magnetics-only EFIT, 2. Magnetics+MSE EFIT, 3. OMFIT-automated kinetic EFITs, and 4. CAKE-automated kinetic EFITs. Here we examine the sensitivity of the solution vector(s) from various EFIT reconstructions to the measurements, which enter EFIT as constraints. These constraints are used in the least-squares square minimization of EFIT to find the optimal fitting coefficients for the pressure and $F=RB_{\phi}$ profiles. We use correlation matrices and singular-value decomposition (SVD) to carry out the uncertainty quantification. Preliminary results show strong correlations between the magnetic measurements. SVD splits the time-dependent measurements into separate spatial and temporal parts, with the singular values indicating the significance of each eigenmode. SVD can be used to filter out the eigenmodes with low singular values. |
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GP11.00036: Improvements to EFIT in Preparation for the Burning Plasma Era Torrin A Bechtel, Joseph T McClenaghan, Cihan Akcay, Lang L Lao, Scott E Kruger, Eric C Howell, Jarrod Leddy, Matthew Leinhauser, Samuel W Williams EFIT was the first and is the most extensively used equilibrium reconstruction code in the world. Although robust, the burning plasma regime will bring new challenges which include adapting to novel operating regimes and incorporating diagnostics that can withstand a harsh, radioactive environment. This regime has motivated the exploration of machine learning techniques to improve the quality of equilibrium reconstructions that can be produced in real-time. To support this development, we are upgrading the core Grad-Shafranov solver. The improvements include clearly separating out the device-specific coding, enhancing code portability, and ensuring thread-safety in preparation for GPU developments. To aid in the development, we have created a test suite for use with continuous integration workflows. |
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GP11.00037: Controlling Plasma Profiles in a Learned Model via Reinforcement Learning Viraj Mehta, Joseph A Abbate, Rory Conlin, Egemen Kolemen, Jeff Schneider Plasmas in a tokamak are well-specified by their profiles alongside parameters describing the shape and magnetic field observed. Many scalar quantities of interest such as $\beta_N$ are derivable from the profiles as well. Therefore, controlling profiles to be close to specified target values well-known and highly useful capability in tokamak experiments. |
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GP11.00038: Novel Techniques for Mathematical Optimization of Black-Box Nonlinear Functions Daniel Raburn, Allan Reiman We have developed and tested a number of novel techniques for mathematical optimization of black-box multidimensional nonlinear real-valued continuous functions, with focus on scenarios where the predominant cost of optimization is the time complexity due to the number of function evaluations, such as is commonly the case with the Princeton Iterative Equilibrium Solver (PIES) code for MHD equilibria calculation. Our research builds on the existing Jacobian-free Newton-Krylov (JFNK) framework. We present a review of existing techniques in addition to our results and analysis. |
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GP11.00039: AE stability of QPS configurations using a Landau closure model Juan Ortiz, Jacobo Varela Rodriguez, Donald A Spong, Luis Garcia, Yashika Ghai The aim of this study is to analyze the linear stability of Alfvén Eigenmodes (AE) in QPS device using the gyro-fluid code FAR3d. AE stability is calculated for different NBI operational regimes, EP β = [0.01 – 0.1] and EP energy Tf = [2 – 180] keV, for the toroidal mode families n=1 to 5 and helical families n=1,3,5 and 2,4,6 as well as for vacuum and finite β configurations. In both configurations the modes with the largest growth rate are triggered by EP with an energy around 30 keV, having n=5 the largest. The AE EP β threshold in the vacuum case is 0.01 and 0.02 in finite β case (except for Tf = 30 keV which EP β threshold is 0.01), showing an increase (decrease) of the AE frequency (GR) with the EP energy above 30 keV. Toroidal Alfvén Eigenmodes (TAEs) have the largest growth rate, unstable in the frequency range of 100 - 230 kHz for the finite β case and 50 - 220 kHz for the vacuum case, triggered plasma periphery (normalized minor radius = [0.7-0.9]). Also, TAE eigenfunctions become slender as the toroidal mode number increases. EPs with Tf > 90 keV trigger Elliptical AE (EAEs) andNoncircular AE (NAE) with a smaller growth rate regarding the TAEs. The helical couplings analyzed are not strong enough to destabilize Helical AE (HAE), only increasing the mode GR around 10 %. |
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GP11.00040: Toroidal torque across layers due to resonant magnetic fields and complex rotational dependencies Jong-Kyu Park, Dylan P Brennan, Richard Fitzpatrick, Nikolas C Logan, Yeongsun Lee, SangKyeun Kim, Yong-Su Na Toroidal torque balance across resonant layers with non-axisymmetric magnetic fields is the key to understanding of perturbed equilibria with rotating or externally forced magnetic islands. Here two-fluid drift-MHD layer formulations have been revisited with numerical integrations in particular using the Riccati transformation. This allows the reliable and efficient finding of the matching condition to the outer layers called the delta-prime in wide multi-variable space, improving the predictions to well-known asymptotic regimes as well as revealing transitions from one to another. The investigations over practical tokamak conditions show the complex rotational dependencies of the torque towards electron and ion diamagnetic frequencies, with characteristic nearby minimums before rising gradually at higher rotation where inertia effects become dominant. This formulation can also be readily coupled with ideal plasma response, giving the reconnected flux and the torque as a function of screening currents and conventional tearing mode indexes at each resonant layer. The predictions of parametric scaling for the field penetration will also be compared with other reduced MHD simulations or empirical regression, to establish the validity as well as the limitations of linear approaches. |
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GP11.00041: Kinetic Effects on Drift Tearing Modes in Slab Geometry Joseph A Spencer, Eric D Held, Jeong-Young Ji, Jacob R King, Scott E Kruger Kinetic effects on drift tearing modes in slab geometry are studied using the extended MHD formulation in the NIMROD code [C. R. Sovinec, et al., J. Comput. Phys. 195, 355-386 (2004)]. In this work, kinetic modifications (beyond that of ion gyroviscosity) enter the fluid equations through closures for the electron parallel heat flux and stress tensor which takes a CGL form. Self-consistent calculation of the electron closures is accomplished by solving the Chapman-Enskog-like form of the drift kinetic equation [J. J. Ramos, Phys Plasmas 17, 082502 (2010)]. We extend results from a previous parametric study that used a strictly fluid model [J. R. King and S. E. Kruger, Phys Plasmas, 21, 102113 (2014); J. R. King and S. E. Kruger, arXiv:1407.3864 [physics.plasm-ph] (2014)]. |
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GP11.00042: Two-phase compressible MHD solver developed in OpenFOAM to simulate compression of magnetized targets by imploding liquid metal liners Victoria Suponitsky, Ivan Khalzov Two-phase compressible MHD solver “mhdCompressibleInterFoam” has been developed in OpenFOAM to study compression of magnetic fields in vacuum and/or magnetized gas targets by imploding liquid metal liners. The primary goal is to simulate diffusion of magnetic field into the liner and how it affects liner dynamics in the regime of the FDP (Fusion Demonstration Plant) that is currently being designed by General Fusion. |
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GP11.00043: Time-discretization of a plasma-neutral MHD model with a semi-implicit leapfrog algorithm Sina Taheri, Jacob R King, Uri Shumlak Interaction between plasma and neutral species can significantly alter the dynamic behavior of magnetically confined devices and incorporating the atomic physics associated with these interactions is required for an accurate plasma-neutral model. Previous work [Taheri, APS-DPP 2020] focused on incorporating the atomic physics in the semi-implicit leapfrog framework of the NIMROD code [Sovinec, JCP 2004] using Crank-Nicolson with Newton iteration for nonlinear terms and an operator-splitting method using Strang splitting. A battery of 0-D test cases showed the accuracy and efficiency of these methods. In this work we expand testing to consider 1-D cases that roughly represent tokamak-plasma edge fueling and z-pinch startup from a plasma-gun injector and show that a Douglas-Rachford inspired operator splitting can reduce the error, over Strang split to a level comparable to that of Crank-Nicolson with nonlinear iteration. |
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GP11.00044: Curvilinear formulation of anti-symmetric plasmas models Federico D Halpern, Tess Bernard, Ronald E Waltz The anti-symmetry formalism is a new representation of the fluid equations that retains important symmetries of the force operator, and that also leads to simple discrete energy conservation theorems by simple analogy with their continuous form [1]. We extend and adapt the anti-symmetry fluid formalism to curvilinear geometry, determining the curvilinear form of the plasma fluid equations, and demonstrating that the anti-symmetry of the force operator is still retained after a coordinate transformation. It is found that the curvilinear equations retain the same structure as in Cartesian coordinates, however recast in terms of generalized variables rescaled by the metric coefficients. Special attention is needed to treat the metrics in a manner such that their derivatives do not introduce artificial forces. The first tests in cylindrical coordinates are carried out using the magnetized Rayleigh-Taylor instability using a Hall-MHD model, demonstrating the numerical robustness of the approach. |
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GP11.00045: Neoclassical transport due to resonant magnetic perturbations in DIII-D Priyanjana Sinha, Nathaniel M Ferraro, Emily A Belli Non-axisymmetric magnetic field perturbations known as resonant magnetic perturbations (RMPs) are applied to mitigate or suppress the instabilities present in the plasma also known as edge localized modes (ELMs) which arise as a result of the steep pressure gradient at the edge in H-mode plasmas. The application of RMPs often leads to a decrease in the plasma density, referred to as density pump-out, which can significantly affect fusion performance. Here we investigate the role of neoclassical transport in density pump-out and heat flux in the presence of RMPs. In this study, the drift kinetic code NEO with the enhanced capability to handle non-axisymmetric magnetic geometry is used to evaluate the neoclassical transport properties in DIII-D plasmas where RMPs are applied. The magnetic field given as an input to NEO is calculated using extended magnetohydrodynamic code M3D-C1 and includes the nonlinear resistive plasma response in realistic geometry and with realistic values of resistivity. The study performed here indicates a dramatic increase of the neoclassical particle and energy fluxes in the presence of the RMPs and are in same range as the diffusive particle fluxes calculated in the ELMing discharges in DIII-D, suggesting that neoclassical transport plays an important role in edge transport in such cases. The calculated neoclassical fluxes in DIII-D plasmas are found to be closely correlated with the observations of density pump-out over a range of RMP spectra. Also, these calculations show that nonlinear MHD simulations are essential at high RMPs to satisfactorily model the perturbed magnetic geometry in the pedestal region. |
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GP11.00046: The Adaptive Toroidal Equilibrium (ATEQ) code and its applications Linjin Zheng, Michael T Kotschenreuther, Francois Waelbroeck, Yasushi Todo The Adaptive Toroidal Equilibrium (ATEQ) code is developed for axisymmetric MHD equilibria. A decomposition with independent solutions is employed in the radial direction and the Fourier decomposition is used in the poloidal direction. The independent solutions are then obtained using an adaptive shooting scheme together with the multi-region matching technique. Using the backward substitution method to the Grad-Shafranov equation, its accuracy is checked and compared with existing equilibrium codes. It shows that the adaptive numerical scheme by ATEQ improves substantially the accuracy of equilibrium calculations, especially in the presence of separatrix. Comparison with existing codes will be detailed and especially how the ATEQ code improves the EFIT equilibrium results will be given. The necessity for better equilibrium solutions will also be discussed for the physics assessment of ITER or other existing devices. |
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GP11.00047: Overview of the C-2W Experimental Program Thomas Roche, Hiroshi Gota, Erik Trask, Sergei Putvinski, Artem Smirnov, Michl Binderbauer, the TAE Team TAE Technologies, Inc. (TAE) pursues an alternate approach to magnetically-confined fusion, relying on field-reversed configuration (FRC) plasmas. TAE’s current experimental device, C-2W [1], has made significant progress in FRC performance. The superior performance of C-2W to its predecessor, C-2U, is due to several factors. 1) NBs and edge-biasing systems, where higher total plasma energy is obtained by increasing NB injection power and applied-voltage on electrodes. 2) A feedback control system implemented to produce consistent FRC performance and reliable machine operation; the system controls magnet currents, electrode voltages/currents, fueling rates, and NB energy; it has also demonstrated stabilization of self-imposed axial instabilities. 3) C-2W divertors have demonstrated excellent electron heat confinement on open field via magnetic mirrors and a highly expanded field in the divertors (ratio > 30); the energy lost per electron ion pair, ??~6-8, is achieved, which is close to the ideal theoretical minimum. 4) A machine-learning framework for experimental optimization, which was developed in collaboration with Google. |
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GP11.00048: Enhanced Stabilization of Internal Tilt Mode by Neutral Beams in C-2W Timothy A DeHaas, Jaeyoung Park, Matt Tobin, the TAE Team TAE Technologies’ current experimental device, C-2W (also called “Norman”), is an advanced, neutral beam-driven, field-reversed configuration (FRC). [1] Traditional FRCs are known to be theoretically unstable to an internal tilt mode whose threshold is characterized by S*/E. Here, S* is the FRC radius divided by the ion inertial length, and E is the FRC elongation. [2] Experimental evidence from numerous FRC devices has suggested that the threshold for the catastrophic tilt is S*/E ~ 3. C-2W has demonstrated an ability to maintain values of S*/E substantially higher than the prior empirical limit, reaching S*/E ~ 5. Theoretical models have validated the assertion that the finite Larmor size for an FRC with β ~ 1 has a stabilizing effect. In C-2W, the presence of a large fast ion population from neutral beam injection maintains plasma stability above the traditional limit. A series of experiments was conducted to probe the effect the fast ion population on FRC stability, utilizing the variable energy neutral beams (15 - 40 keV, total power up to 20 MW). Results will be presented, demonstrating that the maximum achieved value of S*/E scales with the captured neutral beam power. |
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GP11.00049: Evolution and consequences of orbit type distributions in FRCs Francesco Ceccherini, Laura Galeotti, Calvin Lau, Daniel C Barnes, Sean Dettrick, Kevin Hubbard, the TAE Team Field Reversed Configurations (FRCs) are characterized by particular electric and magnetic field geometries that are due to the presence of unique particle orbit types as so-called Betatron, Drift, Fig8 and Type1 orbits. Such a rich orbit type inventory includes a peculiarly significant fraction of large orbits that is of paramount relevance in terms of the global stability of the configuration. Different orbit types may have different toroidal drift velocities and consequently generate toroidal currents in opposite directions. Therefore a change in the orbit type distribution may lead to an important alteration at different scales of the original spatial structure of the plasma current and eventually to an alteration of the magnetic and electric fields geometries. Through simulations carried out with FPIC (TAE's 3D full hybrid PIC code) we investigate the evolution in time of both thermal and fast ion orbit distributions and examine a possible correlation between the observed changes and the onset of global instabilities. |
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GP11.00050: Measuring particle confinement in C-2W advanced FRCs Anthony Cooper, Manjit Kaur, Erik Trask A. Cooper, M. Kaur, E. Trask, and the TAE Team |
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GP11.00051: Inferring Fast Ion Dynamics in the C-2W FRC via Integrated Data Analysis Gabriel Player, Ryan Clary, Sean Dettrick, Martin Griswold, Richard M Magee, Lucas Morton, Toshiki Tajima, the TAE Team In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 – 40 keV, total power up to 20 MW), advanced divertors, end bias electrodes, and an active plasma control system. Fast ions produced via neutral beam injection are critically important for FRC heating, current drive, and stabilization. Due to the high-beta nature of the C-2W FRC, fast ion orbits are large, with orbit radii of the same order as the plasma size. This has significant advantages, offering stability to the plasma and de-coupling the fast ions from local turbulence [2], but it also introduces difficulty in analyzing or measuring their behavior directly. This creates an incentive to leverage fast ion diagnostic data as efficiently as possible though the use of forward modeling and integrated data analysis techniques. We present the framework for an integrated data analysis workflow, utilizing Monte Carlo (MC) simulations of fast ion trajectories to forward model diagnostic data which depend on fast ion behavior – neutral particle analyzers (NPAs), scintillating neutron detectors, fast ion D-alpha (FIDA), and bolometry, among others. These modelled signals are then compared to experimental results, and iterated until cohesive, experimentally-validated models for fast ion behavior can be produced. We also examine the application of this framework to several topics of interest, demonstrating the effectiveness of these techniques. |
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GP11.00052: The study of high power electron beam transport for C-2W experiments Anton Tkachev, Sergey Korepanov, Vladimir Sokolov, Konstantin Pirogov A high-power long pulse electron beam was created at TAE. The beam is designed to be installed in a diverter and injected axially into the C-2W FRC device[1] for additional plasma heating[2]. The beam currently operates at a test stand, and the research on beam injection through a strong magnetic field is carried out. The experimental setup consists of an electron source, a 2-meter long beamline with guiding coils, and a short solenoid with a magnetic field of 1 T. The electron source generates a 30 keV, 100 A electron beam up to 10 ms. The beam is transported in a guiding magnetic field and injected into the short solenoid replicating the magnetic conditions in the mirror region of C-2W. It was understood theoretically and demonstrated in the experiments that the injected beam current is limited primarily by the space charge rather than the initial pitch angle and the magnetic moment conservation. When the electron beam is installed on C-2W, the space charge will be partially compensated by the plasma. At the test stand, space charge compensation was achieved with the additional gas puff into the beamline, and the plasma discharge was ignited and sustained by the beam itself. The parameters of the beam and the plasma were monitored with an extensive set of diagnostics. This paper reports our findings on beam transport through a region with a strong magnetic field. |
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GP11.00053: Experimental investigations of plasma fluctuations and performance on C-2W Kan Zhai, the TAE Team C-2W is an advanced beam driven Field Reversed Configuration (FRC) fusion device, in which plasma sustainment with the total temperature of over 4 keV has been demonstrated. C-2W is equipped with a state-of-the-art Thomson Scattering (TS) system that has 16 spatial cords. The DC output channels of TS polychromators, which are used to calibrate the system spectral response and monitor the background plasma radiation for measurement uncertainty analysis, can also be used to measure plasma fluctuations. The radiation recorded in these DC channels is mainly bremsstrahlung in the range of 850 - 1000 nm, which is determined by the density fluctuations at Te ~ 100 - 300eV. With this setup, fluctuations are observed at the edge of C-2W plasma with peak amplitudes measured where the maximum density gradient occurs. The experimental observation and possible candidate mechanisms will be presented. |
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GP11.00054: Tearing mode observation and stabilization in C-2W Matthew Tobin, Tadafumi Matsumoto, Jaeyoung Park, Loren C Steinhauer, the TAE Team The C-2W device (also known as 'Norman') at TAE Technologies has proven successful at generating stable, long-lived field-reversed configuration plasmas (FRCs) with record temperatures. [1] We report on the observation of axisymmetric periodic deformations of the FRC in external magnetic field and internal electron density measurements in C-2W that appear consistent with the tearing mode. [2] Effects of this mode on particle confinement based on bolometry measurements are presented. Importantly, the mode is found to stabilize when the FRC separatrix radius is close enough to the conducting vessel wall. The empirical stabilization condition for these fluctuations as a function of the ratio of the FRC radius to that of the vessel wall is demonstrated to agree well with theoretical values. Finally, the consequences of these observations for the hypothesis that the tearing mode mediates FRC equilibrium are discussed. |
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GP11.00055: Internal B field measurements using Pulsed Polarimetry on TAE's future Copernicus device Roger J Smith, Ales Necas, Kan Zhai, Laura Galeotti, Gerrit Bruhaug Copernicus will be the next generation field reversed configuration (FRC) device at TAE Technologies. Heated and sustained by neutral beam injection, the plasmas are expected to reach D-T relevant parameters. Recently, the production of intense sub-picosecond far-infrared (FIR) pulses has been demonstrated using 100 TW class lasers on material targets, allowing the implementation of a pulsed polarimeter diagnostic. Pulsed polarimetry is a Lidar technique that combines the two well-known diagnostic techniques of Thomson scattering and polarimetry to provide profiles of ne(s), Te(s) and B||(s) along the laser sightline. The rate of change of the Faraday angle, alpha, with distance, d(alpha)/ds, is directly proportional to the local [neB||(s)] product. An outstanding issue with FRCs is a direct measurement of reversed flux, if the d(alpha)/ds profile changes sign with density constant then B|| is reversed. In Copernicus, line integrated Faraday rotation angles of a few tens of degrees are expected for a laser wavelength of 150 microns allowing a spatial resolution of local B|| to 10cm in a practical system. The diagnostic technique and the intricacies of the laser source, optics and detection of FIR backscatter will be presented and discussed. |
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GP11.00056: Turbulence Spreading and Transport Properties in the Core and Boundary Layer of the C-2W Field-Reversed Configuration Lothar W Schmitz, C. Lau, H. K Leinweber, Thomas Roche, Hiroshi Gota, Roger J Smith, T. Tajima, Michl Binderbauer In the C-2W Field Reversed Configuration (FRC), a closed-fieldline FRC plasma is embedded in an (open fieldline) mirror plasma. We investigate here the interaction of the mirror plasma scrape-off layer, characterized by drift-interchange turbulence with an exponential toroidal wavenumber spectrum, and the much more quiescent FRC core, characterized by very low ion-scale turbulence levels, and a flat or inverted toroidal wavenumber spectrum [1]. Gyrokinetic simulations of the coupled mirror/FRC plasma have predicted turbulence spreading from the (unstable) SOL into the (stable) FRC core [2]. Doppler Backscattering measurements show that radial turbulence correlation properties change significantly near the excluded flux radius. We compare here correlation properties and investigate the radial structure of ion- and electron-scale turbulence and E×B flows, to uncover evidence for the predicted Zonal Flows and spectrally dependent radial turbulence spreading. We also investigate the role of E×B flow shear (controllable within certain limits with electrostatic biasing of the SOL plasma via annular divertor electrodes) for turbulence spreading and radial transport across the FRC excluded flux radius. |
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GP11.00057: Identifying Density Fluctuation Modes with the FIR Interferometer System in C-2W FRCs Chuanbao Deng, M. Beall, M. Kaur, E. Parke, R. J Smith, K. Zhai, the TAE Team In the TAE Technologies current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. The density profile measurements and characterization of fluctuations are made using the powerful 14 chords FIR interferometer system [2]. A variety of coherent density fluctuation modes are observed in different discharge stages and for a range of plasma parameters and machine settings. The attempts to identify these modes by comparing the mode spatial locations and frequencies with the theoretically predicted mode dispersion relation calculated from measured plasma parameters will be presented in this effort. |
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GP11.00058: Automated Phase Counting Corrections for the C-2W Far-Infrared Interferometer Eli Parke, Michael Beall, Roger J Smith, Kan Zhai, and the TAE Team In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. Line-integrated electron density measurements from the multi-chord far-infrared (FIR) interferometer system provide critical information about plasma profiles and performance, and many derived quantities depend on reliable density measurements. Transient reductions in signal power due to refraction through strong plasma density gradients can cause phase counting errors, and system susceptibility to refraction is notably challenging at longer FIR wavelengths. The interferometer phase is calculated from digitized data using post-shot complex phase demodulation algorithms with high bandwidth. Phase counting errors can be identified on either a single channel basis or by combining profile information from all channels. Additionally, techniques successfully utilized by hardware based phase comparators can also be applied through software. The accuracy and robustness of these approaches are compared. |
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GP11.00059: Tomography on C-2W: Time-resolved 2D Plasma Reconstructions via Improved 300-channel Bolometry System Anton S Bondarenko, Juan Aviles, Matt Tobin In TAE Technologies' current experimental device, C-2W (also called ”Norman’’)1, record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 - 40 keV, total power up to 20 MW), expander divertors, end bias electrodes, and an active plasma control system. Tomography offers a valuable and non-invasive diagnostic of the FRC plasma, as tomographic reconstructions of the emission profile yield important information on plasma shape, density, transient MHD behavior, and power loss due to radiation and particle flux. Recently, a bolometer system on the C-2W device has undergone extensive reconfiguration and noise troubleshooting, including a significant redesign of the acquisition electronics grounding and shielding scheme. The overhauled system consists of 300 simultaneously digitized photodiode channels with unique lines of sight that intersect a toroidal plane of the FRC near the mid-plane. The photodiodes respond to a broad range of wavelengths from XUV to NIR as well as energetic particles. Using this system, time-resolved 2D tomographic reconstructions of the near-midplane FRC emission profile are performed via several different methods, including a machine learning approach utilizing de-convolutional neural networks. The reconstructions are compared to well-known FRC equilibrium models, and the global stability of the plasma is qualitatively assessed. |
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GP11.00060: Electrode with Controllable Electron Emission in C-2W FRC Experiments Vladimir Sokolov, Sergey Korepanov, Ivan Isakov In the TAE Technologies current experimental device, C-2W (also called ``Norman'') [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. The end bias electrode system provides stabilization and heating of the high-beta axisymmetric plasmas by inducing azimuthal plasma rotation via the application of radial electric fields [2]. The radial electric fields are produced by flat coaxial bias electrodes. The negative bias voltage of 3-4 kV on the central electrode was proven sufficient for stabilizing C-2W FRC plasmas. |
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GP11.00061: Kinetic Simulation of Recycling Bifurcation in an Expander Divertor Sean Dettrick An expander divertor can be used to expand the Scrape-Off-Layer flux-tube of a linear device to arbitrary size, providing (i) reduced energy flux to the wall, (ii) increased pumping capacity, (iii) formation of a pre-sheath for electrostatic confinement of electrons, and (iv) large surface area for electrode biasing. The magnetic field and plasma density can vary spatially by a factor ~30 or more in the expander, with the result that the ion gyroradius and ion-neutral mean free path may be simultaneously short and long in different regions. To our knowledge this combination of features is not well represented by existing SOL transport models, so we have developed a new kinetic particle transport model where atomic and molecular ions and neutrals co-evolve, with each species acting as the target background for Monte Carlo Collisions with the other species. Magnetic fields are static, with electric fields determined by electron pressure balance and electrode biasing. Using this model a bifurcation from low to high recycling regime has been found which may help to explain operational boundaries of the C-2W experiment [1]. |
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GP11.00062: Spectroscopic study of axial porpagation of bias potentials in C-2W Marcel Nations, Erik Trask, Peter Yushmanov, Deepak Gupta, Erik Granstedt, Manjit Kaur, Ryan S Marshall In TAE Technologies’ current experimental device, C-2W (also called ``Norman'') [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are routinely produced and sustained in steady state. Variations in axial and radial electric fields are determining factors in electron-ion confinement as well as plasma rotation and heating. Axial drops in electric potential along magnetic field lines due to mirror field expansion and in the near-electrode sheath region in the divertors define the effective potentials in the central confinement region relative to the ends. Understanding how the applied biasing voltage is proportioned between axial and radial potential drops is thus a critical area of FRC research at TAE. To this end, the existing Doppler spectroscopy instruments have been extended with sightlines at multiple axial locations. Simultaneous measurements of impurity ion rotation and temperature for different bias and magnetic field configurations will be used to compare with physics models of how bias potentials connect between the divertors and confinement vessel. Here, details of the experimental setup and initial results are presented and discussed. |
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GP11.00063: Power flows from biased end electrodes in the C-2W advanced FRCs Manjit Kaur M. Kaur, E. Trask, D. Gupta, R. Clary, P. Yushmanov, and the TAE team |
Not Participating |
GP11.00064: Edge plasma measurement in open-field line region on C-2W FRC Tadafumi Matsumoto, the TAE Team In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record-breaking, advanced beam-driven field-reversed configuration (FRC) plasmas are produced and sustained in steady-state utilizing variable energy neutral beams, advanced divertors, end bias electrodes, and an active plasma control system. |
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GP11.00065: Multi-sensor data fusion for enhanced analysis of electrode arcing in the C-2W experiment James Sweeney, Erik Granstedt, TAE Team The TAE Technologies C-2W experimental device (also called "Norman") produces advanced beam-driven field reversed configuration (FRC) plasmas. [1] Norman incorporates electrode biasing for improved plasma stability and heating, but under extreme conditions arc discharges can occur on the electrode surface. In an effort to understand the effects of electrode arcing on the C-2W experiment, a system was developed which combines data from several different optical and electrical diagnostics to classify arcing states with enhanced precision. Arcing state data is then combined with data from additional diagnostics, experimental parameters, and machine configuration information in order to enable a comprehensive analysis of electrode arcing. Experimental correlations related to electrode arcing are presented and the physics of arcing is discussed. |
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GP11.00066: Upgraded Diagnostic Array for End Losses from C-2W's Open Field Line Plasma Martin Griswold, Philip McCarroll, Greg N Settles, The TAE Team In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], the FRC core plasma is surrounded by a mirror-confined scrape-off layer plasma on open field lines. An upgraded array of energy analyzers and bolometers has been installed in the divertors of C-2W to measure axial power losses as well as the electron temperature and ion energy distribution function (IEDF) of the plasma at the termination point of the open field lines. The previous system measured a strong ambipolar potential (~4.5 Te) along the open field lines, and value for the energy lost per ion, ηe, of 6-8, indicating good electron heat confinement [2]. The new system has improved radial and azimuthal coverage to investigate two-dimensional effects of open-field-line electron confinement. Design and preliminary measurements will be presented. |
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GP11.00067: Main Ion Temperature and Rotation in the C-2W Field Reversed Configuration (FRC) Plasma Ryan Marshall, Deepak K Gupta, Juan Aviles, Marcel Nations, James Sweeney, the TAE Team TAE Technologies' current experimental device, C-2W (also called "Norman"), produces an advanced beam-driven field reversed configuration (FRC) plasma. One of the newest diagnostics for the C-2W plasma is main ion CHarge Exchange Recombination Spectroscopy (mCHERS) to measure the main ion temperature and velocity. The diagnostic uses a 40kV/8Amp modulated diagnostic neutral beam to produce the charge exchange signal. Here we present the first observations and results from simultaneous measurements along five lines of sight. The radial profile of the measured main ion temperature shows a relatively flat distribution inside the separatrix. It is observed that increasing the bias voltage on the machine end electrodes results in higher main ion temperature. This measured trend builds on previous observations that the electrodes are key to both stability and performance of the C-2W FRC. The measured main ion temperature profile is compared with electron and impurity ion temperatures under various operating conditions. Main ion rotation measurements show a constant angular velocity inside the separatrix. Upgrades to the mCHERS diagnostic are currently underway to decrease measurement uncertainties and increase the number of channels, spectral resolution, and signal-to-noise ratio. |
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GP11.00068: Fast ion Doppler spectroscopy (FIDA) in C-2W Lucas A Morton, Richard M Magee, Ryan S Marshall, Marcel Nations, Deepak Gupta, Gabriel Player, Nathan Bolte, Juan Aviles, the TAE Team In TAE Technologies’ current experimental device, C-2W (also called “Norman”) [1], record breaking, advanced beam-driven field reversed configuration (FRC) plasmas are produced and sustained in steady state utilizing variable energy neutral beams (15 – 40 keV, total power up to 20 MW), advanced divertors, end bias electrodes, and an active plasma control system. We present new results from the Fast Ion D-Alpha (FIDA) on C-2W using modulation of the heating neutral beams, as an alternative to the diagnostic neutral beam. The heating neutral beams provide larger signals, but the close proximity of multiple neutral beams introduces beam emission background. Three viewports were selected to provide a variety of perspectives on the fast ion velocity space. An image intensifier is also employed to increase the signal levels when viewing the diagnostic beam. We have characterized the intensifier to understand its performance. We make use of synthetic diagnostic modeling (using FIDASIM [2,3]) based on Monte Carlo fast ion simulations. The simulations results show highly non-gyrotropic fast ion velocity distributions and large fast ion orbits. These features break the assumptions of standard fast ion orbit tomography techniques & will necessitate use of the more advanced orbit-space reconstruction method (inferring fast ion distribution as parameterized by constants-of-motion.) |
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GP11.00069: Helium line ratio imaging in the C-2W divertor Erik Granstedt, Deepak K Gupta, Kurt Knapp In TAE Technologies' current experimental device, C-2W (also called ``Norman'')[1], voltages applied to concentric annular electrodes located in divertors are used to stabilize beam-driven field reversed configuration (FRC) plasmas. In addition, magnetic field expansion is employed to thermally isolate electrons from the end electrodes. Measurements of electron temperature and density in the divertor are important in order to compare the experiment with models for the axial electrostatic potential and electron confinement. To this end, a three-wavelength, 2D imaging instrument has been deployed to study the plasma in the divertor using the helium line ratio method. The neutral helium target is provided by a super-sonic gas injector located inside the divertor vessel which injects helium toward the magnetic axis and perpendicular to the camera sight-cone. Three wavelength channels are isolated and imaged simultaneously on an image intensifier via an optical system containing dichroic beam-splitters, narrow-band interference filters, and mirrors. |
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GP11.00070: Simulation of Coherent Terahertz Radiation from Relativistic Laser-Solid Interactions ALES NECAS, Roger J Smith, Laura Galeotti, Kan Zhai, Gerrit Bruhaug Experiments to study the interaction between a high-intensity (>1018 W/cm2) laser and various solid targets have been performed at the University of Rochester Laboratory for Laser Energetics (LLE) lab aimed to generate terahertz (THz) radiation. 2D3V particle-in-cell (PIC) simulations have been performed at TAE Technologies, Inc (TAE) in support of the experiment. The PIC simulations reproduce the experimentally observed THz characteristics as well as the electron energy spectrum. Further, we shall report on the scaling of THz energy with laser intensity. We shall also report on a more optimized scenario where a microplasma waveguide (MPW) is irradiated with a relativistic (a0>1) p-polarized sub-pico second laser, resulting in high-charge (10s nC) electron bunches with energies up to 100 MeV. Lastly, we shall present THz radiation application to a novel pulsed polarimetry method to enable the measurement of internal magnetic fields in burning plasmas, with applications to TAE’s next generation field reversed configuration (FRC) device – Copernicus. |
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GP11.00071: A Repetitive Pre-ionizer Using Semiconductor Marx Generator for Compact Toroid Injector Ian A Allfrey, Thomas Roche, Tadafumi Matsumoto, Hiroshi Gota, the TAE Team The C-2W device (also known as 'Norman') at TAE Technologies has proven suc- cessful at generating stable, long-lived field-reversed configuration plasmas (FRCs) with record temperatures. A multi-pulse magnetized coaxial plasma gun has been developed to inject spheromak-like compact toroids to refuel the FRC's core. To minimize excess neutral gas and to improve breakdown characteristics of the compact toroid injector a pre-ionization system has been developed. The pre-ionization plasma is formed by high voltage discharge between two coaxial electrodes and accelerated by the Lorentz force. The discharge can operate at up to 1 kHz rate using the repetitive Marx generator composed of pulsed capacitors, which are charged in parallel and discharged in series by IGBT switches. The Marx generator output is -10 kV operating at a peak power in excess of 1 MW, which is consistent with the design requirements. |
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GP11.00072: MultiPINNs: Using an Ensemble of Physics-Informed Neural Networks to Generalize to Unseen Equilibria Cory B Scott, Sean Dettrick, Laura Galeotti, Calvin Lau Physics-Informed Neural Networks (PINNs) [1], are a flexible family of function approximators which have shown promise as PDE solvers. PINNs can be used to solve boundary value problems, fitting a function on the boundary of a domain (subject to constraints ensuring that predictions satisfy a set of diff. eqns on the interior). We demonstrate that PINNs can reproduce plasma equilibria calculated by TAE's multi-fluid equilibrium code (LR_eqMI) [2]. We train a separate PINN on each equilibrium, showing they can solve our PDEs of interest to comparable levels of numerical error. Once trained, evaluating a PINN is much faster than a run of the equilibrium code. Additionally, we show that once we have a trained ensemble of PINNs, we can generalize to equilibria outside of our training dataset. This ensemble method increases the utility of PINNs, turning them from single-use tools for a particular equilibrium (which find one PDE solution) to a joint solver which finds a solution to an entire class of PDEs. We demonstrate applications of this technique to simulation hyperparameter estimation and surrogate modelling of plasma physics. |
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GP11.00073: Neighboring equilibria and integrity of elongated FRC configurations Leonid Zakharov In the TAE C-2W experiment, confinement and stability of Field |
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GP11.00074: High-density, high-field Field-Reversed Configuration formation scaling John C Boguski, Ian A Bean, Thomas E Weber The formation of dense, long-lived Field-Reversed Configuration (FRC) plasmoids suitable for use in Magnetized Target Fusion (MTF) or kinetic shock physics experiments relies critically on efficient flux-trapping and high values of initial post-formation poloidal flux. New high-voltage, low-inductance pulsed power drivers are under development at Los Alamos National Laboratory (LANL) that will enable formation at several times the density and poloidal flux levels of the former Magnetized Shock eXperiment (MSX) machine. Initial FRC formation results using the new hardware will be presented and performance limits of current techniques discussed. |
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GP11.00075: Pre-ionization for high-density, high-field Field-Reversed Configuration formation Ian A Bean, John C Boguski, Colin S Adams, Thomas E Weber
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GP11.00076: Dependence of magnetic reconnection on ion gyroradius during the super-Alfvénic/sonic collisional merging process of field-reversed configurations Daichi Kobayashi, Taichi Seki, Yasuaki Tamura, Tatsuhiro Watanabe, Tsutomu Takahashi, Jordan Morelli, Tomohiko Asai The observation of the magnetic reconnection during the super-Alfvénic/sonic collisional merging process of field-reversed configurations (FRCs) is being attempted in the FAT-CM device. In the collisional merging formation of FRCs, two FRC-like initial-plasmoids collide at a relative speed that exceeds both the typical Alfvén speed and the ion sound speed, then they merge into one FRC [1]. The fast magnetic reconnection may occur during the collisional merging process which is completed in around 20 microseconds. In collision-less plasmas, the finite Larmor radius effect is considered to be one of the mechanisms that dissipates the magnetic flux and causes fast magnetic reconnection at Alfvén speed. In this work, the low-density FRC formation technique [2] is applied to the formation of dilute and high-temperature initial-plasmoids to control the ion gyroradius. The reconnection process is observed using the fast-framing camera (ULTRA Cam HS-106E, nac Image Technology Inc.) with a bandpass filter. The dependence of global behavior of the magnetic reconnection on ion gyroradius will be presented. |
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GP11.00077: Dependence of Energy Flow on the Translation Velocity in Collisional Merging of Field-Reversed Configuration Taichi Seki, Daichi Kobayashi, Yasuaki Tamura, Tatsuhiro Watanabe, Hiroki Someya, Tsutomu Takahashi, Jordan Morelli, Tomohiko Asai The dependence of energy flow on translation velocity in the collisional merging process of field-reversed configurations (FRCs) is evaluated in the FAT-CM device at Nihon University. Two FRC-like plasmoids formed by field-reversed theta-pinch are translated directly towards each other and collide at super-sonic/Alfvénic velocity. These plasmoids eventually become one FRC by the collisional merging. Assuming that an FRC has little axial velocity after merging, the kinetic energy of initial plasmoids is released to the outside of the confinement or regenerated into internal energy. The energy regeneration in the collisional merging process was experimentally observed [1]. In this work, the regeneration from kinetic energy into internal energy in the collisional merging of initial plasmoids is evaluated by comparing the cases of high and low translation velocity. The initial plasmoids are accelerated due to the magnetic pressure gradient provided by the conical theta-pinch coils. Current density is controlled by changing the pinch coil geometry to a shorter length, then the high magnetic pressure is generated in the formation sections on both ends of the device and the kinetic energy of initial plasmoids is increased. |
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