Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Plasma Physics
Volume 67, Number 15
Monday–Friday, October 17–21, 2022; Spokane, Washington
Session NI01: Magnetic Confinement Fusion IVLive Streamed
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Chair: Stefano Munaretto, PPPL Room: Ballroom 100 A |
Wednesday, October 19, 2022 9:30AM - 10:00AM |
NI01.00001: Runaway current reconstitution after a 3D MHD flush of the runaway plateau Invited Speaker: Chris McDevitt Benign termination of a mega-Ampere level runaway (RE) current has been convincingly demonstrated in JET [1] and DIII-D [2], establishing it as a leading candidate for runaway mitigation on ITER. In particular, these experiments have demonstrated that a global MHD instability triggered by low-Z (deuterium) injection is able to expel the majority of REs and distribute them over a broad area of the vessel wall, thus terminating the RE plateau while avoiding localized damage. Extrapolation of this approach to reactor scale devices hinges on the near complete expulsion of the existing REs by the MHD instability. In this work, we show that the number of REs that survive a global MHD instability depends sensitively on the electron phase space distribution during the current plateau, which in turn is a sensitive function of the impurity content of the plasma. In particular, the strong pitch-angle scattering that coincides with plasmas containing significant amounts of high-Z material leads to a substantial number of magnetically trapped energetic electrons. These trapped electrons remain well confined in the presence of a stochastic magnetic field, and are shown to be sufficient to initiate the partial reformation of the RE plateau via the avalanche mechanism for plasmas carrying current in excess of a few mega-Amperes across a range of plasma conditions. The present work will elucidate strategies through which the phase space distribution of REs during the current plateau can be tailored to minimize this trapped population of electrons, along with determining critical parameters for expediting the decay of the remnant electron population before the magnetic flux surfaces are able to reform. |
Wednesday, October 19, 2022 10:00AM - 10:30AM |
NI01.00002: Off-axis runaway electron seed formation, growth and suppression in MST tokamak plasmas Invited Speaker: Luis F Delgado-Aparicio New observations of the seed formation and dynamics of the birth conditions of runaway electrons (REs) in quiescent density ramp-down experiments have recently been carried out at the Madison Symmetry Torus (MST). The formation of an off-axis RE seed with linear growth rates has been resolved for low energies, a hollow streaming parameter and large electric fields ($E_{\parallel}/E_{D}$) in agreement with theory and simulations; the emergence of the seed population in the plasma periphery instead of that in at the magnetic axis is consistent with a lower electron-density and Dreicer fields. Secondary exponential growth rates have also been spatially resolved for the first time and are consistent with a convective transport of the order of the Ware pinch and energies up to $1000\times T_{e,0}$. The use of a newly developed versatile multi-energy soft x-ray (SXR) pinhole camera provides unprecedented improvement in throughput and signal-to-noise-ratio thus enabling early-detection, imaging ($\Delta r/a\sim2\%$, $Delta t\sim1$ ms) and low-energy discrimination; the latter is of great advantage over conventional REs studies conducted in large tokamaks with electron temperatures of few keV and electron energies up to 1-60 MeV. Seed calculations using a newly developed Backward Monte Carlo code computing the RE generation in space and time dependent dynamic scenarios including radial transport will also be presented. Numerical simulations are shown to reproduce the experimental observations including the off-axis runaway electron generation, radial transport and exponential growth at the core, as well as suppression due to $m=3$ resonant magnetic perturbations even in regions with large ($E_{\parallel}/E_{D}$). |
Wednesday, October 19, 2022 10:30AM - 11:00AM |
NI01.00003: Rotation of non-axisymmetric halo current in disrupting plasmas Invited Speaker: Alex R Saperstein Large rotating halo current (HC) generated during tokamak disruptions may damage next step fusion devices, like ITER and SPARC, if the non-axisymmetric HC rotation resonates and amplifies mechanical stresses on structures surrounding the plasma. An empirical estimate for the expected HC rotation, based on data from C-Mod, NSTX, ASDEX Upgrade, DIII-D, and JET, projected the average HC rotation in ITER will be above 20 Hz [1]. However, the scaling parameters used to define this empirical scaling were insensitive to variations of the toroidal magnetic field. In this presentation, we present a new drift-frequency-based scaling law for the rotation frequency of the asymmetric component of the HC as a function of toroidal field strength and plasma minor radius (frot ∝ 1/(Bt*a2)) [2]. This scaling law is motivated by the faster HC rotation observed in the HBT-EP tokamak, with Bt = 0.35 T, while also being consistent with observations from other tokamaks used previously. The new scaling indicates non-axisymmetric HC will rotate more slowly in ITER (10 Hz) and will rotate near 60 Hz in SPARC, similar to that previously estimated [4]. The new scaling is also important in light of models for the rotation rate [3], because it implies the mechanism for HC rotation must be ExB flow. Even within a single HBT-EP discharge, as the minor radius decreased, the HC rotation increases approximately as 1/a2 demonstrating that the rotation associated with the open field lines of the HC must be primarily poloidal but slower than the speed previously estimated [1]. |
Wednesday, October 19, 2022 11:00AM - 11:30AM |
NI01.00004: (De-)stabilization of sawteeth by anisotropic fast ions in DIII-D negative/positive triangularity plasmas: modeling versus experiments Invited Speaker: Deyong Liu Recent DIII-D experiments show, for the first time, that sawtooth stability is strongly affected by anisotropic fast ions from Neutral Beam Injection (NBI) in negative triangularity plasmas, similar to previous observations in positive triangularity plasmas. Fast ions from co-(counter-) current NBI are stabilizing (destabilizing) for sawteeth, resulting in longer (shorter) sawtooth periods. The relative change of sawtooth period and amplitude is more than a factor of two. Non-perturbative toroidal modeling, utilizing the MHD-kinetic hybrid stability code MARS-K, reveals an asymmetric dependence of the stability of the n = 1 internal kink on the injection direction of NBI, being qualitatively consistent with the experimental observation. The MARS-K modeling results suggest that anisotropic fast ions affect the mode growth rate and frequency through both adiabatic and non-adiabatic contributions. The asymmetry of the internal kink mode instability relative to the NBI direction is mainly due to the non-adiabatic contribution of passing fast ions, which stabilize (destabilize) the internal kink with the co-(counter-) current NBI. On the other hand, Finite Orbit Width (FOW) correction to passing particles partially cancels the asymmetry. Trapped particles are always stabilizing due to precessional drift resonances. Modeling also suggests that fast ions affect the internal kink in a similar manner between negative and positive triangularity plasmas, although being slightly more unstable in the negative triangularity plasmas already in the fluid limit. Furthermore, MARS-K modeling indicates that other factors, such as the plasma rotation and drift kinetic effects of thermal plasmas, weakly modify the mode stability as compared to the drift kinetic resonance effects and FOW correction of fast ions. |
Wednesday, October 19, 2022 11:30AM - 12:00PM |
NI01.00005: Long-radial-range turbulent transport events in high collisionality H-mode plasmas on DIII-D tokamak Invited Speaker: Rongjie Hong We report on the observation of spatially asymmetric long-radial-range transport events in the plasma core when the mean shear layer is reduced, which may explain confinement degradation in high-collisionality H-mode plasmas. In this study, a dimensionless collisionality scan experiment was performed on the DIII-D tokamak, and the turbulent transport events are observed by Doppler backscattering. These events develop from sub-ion-gyroradius turbulence into radially elongated, streamer-like structures, and the intensity spans a wide radial scale in the mid-radius region. The underlying turbulence of these events shows clear intermittency, e.g., large skewness and kurtosis. The underlying turbulence also features a Hurst exponent between 0.7 and 0.8, indicating a long-term memory effect. The wavenumber power spectrum obeys a scale-invariant 1/k power law, which resembles avalanches in self-organized criticality. The amplitude and the radial scale of these transport events increase substantially when the shearing rate of the mean flow is reduced below the turbulent scattering rate. These findings constitute the first experimental observation of long-radial-range turbulent transport events in high-collisionality H-mode fusion plasmas and demonstrate the role of mean shear flows in the formation and propagation of turbulence with long-radial-range correlation. Linear CGYRO simulations suggest that the underlying turbulence is likely driven by the electron temperature gradient. The power balance analysis shows that the core thermal diffusivity increases substantially as collisionality is raised. Such core turbulent transport events may serve as a candidate explanation for the degradation of normalized energy confinement time at high collisionality. These findings are of significance to future fusion power plants which would operate with high density and weak flow shear. |
Wednesday, October 19, 2022 12:00PM - 12:30PM |
NI01.00006: Multi-Z Impurity Transport in DIII-D ITER Similar Shape Plasmas: Experiment, Gyrokinetic Simulation, and Gyrokinetic Based, Flux-Matched Profile Predictions Invited Speaker: Nathan T Howard DIII-D experiments probing impurity transport in ITER similar shape H-mode plasmas have been analyzed using a multi-pronged approach: experimental inference of impurity transport, nonlinear gyrokinetic simulation, and gyrokinetic-based, kinetic profile predictions enabled by novel machine learning techniques. A wide range of impurity species (He, Li, C, F, Al, Ca, and W) were introduced into repeat discharges to gather validation quality profile and spectroscopic data and enable experimental inference of impurity transport coefficients (D and V) and impurity peaking. More than 50 nonlinear gyrokinetic simulations were performed with high physics fidelity and compared with experimental heat (Qi, Qe) and particle flux (gamma_e), allowing for multi-Z impurity transport coefficient predictions to be extracted. These simulations indicate that ITG and grad-n driven TEM dictate heat and particle transport and reveal a clear dependence of impurity diffusion and peaking on Z that varies radially; ITG dominates inside rho = 0.45 with diffusion scaling with 1/Z, and grad-n TEM dominates at larger radii with diffusion scaling with Z. Simulated impurity peaking trends yield only marginal agreement with measured peaking. Attempts to resolve this discrepancy motivated flux-matched profile prediction based on nonlinear gyrokinetic simulation. Previously prohibitively expensive, such analysis has only recently been made tractable using novel optimization techniques. We present experimental transport results (heat fluxes, particle fluxes, and impurity transport coefficients), nonlinear gyrokinetic simulation comparisons spanning rho = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, detailed comparison of gyrokinetic flux-matched profiles with experimental kinetic profiles and impurity peaking, and implications for future FPP operation. |
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