Bulletin of the American Physical Society
60th Annual Meeting of the APS Division of Plasma Physics
Volume 63, Number 11
Monday–Friday, November 5–9, 2018; Portland, Oregon
Session NI3: Flows, 3-D Tokamaks, Pinches |
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Chair: Chris Hegna, University of Wisconsin, Madison Room: OCC Oregon Ballroom 204 |
Wednesday, November 7, 2018 9:30AM - 10:00AM |
NI3.00001: Observations of main-ion toroidal rotation and turbulence fluctuations across the ITG/TEM transition in DIII-D and comparison to gyrokinetic simulations Invited Speaker: Brian A Grierson New full-profile main-ion rotation measurements and gyrokinetic simulations across the transition from ion temperature gradient (ITG) to trapped electron mode (TEM) transport show that main-ion toroidal rotation smoothly progresses from deeply hollow to flat. Furthermore, the impurity toroidal rotation is clearly peaked in the TEM regime in contrast to the very flat main-ion rotation. Main-ion rotation, rather than impurity rotation, is representative of the bulk angular momentum transport and this critical distinction has significant implications on intrinsic rotation studies when only impurity measurements are available. These main-ion measurements were obtained during ohmic deuterium plasma discharges by controlled ramp-down of the plasma density, with electron cyclotron heating in the low-density conditions to access the TEM regime. DBS measurements show long wavelength turbulence in the high density ITG regime and absence of long wavelength fluctuations in the low density TEM regime. In the spatial region where the main-ion rotation gradient changes, GYRO simulations show the linear stability spectrum transitions from ion to electron direction, indicative of an ITG to TEM transition. In the ITG regime, uncertainty analysis indicates close proximity to the ITG/TEM boundary, showing this may be a mixed-mode regime where the most unstable mode depends sensitively on the plasma radius and temperature scale length. However, turbulence measurements indicate the presence of ion-scale fluctuations at this time in the discharge, bolstering the simulation results. Beyond local linear stability, radially global nonlinear simulations have been performed with GTS, and predict hollow and flat profiles in the ITG and TEM regimes, respectively. These measurements and simulations show the need for direct main-ion rotation measurements for validating predictive models of intrinsic plasma rotation. |
Wednesday, November 7, 2018 10:00AM - 10:30AM |
NI3.00002: Impurity transport and zonal flows in improved-confinement reversed field pinch plasmas Invited Speaker: Takashi Nishizawa Trapped-electron-mode (TEM) microturbulence appears in the edge of MST RFP plasmas that have reduced tearing instability and tokamak-level confinement using current profile control. High-frequency density fluctuations (k_perp*rho_s=0.2-0.4) emerge with a critical gradient threshold as the density profile steepens. These features are consistent with gyrokinetic simulations using GENE that include a small magnetic fluctuation mimicking residual tearing activity, which tends to disrupt zonal flow formation. Here we present direct measurements of impurity transport and zonal flows to investigate TEM turbulence saturation and transport. A new method of linearized spectrum correlation analysis for spectroscopic data resolves simultaneously the fluctuations in both the turbulent radial velocity and impurity density. Their correlation reveals an inward flux of C III impurities, which is the first direct evidence for transport associated with TEM turbulence in the RFP. The C III ions are edge-localized and evolve from graphite limiters. The profile of the plasma potential is measured in the edge using two multi-channel capacitive probes, each having 7 mm radial spatial resolution. An edge-localized flow is observed, and with the probes separated 180 degrees toroidally, the flow has a long-range correlation characteristic of zonal structure. The amplitude of the flow is modulated by the turbulence, as occurs in predator-prey-like dynamics. These measurements, together with the gyrokinetic modeling, suggest that transport in RFP plasmas will ultimately be regulated by microturbulence as occurs in tokamak and stellarator plasmas. |
Wednesday, November 7, 2018 10:30AM - 11:00AM |
NI3.00003: Multi-mode plasma response and experimental validation Invited Speaker: Zhirui Wang A multi-mode plasma response model, extended by including rotating three-dimension (3D) field perturbations, has been developed, which provides significant insight into the damping rates and spatial structures of magnetohydrodynamic (MHD) modes in stable fusion plasmas. The enhanced model is validated with dedicated experiments in DIII-D and EAST tokamaks, where, for the first time, the comprehensive capabilities of 3D actuators (magnetic coils) and sensors allow high-fidelity construction of multi-pole plasma transfer functions for stable plasmas, based on simultaneous scans of both the frequency and spatial structure of the applied 3D field. Comparison of MARS-F n=1 multi-mode response modeling with the extracted multiple eigenmodes in DIII-D experiments indicates the importance of the conducting wall for AC response of the plasma, as the rotation frequency of 3D fields exceeds 10 Hz. Based on measured data, reconstruction of multi-mode plasma response model, for the n=2 mode locking experiments in DIII-D L-mode plasmas, appears to reveal a strong correlation between the secondary stable mode and the field penetration threshold. These encouraging results thus demonstrate that this enhancement of the MHD spectroscopy approach not only helps to improve the physics understanding of general stable eigenmodes, but can also offer great potential for understanding and controlling of, e.g. edge localized modes by resonant magnetic perturbations, or for real-time monitoring of the plasma stability, with the latter serving as a key element of an integrated approach for disruption prediction and avoidance in future reactors such as ITER. |
Wednesday, November 7, 2018 11:00AM - 11:30AM |
NI3.00004: Rotation Profile Control Enabled by Multi-modal Response to 3D Fields Invited Speaker: Nikolas C Logan Predictions of the Generalized Perturbed Equilibrium Code (GPEC) have been validated in DIII-D experiments testing the manipulation of the multi-modal plasma response for neoclassical toroidal viscosity (NTV) torque control. A multi-modal matrix formulation was employed to optimize spectra applied using multiple 3D field coil arrays to obtain desired NTV torque profiles. The new formulation [1] solves the anisotropic pressure perturbed equilibrium, representing the nonlinear torque as a “torque response matrix” that has been directly coupled to available experimental coils. The eigenmodes of such a torque response matrix then provide predicted optimal coil configurations for the maximum, minimum, core localized and edge localized NTV torque profiles. Each of these have been predicted for and applied in DIII-D experiments, where the multi-modal n=2 plasma response allows for significant manipulation of the NTV profile using the three 3D field coil arrays. The experiments validated the GPEC model in nonresonant field space, where it provides accurate predictions of quiescent braking profiles that could be used in rotation control algorithms with little impact on the particle or energy confinement. Large edge resonant magnetic perturbations, however, caused large density pumpout not accounted for in the neoclassical model, significantly distorting the equilibrium from the perturbative model prediction and motivating the integration of 3D and 2D transport models. Within their regime of validity, the optimizations have demonstrated the clear ability to manipulate the NTV profile with existing coils and provide the best NTV spectra for the design of future 3D field coils. [1] J.-K. Park and N.C. Logan, Phys. Plasmas 24, 32505 (2017) |
Wednesday, November 7, 2018 11:30AM - 12:00PM |
NI3.00005: Helical core formation and evolution in high-field tokamaks and its extrapolation to ITER Invited Speaker: Andreas Wingen Large, spontaneous m/n = 1/1 helical cores are predicted in tokamaks such as ITER with extended regions of low- or reversed- magnetic shear profiles and q near 1 in the core. The helical core is a saturated internal kink mode; its onset threshold is determined by (dp/dρ)/Bt2 = const. along the threshold. Helical cores occur frequently in Alcator C-Mod [1] during ramp-up when slow current penetration results in a reversed shear q-profile. Using a technique for 3D equilibrium reconstruction, the onset and early development of a helical core in C-Mod was reconstructed in a time series. It is found that a reverse shear q-profile as well as a hollow pressure profile reduce the onset threshold, enabling helical core formation. In C-Mod the pressure profile becomes hollow due to impurity radiation in the plasma core, which is also expected to occur in ITER. Beneficial effects can include sawteeth stabilization; helical cores flatten the q-profile, driving it toward q = 1. Combined with plasma rotation this suggests current redistribution, so called flux-pumping, which has been observed in DIII-D helical cores [2] and modeled theoretically [3]. On the other hand fast ion confinement is predicted to degrade [4]. A cross-machine comparison of the helical core onset threshold for discharges from C-Mod, DIII-D and ITER confirms that while DIII-D is marginally stable, C-Mod and especially ITER are highly susceptible to helical core formation. Predictions for ITER show a large helical core with a size of 50% of minor radius in the 15 MA standard H-Mode scenario. [1] L. Delgado-Aparicio et al., PRL 110, 065006 (2013) [2] P. Piovesan et al., Nucl. Fusion 57, 076014 (2017) [3] S.C. Jardin et al., PRL 115, 215001 (2015) [4] D. Pfefferlé et al., Nucl. Fusion 54, 064020 (2014) |
Wednesday, November 7, 2018 12:00PM - 12:30PM |
NI3.00006: Establishing Stability Conditions for Sheared-Flow-Stabilized Z-Pinch Plasmas via Fully Kinetic 2-D Simulations1 Invited Speaker: Kurt Tummel Z-pinch configurations with improved stability are a topic of considerable interest, both for understanding the physics underlying the stabilization and for the possibility of extending performance to fusion relevant conditions. The sheared-flow-stabilized (SFS) Z-pinch[1,2] has demonstrated this behavior with stable plasma columns persisting for over 1000 Alfven radial transit times, and the on-going FuZE experiments[3] are scaling this design to higher current. First-ever 2-D fully-kinetic simulations have been performed to determine growth rates and map out stability boundaries for the most destructive instabilities. The simulations use a realistic mass ratio with a direct implicit scheme and a spatial resolution ~1/10 the minimum ion gyroradius, which can resolve ion-scale turbulence. The simulations are performed with plasma conditions that match the on-going experiments, and with the projected conditions of an SFS Z-pinch reactor. In both equilibria stabilization and damping of m = 0 sausage instabilities is achieved when peak flow speeds are less than the plasma sound speed. This is a more favorable result when compared to previous simulations based on fluid approximations that indicated supersonic flows are necessary: a requirement that would preclude scaling the concept to Q>1 reactor conditions.
[1] U. Shumlak, C. W. Hartman, Phys. Rev. Lett. 75 3285 (1995). [2] U. Shumlak, R. P. Golingo, B. A. Nelson, D. J. Den Hartog, Phys. Rev. Lett. 87 205005 (2001) [3] E. L. Claveau et al., this conference |
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