Bulletin of the American Physical Society
56th Annual Meeting of the APS Division of Plasma Physics
Volume 59, Number 15
Monday–Friday, October 27–31, 2014; New Orleans, Louisiana
Session TI1: 3D Magnetic Confinement Physics |
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Chair: Stephen Jardin, Princeton Plasma Physics Laboratory Room: Acadia |
Thursday, October 30, 2014 9:30AM - 10:00AM |
TI1.00001: Simulation of 3D effects on partially detached divertor conditions in NSTX and Alcator C-Mod Invited Speaker: Jeremy Lore Establishing a validated, predictive capability for the divertor plasma is critical for future fusion reactors, which must operate with detached divertors to reduce peak heat fluxes to the plasma facing components (PFC) and to mitigate net material erosion. This is challenging even for existing 2D codes, and is complicated further by non-axisymmetric effects due to divertor-localized gas injection, 3D magnetic fields, and 3D PFCs, modeling of which requires 3D simulations. New experiments performed on C-Mod at the request of the ITER organization to examine the consequence of localized nitrogen gas injection, show clear toroidal asymmetries in radiated power, impurity radiation, and divertor pressure. The 3D plasma/neutral transport code EMC3-EIRENE has been applied to model these experiments in the first attempt to benchmark the code against tokamak experimental data under detached conditions. The measured pressure modulation and the impurity radiation trends in the edge are qualitatively reproduced by the simulations, which also predict a $\sim$ 2x modulation in heat flux at the outer strike point. Discrepancies are found in comparison with the measured private region radiation, and the simulations also indicate colder, denser divertor conditions than measured, suggesting that drifts and kinetic corrections may be required for more quantitative agreement. In separate experiments on NSTX, detached divertor plasmas are observed to reattach when 3D fields are applied. Modeling of the NSTX experiments reproduces these trends, with an increased peak heat flux with 3D fields and qualitative agreement of the striated flux patterns. The experimental identification of toroidal asymmetries in detached plasmas highlights the need for reliable 3D models for projecting the impact for ITER and beyond. Support from USDOE DE-AC05-00OR22725, DE-AC02-09CH11466, DE-FC02-99ER54512. [Preview Abstract] |
Thursday, October 30, 2014 10:00AM - 10:30AM |
TI1.00002: Comparison of Measurements of Profile Stiffness in HSX to Nonlinear Gyrokinetic Calculations Invited Speaker: Gavin Weir Tokamaks and stellarators have observed significant differences in profile stiffness, defined as the ratio of the transient thermal diffusivity obtained from heat pulse propagation to the diffusivity obtained from steady-state power balance. Typically, stellarators have measured stiffness values below 2 and tokamaks have observed stiffness greater than 4. In this paper we present the first results on stiffness measurements in the quasihelically symmetric experiment HSX in which the neoclassical transport is comparable to that in a tokamak and turbulent transport dominates throughout the plasma. Electron Cyclotron Emission (ECE) is used to measure the local electron temperature perturbation from modulating the ECRH system on HSX. Spectral analysis of the ECE data yields a profile of the perturbed amplitude and a resulting transient electron thermal diffusivity that is close to the steady-state diffusivity. This evidence of a lack of stiffness in HSX agrees with the scaling of the steady-state heat flux with temperature gradient. The experimental data is compared to gyrokinetic calculations using the GENE code with two kinetic species. Linear calculations demonstrate that the Trapped Electron Mode (TEM) is the dominant long-wavelength microturbulence instability with growth rates that scale linearly with electron temperature gradient. Nonlinear gyrokinetic flux tube simulations indicate that the TEM contributes significantly to the saturated heat fluxes in HSX, shifting the transport-carrying wavenumbers to larger values than in typical Ion Temperature Gradient (ITG) turbulence. A set of nonlinear simulations are being executed, examining the saturated nonlinear heat flux as a function of the electron temperature gradient, to obtain a stiffness value from the simulations to compare with experimental results. This work is supported by DOE grant DE-FG02-93ER54222. [Preview Abstract] |
Thursday, October 30, 2014 10:30AM - 11:00AM |
TI1.00003: Drift Kinetic Effects on 3D Plasma Response in High-beta Tokamak Resonant Field Amplification Experiments Invited Speaker: Z.R. Wang Through theory and simulation of drift kinetic effects, modeling with the MARS-K code has for the first time explained the linear plasma response to 3D fields in the vicinity of the ``no-wall'' ideal beta limit. A longstanding issue in understanding resonant field amplification (RFA) of plasma to 3D fields is that the ideal magnetohydrodynamics (MHD) theory predicts an unlimited amplification near the no-wall stability limit. However, in many experiments such as DIID-D and NSTX, the plasma response increases almost monotonically along with the plasma beta across the ideally predicted no-wall limit. This disagreement is now explained by perturbed drift kinetic theory and associated with distorted particle orbits by 3D fields. The upgraded MARS-K code, which has the capability to solve linearized hybrid MHD equations with drift kinetic effects self-consistently, is applied to study the DIII-D RFA experiments through the quantitative comparison. It reveals the kinetic effect due to thermal particles plays a major role in modifying the response structure throughout plasma and keeps the finite amplification of response, as the experimental measurements, around the no-wall beta limit. The perturbed energy analysis shows the modification of plasma response is mainly contributed by the precession, bounce and transit resonances of thermal ions. The kinetic effect of isotropic energetic particles with slowing down distribution can further slightly change the plasma response without significant contribution. RFA experiments in NSTX plasmas are also analyzed to affirm the role of drift kinetic effect on modifying the plasma response. This study shows good agreements between theoretical results and various RFA experimental measurements, providing the possible physics explanation of RFA phenomena observed in many tokamaks. The results also indicate the validity of self-consistent calculation of hybrid drift kinetic-MHD model with drift kinetic effect in high beta tokamaks. [Preview Abstract] |
Thursday, October 30, 2014 11:00AM - 11:30AM |
TI1.00004: Experimental Tests of Linear and Nonlinear 3D Equilibrium Models in DIII-D Invited Speaker: J.D. King A major upgrade to the DIII-D magnetic diagnostics has been used to show that linear ideal MHD captures the key features of plasma response to 3D fields over a wide range of conditions, while a nonlinear model disagrees with observations. New measurements on the low and high field sides of the torus allow detailed comparisons with ``synthetic diagnostic'' predictions of several MHD models, in plasmas with edge safety factor ($q_{95}$) and plasma pressure ($\beta$) spanning the range expected for ITER's baseline scenario. Model comparisons with DIII-D data confirm that the linear ideal MHD code MARS-F provides a quantitative description of the plasma response to applied fields with toroidal mode numbers n=1 and n=3. Similarly, good experimental agreement is seen for the linearized two-fluid M3DC-1 code and ideal MHD IPEC code. In contrast, the n=1 plasma response predicted by the nonlinear 3D equilibrium code VMEC is found to disagree with measurement in both amplitude (by a factor of 3 or more) and qualitative structure - perhaps related to VMEC's strong sensitivity to the current density near the plasma edge. The measured plasma response to n=3 perturbations is characterized by strong variations in amplitude at the high field side as the edge safety factor is varied, in good qualitative agreement with ideal MHD predictions of resonant behavior as rational surfaces reach the edge ($q_{95}=m/3$). However, the resonance at $q_{95}=11/3$ appears significantly weaker than the others, which suggests the presence of non-ideal effects - possibly loss of screening currents and formation of islands. This observation may shed new light on the physics of the suppression of edge localized modes (ELMs) by n=3 magnetic perturbations, which in DIII-D occurs most easily with $q_{95}$ near 11/3. The success of linear, ideal MHD models, as well as the understanding of the limits of their validity (e.g. edge resonances, and the well-known role of kinetic effects in high $\beta$ plasmas), will be important in predicting the response of future burning plasmas to small 3D perturbations. [Preview Abstract] |
Thursday, October 30, 2014 11:30AM - 12:00PM |
TI1.00005: Observing the plasma response to applied non-axisymmetric fields in the presence of an adjustable ferritic wall Invited Speaker: Jeffrey Levesque We report high-resolution detection of the time-evolving, three-dimensional (3D) plasma response to applied non-axisymmetric magnetic fields in a tokamak with an adjustable ferromagnetic wall and with a variably-shaped equilibrium. Ferritic tiles (5mm thick, saturated $\mu / \mu_0 \sim 8$) have been added to the plasma-facing side of half of the in-vessel movable wall segments in the High Beta Tokamak -- Extended Pulse (HBT-EP) device\footnote{J.P. Levesque \textit{et al.}, \textbf{Nucl. Fusion} \textbf{53}, 073037 (2013).} in order to explore Ferromagnetic Resistive Wall Mode (FRWM) stability.\footnote{V.D. Pustovitov and V.V. Yanovskiy, \textbf{Phys. Plasmas} \textbf{21}, 022516 (2014).} Low-activation ferritic steels are a candidate for structural components of a fusion reactor, and these controlled experiments examine MHD stability of plasmas with nearby ferromagnetic material. Plasma-wall separation for alternating ferritic and non-ferritic wall segments can be adjusted between discharges without opening the vacuum vessel. Amplification of applied resonant fields is observed to increase when the ferromagnetic wall is close to plasma surface instead of the standard stainless steel wall. Experiments with rapidly rotating external kink modes show wall stabilization despite the presence of the close ferritic wall ($b/a \sim 1.07$), extending previous observations in JFT-2M.\footnote{K. Tsuzuki \textit{et al.}, \textbf{Nucl. Fusion} \textbf{46}, 966 (2006).} Plasmas are observed to have reduced wall stabilization when a biased electrode is used to slow the mode rotation. Resonant fields are also applied while the plasma evolves from circular limited cross-sections to shaped, single-null cross-sections in order to study the effects of shaping on multimode interactions. Multimode activity in diverted and limited plasmas is compared with DCON predictions. [Preview Abstract] |
Thursday, October 30, 2014 12:00PM - 12:30PM |
TI1.00006: Observations of Transitions Between Nested and Braided Magnetic Island in DIII-D and LHD Invited Speaker: K. Ida Measurements of modulated heat pulse propagation in DIII-D have revealed the existence of self-regulated oscillations in the radial energy transport that are indicative of bifurcations in the structure of a q=2 magnetic island. Strong screening of the heat pulse is seen in one state followed by weak screening later in the discharge. The magnetic island with strong screening is interpreted as having a narrow stochastic region near X-point with nested flux surfaces occupying most of the island (a completely nested island), where very slow heat pulse propagation suggests a reduction of transport. Weak screening of heat pulse suggests a wide stochastic region near the X-point and a small region of nested flux surfaces inside the island (partially braided magnetic island). The reduction of heat transport observed inside magnetic island has recently been reported both in helical and tokamak plasmas [1,2]. In the LHD experiment, there are two patterns of heat pulse propagation observed in the flat temperature region. One is a bi-directional slow heat pulse propagation and the other is a fast heat pulse propagation. This bifurcation in the heat pulse propagation is consistent with a topological transition between a nested magnetic island and a completely braided magnetic island in LHD and is consistent with the DIII-D measurement. In the DIII-D experiments, the radial heat transport is either enhanced or reduced depending on the state of the island, because the radial transport is reduced inside O-point region while it is enhanced near the X-point region. These results demonstrate that the structure of magnetic islands can spontaneously transition during a discharge and modulate the local energy transport inside magnetic island by at least a factor of three.\par \vskip6pt \noindent [1] S.~Inagaki et al., Phys.\ Rev.\ Lett.\ {\bf 92}, 055002 (2004).\par \noindent [2] K.~Ida et al., Phys.\ Rev.\ Lett.\ {\bf 109}, 065001 (2012). [Preview Abstract] |
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