Bulletin of the American Physical Society
55th Annual Meeting of the APS Division of Plasma Physics
Volume 58, Number 16
Monday–Friday, November 11–15, 2013; Denver, Colorado
Session PI2: Heating, Flows and Transport |
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Chair: John Rice, Massachusetts Institute of Technology Room: Plaza E |
Wednesday, November 13, 2013 2:00PM - 2:30PM |
PI2.00001: Fast wave heating and edge power losses in NSTX and NSTX-U Invited Speaker: Nicola Bertelli Experimental studies of high harmonic fast wave (HHFW) heating on the National Spherical Torus Experiment (NSTX) have demonstrated that substantial HHFW power loss can occur along the open field lines in the scrape-off layer (SOL), but the mechanism behind the loss is not yet understood [1]. Extended ray tracing and full wave codes are being applied to specific NSTX discharges in order to predict the causes of this power loss. Previous full wave simulations predict that cavity-like modes may form outside of the LCFS [2]. We find that inserting a collisional loss in the SOL of AORSA to represent a damping process indicates an effective collisional term of $\nu/\omega\sim [0.05 - 0.1]$ which is considerably larger than the $\nu/\omega\sim 0.005$ obtained with Spitzer resistivity, suggesting the damping scale of the loss mechanism. The magnitude of the edge collisional losses are being used to evaluate possible potential damping mechanisms in the SOL. Initial numerical analyses show that the presence of the SOL has a significant impact on the launched antenna spectrum. The upgrade of NSTX, NSTX-U, will operate with toroidal magnetic fields ($B_{\rm T}$) up to 1 T, nearly twice the values used on NSTX. The doubling of $B_{\rm T}$ while retaining the 30 MHz RF frequency moves the heating regime for NSTX-U to the mid harmonic fast wave (MHFW) regime [3], which will be analyzed and contrasted with the HHFW regime on NSTX. These studies indicate that direct ion damping might be more significant in NSTX-U under TRANSP predicted full performance conditions. Modifications of fast ion distributions due to the interaction of fast waves with NBI will be presented in both MHFW and HHFW regimes.\\[4pt] [1] R. J. Perkins et al, Phys. Rev. Lett. 109, 045001 (2012).\\[0pt] [2] D. L. Green et al, Phys. Rev. Lett. 107, 145001 (2011).\\[0pt] [3] N. Bertelli et al, 20th Topical Conf. on RF Power in Plasma (2013), to be published in AIP Conference Proceedings. [Preview Abstract] |
Wednesday, November 13, 2013 2:30PM - 3:00PM |
PI2.00002: Physics of fast flux closure in coaxial helicity injection experiments in NSTX Invited Speaker: Fatima Ebrahimi Advancing toward non-inductive start-up and current drive for tokamaks, a solenoid-free plasma start-up method called transient coaxial helicity injection (CHI), first developed on the small HIT-II device, has been extended to the large NSTX device, in which up to 300kA of plasma current has been generated. Unlike driven CHI (edge current drive) where non-axisymmetric MHD activity relaxes the current inward, in transient CHI only axisymmetric reconnection generates a high quality closed flux start-up equilibrium, as found in resistive MHD simulations of CHI in NSTX using the NIMROD code (nimrodteam.org). Closed flux surfaces during simulations of transient CHI can be explained through 2-D Sweet-Parker type reconnection. Non-axisymmetric 3-D modes do not appear to play a dominant role at present experimental parameters. Our simulations have used fixed boundary flux (including NSTX poloidal coil currents) and the NSTX experimental geometry. We find that, as in the experiment, an X point followed by a fairly large volume of closed flux surfaces is rapidly formed; within 0.5 ms after the injector voltage and current begin to rapidly decrease. These direct numerical simulations reveal the fundamental mechanism for the reconnection process in transient CHI. Through direct numerical calculations, we find that as the injector voltage is turned off, the fields lines tend to untwist in the toroidal direction and magnetic field compression exerts a radial JXB force to bring oppositely directed field lines closer together to reconnect. A hierarchy of models from a zero pressure model to simulations with temperature evolution, allow us a full and more detailed understanding of the reconnection and closed flux surfaces. We find that magnetic fluxes are only reconnected at low magnetic diffusivity (high Lundquist number). In these simulations, narrow current layers form and cause the flux to close at a fast time scale when pinch flows are generated near the injector flux foot-print location.\\[4pt] This work has been done in collaboration with R. Raman, E. B. Hooper and C. R. Sovinec. Supported by DOE-FG02-12ER55115. [Preview Abstract] |
Wednesday, November 13, 2013 3:00PM - 3:30PM |
PI2.00003: Testing Neoclassical and Turbulent Effects on Poloidal Rotation in the Core of DIII-D Invited Speaker: C. Chrystal A new method of measuring impurity poloidal rotation with charge exchange recombination spectroscopy has been developed for DIII-D [1], and is used to show poloidal spin-up at the formation of an internal transport barrier as well as poloidal rotation which is not neoclassical. Rotation is an essential part of tokamak plasma dynamics, and poloidal rotation, though typically smaller than toroidal rotation in present-day experiments with current-aligned neutral beam injection, can be particularly important when toroidal rotation is small, as is expected to be the case in future tokamaks such as ITER. These new measurements are focused on the core of DIII-D plasmas and their high spatial resolution in this region has been instrumental to showing that poloidal rotation makes a significant contribution to the high $E\times B$ shearing rate needed to sustain an internal transport barrier. In addition, measurements for a variety of low- and high-performance plasmas have been compared to neoclassical theory and shown both agreement and disagreement. All these measurements are aided by the fact that the new method does not require calculations to correct for the energy dependence of the charge exchange cross section or gyro motion during the finite excited state lifetime. Disagreement with neoclassical theory is not easily explained by a single global parameter and agreement was seen in unexpected regimes such as QH-mode. A possible explanation for disagreement between experiment and neoclassical theory is the presence of turbulence and turbulence-driven poloidal rotation. The role turbulence plays in poloidal rotation dynamics is important for future tokamaks and it is investigated through the use of turbulence diagnostics and GYRO simulation.\par \vskip6pt \noindent [1] C.~Chrystal, et al., Rev.\ Sci.\ Instrum.\ {\bf 83}, 10D501 (2012). [Preview Abstract] |
Wednesday, November 13, 2013 3:30PM - 4:00PM |
PI2.00004: Neoclassical Flows, Transport, and Non-Axisymmetric Effects in the Tokamak Plasma Edge Invited Speaker: E.A. Belli The drift-kinetic code NEO is used to explore the neoclassical transport and flows for parameters relevant in the plasma edge. NEO includes multiple ion species, general geometry, strong rotation effects, and full-linearized Fokker-Planck collisions. Comparisons are made with measurements of the deuterium and carbon flows for DIII-D L-mode discharges. An assessment of the accuracy of analytic models for the bootstrap current finds that NEO provides a 15\% correction to the Sauter model for experimental plasmas. Analysis of the recent XGC0-based modification by Koh et al., finds that while the Koh modification is negligible for typical DIII-D plasmas, there is a large discrepancy from the NEO results in the pedestal for NSTX plasmas due to a failure in the formula at large inverse aspect ratio for large collision frequency ($\nu_{*_e}\sim 1$), and thus the Koh formula is not accurate in regions where it differs from the Sauter model. Overall, the resulting implication that NEO could significantly improve the accuracy of peeling ballooning and kinetic ballooning mode stability calculations in the edge barrier region is explored through coupling with the EPED model. Finally, NEO is extended to include toroidal non-axisymmetric effects for studies of magnetic field ripple and resonant magnetic perturbations. The equilibrium is generated using a new 3D local analytic equilibrium solver, analogous to a 3D extension of the Miller formalism for shaped axisymmetric equilibria, based on the formalism by Hegna. Unlike a global solver, the method allows for systematic studies of the effects of 3D flux-surface shaping parameters. With the solver, the onset of stochasticity for general 3D flux surface configurations is studied. Combined with NEO, the effects of enhanced neoclassical transport due to the formation of superbanana orbits and the development of a more accurate kinetic-based NTV are explored. [Preview Abstract] |
Wednesday, November 13, 2013 4:00PM - 4:30PM |
PI2.00005: Ion intrinsic rotation in a tokamak caused by momentum transport due to diamagnetic flow effects Invited Speaker: Jungpyo Lee Ion toroidal angular momentum is redistributed in a tokamak by turbulence, and the momentum redistribution determines the radial profile of rotation. The momentum transport due to diamagnetic flow effects is an important piece of the radial momentum transport for sub-sonic rotation (Mach $\sim$ 0.1-0.2), which is often observed in experiments. In a non-rotating state, the diamagnetic flow and the ExB flow must cancel. The diamagnetic flow and the ExB flow have different effects on the turbulent momentum flux, and therefore induce intrinsic rotation. This momentum flux is evaluated using gyrokinetic equations that are higher order in poloidal rhostar, which include the diamagnetic correction to Maxwellian equilibria. To study the momentum transport due to diamagnetic flow effects, three experimental observations of ion rotation are examined. First, we found that a strong pressure gradient at the plasma edge results in a significant inward momentum transport due to the diamagnetic effect, which may explain the observed peaking of rotation in a high confinement mode. Second, a different direction of the momentum transport in terms of collisionality is found, which is qualitatively consistent with the observed reversal of intrinsic rotation by plasma density and current. Last, the dependence of the momentum flux on the current profile is found, and it may explain the rotation change in the presence of lower hybrid current drive, which is different in low current and high current discharges. [Preview Abstract] |
Wednesday, November 13, 2013 4:30PM - 5:00PM |
PI2.00006: Electron Particle Transport on DIII-D - Testing Theory-Based Models and Implications for ITER Invited Speaker: E.J. Doyle A series of experiments on the DIII-D tokamak show peaked (non-flat) density profiles which are invariant as a function of collisionality, with experimental measurements in good agreement with theory-based predictions by TGLF and GYRO. The experiments were designed to test the $\nu^\ast$ scaling of particle transport, a key issue for ITER, with measurements of perturbative transport and complete turbulence data sets. In both L- and H mode experiments, similarity scan techniques were employed to vary collisionality ($\nu^\ast$) by a factor of 3-5, while holding other dimensionless parameters constant. No change was observed in either case in the measured density profiles and density profile peaking, in contrast to published H-mode database results in which density peaking clearly scales with $\nu^\ast$. The experiments were also specifically designed to compare measured particle diffusion coefficients (D) and particle pinch velocities (v), as well as turbulence characteristics, with TGLF and GYRO. The experimental results are in qualitative agreement with GYRO predictions of little change in particle transport across the collisionality range probed. Detailed GYRO analysis of the actual experimental discharges is underway in order to assess quantitative behavior. We have also compared the experimental results to TGLF modeling in three separate ways, including a significant new capability to directly compare experimental perturbative particle transport measurements with the predictions of TGLF modeling. The first such comparisons, presented here, show good agreement for perturbative particle transport rates. TGLF modeling is also in good agreement with experimental measurements for equilibrium density profile and density fluctuation response as a function of collisionality. Neutral particle modeling has been performed with SOLPS5, which can replicate the core density response to perturbative gas puffing. [Preview Abstract] |
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