Bulletin of the American Physical Society
50th Annual Meeting of the Division of Plasma Physics
Volume 53, Number 14
Monday–Friday, November 17–21, 2008; Dallas, Texas
Session BI2: Rotation in Tokamaks |
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Chair: John de Grassie, General Atomics Room: Landmark B |
Monday, November 17, 2008 9:45AM - 10:15AM |
BI2.00001: Spontaneous Toroidal Rotation Profiles and Observations of Momentum Pinch in Alcator C-Mod Plasmas Invited Speaker: Spontaneous toroidal rotation, self-generated in the absence of external momentum input, exhibits a rich phenomenology. In Ohmic L-mode plasmas, the rotation is predominantly in the counter-current direction and varies in a complicated fashion with electron density, magnetic configuration and plasma current. In contrast, the rotation in H-mode plasmas is mainly directed co-current, and has a relatively simple parameter dependence, with the magnitude of the core velocity proportional to the stored energy normalized to the plasma current. The co-current rotation is observed to propagate in towards the center from the plasma edge following the H-mode transition, on a time scale similar to the energy confinement time. The profile shapes, determined with a new imaging x-ray spectrometer system, range from relatively flat to centrally peaked, which in the latter case is indicative of the presence of an inward momentum pinch. The velocity gradient region is typically in the outer 2/3 of the plasma. Addition of LHCD power into ICRF H-mode discharges causes the rotation profile to become hollow in the core region, suggesting that the pinch has changed sign. In pure LHCD plasmas, the rotation is strongly peaked in the counter-current direction in the central half of the plasma, with the strong gradient region near r/a=0.3, demonstrating that the momentum pinch operates on rotation in either direction. Comparisons of these diverse observations of momentum pinches will be compared with a variety of recent theories. [Preview Abstract] |
Monday, November 17, 2008 10:15AM - 10:45AM |
BI2.00002: Plasma Rotation Driven by Static Nonresonant Magnetic Fields Invited Speaker: Recent experiments in high temperature DIII-D tokamak plasmas have reported the first observation of plasma acceleration driven by the application of static non-resonant error fields, with resulting improvement in the global energy confinement time. Toroidal rotation benefits tokamak experiments by flow shear stabilization of turbulence, screening of error fields, and stabilization of neoclassical tearing modes and resistive wall modes. However, a self-heated burning plasma will have little toroidal momentum injection. Toroidal momentum sinks will exist in a burning plasma, including torques from unavoidable magnetic field errors. Although the braking effect of static magnetic field asymmetries is well known, recent theory [A.J.\ Cole, et al., Phys. Rev. Lett. {\bf 99}, 065001 (2007)] predicts that they can lead instead to an increase in rotation frequency toward a ``neoclassical offset'' rate. We report the first experimental confirmation of this surprising result. When a large nonresonant $n=3$ field is applied to a steady plasma with small neutral beam torque, the measured toroidal rotation of impurity ions and the calculated main ion toroidal rotation both increase in the electron diamagnetic drift direction. The magnitude, direction, and radial profile of the observed offset rotation are consistent with theory, as is the reduction of the rotation increase at low beta. The offset rotation rate is about 1\% of the Alfv\'en frequency, more than double the rotation needed for stable operation at high beta above the no-wall kink limit in DIII-D. An important consequence is that the nonresonant field torque on a fusion plasma from high-n fields for ELM suppression may not represent a problem, but instead may generate a rotation rate sufficient to benefit confinement and stability. [Preview Abstract] |
Monday, November 17, 2008 10:45AM - 11:15AM |
BI2.00003: Progress in Physics and Control of the Resistive Wall Mode in Advanced Tokamaks Invited Speaker: The resistive wall mode (RWM) instability limits the achievable plasma pressure in present and future tokamaks operated in advanced scenarios. Motivated by the recent observation of enhanced stability of the RWM in low rotation plasmas in the collision-less regime, we extend the physics modeling of the RWM to include the drift kinetic resonance damping caused by thermal plasma particles. Starting from the drift kinetic equations, a self-consistent toroidal kinetic damping model of general plasma electromagnetic perturbations is developed and incorporated into the MARS stability code. This extended MARS-K code allows the study of the kinetic effects as both a perturbation to the basic MHD model resulting in a perturbed kinetic potential energy or in a non-perturbative manner resulting in a self-consistent kinetic potential energy based on a kinetic-MHD eigenfunction. MARS-K has been successfully benchmarked against other perturbative codes. With the unique capability of performing self-consistent kinetic calculations, MARS-K find that, in many cases, the non-perturbative approach leads to less stabilization than the perturbative approach. Detailed modeling of the plasmas in DIII-D, JET and ITER will be presented. We also extend our modeling capability to allow interaction of the 2D MHD code with the 3D eddy current structure. The new code, called CarMa, is capable of describing in much more detail the realistic geometry of the ITER walls and blanket modules, during feedback control of the RWM. [Preview Abstract] |
Monday, November 17, 2008 11:15AM - 11:45AM |
BI2.00004: Feedback Suppression of Rotating External Kink Modes in the Presence of Noise Invited Speaker: We report on the first experimental demonstration of active feedback suppression of rotating external kink modes near the ideal wall limit in a tokamak using Kalman filtering to discriminate the $n=1$ kink mode from background noise. In order to achieve the highest plasma pressure limits in ITER, resistive wall mode stabilization is required,\footnote{T.\thinspace C. Hender, J.\thinspace C. Wesley, J. Bialek, \textit{et al}., \textit{Nuclear Fusion} \textbf{47}, S128 (2007).} and feedback algorithms will need to distinguish the mode from noise due to other magnetohydrodynamic activity. The Kalman filter contains an internal model that captures the dynamics of a rotating, growing $n=1$ mode. This model is actively compared with real-time measurements to produce an optimal estimate for the mode's amplitude and phase. On the HBT-EP experiment, the Kalman filter algorithm is implemented using a set of digital, field-programmable gate array controllers with 10 microsecond latencies. Signals from an array of 20 poloidal sensor coils are used to measure the $n=1$ mode, and the feedback control is applied using 40 poloidally and toroidally localized control coils. The feedback system with the Kalman filter is able to suppress the external kink mode over a broad range of phase angles between the sensed mode and applied control field, and performance is robust at noise levels that render proportional gain feedback ineffective. Suppression of the kink mode is accomplished without excitation of higher frequencies as was observed in previous experiments using simple lead-lag loop compensation.\footnote{A.\thinspace J. Klein, D.\thinspace A. Maurer, T.\thinspace S. Pedersen, M.\thinspace E. Mauel, \textit{et al}., \textit{Phys. Plasmas} \textbf{12}, 040703 (2005).} [Preview Abstract] |
Monday, November 17, 2008 11:45AM - 12:15PM |
BI2.00005: Penetration of Resonant Magnetic Perturbations in Rotating Plasma Invited Speaker: Mode penetration occurs when a magnetic perturbation changes the plasma rotation so as to eliminate the screening effect. This enables the induced island to grow to a size comparable to its vacuum size. In the core, mode penetration gives rise to disruptive locked modes and must be avoided. In the edge, by contrast, resonant magnetic perturbations (RMP) have been shown to mitigate the edge localized modes (ELM), which present a serious threat to the ITER divertor. It is unclear, however, if mode penetration (and thus magnetic stochasticity) is in fact achieved in ELM mitigation experiments or if the favorable effect of the applied perturbations is due to the influence of the screened modes on edge turbulent transport. This is a critical question for ITER, since design constraints make it very difficult to place coils sufficiently close to the plasma so as to produce mode penetration at the edge while avoiding locked modes in the core. We have used a two-fluid model to investigate the effects of pedestal drifts and turbulence on mode penetration. We find that penetration occurs when the electric (ExB) drift compensates the electron diamagnetic drift, so that the electron fluid is at rest with respect to the perturbation. If the electrons are flowing, however, there is a marked asymmetry in the plasma response according to the direction of their flow. When their total velocity is opposite to that of the ion fluid, the magnetic perturbation excites drift-acoustic waves that cause anomalous transport. These waves propagate beyond the screened islands, so that the drift- acoustic wakes for nearby islands will overlap before the islands do. This suggests that the beneficial effects of RMP could accrue for much lower amplitudes than those required for mode penetration. [Preview Abstract] |
Monday, November 17, 2008 12:15PM - 12:45PM |
BI2.00006: Observation of ICRF Mode Conversion Plasma Flow Drive on Alcator C-Mod Invited Speaker: Plasma flow driven by externally launched rf waves could be important in stabilizing micro- and macro-instabilities in tokamaks. We report the first observation of both toroidal (V$_{\phi })$ and poloidal (V$_{\theta })$ flows driven via an ICRF mode conversion (MC) process in D($^{3}$He) plasmas. At modest $^{3}$He levels (n$_{3He}$/n$_{e}\sim $ 8{\%}), in relatively low density plasmas, $<$n$_{e}>\le $ 1.3$\times $10$^{20}$m$^{-3}$, heated with 50 MHz rf power (B$_{t0}\sim $ 5.1 T), strong V$_{\phi }$ in the co-current direction is observed by high-resolution x-ray spectroscopy. The central V$_{\phi }$ scales with the applied rf power ($\le $ 30 km/s per MW), and is at least a factor of 2 more than the empirically determined intrinsic plasma rotation [1]. The rotation near the plasma center (r/a $<$ 0.3) responds more quickly to the applied rf power than the outer region, indicative of a local flow drive source. Localized poloidal rotation (0.3 $\le $ r/a $\le $ 0.5) in the ion diamagnetic drift direction is observed when P$_{rf }\ge $ 1.5 MW and increases with power ($\sim $ 2 km/s at 3 MW). Turbulence spectrum broadening seen by a phase contrast imaging (PCI) system indicates strong flow also exists in the main ions. The mode converted ion cyclotron wave (MC ICW) is observed by PCI and confirmed by 2-D full wave TORIC code simulation. The simulation result shows that due to the up-shifted k$_{\vert \vert }$,$_{ }$the MC ICW is strongly damped on $^{3}$He ions in the vicinity of the MC layer, approximately on the same flux surfaces where poloidal flow is observed. The involvement of ion heating and short-wavelength slow wave is consistent with theoretical considerations for efficient rf flow drive. Our experimental results are comparable to the predictions [2], assuming similar ion interaction mechanism for the MC ICW and direct launch ion Bernstein wave. The feasibility of ICRF flow drive on ITER will be discussed. [1] J. E. Rice, et al, Nucl. Fusion \textbf{47}, 1618 (2007). [2] J. R. Myra and D. A. D'Ippolito, Phys. Plasmas \textbf{9}, 3867 (2002). [Preview Abstract] |
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