Bulletin of the American Physical Society
49th Annual Meeting of the Division of Plasma Physics
Volume 52, Number 11
Monday–Friday, November 12–16, 2007; Orlando, Florida
Session YI1: Ripple and Rotation Studies |
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Chair: Steve Scott, Princeton Plasma Physics Laboratory Room: Rosen Centre Hotel Junior Ballroom |
Friday, November 16, 2007 9:30AM - 10:00AM |
YI1.00001: Effects of toroidal field ripple on H-modes in JET and implications for ITER operations Invited Speaker: The effects of toroidal field ripple (dB) on H-mode confinement are not well understood and were not included in the criteria for the design of the ITER toroidal field coil system. ITER has 18 TF coils, and dB at the plasma outer midplane separatrix position is about 1{\%} or, with the present design of correcting ferritic inset (designed to limit first wall loads from fast ion losses), is reduced to about 0.5{\%} at full field. Experiments were carried out in JET to study the effect of toroidal field ripple dB on ELMy H-modes confinement and pedestal parameters, as well as on ELM type and size. In the experiment, dB at the separatrix was increased to 0.3{\%}, 0.5{\%}, 0.75{\%} and 1{\%} in separate plasma discharges, at approximately constant absorbed power. The main effect of increasing dB is a loss of plasma density (approx 30{\%} to 40{\%}), across the whole plasma profile, observed even at the minimum dB. The H-mode enhancement factor is reduced by up to approximately 20{\%} at 1{\%} ripple compared to reference (dB=0.08{\%}). VTOR decreases by a factor of 4 in the pedestal region when dB$_{ }$goes from 0.08{\%} to 0.5{\%} At higher dB, VTOR at the plasma edge goes to zero and eventually becomes negative (dB =1{\%}). The H-mode density can be increased by external gas puff and the response of the density to external fuelling is similar for plasmas with and without ripple. Ripple also affects Type I ELM behavior. For plasmas without external gas fuelling, the ELM energy loss is smaller by a factor of 2 with 1{\%} ripple compared to the reference case, due to a reduction of the prompt temperature drop at the ELM. The confinement degradation and loss of density may have worrying implications for the performance of the ITER baseline scenario. The extrapolation of JET results to ITER and possible design solutions for further reduction of dB in ITER will be discussed. [Preview Abstract] |
Friday, November 16, 2007 10:00AM - 10:30AM |
YI1.00002: Plasma Rotation Research for Advanced Tokamak Plasma Control in JT-60U Invited Speaker: The fusion research has to establish an efficient control of the self-regulating plasmas for achieving a high integrated performance. Toward this goal, the mechanisms determining the plasma rotation profile and effects of the rotation on transport and stability are the central issues. Reduction of the toroidal field ripple by installing ferritic steel tiles in JT-60U reduced the fast ion losses and resultant counter plasma rotation drive. By combining this newly achieved freedom of rotation with co-, counter- and perpendicular NBs, JT-60U has been promoting an integrated research project focusing on the plasma rotation covering the research areas of transport, stability, pedestal, and steady state operation. With a new perturbed transport analysis, we successfully separated the diffusive and the non-diffusive terms of the momentum transport in L and H-mode plasmas, clarified their dependences, reproduced the toroidal rotation profile utilizing these evaluated transport coefficients and external torque input, and identified the drive for intrinsic rotation which is determined by the pressure gradient locally. In the type-I ELM regime, we found that the shift of the rotation into the co-direction reduces the inter-ELM transport, enhances the pedestal width and height. The grassy ELM frequency increases almost linearly with increasing counter rotation. Even at zero-rotation, a high ELM frequency with sufficiently small ELM energy loss is obtained. This result encourages applicability of the grassy ELM to ITER. As for the high-beta stability, the critical rotation speed for the resistive wall mode stability was found to be 0.3{\%} of the Alfven speed almost up to the ideal-wall limit. Rreversed shear plasmas with bootstrap fraction $\sim$70{\%} was sustained for 8 s, where the beta-collapse was avoided through reduction of the pressure gradient at ITB by the rotation control. [Preview Abstract] |
Friday, November 16, 2007 10:30AM - 11:00AM |
YI1.00003: Spontaneous Plasma Rotation Scaling in the TCV Tokamak Invited Speaker: Predicting intrinsic plasma rotation that helps stabilize various plasma instabilities that can adversely affect the plasma performance is important for prospective fusion grade devices. ITER-like scenarios have been extrapolated from measured experimental plasma rotation data but little is understood about the underlying mechanisms governing either the generation or dissipation of momentum in a Tokamak plasma. On ITER, rotation is expected to be dominated by intrinsic plasma processes whereas most experimental observations use strong momentum injection sources such as heating Neutral Beams. With a Diagnostic Neutral Beam, driving negligible toroidal velocity, CXRS in TCV provides a high quality direct measurement of the intrinsic plasma toroidal and poloidal rotation profiles for Ohmic and EC-heated plasmas in diverted and limited configurations for a wide range of plasma shaping. The plasma behavior can be separated by the core and edge regions. For limited configurations, core counter-current toroidal rotation scales inversely with plasma current (Scarabosio PPCF 2006) and exhibits a reproducible direction inversion with a $<$10{\%} rise in plasma density (Bortolon PRL 2006). In diverted configurations, a co-current toroidal velocity reverses direction with a $<$10{\%} rise in plasma density. Edge toroidal rotation is strongly frictional for limited configurations whereas an edge velocity scaling with core density is observed for diverted configurations. Core toroidal momentum is strongly distributed by sawteeth but the rotation torque evolves and inverts separately from the edge. The behavior of the rotation and deduced radial electric field profiles are shown as a function of plasma shape and compared to changes in other plasma parameters. [Preview Abstract] |
Friday, November 16, 2007 11:00AM - 11:30AM |
YI1.00004: Dependence of Edge Turbulence Dynamics and the L-H Power Threshold on Toroidal Rotation Invited Speaker: The injected power required to induce a transition from L-mode to H-mode plasmas is found to depend strongly on the injected neutral beam torque and consequent plasma toroidal rotation. Edge turbulence and flows, measured near the outboard midplane of the plasma (0.85 $<$ r/a $<$ 1.0) on DIII-D with the high-sensitivity 2D beam emission spectroscopy (BES) system, likewise vary with rotation and suggest a causative connection. The L-H power threshold in plasmas with the ion $\nabla $B drift away from the X-point decreases from 4-6 MW with co-current beam injection, to 2-3 MW near zero net injected torque, and to $<$2 MW with counter injection. Plasmas with the ion $\nabla $B drift towards the X-point exhibit a qualitatively similar though less pronounced power threshold dependence on rotation. 2D edge turbulence measurements with BES show an increasing poloidal flow shear as the L-H transition is approached in all conditions. As toroidal rotation is varied from co-current to balanced in L-mode plasmas, the edge turbulence changes from a uni-modal character to a bi-modal structure, with the appearance of a low-frequency (f=10-50 kHz) mode propagating in the electron diamagnetic direction, similar to what is observed as the ion $\nabla $B drift is directed towards the X-point in co-rotating plasmas. At low rotation, the poloidal turbulence flow near the edge reverses prior to the L-H transition, generating a significant poloidal flow shear that exceeds the measured turbulence decorrelation rate. This increased poloidal turbulence velocity shear may facilitate the L-H transition. No such reversal is observed in high rotation plasmas. This reduced power threshold at lower toroidal rotation may benefit inherently low-rotation plasmas such as ITER. [Preview Abstract] |
Friday, November 16, 2007 11:30AM - 12:00PM |
YI1.00005: Measurements and implications of particle and momentum transport from magnetic stochasticity in MST Invited Speaker: Magnetic stochasticity associated with radial magnetic field fluctuations ($\delta b_r )$ is expected to have significant effects on plasma transport. Particle and momentum transport due to stochastic magnetic fields are defined as $\frac{<\delta j_{//,e} \delta b_r >}{eB_0 }$ and $\frac{<\delta p_{//,i} \delta b_r >}{B_0 }$, respectively, where $\delta j_{//,e} $ and $\delta p_{//,i} $ are parallel electron current density fluctuations and parallel ion pressure fluctuations. A recently developed differential interferometer method is used to measure local density fluctuations, while a fast Faraday rotation diagnostic measures radial magnetic field fluctuations and current density fluctuations. Direct measurements of particle and momentum transport during reconnection events (the crash phase of a sawtooth oscillation) in the MST reversed field pinch show that; (1) the magnetic fluctuation-induced particle flux accounts for the change in the core equilibrium density, and (2) the convective component of the \textit{momentum} transport from stochasticity is of sufficient magnitude to contribute to the known anomalous momentum transport in the plasma core. Furthermore, the difference between magnetic fluctuation-induced electron flux and ion flux, ($\frac{<\delta j_{//} \delta b_r >}{eB_0 })$, has been experimentally determined by measuring Maxwell stress directly in the plasma core. It is nonzero (transport is locally nonambipolar) and produces a large radial electric field (and field shear) localized to the reconnection (resonant) surface. This electric field implies the existence of a localized \textit{zonal flow} that reverses direction about a reconnection surface -- a new mechanism for zonal flow generation. Author acknowledges contributions from D.L. Brower, B.H. Deng, T.F. Yates, UCLA, and the MST team. Work is supported by DoE and NSF. [Preview Abstract] |
Friday, November 16, 2007 12:00PM - 12:30PM |
YI1.00006: Turbulent Equipartition Theory of Toroidal Momentum Pinch Invited Speaker: The turbulent convective flux (pinch) of the toroidal angular momentum density is derived using the nonlinear toroidal gyrokinetic equation which conserves phase space density and energy[1], and a novel pinch mechanism which originates from the symmetry breaking due to the magnetic field curvature is identified. A net parallel momentum transfer from the waves to the ion guiding centers is possible when the fluctuation intensity varies on the flux surface, resulting in imperfect cancellation of the curvature drift contribution to the parallel acceleration. This pinch velocity of the angular momentum density can also be understood as a manifestation of a tendency to homogenize the profile of ``magnetically weighted angular momentum density,'' $nm_{i}RU_{\parallel}/B^{2}$. This part of the pinch flux is mode-independent (whether it's TEM driven or ITG driven), and radially inward for fluctuations peaked at the low-$B$-field side, with a pinch velocity typically, $V^{TEP}_{Ang} \sim - 2 \chi_{\phi}/R_{0}$. We compare and contrast the pinch of toroidal angular momentum with the now familiar ``turbulent equipartition'' (TEP) mechanism for the particle pinch[2] which exhibit some relevance in various L-mode plasmas in tokamaks. In our theoretical model[3], the TEP momentum pinch is shown to arise from the fact that, in a low-$\beta$ tokamak equilibrium, $B^{2}{\bf u}_{E} = c{\bf B \times \nabla} \delta \phi$ is approximately incompressible, so that the magnetically weighted angular momentum density ($m_{i}nU_{\parallel}/B^{3} \propto m_{i}nU_{\parallel}R/B^{2}$) is locally advected by fluctuating $\bf E \times B$ velocities, to the lowest order in $O(a/R)$. As a consequence $m_{i}nU_{\parallel}R/B^{2}$ is mixed or homogenized, so that $\frac {\partial}{\partial \psi} m_{i}nU_{\parallel}R/B^{2} \rightarrow 0.$ \newline \newline [1] T.S. Hahm, Phys. Fluids {\bf 31}, 2670 (1988) \newline [2] V.V. Yankov, JETP Lett. {\bf 60}, 171 (1994); M.B. Isichenko {\it et al.,} Phys. Rev. Lett. {\bf 74}, 4436 (1995); X. Garbet {\it et al.,} Phys. Plasmas {\bf 12}, 082511 (2005). \newline [3] T.S. Hahm, P.H. Diamond, O. Gurcan, and G. Rewoldt, Phys. Plasmas {\bf 14}, 072302 (2007). [Preview Abstract] |
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