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 UI1: MHD |
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Chair: Francois Waelbroeck, University of Texas Room: Rosen Centre Hotel Junior Ballroom |
Thursday, November 15, 2007 2:00PM - 2:30PM |
UI1.00001: Progress in understanding error-field physics in NSTX spherical torus plasmas Invited Speaker: The low aspect ratio, low magnetic field, and wide range of plasma beta of NSTX plasmas provide new insight into the origins and effects of magnetic field errors. An extensive array of magnetic sensors has been used to analyze error fields (EFs), to measure error field amplification (EFA), and to detect resistive wall modes (RWMs) in real time. The measured error-field threshold for the onset of locked modes shows a linear scaling with plasma density, a weak dependence on B$_{T}$, and a positive scaling with magnetic shear. These results extrapolate to a favorable threshold $\delta $B$_{21}$/B$_{T} \quad >$ 1$\times $10$^{-4}$ for ITER. For these low-beta locked-mode plasmas, perturbed equilibrium calculations find that the plasma response must be included to explain the empirically determined optimal correction of NSTX error fields [1]. In high-beta NSTX plasmas exceeding the n=1 no-wall stability limit where the RWM is stabilized by plasma rotation, active suppression of n=1 EFA and correction of newly discovered n=3 error fields have led to sustained high rotation and record durations free of low-frequency core MHD activity. For sustained rotational stabilization of the RWM, both the rotation threshold and magnitude of EFA are important. At fixed normalized dissipation, kinetic damping models predict rotation thresholds to scale nearly linearly with particle orbit frequency. Studies for NSTX find orbit frequencies at large minor radius are a factor of two higher than used in the present kinetic damping theory derived in the limit of high aspect ratio and circular plasma cross-section. Such discrepancies may explain the recent observation of kinetic damping models under-predicting the critical rotation [2]. \newline \newline [1] J.K. Park, et al., ``Correction of magnetic field errors in tokamaks'', submitted to PRL (2007) \newline [2] H. Reimerdes, et al., Phys. Rev. Lett. 98, 055001 (2007) [Preview Abstract] |
Thursday, November 15, 2007 2:30PM - 3:00PM |
UI1.00002: Neoclassical toroidal viscosity and error-field penetration in tokamaks Invited Speaker: A model for field error penetration is developed that includes both resonant and non-resonant perturbed 3-D magnetic fields [1]. The non-resonant components give rise to a global neoclassical toroidal viscous [NTV] torque while a single resonant component produces a localized electromagnetic braking torque on its respective resonant surface. The NTV torque tries to keep the plasma flowing at a rate comparable to the ion diamagnetic flow. A phenomenological cross-field viscosity is included which resists the resonant electromagnetic torque in the vicinity of the resonant surface. Steady-state toroidal momentum balance across the resonant layer gives a solubility condition determining the ``critical'' resonant error-field strength---termed the \emph{penetration threshold}---above which rotational shielding is lost and the resonant surface locks to the lab frame. Such locking occurs in low-density start-up tokamak plasmas [2], leading to plasma disruptions or confinement degradation and is a key issue for ITER. The toroidal momentum balance equation admits a WKB-type solution which implies that NTV acts to enhance cross-field viscosity in the vicinity of the resonant surface. This enhancement makes the plasma less sensitive to error-field penetration than previously predicted [3]. In particular, if $\tau_E \propto n_e$ (neo-Alcator-like) and the perpendicular momentum confinement time has no density dependence, we find the penetration threshold scales linearly with electron density---a result giving quantitative agreement for the first time between theory and experiment [2]. \newline \newline [1] A.J.~Cole, C.C.~Hegna, and J.D.~Callen, to be published in PRL (2007). \newline [2] S.M.~Wolfe, I.R.~Hutchinson, et al., Phys.\ Plasmas \textbf{12}, 056110 (2005) and refs.~cited therein. \newline [3] A.J.~Cole and R.~Fitzpatrick, Phys.\ Plasmas \textbf{13}, 032503 (2006) and refs.~cited therein. [Preview Abstract] |
Thursday, November 15, 2007 3:00PM - 3:30PM |
UI1.00003: Extrapolating Neoclassical Tearing Mode Physics to ITER -- Physics Basis and Experimental Comparison Invited Speaker: Neoclassical Tearing Modes (NTMs) represent one of the most serious concerns for baseline and hybrid scenario performance in ITER. Already on present devices they limit attainable $\beta $, degrading confinement and causing disruptions. The concern is increased for ITER where stabilising small island and rotation effects are likely to be reduced. In this paper we review the physics basis for NTM scalings, and compare to experimental behaviour, to deduce the key effects and impact on ITER prediction. The principal criteria for NTM onset is dictated by a competition between stabilising small island effects, and the drive from NTM-triggering MHD (eg. sawteeth). Typically the former arise from orbit and transport effects when island sizes are comparable to ion banana widths. This suggests a lowering of NTM $\beta $ thresholds as ITER-like \textit{$\rho $}$_{i}^{\ast }$s are approached. In addition, reduced plasma rotation will increase NTM coupling to other instabilities and decrease stabilising effects due to wall and rotation shear. New studies on JET and DIII-D have highlighted this with falls of $\sim $30{\%} in both m/n=3/2 and 2/1 NTM $\beta $ thresholds as momentum injection is removed. Indeed, a wide body of work confirms many aspects of the theory, particularly the expected small island effects and \textit{$\rho $}$_{i}^{\ast }$ scalings, while more detailed examinations, for example locally perturbing rotation with error fields, begin to distinguish particular physics mechanisms such as ion polarisation current effects. Thus consideration of the stabilising elements points to a lower metastability threshold for the NTM in ITER. Nevertheless, the triggering mechanisms provide grounds for optimism. For the most serious 2/1 NTM, onset in hybrid, and possibly baseline, scenario appears related to proximity to ideal $\beta $ limits. Conversely, modes triggered by core MHD may be managed by proven control techniques for the core MHD itself. [Preview Abstract] |
Thursday, November 15, 2007 3:30PM - 4:00PM |
UI1.00004: Advancing Tokamak Physics with the ITER Hybrid Scenario on DIII-D Invited Speaker: Recent DIII-D experiments using hybrid scenario plasmas (hybrids) have furthered our understanding of transport and stability in high beta tokamaks, leading to the possibility of high fusion performance on ITER. The hybrid is a stationary, inductively driven, $q_0 \sim1$ discharge with better confinement and stability than standard H-mode. Providing stationary, high beta conditions, the hybrid is an excellent configuration for study of tokamak plasma physics under conditions of interest to burning plasmas, such as low rotation, balanced $T_{\mbox{e}}$ and $T_{\mbox{i}}$, shaping, and pedestal behavior. Compared to a standard H-mode, the hybrid has a broader current profile, reducing or eliminating the deleterious effects of sawteeth, and is less susceptible to $m/n = 2/1$ NTMs, allowing higher $\beta$ operation. Our experiments have conclusively shown that the current profile is broadened by a relatively benign $m/n = 3/2$ NTM. Power balance in hybrids is dominated by electron heat conduction, but the observed electron thermal diffusivity is relatively small, and the ion thermal diffusivity is consistently at or close to the neoclassical value. Using the recent modification to the DIII{\-}D neutral beam configuration, we have been able to reduce the toroidal rotation velocity to a central Mach number $<$0.1, under stationary conditions. We find that confinement improves with increasing rotation. Gyrofluid simulations indicate that this is associated with the change in ExB flow shear. The width of the NTM island decreases as rotation and rotation shear are increased. However, the difference in the fusion performance parameter G (=$\beta_{\mbox{N}}\cdot H$/$q^{2})$ at low and high rotation is only 10{\%}-30{\%}. Thus, although rotation and rotation shear are important parameters for improving tokamak performance, good confinement and stability can be maintained even in their absence. [Preview Abstract] |
Thursday, November 15, 2007 4:00PM - 4:30PM |
UI1.00005: MHD simulations of disruption mitigation on DIII-D and Alcator C-Mod Invited Speaker: The three potential threats posed by disruptions---halo currents, heat fluxes and runaway electrons---scale unfavorably from present tokamaks to ITER. Disruption mitigation experiments on several tokamaks have shown massive gas injection (MGI) to be an effective means of reducing poloidal halo current and heat flux. However, both theory and measurements support the conclusion the penetration of the neutral jet is weak. Thus the core thermal quench relies on MHD, both to mix impurities into the core, and to conduct heat to the impurity-dense edge. NIMROD simulations of C-Mod have shown that enhanced transport alone---due to large 1/1 and 2/1 modes triggered by edge cooling---can quench the core plasma [1]. These simulations show similarity to C-Mod temperature measurements [2], and the role of the 1/1 and 2/1 modes is supported by observations in DIII-D [3]. However, to determine the relative importance of thermal transport versus impurity mixing simulations that include both mechanisms are needed. An extension of the NIMROD code has been developed which includes both accurate atomic physics from the 0D KPRAD code and separate continuity equations for each species. C-Mod simulations for both helium and argon impurities are compared with earlier simulations and experimental data to assess the extent of impurity mixing and evaluate MGI as a mitigation technique for ITER. DIII-D simulations are carried out with different radial neutral fueling profiles to understand the thermal quench when impurity injection is more uniform, or centrally peaked, as would be the case for designer pellets or liquid jets. \newline \newline [1] V.A. Izzo, Nucl. Fusion \textbf{46} (2006) 541. \newline [2] R.S. Granetz, et al., Nucl. Fusion \textbf{46} (2006) 1001. \newline [3] E.M. Hollmann, et al., Nucl. Fusion \textbf{45} (2005) 1046. [Preview Abstract] |
Thursday, November 15, 2007 4:30PM - 5:00PM |
UI1.00006: Momentum transport from current-driven reconnection Invited Speaker: In rotating toroidal plasmas in both laboratory and astrophysical settings, toroidal angular momentum is observed to be transported radially outward. In both cases the transport is much greater than can be explained by collisional viscosity. In the reversed field pinch (RFP), the toroidal rotation profile flattens abruptly during a reconnection event. To explain the RFP transport, we have performed a theoretical and computational study of momentum transport from reconnection - from tearing modes in the presence of sheared flow. We find that, whereas a single mode produces transport, a strong enhancement in transport arises from the nonlinear coupling of multiple modes. A single tearing mode, in the presence of equilibrium flow, produces momentum transport in the vicinity of the reconnection layer. This is demonstrated from quasilinear calculation of Maxwell and Reynolds stresses. However, nonlinear, resistive MHD computation of the full, multi-mode dynamics reveals an additional effect. In the presence of multiple tearing modes, nonlinear coupling strongly enhances the torques. The effect of multiple tearing modes is not merely the superposition of independent, radially separated effects. Rather, the torque from one spatial mode is itself increased by the presence of other modes. The resulting transport is much more rapid than that from viscosity only. Theoretical results will be compared to momentum transport measurements in the MST experiment. Momentum transport in astrophysical plasmas (such as accretion disks) is generally thought to arise from flow-driven MHD instability. Our work raises the question of whether current-driven instability can play a role. Preliminary application to astrophysical disks will be discussed. Work supported by NSF and DOE. [Preview Abstract] |
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