Bulletin of the American Physical Society
2005 47th Annual Meeting of the Division of Plasma Physics
Monday–Friday, October 24–28, 2005; Denver, Colorado
Session GI1: MHD and Stability |
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Chair: Eliezer Hameiri, New York University Room: Adam's Mark Hotel Plaza Ballroom ABC |
Tuesday, October 25, 2005 2:00PM - 2:30PM |
GI1.00001: Advances in the Numerical Modeling of Field Reversed Configurations Invited Speaker: The field-reversed configuration (FRC) is a compact toroid with little or no toroidal field. It offers a unique fusion reactor potential because of its compact and simple geometry, translation properties, and high plasma beta. Theoretical understanding of the observed FRC equilibrium and stability properties presents significant challenges due to high plasma beta, flows, large ion gyroradius and stochasticity of the particle orbits. Advanced numerical simulations are generally required to describe and understand the behavior of FRC plasmas. Results of such simulations are presented in this paper. It is shown that 3D nonlinear hybrid simulations using the HYM code reproduce all major experimentally observed stability properties of elongated (theta-pinch-formed) FRCs. Namely, the scaling of the growth rate of the $n=1$ tilt mode with $S^*/E$ parameter ($S^*$ is the FRC kinetic parameter, $E$ is elongation, and $n$ is toroidal mode number), the nonlinear saturation of the tilt mode, ion toroidal spin-up and the growth of the $n=2$ rotational mode have been demonstrated. The HYM code has also been used to study FRC formation by counter-helicity spheromak merging, and stability properties of FRCs formed by this method have been investigated. A new stability regime has been found for FRCs with $E\sim 1$ and $S^* \sim 20$, which requires a close-fitting conducting shell and energetic beam ion stabilization. It is shown that the $n=1$ and $n=2$ MHD modes can be effectively stabilized by combination of conducting shell and beam ion effects, and residual weakly unstable $n>2$ modes saturate nonlinearly at low amplitudes. The resulting configuration remains stable with respect to all global MHD modes, as long as the FRC current is sustained. [Preview Abstract] |
Tuesday, October 25, 2005 2:30PM - 3:00PM |
GI1.00002: Modelling of FRC experiment with large safety factor Invited Speaker: A field-reversed configuration (FRC) with a modest toroidal field can have a large safety factor. Although the toroidal field is relatively small, the flux-surface elongation can be large. Both factors contribute to q, and large elongation can compensate for small toroidal field. Internal field measurements in FRCs formed by ejection in the Translation, Confinement and Sustainment (TCS) facility at the University of Washington indicate both q $>$ 2 at the edge and significant forward magnetic shear (gradient of q). With this q profile and the low-aspect ratio the FRC bears a strong resemblance to a spherical tokamak, albeit without a center column. These results are interpreted using the ``nearby-fluids'' model, a flowing, two-fluid equilibrium where the ion flow surfaces and the magnetic surfaces are ``nearby'' but do not coincide. Flow is necessary because the plasma exhibits a significant rotational speed of $\sim $ 40 km/s. A two-fluid model is needed because a single-fluid model cannot reproduce the observed toroidal field structure. Two-dimensional computations are presented relevant to both the TCS experiments (relatively large ion skin depth) and in larger-size plasmas (small skin depth). The former have significant forward shear throughout the plasma, and a q = 1 surface in the interior, as inferred from TCS. The latter show significant reversed shear and q $>$ 1 throughout the plasma. The computations also indicate that strong poloidal flows of the same order as the toroidal flows appear in the TCS experiments. The poloidal flow may figure in the observed stability. The stability of FRC experiments has generally been attributed to finite-Larmor-radius (FLR) effects. FLR becomes ineffective when the plasma is scaled up to fusion--relevant size. However, the possibility that a very high-beta plasma can satisfy conditions for global kink (q $>$ 1) and local instability (sufficient gradient of q) is an exciting new prospect for FRCs and fusion research. [Preview Abstract] |
Tuesday, October 25, 2005 3:00PM - 3:30PM |
GI1.00003: MHD dynamo in Reversed Field Pinch Plasmas: electrostatic drift nature of the dynamo velocity field Invited Speaker: Within the framework of MHD numerical modelling, the Reversed Field Pinch (RFP) has been found to develop turbulent or laminar regimes switching from the former to the latter in a continuous way depending on the strength of dissipative forces. The laminar solution corresponds to a simple global helical deformation of the current channel and is associated to an electrostatic dynamo field. In this work we show that the associated drift yields the main component of the dynamo velocity. While quite natural in the stationary helical state, this analysis is shown to extend also to the dynamic turbulent regime for a sustained RFP. The continuity of the transition between the two regimes suggests that the simple helical symmetric solution can provide a fruitful intuitive description of the RFP dynamo in general. Many of the MHD predictions are in good agreement with experimental findings. \newline \newline References: \newline [1] S. Cappello and D.F. Escande, ``Bifurcation in viscoresistive MHD: the Hartmann number and the RFP,'' Phys. Rev. Lett. 85, 3838 (2000) \newline [2] S. Cappello, ``Bifurcation in the MHD behaviour of a self-organizing system: the RFP,'' Plasma Phys. Control. Fusion 46, B313 (2004) \newline [3] D. Bonfiglio, S. Cappello, D.F. Escande, ``Dominant electrostatic nature of the Reversed Field Pinch dynamo,'' Phys. Rev. Lett. 94, 145001 (2005) In collaboration with D.F. Escande and D. Bonfiglio. [Preview Abstract] |
Tuesday, October 25, 2005 3:30PM - 4:00PM |
GI1.00004: 2-D Real Time Images of Self-Organized T$_e$ Redistribution of Sawtooth Oscillation (m=1 mode) on TEXTOR Invited Speaker: A novel 2-D Electron Cyclotron Emission Imaging (ECEI) system [1] for measuring electron temperature fluctuations has been applied to study the physics of the sawtooth oscillation (m=1mode) in TEXTOR. The real time 2-D images with high spatial resolution [128 pixels covering 8 cm (radial) x 16 cm (vertical)], and high temporal resolution (up to $\sim$5 microsec) are ideal for the physics study of complex and multi- dimensional plasma phenomena. The observed 2-D dynamics of T$_e$ fluctuations during the sawtooth period revealed physics information not accessible through conventional methods (1-D ECE and/or X-ray tomography). This paper describes the key technologies for the state-of-the-art diagnostic tool as well as new physics results from TEXTOR sawtooth oscillation studies. The observations revealed that the magnetic field reconnection (puncture) could occur everywhere along the q$\sim$1 surface regardless of whether it is the high or low field side. The measured poloidal extent of the magnetic puncture size is finite and the finite extent of the toroidal magnetic field puncture size has been estimated based on the measured speed of the heat flow and the heat flow pattern from the core to outside of the inversion radius. The physical mechanisms that might be responsible for the magnetic reconnection processes such as stochasticity, fractals, magnetic islands, ballooning modes and pressure driven fluctuations will be discussed. The heat of the m=1 mode transported to the region outside the inversion radius initially follows the local magnetic pitch. Shearless magnetic zone arises from the current sheet [2] may play a role accelerating the heat transport process within the area known as the mixing zone. \newline \newline [1] H. Park et al., Rev. Sci. Instrum. \textbf{75}, 3787 (2004) \newline [2] H. Soltwisch et al., Plasma Phys. \& Control. Fusion \textbf {37}, 667 (1995) [Preview Abstract] |
Tuesday, October 25, 2005 4:00PM - 4:30PM |
GI1.00005: Cross-Machine Comparison of Resonant Field Amplification and Resistive Wall Mode (RWM) Stabilization by Plasma Rotation Invited Speaker: Dedicated experiments in the \hbox{DIII-D} and JET tokamaks and the NSTX spherical torus reveal an aspect ratio A and safety factor q dependence of the stabilizing effect of plasma rotation on the RWM, which is used to discriminate among theories and improve stabilization strategies in future devices including ITER. Despite the different distance of the \hbox{DIII-D} and JET walls, the critical rotation for RWM stabilization normalized to the inverse Alfven time is the same. Similar NSTX plasmas at lower A, however, yield a significantly higher critical rotation. A decrease of the critical rotation with increasing q$_{95}$ in \hbox{DIII-D} and JET is consistent with the 1/q$^2$ dependence observed in NSTX. This increase of rotational stabilization with increasing q is predicted by a kinetic damping model [Bondeson, Phys.\ Plasmas ${\bf 3}$ (1996) 3013] and can be derived from a viscous damping model [Fitzpatrick, Phys.\ Plasmas ${\bf 9}$ (2002) 3459] when taking into account neoclassical viscosity [Shaing, Phys.\ Plasmas ${\bf 11}$ (2004) 5525]. The observed A dependence of the critical rotation follows from this theory. In each device the weakly damped n=1 mode manifests itself in resonant field amplification (RFA) above the no-wall stability limit. Applying n=1 fields in similar \hbox{DIII-D} and JET plasmas yields the same amplification confirming that the damping process is independent of the wall properties. The RFA is well described by a single weakly damped mode and can be used to infer the RWM damping rate and rotation frequency. Measurements of RFA induced rotation damping in \hbox{DIII-D} will be compared to NSTX observations that agree with neoclassical toroidal viscosity induced drag.\par \vspace{0.5em} \noindent In collaboration with the DIII-D Team, JET-EFDA contributors and the NSTX Team. [Preview Abstract] |
Tuesday, October 25, 2005 4:30PM - 5:00PM |
GI1.00006: Modeling of Resistive Wall Mode and its Control in Experiments and ITER Invited Speaker: To maintain a high beta advanced tokamak in steady state, the plasma has to be stabilized against the resistive wall mode (RWM) --- an external kink mode with growth rate slowed down to the resistive diffusion time of the surrounding wall. This work presents new theory development and modeling results using the MARS-F code, for the control of the RWM in a rotating plasma, with specific emphasis on the comparison with experiments on JET/DIII-D and the predictions for ITER. Two unique features are included in our work. The first is to adopt a physics based kinetic damping model, derived from the drift kinetic energy principle, to describe the transfer of toroidal angular momentum between the RWM and the rotating plasma. The second is to employ a compact description for the response of the plasma and also the external hardware to the feedback signal. These response characteristics are included in terms of their frequency dependent transfer functions. Extensive comparison with both DIII-D and JET experiments has revealed that the present kinetic damping model is adequate for the description of the experiments. Important results include the parametric dependence of the critical rotation required for stabilization of the RWM, demonstration of the advantages of inside versus outside placed feedback coils and sensor geometries, and the frequency dependent plasma response to the prescribed external perturbation. Single mode approximation describes well the stable RWM dynamics; unstable RWM are better described by a superposition of multiple modes. The combination of plasma rotation and active feedback provides a viable way to stabilize the RWM in ITER. [Preview Abstract] |
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