Bulletin of the American Physical Society
53rd Annual Meeting of the APS Division of Plasma Physics
Volume 56, Number 16
Monday–Friday, November 14–18, 2011; Salt Lake City, Utah
Session VI3: Tokamak Disruption Physics; Stellarator Progress |
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Chair: Andrew Ware, University of Montana Room: Salt Place Convention Center Ballroom AC |
Thursday, November 17, 2011 3:00PM - 3:30PM |
VI3.00001: Control of Post-disruption Runaway Electron Beams in the DIII-D Tokamak Invited Speaker: Recent experiments on DIII-D have demonstrated real-time control of post-disruption runaway electron (RE) beams, presenting the possibility for slow, controlled dissipation of the beam energy. RE beams will present a greater challenge to ITER than present tokamaks due to ITER's high RE avalanche gain constant [Nucl.Fusion {\bf 37}, 1355-62 (1997)] and the difficulty repairing potential damage to its first wall. In the rare event that disruption control and mitigation schemes fail to suppress RE generation, active control of the RE beam may be an important line of defense to prevent rapid, localized deposition of RE beam energy on the first wall. Initially, sustaining a RE beam plateau requires avoiding radial collapse of the beam into the inner wall during the first 1-2 wall penetration times following the current quench (CQ). This collapse is caused by attractive induced currents in the wall and a lack of radial equilibrium with slow vertical field coils. The collapse is avoided by slewing the inner PF coils to push the RE beam off the wall while reducing the outer PF coil currents. Beam survival through this phase requires sufficient RE plateau current ($I_{RE}$) and power supply slew rates to re-establish equilibrium. Following that transient period, RE beam vertical position was dynamically controlled, and stabilization was maintained in an elongated ($\kappa \leq 1.8$) DND configuration for up 250$\,$ms. Most controlled RE beams end in a rapid vertical displacement event (VDE), indicating that the profiles evolve even as the position is controlled. Experimental radial evolution and VDE onset are shown to be consistent with theoretical calculations of controllability boundaries. However, ohmic regulation of $I_{RE}$ has been shown to delay VDEs to the pre-programmed ramp-down time, indicating that steady-state control may be achievable. [Preview Abstract] |
Thursday, November 17, 2011 3:30PM - 4:00PM |
VI3.00002: Understanding disruptions in tokamaks Invited Speaker: Disruptions in tokamaks are known since 1963 but even now some aspects of them remain a mystery. This talk describes progress made recently in understanding disruptions. A major step forward occurred in 2007 when the importance of galvanic contact of the plasma with the wall in plasma dynamics was pointed out. The toroidal asymmetry of plasma current, observed in JET vertical disruptions, was explained by the theory of the wall touching kink mode [1]. The currents shared by the plasma with the wall and responsible for the asymmetry were identified as generated by the kink mode. Such currents are referred to as Hiro currents. They have shown exceptional consistency with the entire JET disruption data base (more than 5500 cases) and ruled out the long lasting interpretation based on ``halo currents,'' which contradict experiments even in the sign of the measured asymmetry. Accordingly, the sideways forces are understood and their scaling from JET to ITER was justified. Hiro currents provide also a plausible explanation of the current spike at the beginning of the disruptions. The important role of the plasma edge and its interaction with the wall was revealed. Based on this new understanding of disruptions, dedicated experiments on the current spike (J-TEXT, Wuhan, China) and runaway prevention by the repetitive triggering of kink modes (T-10, AUG, Tore Supra) were motivated and are in progress. Accordingly, the need for new, adaptive grid approaches to numerical simulations of disruptions became evident. In addition to the core MHD, simulations of realistic wall geometry, disruption specific plasma edge physics, plasma-wall interaction, and energetic particles need be developed. The first results of simulations of the fast MHD regime, Hiro current generation, and slower plasma decay due to a wall touching kink mode made with the new DSC code are presented. \\[4pt] [1] L.E.Zakharov. Phys. Plasmas, 15, 062507 (2008) [Preview Abstract] |
Thursday, November 17, 2011 4:00PM - 4:30PM |
VI3.00003: 3-D Equilibrium Reconstruction in the HSX Stellarator Invited Speaker: Axisymmetric toroidal devices reconstruct the MHD equilibrium properties from measured pressure, magnetic field components, external field coil currents, and other diagnostics, by solving the Grad-Shafranov equation. For modern toroidal systems including advanced stellarators and tokamaks with asymmetric fields, such as those that arise from finite toroidal ripple or ferromagnetic blanket materials, a 3-D equilibrium reconstruction is required to account for non-axisymmetric effects and accurately determine the plasma profiles. The 3-D equilibrium reconstruction of plasma current and pressure profiles in the quasi-helically symmetric stellarator HSX is presented. The equilibrium currents in the HSX stellarator are measured with a set of magnetic diagnostics, which includes Rogowski coils, diamagnetic loops, two poloidal `belts' that are separated by 1/3 of a field period, and internal coils. Each belt consists of 16 3-axis magnetic pick-up coils to measure the local magnetic field, and 15 internal coils measure the poloidal field. V3FIT [1], a 3-D equilibrium reconstruction code, is used to reconstruct the pressure and current profile from the measured fields and fluxes. Reconstructions based on the external diagnostics confirm that the Pfirsch-Schl\"{u}ter current is helical due to the lack of toroidal curvature in HSX. The reconstruction of the pressure profile and stored energy based on the internal poloidal array agrees well with that measured by Thomson scattering and the flux loop. Later in time, the measurements are dominated by the bootstrap current which rises on a timescale comparable to the length of the discharge. The reconstruction of the current profile is consistent with the neoclassical bootstrap current when the effects of momentum conservation between plasma species [2] and the 3-D inductive response of the plasma column [3] are considered. The magnitude of the Pfirsch-Schl\"{u}ter and bootstrap currents are reduced by the high effective transform ($\sim $3), which is characteristic of quasi-helically symmetric systems. The level of uncertainty in the reconstructed pressure and current profiles is largest near the core of the plasma. \\[4pt] [1] J.D. Hanson, et al, Nucl. Fusion 49 (2009) 075031. \\[0pt] [2] D.A. Spong, Phys. Plasmas 12, (2005) 056114. \\[0pt] [3] P.I Strand and W.A. Houlberg, Phys. Plasmas 8 (2001) 2782. [Preview Abstract] |
Thursday, November 17, 2011 4:30PM - 5:00PM |
VI3.00004: Healing of magnetic islands in stellarators by plasma flow Invited Speaker: Recent experiments from the Large Helical Device (LHD) demonstrate a correlation between the ``healing'' of vacuum magnetic islands in stellarators and changes in the plasma flow. In the LHD experiments, external 3-D coils are intentionally applied to produce magnetic islands in the vacuum configuration. With plasma, both island growth and healing is seen with the two disparate plasma responses distinguished by a sharp boundary in a parameter space defined by the plasma $\beta$ and collisionality at the rational surface. While island growth is observed at low $\beta$ and high collisionality, at sufficiently high $\beta$ and/or low collisionality, the plasma abruptly changes to a configuration with no island. A model explaining this phenomenon is developed reminiscent of ``mode locking/unlocking'' theory of tokamak physics. The theory describes transitions between two asymptotic solutions, a state with a large nonrotating island and a state where rotation shielding suppresses island formation. Transitions between these two states are governed by coupled torque balance and island evolution equations. In conventional stellarators, neoclassical damping physics plays an important role in establishing the flow profiles. The balance of neoclassical damping and cross-field viscosity produces a radial boundary layer for the plasma rotation profile outside the separatrix of a locked magnetic island. The width of this boundary layer decreases as the plasma becomes less collisional. This has the consequence of enhancing the viscous torque at low collisionality making healing magnetic islands occur more readily in high temperature conventional stellarators. The analytic theory produces a critical $\beta$ for healing [$\beta_{crit} \sim (\nu^*)^{1/4}$] that is in qualitative agreement with LHD observations. [Preview Abstract] |
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