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NERSC 3 Greenbook

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3d MHD Calculations in Toroidal Geometry

S. C. Jardin, D. Monticello, W. Park and A. Reiman
Princeton Plasma Physics Laboratory
Princeton NJ 08543

High-$\beta$ Disruption Modeling: In the record-setting deuterium-tritium discharges in TFTR, performance is limited by magnetohydrodynamic (MHD) activity, the most dangerous type being the major disruption. This event causes the loss of the confined thermal energy during the thermal quench phase and a subsequent loss of plasma current during the current quench phase. Ballooning modes are expected in plasmas with pressure gradients exceeding a threshold set by the structure of the confining magnetic field. Recently, ballooning modes were clearly observed on TFTR for the first time.

The first dynamic calculation of a non-axisymmetric ballooning instability was performed at PPPL using the 3D nonlinear MHD initial value code, MH3D. A 2D equilibrium and its low-n linear ballooning mode were first calculated using the experimental pressure and current profiles prior to the onset of a disruption. Nonlinear evolution of the mode results in a local pressure steepening, which in turn drives a linear toroidally localized high-n ballooning mode. Nonlinearly, the mode becomes even more localized producing a local pressure bulge which destroys the flux surfaces, resulting in a thermal quench. Figure 24(a) shows the simulated Electron Cyclotron Emission (ECE) signal obtained. The calculated signals are in good agreement with the experimental ECE data shown in Fig. 24(b).

  

Figure 24: ECE signals; (a)theoretical and (b)experimental.

Three-Dimensional Equilibrium: Stellarator configurations, which are receiving substantial financial support and construction money in Japan and Europe, are intrinsically non-axisymmetric three-dimensional plasma confinement systems. Because of the lack of symmetry, good flux surfaces (KAM surfaces of the magnetic field line trajectories) are not guaranteed. Non-integrable field line trajectories may wander through a volume, and cause loss of confinement. The magnetic fields produced by stellarator coils are designed to minimize this effect for a low pressure plasma, but the self-consistent plasma currents that arise with increasing pressure will in general degrade the flux surfaces. The three-dimensional equilibrium PIES code has been used in several studies of this effect, which have aimed to understand the underlying physics, and to assess the significance of the effect for several proposed stellarator devices. PIES is one of only two codes in the world that are capable of addressing this key physics issue, and PIES is the only such code that can handle net plasma currents, and is also the only such code that can realistically specify the equilibrium through the currents in the external coils. Figure 25 shows the output of the PIES code for a reactor proposal, MHH, based on the Stellarator concept. At $<\beta\gt = 2.35\%$, Fig. 25, shows magnetic islands developing over much of the reactor's cross-section.

  

Figure 25: Poincare plot of magnetic field for the MHH Stellarator

NERSC 3 Greenbook

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