Magnetic Field Structure in a Finite-Beta Toroidal Plasma.
Toroidal plasmas with finite beta are produced in the TERP II device using fast rising applied fields. The formation and equilibrium phases are diagnosed using magnetic field probes. During formation, the plasma is rapidly heated by diamagnetic plasma currents. The diamagnetism decreases rapidly in the first few (mu)sec due to an outward radial drift of the magnetic well. The loss of diamagnetism is attributed to the observed decrease and reversal of the toroidal current. The current decrease removes the inward I x B(,Z) force necessary for radial force balance. The toroidal current behavior seems to be caused by coupling between the toroidal field coils and the plasma current. Several coupling mechanisms are examined, including anisotropic resistivity and plasma velocity. The observed current decrease can be explained by a resistive MHD model with enhanced resistivity and nearly force-free currents. Current reversal may be a toroidal effect caused by changing major and minor plasma radii. Current density in the reverse direction is seen inside the major radius during the formation. This reverse current is unstable and is lost to the chamber walls in about 10 (mu) sec. The forward current shows vertical motion but is confined in the outer half of the chamber. These observations agree with measurements of electron density. The current reversal phenomenon contributes to the observed rapid energy loss, limiting the beta value in the subsequent equilibrium phase, which lasts about 100 (mu)sec. The field structure of the equilibrium plasma shows nested flux contours and q profiles typical of tokamak configurations. Within experimental uncertainty, the poloidal beta is 1 and the volume average beta is about 3%. This beta value is near the predicted stability limit for ballooning modes. Experiments are performed to investigate field structures on scale lengths smaller than the plasma radius, but no indications of unstable activity are seen.
- Pub Date:
- Physics: Fluid and Plasma