Simulation of current-filament dynamics and relaxation in the Pegasus ST
ORAL
Abstract
The dynamics of magnetic relaxation of during non-inductive startup in the Pegasus spherical tokamak are investigated with nonlinear MHD-based numerical computation. A new phenomenology, evident from our modeling, is the attraction and reconnection of adjacent passes of the helical filament, releasing axisymmetric rings of current [O'Bryan, Phys.\ Plas.\ 19, 080701 (2012)]. Accumulation of poloidal flux over multiple relaxation events leads to toroidal configurations with hollow average $J_{||}$ profiles. Similar to experimental results, the magnetic fluctuation during helicity injection is approximately 5\% of the toroidal field with significant activity in the 10-20 kHz range. The inclusion of two-fluid effects in Ohm's law produces qualitatively similar plasma evolution. Temperature and current profiles broaden and closed flux surfaces form rapidly upon cessation of the simulated current drive, leaving a tokamak-like discharge suitable for transition to other forms of current drive. Investigation of time-dependent evolution of externally driven poloidal flux is underway.
*Work supported by the US Dept. of Energy
–