Numerical simulation of current evolution in the Compact Toroidal Hybrid

Nonlinear magnetohydrodynamic computation is applied to study magnetic topology evolution for current-driven discharges in the Compact Toroidal Hybrid (CTH). In these simulations, ohmic currents are driven with a constant applied loop voltage in zero-beta plasmas. These driven currents initially are aligned with the vacuum flux surfaces. The rotational transform increases with rising plasma current, exhibiting a broad profile during the current rise. Island structures appear as low-order rational surfaces come into resonance. Because CTH is a five field period device, island structures have a toroidal periodicity of n = 5. There is special significance when the rotational transform exceeds 1/2. At this point m/n = 10/5 island chains appear that subsequently evolve to m/n = 2/1 islands through a coalescence process. With more driven current, a region of magnetic stochasticity grows that ultimately destroys flux surfaces. UW-CPTC Report 12-5.