A power-balance model for local helicity injection startup in a spherical tokamak. (15th May 2018) - Barr, J.L.
Abstract: A 0D circuit model for predicting in Local Helicity Injection (LHI) discharges is developed. Analytic formulas for estimating the surface flux of finite- plasmas developed by Hirshman and Neilson (1986 Phys. Fluids 29 790) are modified and expanded to treat highly shaped, ultralow- tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the Pegasus spherical tokamak, and are found to agree within 15% of experimental . High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the first half of the current ramp, or 75 kA. The second half of the current ramp follows a limit imposed by power-balance as plasmas expand from high- A to ultralow- A . This shape evolution generates a significant drop in external plasma inductance, effectively using the plasma's initially high inductance to drive the current ramp and provide >70% of the current drive V-s. Projections using this model indicate the relative influences of higher helicity input rate and injector current on the attainable total plasma current.