A power-balance model for local helicity injection startup in a spherical tokamak

作者：Barr J L; Bongard M W; Burke M G; Fonck R J; Hinson E T; Perry J M; Reusch J A

来源：Nuclear Fusion, 2018, 58(7): 076011.

DOI：10.1088/1741-4326/aac0a0

• plasma startup
• spherical tokamaks
• current drive
• helicity injection
• Taylor relaxation
• tokamaks

A 0D circuit model for predicting I-p(t) in Local Helicily Injection (LHI) discharges is developed. Analytic formulas for estimating the surface flux of finite-A plasmas developed by Hirshman and Neiison (1986 Phys. Fluids 29 790) are modified and expanded to treat highly shaped, ultralow-A tokamak geometry using a database of representative equilibria. Model predictions are compared to sample LHI discharges in the A similar to 1 Pegasus spherical tokamak, and are found to agree within 15% of experimental I-p(t). High performance LHI discharges are found to follow the Taylor relaxation current limit for approximately the first half of the current ramp, or I-P less than or similar to 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.

• 出版日期2018-7