Optimization of Outer Poloidal Field (PF) Coil Configurations for Inductive PF Coil-only Plasma Start-up on Spherical Tori
Authors: Wonho Choe, Jayhyun Kim, and Masayuki Ono
Date of PPPL Report: April 2004
Submitted to: Nuclear Fusion
The elimination of in-board ohmic heating solenoid is required for the spherical torus (ST) to function as an attractive fusion power plant. An in-board ohmic solenoid, along with the shielding needed for its insulation, increases the size and, hence, the cost of the plant. Here, we investigate using static as well as dynamic codes in ST geometries a solenoid-free start-up concept utilizing a set of out-board poloidal field coils. By using the static code, an optimization of coil positions as well as coil currents was performed to demonstrate that it is indeed possible to create a high quality multi-pole field null region while retaining significant flux (volt-seconds) needed for the subsequent current ramp-up. With the dynamic code that includes the effect of vacuum vessel eddy currents, we then showed that it is possible to maintain a large size field null region for several milliseconds in which sufficient ionization avalanche can develop in the applied toroidal electric field. Under the magnetic geometry typical of a next generation spherical torus experiment, it is shown that the well-known plasma breakdown conditions for conventional ohmic solenoid start-up of ETBT/BP ~ (0.1 - 1) kV/m with Vloop ~ 6 V can be readily met while retaining significant volt-seconds ~ 4 V-S sufficient to generate multi-MA plasma current in STs.