Solenoid-free toroidal plasma start-up concepts utilizing only the outer poloidal field coils and a conducting centre-post

Abstract

Eventual elimination of the in-board Ohmic heating solenoid is required for the spherical torus (ST) to function as a compact component test facility and as an attractive fusion power plant. An in-board Ohmic solenoid, along with the shielding needed for its insulation, can dramatically increase the size and, hence, the cost of the plant. Advanced tokamak reactor designs also assume no or a small in-board solenoid to reduce the size and cost of the plant. In addition, elimination of the in-board solenoid greatly reduces the coil stresses and simplifies the coil design. Here, we investigate using static as well as dynamic codes in ST geometries with two complementary solenoid-free plasma start-up approaches: one utilizes only the outer poloidal field coils to create a relatively high quality field null region while retaining significant poloidal flux, and the other takes advantage of the poloidal flux stored in the conducting centre-post to create a start-up condition similar to that of the conventional Ohmic solenoid method. We find that it is therefore possible to come up with a promising configuration, which produces a quality multi-pole field-null and sufficient loop-voltage needed for plasma initiation and significant poloidal flux for subsequent current ramp-up. The present solenoid-free start-up concept, if proved feasible, can be readily extended to higher field devices due to relatively simple physics principles and favourable scaling with the device size and toroidal field.