Prospect of Spherical Tokamak towards a Power Reactor -Challenge towards the Lowest Aspect Ratio Tokamak- 4. Is an Operation Scenario for CS-Less Current Ramp-Up to Steady-State Burning Feasible? 4.1 Is ”Current Ramp-Up without a Center Solenoid” Possible?

Abstract

The present status of experimental and theoretical efforts and future prospects on current start-up and ramp-up methods for ST reactors to reach fusion burning without a center solenoid are discussed. First, we need to form an initial magnetic surface and next to ramp-up the plasma current to a sufficient level for energetic neutral beam injection (NBI). The second heating and current drive by NBI with the ramp-up of the external vertical field may bring the plasma to fusion ignition. The first stage will be realized by refinement of various methods including radio frequency heating and current drive, coaxial helicity injection, plasma merging, and poloidal flux swing. In the second stage, α heating and the bootstrap current dominate as the plasma gets close to burning condition. This is a totally new regime and we need further experimental and theoretical efforts involving novel approaches to ramp-up such a self-regulated plasma successfully.