Modelling Microwave Start-up in Spherical Tokamaks

We present the development of a time dependent, self-consistent model for microwave start-up in spherical tokamaks. Microwave start-up is a solenoid-free method of driving the plasma current during the early stages of tokamak operation, from ionisation to the formation of closed flux surfaces. It is a critical issue for tokamak power plants with limited space for a neutron-shielded solenoid. Experiments on MAST achieved 73 kA of current with 100 kW of input microwave power via OXB mode conversion and a mirror polarizer on the centre column [1]. However, there has been limited modelling on this topic so far as most ray-tracing and Fokker-Planck codes assume closed flux surfaces. Including the open field line configuration is important as modelling has shown that the asymmetric confinement of electrons plays a significant role in the current drive [2]. Additionally, the current drive mechanisms and microwave-plasma interactions change over time due to the increasing density and changing poloidal magnetic field, introducing the need for time-dependent simulations.



[1] Shevchenko V.F. et al 2015 EPJ Web Conf. 87 02007

[2] Maekawa T. et al 2012 Nucl. Fusion 52 083008