Long Pulse EBW Start-up Experiments in MAST

TL;DR

This paper reports on long pulse electron Bernstein wave start-up experiments in MAST, demonstrating high plasma currents driven solely by RF power through a double mode conversion scheme, advancing solenoid-free plasma initiation.

Contribution

It introduces a novel long pulse EBW start-up method with successful experimental validation, achieving record plasma currents without solenoid assistance.

Findings

Plasma currents up to 73 kA achieved with 450 ms RF pulses.

Extended RF pulses lead to increased plasma current.

Optimized start-up scenario enhances current drive efficiency.

Abstract

The non-solenoid start-up technique reported here relies on a double mode conversion for electron Bernstein wave (EBW) excitation. It consists of the mode conversion of the ordinary mode, entering the plasma from the low field side of the tokamak, into the extraordinary (X) mode at a mirror-polarizer located at the high field side. The X mode propagates back to the plasma, passes through electron cyclotron resonance and experiences a subsequent X to EBW mode conversion near the upper hybrid resonance. Finally the excited EBW mode is totally absorbed at the Doppler shifted electron cyclotron resonance. The absorption of EBW remains high even in cold rarefied plasmas. Furthermore, EBW can generate significant plasma current giving the prospect of a fully solenoid-free plasma start-up. First experiments using this scheme were carried out on MAST [V. Shevchenko et al, Nuclear Fusion 50, 022004 (2010)]. Plasma currents up to 33 kA have been achieved using 28 GHz 100kW 90ms RF pulses. Recently experimental results were extended to longer RF pulses showing further increase of plasma currents generated by RF power alone. A record current of 73kA has been achieved with 450ms RF pulse of similar power. The current drive enhancement was mainly achieved due to RF pulse extension and further optimisation of the start-up scenario.