L-H transition studies on MAST: power threshold and heat flux analysis

Abstract

Spherical tokamaks are known to differ from conventional tokamaks in a number of physics areas, but our understanding is limited by a comparative lack of experimental data. A comprehensive study of the density dependence of the H-mode power threshold P LH on the mega-amp spherical tokamak (MAST) is presented, mapping out the low-density and high-density branches, and describing different types of L-H transitions and intermediate behaviours. Transitions at low densities are characterised by longer preceding 3–4 kHz I-phase oscillations or intermittent periods. Commonly used P LH scalings underestimate the experimental values in this data set by at least an order of magnitude, and a fit to the high-density branch gives . To investigate the possibility of a critical edge ion heat flux, transitions of different densities and neutral beam heating powers were analysed with the interpretative transport code TRANSP, revealing that the heat flux possesses a density dependence independent of the net power. The density dependence of the electron heat flux and ion heat flux is caused by a decrease in beam heating efficiency for lower densities, with the fraction of injected power heating the plasma decreasing from 80% to 20% at low densities. Low-density discharges have greater fast ion orbit and charge-exchange losses, which are seen in Mirnov signals as chirping 50–20 kHz modes and broadband MHD at 150–250 kHz. The captured beam power estimate by TRANSP, which only corrects for shine-through losses, is a significant overestimate for low-density plasmas on MAST. If the P LH study is adjusted to account for all losses, the scatter in H-mode points is reduced and the density exponent is increased to . A residual low-density branch remains due to the variation.