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

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 P-LH[MW] = 11.35x (n) over bar (1.19)(e20). 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 Q(e) similar to 0.57(n)over bar>(e19) and ion heat flux Q(i) similar to 0.1(n)over bar>(e1) is caused by a decrease in beam heating efficiency for lower densities, with the fraction of injected power P-inj(NBI) 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 P-LH = 12.8x (n)over bar>(1.5)(e20). A residual low-density branch remains due to the dW/dt variation.