The impact of transient mitigation schemes on the MAST edge plasma

A disruption is the sudden and uncontrolled loss of plasma confinement in a tokamak. Disruptions on the Mega Amp Spherical Tokamak (MAST) are characterised in
terms of thermal quench timescales, energy balance and pre disruption energy loss. Analysis of the energy balance during disruptions on MAST has shown that approximately 10% of the stored energy is radiated during a disruption and 80% is deposited onto the divertor. The energy loss prior to the thermal quench is found to be 50% of the maximum energy in the plasma, which is half the value assumed
for the ITER design.
Disruptions occur when operational boundaries, in terms of current, pressure and density, are exceeded. An analysis of the operational boundaries in MAST shows that the frequency of disruptive events increases as the density is raised to
1.5 times the Greenwald density limit and that the pressure limit is consistent with empirical scalings. The current limit on MAST is triggered before the expected value
of q95 is reached. Further analysis of the disrupting discharges in MAST shows that there is substantial energy loss prior to the thermal quench of up to 50%, however,
disruptions at full performance are frequent.
Disruption mitigation on MAST, via massive gas injection, has been performed using 0.32 bar litres (7.7x1021 particles, 10 times the plasma inventory) of a 90%
helium and 10% argon mixture. The evolution of the plasma during mitigation is followed using high speed (up to 50kHz) imaging and high temporal (0.2ms) resolution Thomson scattering. High speed imaging of the plasma shows that the
neutral impurities are confined to the plasma periphery. Impurity ions penetrate to the q=2 surface and mix with the bulk plasma during the thermal quench. Thomson
scattering data shows significant (double the initial core density) build of density on rational surfaces, specifically q=2, prior to the thermal quench.
Analysis of the power load to the divertor during mitigated disruptions shows reductions of 60% in peak power loadings compared to unmitigated. The energy balance during mitigated disruptions shows an increase in the radiated energy to 40% of the total stored energy and a decrease in the energy to the divertor of 40%. The effect of mitigation is to increase the current quench time and decrease the
magnitude of halo currents by 80%