Otto Asunta, Wilfred Anthony Cooper, Jonathan Graves, Jan Horacek, Ben McMillan, Ricardo Pérez Suárez, Mario Podesta
The Mega Ampere Spherical Tokamak (MAST) programme is strongly focused on addressing key physics issues in preparation
for operation of ITER as well as providing solutions for DEMO design choices. In this regard, MAST has provided key
results in understanding and optimizing H-mode confinement, operating with smaller edge localized modes (ELMs), predicting
and handling plasma exhaust and tailoring auxiliary current drive. In all cases, the high-resolution diagnostic capability on
MAST is complemented by sophisticated numerical modelling to facilitate a deeper understanding. Mitigation of ELMs with
resonant magnetic perturbations (RMPs) with toroidal mode number
n
RMP
2
,
3
,
4
,
6 has been demonstrated: at high and
low collisionality; for the first ELM following the transition to high confinement operation; during the current ramp-up; and
with rotating n=3 RMPs.
n=4,6 fields cause less rotation braking whilst the power to access H-mode is less
with
n = 4 than n=3,
6. Refuelling with gas or pellets gives plasmas with mitigated ELMs and reduced peak heat
flux at the same time as achieving good confinement. A synergy exists between pellet fuelling and RMPs, since mitigated
ELMs remove fewer particles. Inter-ELM instabilities observed with Doppler backscattering are consistent with gyrokinetic
simulations of micro-tearing modes in the pedestal. Meanwhile, ELM precursors have been strikingly observed with beam
emission spectroscopy (BES) measurements. A scan in beta at the LâH transition shows that pedestal height scales strongly
with core pressure. Gyro-Bohm normalized turbulent ion heat flux (as estimated from the BES data) is observed to decrease
with increasing tilt of the turbulent eddies. Fast ion redistribution by energetic particle modes depends on density, and access to
a quiescent domain with âclassicalâ fast ion transport is found above a critical density. Highly efficient electron Bernstein wave
current drive (1 A W
â
1
) has been achieved in solenoid-free start-up. A new proton detector has characterized escaping fusion
products. Langmuir probes and a high-speed camera suggest filaments play a role in particle transport in the private flux region
whilst coherence imaging has measured scrape-off layer (SOL) flows. BOUT++ simulations show that fluxes due to filaments
are strongly dependent on resistivity and magnetic geometry of the SOL, with higher radial fluxes at higher resistivity. Finally,
MAST Upgrade is due to begin operation in 2016 to support ITER preparation and importantly to operate with a Super-X
divertor to test extended leg concepts for particle and power exhaust.IOP2015