On the Neutral Beam Heating of MAST

We present a brief synopsis of the phenomenology associated with Neutral Beam heating (NBI) of the MAST Spherical Tokamak. Results, although at an early stage of analysis, are highly encouraging for the future auxiliary heating capabilities of the device. In particular, only 500-800kW of NBI power (30keV H injection, ~10% of design total) is sufficient to significantly increase Te and to double the thermal electron pressure. Further, preliminary results indicate that the ion temperature increases by a factor of ~3, qualitatively consistent within systematic errors with that expected, given tolerable fast ion loss and ITER confinement scaling IPB98(y,1). NBI data recorded so far exhibit suprathermal ion tail formation and ohmic discharges a phenomenologically similar tail following Internal Reconnection Events. The injected fast ions, corresponding to 10-50% of the total stored energy, are responsible for driving a wide variety of sporadic, high frequency MHD modes, the low magnetic fields (and hence low Alfvén speeds) inherent to the Spherical Tokamak geometry providing an ideal testing ground for fast-particle MHD theory.