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Abstract. Understanding alpha particle confinement in spherical tokamaks (STs) is crucial for assessing the feasibility of ST burning plasmas, and provides a testbed for theoretical modelling under conditions of strong toroidicity and shaping, and high beta. Enhanced fast particle transport may result from Alfvénic modes such as TAEs, EAEs and GAEs. Modes with frequencies in the TAE and EAE ranges have been observed during beam heating in the MAST ST. Analysis of TAEs in MAST reveals wide gaps in the Alfvén continuum, containing many discrete modes. The fast particle drive of TAEs in MAST has also been computed. The mode frequencies, poloidal mode structure, and variation of fast particle drive with mode number are all broadly consistent with analytical predictions made for conventional tokamaks. Alfvénic mode activity has also been detected during ohmic discharges in MAST; similar activity was observed in the conventional tokamak TFTR. In TFTR and MAST discharges for which mode number information is available the dominant toroidal mode number was found to be zero. These observations are consistent with the excitation of GAEs with n=0 and low m, driven impulsively by low frequency MHD.
IAEA 2003
