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Description
ST40 is a high-field spherical tokamak with a major radius up to 0.55 m and toroidal magnetic field up to 2.1 T, operating at plasma currents ranging from 0.2-1.0 MA. It is equipped with two neutral beam injectors (NBI) with 1.0 MW at 55 keV and 0.8 MW at 24 keV. Across the wide toroidal field range the fast ions from NBI can range from sub- to super-Alfvénic, driving a variety of Alfvénic modes including toroidal Alfvén eigenmodes (TAE). These are commonly observed at the start of the discharges but are often suppressed later in the flat-top, in particular after sawtooth oscillation begins.
In this contribution we present experimental and modelling results on interaction between NBI fast ions and TAEs and sawteeth. Discharges have been analysed across the ST40 operating range where TAEs have been observed. The fast particle code ASCOT has been used to assess the drive from fast ions and fast ion transport in various scenarios. While prevalent, TAEs on ST40 typically do not appear to cause significant fast ion losses based on measured neutron rates and neutral particle analyser (NPA) fluxes.
In contrast, sawtooth crashes are often associated with dips in neutron and NPA measurements. The effect of sawteeth has been investigated with ASCOT using an ad-hoc model of flattening the fast ion distribution within the mixing radius. These results suggest significant fast ion redistribution, potentially suppressing the drive for TAEs. This is also supported by integrated modelling with ASTRA, where anomalous fast ion diffusion up to 1 m²/s is required to match the experiment also in the absence of TAEs.