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Spontaneous, strongly supra-thermal ion cyclotron emission (ICE) at or close to multiple ion cyclotron harmonics has been detected in many magnetically-confined plasmas, and is usually attributed to velocity space inversion in an energetic ion population. It thus provides a passive diagnostic of these ions that can be combined with other measurements to enhance understanding of their behaviour. Energetic particle-driven ICE in the Mega-Amp Spherical Tokamak Upgrade (MAST-U) has been detected following the addition of a high sampling rate capability to coils in an existing Mirnov array [1,2]. These measurements were carried out in pulses with either on-axis neutral beam injection (NBI) only, or a combination of on-axis and off-axis NBI. The fuel and beam ion species in MAST-U is deuterium, and ions born in the plasma at the primary injection energy are usually super-Alfvénic. ICE has been detected in MAST-U using both the high sampling rate coils and Doppler backscattering (DBS) reflectometry, thereby making it possible to obtain information on the spatial source of the emission. In one recent pulse sawtooth crashes terminated global Alfvén eigenmodes (GAEs) at 1.5 – 2.0 MHz and then excited short bursts of 2nd harmonic ICE at around 9 MHz, suggesting sawtooth-induced redistribution of fast ions driving the GAEs to the ICE-emitting region. In another case quasi-continuous, broadband ICE was excited up to the 6th harmonic, correlated with recurring instabilities in the shear Alfvén range, 100 - 300 kHz. ICE in MAST-U typically has a wide range of toroidal mode numbers n, including co- and counter-current as well as n=0, suggesting that radial gradients do not play a significant role in the instability drive. Although uncertainties remain in the ICE mode identification, preliminary linear modelling, performed by solving the local dispersion relation for compressional Alfvén waves (CAWs) in a plasma with a ring-beam distribution of energetic ions [3], show instability at frequencies and wavenumbers that are broadly consistent with the MAST-U ICE measurements.
[1] M J Hole, L C Appel, R Martin Rev. Sci. Instrum. 80, 123507 (2009)
[2] M B Dreval et al. Plasma Phys. Control. Fusion 68, 035014 (2026)
[3] J W S Cook Plasma Phys. Control. Fusion 64, 115002 (2022)