Description
Spontaneous, strongly supra-thermal ion cyclotron emission (ICE) has been detected in many laboratory and space plasmas, and is usually attributed to some form of velocity space inversion in an energetic ion population. As such, it provides a natural, passive diagnostic of these ions that can be combined with other measurements to provide a more comprehensive understanding of their behaviour. Energetic particle-driven ICE in the Mega-Amp Spherical Tokamak Upgrade (MAST-U) was first detected in the 2024 - 2025 experimental campaign (MU04) by adding a high sampling rate (250MHz) capability to one coil of an existing Mirnov array [1], oriented to measure magnetic fluctuations in the magnetic field direction. 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. Two types of energetic particle-driven ICE were detected in MU04 [2]: narrow band emission with some frequency splitting at low integer harmonics of the on-axis deuterium cyclotron frequency $f_{cD}$ and broader-band but still harmonically-structured emission with peak frequencies corresponding to low integer harmonics of $f_{cD}$ at the low field side plasma edge. In the current MAST-U campaign (MU05), a similar high frequency capability is being added to several other toroidally-distributed coils in an attempt to enable toroidal mode numbers of the ICE to be measured. Depending on the signal/noise ratio, it may also be possible to determine mode polarity using a pair of coils with different orientations at one toroidal location. In this presentation ICE measurements from MU04 and MU05 will be reported and their implications for the behaviour of energetic beam ions in MAST-U will be discussed.
This work has been carried out within the framework of the EUROfusion Consortium, funded by the European Union via the Euratom Research and Training Programme (Grant Agreement No. 101052200—EUROfusion). It has also been supported by the EPSRC Energy Programme (grant number EP/W006839/1), by the Simon Foundation and by the National Research Foundation of Ukraine (grant 2023.03/0101).
[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, submitted (2026)