Observation of Ion Cyclotron Emission in MAST-U Ohmic Plasma

Coherent fluctuations near harmonics of the ion cyclotron frequency, known as ion cyclotron emission (ICE), are often driven by energetic ions in tokamak plasmas. Unexpectedly, we have observed robust ICE bursts near the fundamental ion cyclotron frequency in Ohmic plasmas on MAST-U, where energetic particles are expected to be negligible (f_ci = 2.9 - 3.7 MHz for B_T = 0.38 - 0.48 T at R = 1 m). Doppler Backscattering measurements reveal that this ohmic ICE corresponds to a high-k_⊥ ion cyclotron wave ( k_⊥ρ_s ≥ 1, ρ_s is the ion acoustic gyroradius) extending over 0.6 < r/a < 0.8 (a is minor radius) with a normalized lifetime f_cit_lifetime ≈ 100 which is confirmed via multiple Doppler Backscattering frequency channels. Cross-polarization scattering and edge Mirnov loop measurements show no corresponding ICE features in the magnetic fluctuation spectra, suggesting electrostatic nature of ICE. Notably, sawtooth activity observed in soft X-ray measurements does not influence ICE stability. A parametric scan of the B_T/I_P ratio at constant electron density and temperature indicates that ICE is unstable for B_T/I_P ≤ 0.9 T/MA ( B_T is the toroidal magnetic field and I_P is the toroidal plasma current) and also for elongation below 2 at constant B_T/I_P, electron density, and temperature. These results suggest that ICE stability depends on plasma safety factor, q, which increases with B_T/I_P and with elongation at fixed B_T/I_P . ICE is unstable only for |q₉₅| < 7, where q₉₅ is the safety factor at the 95% poloidal flux surface.