Ion cyclotron emission from ECRH-heated plasmas in Wendelstein 7-X
We report the observation and analysis of ion cyclotron emission (ICE) in the Wendelstein 7-X (W7-X) stellarator during discharges without a significant fast-ion population. In these experiments, ICE is observed using the strap of the ICRF antenna, while remaining undetectable on the B-dot probes during ECRH-only phases. At low electron densities, the ICE signal on the ICRF strap exhibits an exponential increase in time, ultimately leading to diagnostic saturation, coinciding with the absence of low-frequency MHD activity and a rise in energy transport. Although a causal relationship cannot be firmly established, the correlation is striking. The ICE spectrum also displays multiple local maxima spaced by ≈700 kHz; simulations based on Kolesnichenko’s model suggest that the origin of the ICE is outside of the last closed flux surface (LCFS). While the model can be used for studying the observations in hydrogen, it fails to describe ICE in helium discharges, where the signal peaks at 41 MHz instead of the expected 17MHz.