Comment on ‘Comparison of runaway dynamics in LH and ECRH heated discharges in the Frascati Tokamak Upgrade’

In their paper, the authors used the injected parallel refractive index of lower hybrid waves (LHWs) to evaluate the anomalous Doppler resonant interaction of runaway electrons with LHWs. This led to the conclusion of no wave–particle resonant effect in FTU. This is not true when the upshift effect in the LHW spectrum is considered. The wave–particle resonant energy for the upshifted LHW spectrum is consistent with the measured runaway energy during the lower hybrid current drive phase, while the predicted runaway energy based on the loop voltage is much higher than the measured runaway energy after the injection of LHW power. This indicates that the rapid decrease in runaway energy after the injection of LHW power possibly results from the anomalous Doppler resonant effect.