Measurement of electromagnetic waves from runaway electrons

Abstract

Electromagnetic waves emitted during a tokamak discharge can be partially ascribed to coupling with plasma waves. In particular, in the presence of runaway electrons, the electromagnetic waves deliver information, otherwise inaccessible, about kinetic instabilities excited by the fast particles. Experiments aimed at studying radio frequency emissions from runaway electron scenarios during different stages of plasma discharge have been carried out at the Frascati Tokamak Upgrade. Frequencies in the range of lower hybrid and whistler waves have been explored, in the presence of relativistic electrons with different energies, ranging from a few to tens of MeV. A pronounced sensitivity of the radio frequency measurements in detecting driven instabilities is observed, providing the possibility to exploit this kind of technique as a monitor of the instability processes and for studies of the fast electron activity. In particular, in this work, we propose a simplified analysis of the frequency scaling of a specific family of kinetic instabilities arising at the lower hybrid frequency range during the current ramp-up stage. The study is performed with respect to the density profile and the wave vector coupling conditions and is aimed at obtaining a rough estimate of the most likely radial location of the interaction between the runaway electron beam and plasma waves at the emission times of the observed signals.

Stability analysis of plasma waves driven by runaway electrons in tokamak hot plasmas

“…This set ensured a maximum acquisition time window of 200 ms, sufficient, for example, to measure signals in a continuous way for most of the FTU current rump-up phase. Details of the diagnostic system are illustrated in [8]. A first account of RF emissions detected in FTU for various plasma regimes with RE is given in [6].…”

“…RE driven instabilities identified as lower hybrid waves (LHWs) were observed for the first time in FTU under ITER-relevant conditions [7]. Details of the relevant detection system are reported in [8]. RF emissions during plasma discharges with RE have been observed in COMPASS [9][10][11] and KSTAR [12].…”