First Intrashot Observation of Runaway-Electron-Driven Instabilities at the Lower-Hybrid Frequency Range under ITER-Relevant Plasma-Wave Dispersion Conditions.

Kinetic instabilities driven by runaway electrons (REs) have recently received attention in the fusion community as a means to control and diagnose REs in a tokamak. Experiments aimed at studying such kinetic instabilities have been performed at the Frascati Tokamak Upgrade (FTU), where different families of waves have been identified, from wide-band bursting emissions to quasi-monochromatic waves and sharp lines, in the presence of REs with energies from a few to tens of MeV. A specific family of waves with intense kinetic drive was directly observed for the first time, during both the early Ohmic plasma start-up and the current ramp-up. A clear wave frequency scaling with respect to the electron density was demonstrated. This scaling, with the complementary analysis of signals observed at different magnetic fields, allowed the identification of these instabilities as lower-hybrid waves. The relevant analysis shown in this Letter is based on a continuous intrashot detection of the RE-driven wave, which is reported for the first time for this kind of instability. We demonstrated that unstable waves are excited already at the very beginning of a tokamak discharge, opening the way to new possible research on the exploitation of this kind of measurement for monitoring seed REs formation at the early plasma stage, while most diagnostics still have limited capabilities. The conditions for plasma wave dispersion at the early phase of the FTU discharge are very similar to the ones expected during the ITER start-up, when analogous instabilities might, hence, come to light, in case of formation of suprathermal populations.