PDI observation in RF plasma generation and current drive experiments on Globus-M tokamak

This work presents first results of observation of parametric decay instabilities (PDI) in plasma formation and current start-up experiments on the spherical tokamak Globus-M by radiofrequency (RF) waves without use of induction electric field. In these experiments RF power was used for plasma discharge, as well as for current ramp-up. The current direction was determined on the direction of vertical magnetic field. We used RF generator with operational frequency 900 MHz. Spectral analysis of electromagnetic radiation from plasma showed appearance of big number of low frequency oscillations which was excited by pump wave. These spectra are typical for non-linear parametric instabilities. PDI may lead to the redistribution of the incident RF power between secondary waves with frequencies, differing from pump, to abnormal reflection and even to significant change of plasma-wave interaction mechanism. PDI mostly appeared on the current ramp-up phase. Both sidebands with small shift and with large, about half of pump frequency shift were excited. Relation between PDI and discharge parameters was defined.