89/42 PARAMETRIC INSTABILITIES EXCITED BY ION SOUND AND ION CYCLOTRON QUASI-MODES DURING LOWER HYBRID HEATING OF TOKAMAK PLASMAS R. CESARIO, A. CARDINALI Centro Ricerche Energia Frascati, Associazione Euratom-ENEA sulla Fusione, Frascati, Rome, Italy ABSTRACT. The parametric decay of lower hybrid (LH) pump waves is studied in a wide range of low frequency quasi-modes which drive the instabilities. The numerical results of the parametric dispersion relation show that an ion sound quasi-mode with frequency WIR can excite a parametric instability with a growth rate 'Y ;:: WIR or WIR <C we; at the plasma edge, even at low power levels of the pump wave. In this condition, solutions can be found where the two LH sidebands are resonant within a small frequency shift which is comparable to the growth rate, and also the upper sideband contributes to the instability. The estimated convective thresholds are in the range of the radiofrequency (RF) power used for heating experiments of tokamak plasmas. The parametric instability is related to the frequency broadening of the injected RF power observed during experiments with additional LH heating in FT and in other devices. In the range WIR '" iwe; the analysis shows that in the outer plasma region ion cyclotron quasi-modes can drive parametric instabilitieswith highergrowthrates for i =< 8-10 if a wave numberbroadeningof the launchedparallelwave is assumed. This kind of instability may explain the RF spectra observed in high density plasma regimes, which show a typical non-monotonic envelope of the sideband peaks. 1. INTRODUCTION In heating and current drive experiments in tokamaks with radiofrequency (RF) power near the lower hybrid (LH) frequency, a typical activity of the probe RF signals is observed when the plasma density is increased to above a certain threshold value. The frequency spectra of these signals (see Fig. 1) are mainly characterized by a line broadening of a few megahertz around the fre- quency of the injected RF power and by many side- bands, showing a typical non-monotonic envelope, which are separated by a gap corresponding approxi- mately to the ion cyclotron frequency at the plasma border. These phenomena seem to be related to the forma- tion of fast ion tails and a strong loss of heating or current drive, which characterizes the 'RF density limit'. However, a different behaviour has been observed in various experiments [1-7]. In Alcator C, ion cyclotron sideband spectra and a conspicuous pump wave broadening are observed when the line averaged plasma density exceeds the RF density limit [8]. It should be noted that in deuterium discharges the onset of ion tails and ion cyclotron sidebands occurs at a plasma density slightly higher than in hydrogen, but the width of the pump wave broadening is about the same in both cases. NUCLEAR FUSION. Vo1.29. No.tO (1989) In the 2.45 GHz experiment in FT, the ion cyclotron sidebands appear at a line averaged plasma density which is about twice the value of the RF density limit [4]. In the plasma regimes with a density around this value, only a slight increase of the pump wave broadening is observed. Nevertheless, as in Alcator C, the broadening becomes very large when the density threshold of the ion cyclotron sidebands is exceeded. In PLT [2], Asdex [3] and Petula B [6], ion cyclotron sideband spectra have been observed at values of the plasma density near the density limit, but data about the pump wave broadening are not available. It may be assumed that the pump wave broadening and the ion cyclotron sidebands are phenomena that are connected with some process which is responsible for the degradation of the heating and current drive efficiency. It has been pointed out [8] that the sidebands of the RF probe spectra in Alcator C are due to parametric instabilities driven by ion cyclotron quasi-modes. In a previous study [9] it has been attempted to explain the occurrence of the first harmonic ion cyclotron lower sideband in FT, which is almost always the dominant peak in the RF probe spectra. By comparing the experimental and calculated threshold values of different decay channels, it has been shown that para- metric instabilities driven by ion cyclotron and electron 1709