Experimental Study of Parametric Instabilities Driven By Finite Extent Lower Hybrid Waves in a Plasma.

Abstract

The parametric decay of a lower hybrid wave into a lower hybrid sideb and and a nonresonant low frequency quasimode was observed in a linear magnetized plasma device. The new contribution of this work is a measurement of the dispersion properties of this process in a realistic geometry for lower hybrid wave heating. The dispersive properties of the parametric instability were measured inside the spatially localized region formed by the driving lower hybrid wave. The driving lower hybrid wave was generated by a four ring wave coupler. Helium gas was used in a microwave generated plasma with peak densities up to 4 x 10('10) cm('-3) and 850 Gauss magnetic field. The quasimode decay process was identified by a broad decay spectrum with frequencies approaching thirty percent of the pump frequency and by agreement with the condition (omega)/k(,(PARLL)) = v(,te)/SQRT.(2. The wavenumber versus frequency of the sideb and decay waves was measured by a test wave technique. It was found that the parallel wavenumber component k(,(PARLL)) of the sideb and was different depending on whether the sideb and group velocity was in the same or opposite direction to the pump wave. The pump wavenumber has components k(,x0) and k(,(PARLL)0) where (PARLL) parallel refers to the direction of the magnetic field. The difference is due to the finite wavenumber of the pump, which adds to the sideb and wavenumber in one case and subtracts in the other. There was no difference in the measured (theta)(y) component of sideb and wavenumber, since the pump wave is symmetric in (theta). The measured values of sideb and wavenumber k(,(theta)2) versus quasimode frequency agreed with theory except for a numerical factor of 2/3. The same was true for measurements of the sideb and parallel wavenumber k(,(PARLL)2). The threshold electric field was measured by a calibrated RF double probe and found to agree with theory. A continuous transition from ion-acoustic mode to quasimode decay spectra was observed by varying the plasma density or pump wave frequency. A lower limit for the sideb and and quasimode growth rate is established by pulsing the RF pump. The saturated amplitude of the decay wave increased linearly with pump power ((PROPORTIONAL)E('2)). Above a certain power level the pump wave power inside the plasma increased more slowly as a function of applied power. Convection of waves out of the unstable region is expected to saturate this decay process, and measurements of decay wave saturated amplitudes are consistent with this expectation. Computer calculations of (omega) + i(gamma) versus k(,(PARLL)) and k(,(PERP)) of the decay waves were made to compare with the experimental results. The effects of finite pump width were included in the calculations. Thresholds, wavenumbers, and final amplification values agreed qualitatively with experiment. The results, displayed on contour plots, show the wide range of unstable wavenumbers and frequencies for the nonresonant (quasimode) decay process. Calculations of the number of e-foldings ((GAMMA)) of the decay waves were made, based on a model of convective saturation.