Lower-Hybrid-Wave Heating in the Alcator-ATokamak

Abstract

We report the results of injecting 90 kW of microwave power near the lower hybrid frequency into the Alcator-A tokamak through a two-waveguide array. The observed plasma heating is in disagreement with that expected from linear waveguide-plasma coupling theory. From these results and auxiliary rf probe measurements we infer the nonlinear formation of a high-&n wave power spectrum at the plasma edge.

Effect of waveguide boundary conditions on the backscatter of lower hybrid waves

“…These results are in qualitative agreement with the observations of the Alcator A lower hybrid heating experiment, 7 which inferred a kz spectrum peaked. at i/b.…”

Nonlinear Coupling of Lower-Hybrid Waves at the Edge of Tokamak Plasmas

“…At these power levels significant deviations from linear coupling have been observed. The reflection coeflicient depends on the power density and becomes less sensitive to the phasing of adjacent waveguides [3,41. In other experiments one has observed a change in the power spectrunm, which is shifted to larger values of the longitudinal refractive index (n.), [5]. The purpose of this paper is to present a theory which will explain, at least qualitatively, these results.…”

Accessibility and Energy Depositon of Lower-Hybrid Waves in a Tokamak with Density Fluctuations

“…The model is electrostatic and the drift kinetic approximation is used for the electrons while full dynamics of ions are followed in time. 6 Figure 1 shows the time development of the potential, ecp k (t)/T e , (a) for a long wavelength, £^^ = 0.16, and (b) for a short wavelength, k ± p { = 1.2, and the frequency spectrum obtained from one of the one-dimensional simulations. Here, w i /m^ = 1837, T e /T { = l 9 tt e /u pe =5 (ratio of the electron gyrofrequency to the plasma frequency), k n /k ± = 0.1, v d /v the = lA, and L= 1024A are used where v d is initial electron-drift speed along the magnetic field, v the is electron thermal speed, and A is the computational mesh size.…”

“…Density fluctuation scattering becomes especially interesting in light of recent puzzling results of the Alcator-A lower-hybrid-heating experiment, where it was found that the power coupled to the plasma was independent of the phasing of the two wave-guide grills used to launch the microwaves, 6 thus indicating that the inaccessible part of the power spectrum was able to propagate to the plasma center and heat. Furthermore, a high level of density fluctuations, peaked at the limiter radius, 1 was observed in Alcator-A.…”