Electrostatic ion cyclotron waves in a steady-state toroidal plasma

Thorello is a steady-state toroidal plasma, produced in different neutral gases by hot filament electron emission and acceleration by a constant bias voltage. Electron temperature and density, evaluated by Langmuir probes, are of the order of 5 eV and 1010cm-3, respectively. The maximum toroidal magnetic field on axis is about 2 kG. In this environment, the dispersion relation of electrostatic ion cyclotron waves has been studied, in hydrogen and helium plasmas. These waves are excited by a pair of thin metallic blades, fed with opposite phase by a low power (<10 W) r.f. signal in the range of the ion cyclotron frequency and its first harmonics. The antenna system, installed at the plasma center, in conjunction with a movable r.f. probe, forms a continuous-wave interferometer suitable for the experimental reconstruction of the ion wave dispersion relation. A phase comparison method, for any given frequency, allows the evaluation of the perpendicular wavenumber. The fit of experimental data to the theoretical curves, generated by a numerical analysis of the complete e.s. dispersion function, gives the possibility of evaluating the order of magnitude of the ion temperature. A good agreement is obtained in the hydrogen case by applying the stationary phase method to the normal mode k''parallelto'' integral, while, in helium, by considering collisional effects, not negligible in weakly ionized, low temperature plasmas. (author)