Very Efficient Plasma Generation by Whistler Waves near the Lower Hybrid Frequency

Experimental results are presented which show that r.f. power at frequencies near the lower hybrid frequency couples resonantly into a standing whistler wave. For an input power flux of less than 5 W densities above 10” cm-3 with close to 100% ionization have been achieved. Measured density, temperatures and wave fields are presented and are used as input parameters for a theoretical model. 1. I N T R O D U C T I O N IT HAS been long known that it is possible to ionize a gas by radio frequency power using either inductive or capacitive coupling methods. Rather than simply relying on the large oscillatory electric fields of the antenna to accelerate electrons to ionizing energies, several resonant excitation methods have been found to be more effective. These often operate at microwave frequencies and typically use some form of slow wave structure such as that described by LISITAKO et al. (1970). Recently, interest has been shown in coupling energy into magnetoplasmas at or near the lower hybrid frequency. This interest has been spurred by the desire to heat the ions in toroidal plasma devices where the radial variation of the magnetic field allows the propagation of lower hybrid waves along resonance cones to penetrate into the centre of the plasma. In this paper we describe a technique fo: producing essentiaiiy fuiiy ionized plasma which uses a special antenna structure to couple into a whistler mode wave propagating near the lower hybrid frequency. Average electron densities 10l2 cm-3 have been obtained in a 10 cm diameter plasma with an initial filling pressure torr using much lower r.f. powers and corresponding fluxes of 5 W cm-2. For comparison, the experiment of MOTLEY et al. (1979) involved fluxes of up to several kilowatts cmW2. We have in fact recently demonstrated (BOSWELL et al., 1982) that in a 5 cm diameter magnetoplasma, centrai densities approaching cmare readily achieved in steady state operation using the method described here; however, in the latter case the electron temperature was 3 eV compared with 10 eV for the plasma described by MOTLEY et al. (1979). In this type of discharge one of the main power losses is by electron heat conduction along the magnetic field, so that in a linear device, once the plasma is fully ionized, the effect of increasing the r.f. power is to heat the electrons, the equilibrium temperature being determined primarily by the rapid end losses. At frequencies higher than the lower hybrid frequency, MOTLEY et al. (1979) report a significantly improved performance approaching that of the source described here. However, our main interest here is‘r.f. excitation near the lower hybrid frequency and not excitation of an “overdense” plasma column. torr, LEHAh‘E and THONEMANN (1965) have obtained a 10 cm diameter plasma with density cm-3 generated by inductively coupled r.f. at a power flux of about 50 W cm-2. The plasma was, however, 1147 Using a higher filling pressure -4 x