DIAGNOSTICS AND MODELING OF ECRH MICROWAVE DISCHARGES

Stationary plasmas in various gases were generated at pressures of some 10 Pa in weak magnetic fields by microwave heating. Quantitative spectroscopy of atomic lines and molecular bands was applied for diagnostics of plasmas in hydrogen, nitrogen, methane and argon-helium mixtures. Simple model calculations were based on the well-known balance equations of glow discharges including chemical reactions especially for methane. Some tests were also performed nitrogen glow discharges. Excitation, ionization and dissociation rate coefficients for diagnostic or modelling purposes were taken from nuclear fusion research or calculated from experimental cross-sections assuming a Maxwellian energy distribution. The electron temperatures of the discharges, predicted from electron confinement times and ionization rate coefficients (almost-equal-to 1-4 eV), were well confirmed by the experiment. Due to additional energy losses into vibrational excitation and dissociation, there is a substantial difference in plasma size between atomic and molecular plasmas at a given input power. The electron density in molecular plasmas is of the order of the cut-off density, i.e. almost-equal-to 10(11) cm-3, almost independent of the pressure. The kinetic temperature of the heavy particles was determined from the N2 band rotational intensity distribution (almost-equal-to 500 K), while the rotational temperature of CH bands and the hydrogen gas temperature are higher-a proof of energy gain during CH4 dissociation. Hydrogen and CH densities were measured in methane plasmas by adding a small amount of argon and comparing H-alpha and CH molecular bands to Ar line intensities. H and CH particle densities were also derived from model calculations using electron dissociation rate coefficients and respective confinement times for the dissociation products. Some neutral-neutral reactions were also included. The measured H and CH densities agree with the model with and without neutral-neutral reactions. A comparison of nitrogen and methane power balances shows that there are no other important energy loss channels in CH4 plasmas.