Toroidal Microwave Discharge Heating of Gas

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Abstract
In the series of experiments to he described we had as our objective the coupling of large amounts of microwave power into gas at pressures of around one atmosphere. The gas used in all cases reported here was commercial helium or argon at pressures of 1 to 3 atmospheres. Both continuous wave and pulsed microwaves at S-band were used. A toroidal discharge was obtained by means of the circular electric (TE01°) mode in a circular waveguide. This configuration minimized wall losses and provided radial stability. Axial stability was obtained in two different ways. First, the gas pressure and microwave power level were adjusted in a tapered, circular waveguide such that a discharge could be maintained only in a certain axial region. The end of the waveguide was effectively short-circuited. Coupling efficiency was measured by comparing the reflected power with and without a discharge. This efficiency was in excess of 90% for pulses of 250 kW, and ranged from 25% to 60%o for CW power of 23 kW. Second, an ellipsoidal antenna system operating in the TE01° mode was used to transmit power to the discharge chamber through a large domed window. The strong focusing action of the antenna allowed breakdown only inside the waveguide chamber, and not near the window. The temperature rise in flowing gas observed beyond the discharge was used to determine the coupling efficiency in this case. Pulsed experiments were carried out at power levels of 1 MW peak, 6 kW average, with antenna transfer efficiencies up to 25% and heating efficiencies from 20% to 50%.

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