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Abstract
In the series of experiments to be 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  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% for CW power of 23 W. Second, an ellipsoidal antenna
system operating in the  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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