High pressure, high temperature microwave discharges for application in plasma chemistry

The development of the microwave electrothermal thruster has created the technology to produce high pressure (50 Torr-2 atmosphere) electrodless microwave discharges in flowing inert and molecular gases. These discharges are located inside a quartz tube which passes coaxially through a cylindrical microwave cavity. The discharge is created with 100 to 2,000W input power at 2.45 GHz by exciting the cavity in the TM/sub 012/ or TM/sub 011/ mode. If the discharge pressure is increased from 20 or 30 Torr to higher pressures the discharge separates from the enclosing quartz walls and floats in the center of the cavity. The center of the discharge has a gas temperature that is considerably higher than the temperature of the gas adjacent to the quartz walls. Thus, the discharge is thermally inhomogenous with a hot central core, and sharp temperature gradients exist between the center and the surrounding cooler gas outside the discharge. These discharges, called microwave arcs, take on volumes that increase with input power and decrease with increases in pressure. Measured microwave coupling efficiencies are in excess of 95% over an operating pressure range of 100 Torr - 2 atmosphere. These high pressure, high temperature discharges may have uses in plasma chemistry. This paper reviews the basic principles of ignition and operation of these discharges and compares their known behavior to the lower frequency induction plasma discharges