Abstract
The authors briefly review the theory of microwave gas breakdown and compare theoretical and literature breakdown curves for air and noble gases to show the effects of pressure, temperature, frequency, gas composition, size and shape of cavity, and electric field strength. The authors then derive an equation which shows the effect of a dielectric load in a cavity on gas breakdown, and shows how this relates to heating of the dielectric load and to electric field strength. Experimental breakdown curves for air, Water and carbon dioxide are given and related to theory at 2450 MHz, the most commonly used frequency for food materials.Pressures studied were in the range of 0.1 torr to 20 torr. Temperatures were essentially ambient (24°)C. Electric field strengths were varied from 150 to 600 volts/cm. The agreement between theory and experiment is good, both for air breakdown and the effect of a dielectric load. The results obtained show a given microwave freeze-dryer with an arbitrary operating pressure and food load, just how much power can be applied and absorbed by the food without gas breakdown.
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