Reentrant cavity as a low-power plasma source

A low-power source of plasma employing ECRH microwave breakdown in a reentrant cavity is described. The cavity has the following features: its dimensions can be made much smaller than its resonant wavelength, cavity Q values of several hundred are not difficult to achieve, and the vacuum rf electric field is uniform near its center. The plasma density to power ratio is roughly 109 cm-3 per watt for powers up to about 10 W. A smooth radial profile and a temperature of 5--10 eV are observed, and the plasma is very quiescent (δn/n is less than 1%). The cavity's construction and performance is described and compared to the performance of a simple loop antenna in a nonresonant structure