A TUNABLE MICROWAVE PLASMA SOURCE FOR SPACE PLASMA SIMULATION EXPERIMENTS

In laboratory experiments related to space plasma physics it is often desirable to produce plasmas with characteristics as close as possible to various naturally occurring plasma regimes. In the near-earth regi on space plasma densities typically vary from 10(3)-10(7) cm-3 and tem peratures range from a few tenths of an eV to the order of 1 eV. The p lasma parameters of electron density, electron temperature, and ion sp ecies are primary variables which are often not easy to reproduce in a chamber environment which is dependent upon conventional gas discharg e or arc sources for plasma production. A simple microwave discharge d evice was developed which is easily tunable and capable of producing t he moderate range of electron densities without an external magnetic f ield. The Asmussen-type microwave plasma source described here covers and exceeds the parameter ranges required, is relatively easy to const ruct, and is inexpensive. The device makes use of an air dielectric co axial coupler to couple magnetron output to a resonant cavity. Estimat es of effective electric fields and source densities and temperatures suggest that similar devices can easily be constructed and fashioned t o produce these parameters, depending upon requirements, over a wide r ange of values. The use of widely available commercial magnetrons manu factured for microwave ovens allows a certain ease in the construction of these devices in that available cavity Q's can range to lower leve ls and therefore resonant lengths can be adjusted more easily. The des ign is discussed relative to desired experimental parameter ranges and some discussion is given of expected source current densities, electr ic fields, and temperature ranges.