Microwave Diagnostic Results From the Gaseous Electronics Conference Rf Reference Cell

Abstract

Electron density measurements with even the simplest microwave interferometry tech-niques can range over three to four orders of magnitude, can be responsive on time scales as fast as 50 ns, and are simple to obtain and interpret. Three groups have published electron density data taken in the Gaseous Electronics Conference (GEC) ref-erence reactor using microwave interfer-ometry. The agreement in the data from these groups at higher pressures is excellent especially when one considers that the GEC reactors involved have some key dif-ferences. These may have been the cause of some differences between the results ob-tained at low pressures, although, the man-ner in which the measurements were inter-preted may also have contributed. The electron densities compare favorably in argon, helium, and nitrogen above 33.3 Pa (250 mTorr); but, the measurements tend to diverge some at 13.3 Pa (100 mTorr) and in 133 Pa helium above approximately 200 mA. It is speculated that the latter difference occurs as the discharges change from a bulk ionization or ␣ -mode to a sec-ondary electron emission or ␥ -mode, and that this transition occurs at lower voltages and currents for reactors with aluminum electrodes than it does for those with stain-less steel electrodes. In addition, time resolved electron densities are presented. There is agreement between time resolved measurements in the two reactors, in par-ticular, the electron density in helium dis-charges is found to rise dramatically after the rf excitation is turned off while the electron densities in argon and nitrogen glows exhibit only slight increases.