Abstract
Electron density measurements of steady state helium plasmas in glass cylinders 17 to 96 mm in diameter are made at 34 GC and at pressures of 0.08 to 1.0 mmHg and densities of about 5×1010 to 1012 cm-3. Standard pyramidal horns of 20 db gain and teflon lenses about 75 mm in diameter are used. Errors arising from the multiple reflection between antennas and the spreading of microwave beams incident on plasmas are investigated. It is shown that over 90% reliability by Langmuir probe method criterion is attainable for low density plasmas when an appropriate use is made of the free space method.