90 GHz Phase-Bridge Interferometer for Plasma Density Measurements in the Near Field of a Hall Thruster.

摘要

Preliminary measurements are described of electron number density obtained in the near field of a 200W Busek Hall thruster. The approach taken is ultra-high high frequency (3.3 mm wavelength, 90 GHz) microwave interferometry that affords high spatial resolution suitable for studying the near exit region, where the plasma density can be as high as 10 (11)cm-3. A 90 GHz beam can be focused down to a waist of approximately 7 mm, and over a 50 mm plasma path length, a l0(11)cm-3 plasma density gives rise to an easily measurable 30 phase shift. The system is of a phase-bridge design, utilizing two signal anns split from a fixed frequency source (one passing through the plasma) that recombine at two balanced nuxers. A line-integrated electron density is obtained by comparing the two signals from the mixers, This interferometer is suitable for measuring time-dependent plasma density fluctuations offering unprecedented information about plasma turbulence in the near exit region, and the opportunity to study turbulence-enhanced electron transport in regions where the plasma is presumably collisionless and free of interactions with the channel wall.