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Millimetre wave plasma interferometry in the near field of a Hall plasma accelerator

Measurements are described of electron number density in the near-field of a 200 W Hall plasma accelerator using a 90 GHz microwave interferometer. The system is of a phase-bridge design, utilizing two signal arms from a fixed frequency source (one passing through the plasma) that recombine at two balanced mixers. A line-integrated electron density is obtained by comparing the in-phase and quadrature signals from the mixers. Measurements across chords through a ‘symmetric’ plasma plume are Abel inverted to give the radial variation in the plasma density, which is found to be toroidal in shape, reaching values as high as 4 × 1017 m−3. The first direct non-intrusive measurements of the electron density in the near-field of this source reveal structure not previously resolved with electrical probes. This interferometer is suitable for measuring time-dependent plasma density fluctuations offering non-intrusive information about plasma oscillations and instabilities in the near exit region where electrical probes are known to interfere with the discharge operation.