A 2mm microwave transmission interferometer is used to measure the electron density, collision frequency and electron temperature in order to study the expanding plasma of BSG I device. The electron density is above 2.5×10
20m-
3 at the peak and the microwave beam is cut off. The density decay below cut off is obtained from the phase shift and the density profile measured by floating double probes. Oversized waveguide of 10Ge/s, standard pyramidal horns of 25db gain andteflon lenses about 60mm infocal distance are used.
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