Microwave diagnostics of the plasma sheath produced by slot antenna breakdown

The plasma sheath caused by pulsed RF breakdown in helium outside the aperture of a waveguide radiator has been probed. The highest measurable phase shifts and attenuation changes caused by the plasma were compared with those calculated on the basis of a slab model. The results are consistent with an effective thickness of the plasma sheath of about 1 cm, and electron densities in the active sheath being considerably in excess of the critical value.