Studies of terahertz discharge in noble gases using a Michelson interferometer

The measurements of the spatial-time evolution of the discharge in argon and krypton at a wide range of pressures were performed. The gyrotron was used for plasma ignition. The radiation was 40 kW with frequency 0.67 THz. The pulse duration was 20 μs. A Michelson interferometer has been exploited for investigation of the discharge in the noble gases in a focused beam of terahertz waves. The diameter of the laser beam near the plasma cloud was 25 mm at 1/e² level, λ = 532 nm. The rectangle area (about 20 × 15 mm) near the focus of THz beam was examined synchronously by the interferometer and a CCD camera. The spatial-time evolution of laser beam phase-shift of THz discharge in Kr and Ar from atmospheric pressure down to a few torrs was obtained. The intensity of the discharge glow in optical range correlates with the phase shift map. Previous measurements of plasma density made in Ar under a background gas pressure at a level of a few torrs, based on the Stark effect, are in good agreement with the gained results.