Imaging of a High-Power Millimeter Wave Beam Using a Millimeter Wave-Induced Gas Breakdown Initiated by a Metal-Dielectric Screen

A semiquantitative technique for imaging of high-power millimeter wave (MMW) beam using visible light from the plasma of an MMW-induced gas breakdown, which was initiated by a metal-dielectric (MD) screen, is discussed. The design of the gas breakdown sensor for imaging of the wave beam is presented. A plane MD screen was placed in a shallow airtight chamber with a transparent window of a thin mylar film. The chamber was filled with gas at a near atmospheric pressure. An MMW-induced breakdown in air, nitrogen, helium, and krypton, initiated by an MD screen, has been experimentally studied. In the experiments, a high-power 250-GHz pulsed gyrotron served as the source of MMWs. In a chamber filled with noble gases, bulk plasma was formed at an MMW intensity above the gas breakdown threshold. A single-pulse wave beam was imaged using a profile of the flux density of light from the breakdown plasma in helium and krypton. It was shown that using the presented technique, one can determine the position of the intensity maxima and the beam profile of an MMW beam in the range from one to two threshold intensities of an MMW-induced gas breakdown initiated by an MD screen.