1. Powerful sources of millimeter wavelength (MMW) radiation are developed nowadays for plasma investigation, particularly for thermonuclear plasma heating. Powerful oscillators for plasma heating - gyrotrons - at wavelengths down to 2 mm with output of several hundreds kilowatt at CW operation and over 1 MW at pulsed operation (pulse duration up to `1 s) are created or being created.1 The existence of CW multi kilowatt gyrotrons allows for new possibilities for material processing. The MMW range is situated at the end of the microwave range close to infrared and naturally allows applications either in the multimode microwave-oven manner or in the form of wave beam energy fluxes similar to laser radiation. Gyrotrons are close to technological lasers from the view-point of high flux densities up to `106W/cm2. By their efficiency (`40%), gyrotrons are among the most effective electromagnetic devices and significantly exceed technological lasers in efficiency as well as in CW outputs. So gyrotrons can be used for various material transformation processes requiring great amounts of energy such as plasma chemistry2 and processing of solid materials.