High power microwave plasma discharge

Abstract

High-power microwave (HPM) has broad applications in scientific research, civil and military community. HPM has the character of peak power of tens of Giga-Watt and the pulse width from tens of to hundreds of nanosecond. The intense electromagnetic breakdowns in the HPM generation, transmission and radiation systems, seriously limit the HPM systematic power capacity, and become the bottle neck of the HPM technology-development and the international technical challenge. In this paper, the recent developments of intense electromagnetic breakdown at vacuum and air sides of output window and on metal surfaces are reviewed and discussed. To understand the breakdown mechanism at window, several multipactor dynamic models are developed, especially involving electron-neutral collision and ionization in the desorbed gas layer, analytically obtaining the influence of desorption gas on multipactor saturation, and space charge field and potential distribution above dielectric surface formed by multipactor and plasma. By diagnosing the time- and space-evolution optical emissions, the mechanisms of nano-second microwave-driven discharges near the dielectric/vacuum and dielectric/air interface were discussed. For breakdown at the dielectric/vacuum interface, multipactor and plasma developing in a thin layer of several millimeters above interface, revealing intense ionization concentrated in a desorbed high-pressure layer. For breakdown at the dielectric/air interface, nonlinear positive feedback of formation of a space-charge microwave sheath near the dielectric surface, accelerated by the normal components of the microwave field, significantly enhances the local-field amplitude and hence ionization near the dielectric surface. The mechanism and methods of using magnetic field satisfying specific amplitude and direction perpendicular to E rf x E dc to suppress microwave multipactor were theoretically studied and experimental demonstrated. The methods of the external resonant magnetic field have been demonstrated by proof-of principle S-band large power experiments to significantly improve the power capacity by 9 times. The Halbach-like magnets to generate the transverse homogeneous B-field in a large scale was designed to suppress multipactor, and the window breakdown threshold was significantly enhanced at multi-Giga-watt. The methods of periodic surface profiles on suppressing microwave multipactor were discussed. The three-dimensional periodic ripple profile with each unit of rotational symmetric surface is proposed and theoretically and experimentally demonstrated to suppress multipactor for arbitrary electromagnetic mode with any polarization. In theoretical aspect, basic physics courses are described qualitatively, the basic models of secondary electron multipator, gas desorption and plasma avalance are established, but lack of quantitive results and experimental diagnostic of plasma energy and density. The physical and chemical methods of periodici surface, resonant magnet, and surface fluorination are successfully used in suppressing breakdown and improving the power capacity, but there is distance away from practical application.