Wakefield excitation by a powerful sub-nanosecond 28.6 GHz microwave pulse propagating in a plasma filled waveguide

Abstract

High-power microwave pulse generation ($1.2 GW, $0.4 ns, 28.6 GHz) by a super-radiant backward wave oscillator (SR-BWO) and the feasibility of wakefield-excitation with this pulse in a plasma-filled waveguide are presented. The SR-BWO is driven by an electron beam ($280 keV, $1.5 kA, $5 ns) generated in a magnetically insulated foilless diode and propagating through a slowwave structure in a guiding magnetic field of 8 T. The plasma produced by an array of flashboards filling a cylindrical wire-array waveguide attached at the exit of the SR-BWO is also characterized. 1D and 3D numerical simulations demonstrate that for the experimental parameters of the microwave pulse and the flashboard plasma filling the waveguide, a wakefield forms accompanied by significant periodic density modulations such that their radial location and depth can be controlled by the waveguide radius, plasma density, and microwave power.

Experiments Designed to Study the Non-Linear Transition of High-Power Microwaves through Plasmas and Gases

“…Two SR-BWOs were developed and built in our laboratory. One operating in the X-band at 9.6 GHz (referred to below as X-SR-BWO) [29,35] and the second at 28.6 GHz in the Ka band (Ka-SR-BWO) [36,37]. Drawings of the X-band SR-BWO is presented in Figure 1.…”

“…At the same time, the distance between the wires is small enough so that the electromagnetic power is contained with minimal losses. The flashboard plasma was characterized to be stable in time and space [37]. The chamber has longitudinal observation windows where external diagnostics can be placed.…”

“…Depending on the waveguide radius, the total radial force can either be in the direction of the Lorentz force, outwards from the axis of the waveguide, or in the opposite direction forcing the electrons towards the waveguide axis. For the radial force to be positive everywhere, that is, all electrons are forced towards the waveguide walls, it is required that the waveguide radius be [37]. If > , some electrons move towards the axis and some towards the walls.…”

“…For the radial force to be positive everywhere, that is, all electrons are forced towards the waveguide walls, it is required that the waveguide radius be r WG ≤ r cr [cm] = 19.85 f [GHz] [37]. If r WG > r cr , some electrons move towards the axis and some towards the walls.…”

“…Plasma 2019, 2 FOR PEER REVIEW 10 Figure 10. Radial distributions of the normalized density ( )/ 0 for different waveguide radii, for P = 0.5 GW, 0 = 2.5 × 10 10 cm −3 [37].…”