Ionization-induced self-channeling of a $\le 500\mathrm{MW}$, 9.6 GHz, $<1\mathrm{ns}$ microwave beam injected into air at $\sim 4.5\times {10}^3\mathrm{Pa}$ or He at $\sim {10}^3\mathrm{Pa}$ is experimentally demonstrated for the first time. The plasma, generated by the impact ionization of the gas driven by the microwave beam, has a radial density distribution reducing towards the beam axis, where the microwave field is highest, because the ionization rate is a decreasing function of the microwave amplitude. This forms a plasma channel which prevents the divergence of the microwave beam. The experimental data obtained using various diagnostic methods are in good agreement with the results of analytical calculations, as well as particle in cell Monte Carlo collisional modeling.

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 slow-wave 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.

We present the results of an experimental, theoretical, and numerical study of short and long time wake, produced by a 0.6 ns, 0.5 GW, 9.6 GHz high power microwave (HPM) pulse propagating in a plasma-filled cylindrical waveguide. The perturbation of the plasma density caused by the ponderomotive force prevents not only the pulse from spreading due to dispersion, but also leads to pulse compression. The high power pulse leaves far behind it a long lived positively-charged plasma whose electrons oscillate in the Coulomb potential well and ionize the background neutral gas over several tens to hundreds of nanoseconds. This leads to long time light emission observed in the experiment. The density of this newly created plasma can exceed many-folds its initial value. The theoretical model shows that as a result of the wake excitation by the propagating HPM pulse, fast electrons are ejected and collected on the waveguide wall. These high energy electrons, pulse compression, and long time light emission are evidence of the wake formation. The results of the experiment, the analytical model, and the numerical simulations are in good agreement.

We report the successful operation of an advanced relativistic magnetron made up of separate anode segments and fed by a split cathode as suggested by Leopold et al. [Phys. Plasmas 27, 103102 (2020)]. A split cathode has the advantage that the emitter is placed outside the interaction region so that the cathode plasma does not cause pulse shortening. We find that the split cathode-fed magnetron performs as well as a common explosive emission plasma cathode but without the appearance of microwave pulse shortening during ∼200 ns while the pulsed generator is at its maximum power. The angular magnetron segments facilitate longitudinal slits in the magnetron's anode block, which allows for fast magnetic field penetration. This drastically decreases the power requirements of the pulsed generator feeding the axial insulating magnetic field producing solenoid and does not affect the operation of the magnetron. This results not only in a compact system but also in overall high electrical efficiency and the possibility to operate the relativistic magnetron in a repetitive mode.

Frequency up-conversion ($\sim 10\%$) and compression (almost twofold) of a powerful ($\le 250$ MW) microwave pulse in the propagating ionization front produced by the pulse itself in a gas-filled waveguide, is investigated experimentally and analyzed theoretically. Pulse envelope reshaping and group velocity increase manifest themselves in a propagation of the pulse faster than in the empty waveguide. A simple one-dimensional mathematical model allows the adequate interpretation of the experimental results.

We observe complete absorption of an ∼1.2 GW, 0.5 ns, 25.6 GHz high power microwave pulse propagating in a plasma-filled waveguide when the plasma density dependent waveguide cutoff frequency is close to the pulse frequency. Some of the plasma electrons are ejected to the walls, leaving in the waveguide an uncompensated ion charge which forms a potential well where the remaining electrons oscillate in the pulse field. Due to the decreased group velocity of the wave, these trapped electrons have sufficient time to collide with ions, while their regular oscillatory motion becomes chaotic and thermal. Almost all the energy of the electromagnetic pulse is transferred to the kinetic energy of the electrons. This mechanism of absorption is absent when the pulse power is low, and a potential well does not form in the waveguide.