When a magnetized annular relativistic electron beam propagating in a conducting tube carries a charge higher than the space charge limit, it can stabilize at a lower energy and higher density state. Such a charge distribution can be used as an electron source in high power microwave devices, a relativistic magnetron in particular, and in other applications. The limiting current transmitted by the beam decreases in tubes with larger radii, so in a tube with a radial transition from a small to large radius, the current can over-inject the downstream tube. This can start a dynamical process which stabilizes as a high density state. The same effect can be achieved by increasing the magnetic field in a magnetic mirror-like scheme or by adding a slowing down potential in the electron beam's route. Here, we propose a simpler, more practical way to produce such a dense state by splitting the cathode into an emitter and a reflector. This scheme is tested in simulation and experiment.

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.

The relativistic magnetron is one of the most efficient high power microwave (HPM) sources but pulse shortening, the result of explosive cathode plasma's radial expansion toward the anode, makes it impractical because the HPM pulse terminates much earlier than the applied voltage. We present experimental results of the operation of a relativistic magnetron fed by a split cathode. A split cathode [Leopold et al., Phys. Plasmas 27, 103102 (2020)] consists of a cathode placed upstream and outside the anode, connected by an axial rod to a reflector (a transverse conducting circular plate) placed downstream from the anode. The electron charge, emitted by an annular explosive cathode emitter, accumulates in the space between the cathode and the reflector and at the same time, screens the rod from explosive plasma formation. This accumulated space charge serves as the electron source for the magnetron. The explosive plasma developing on the emitter remains outside the magnetron and does not propagate into the anode while it operates. We compare the performance of the magnetron operating with a standard explosive emitting solid carbon cathode to that with a split cathode. The experiments demonstrate that whereas for the solid cathode, the microwave pulse developing in the magnetron suffers from pulse shortening, with a split cathode, the pulse survives as long as the amplitude of the applied voltage is sufficient for the magnetron's operation. We support the experiment by particle-in-cell simulations.

The results of an experimental and a numerical study of a relativistic A6 magnetron with single radial microwave power output slot operating with angular segment (AS) NdFeB permanent magnets placed inside the anode vanes and cylindrical rod magnets inside the cathode are presented. With permanent magnets, ~10 ^-7 s long pulses of tens of megawatts of microwave power with an electronic efficiency of ~7% are generated. The results of experiments and simulations show that electron losses near the poles of the anode magnets where magnetic field lines cross the anode-cathode gap limit the microwave generation efficiency.

An experimental setup to demonstrate a recent scheme [Leopold et al., Phys. Plasmas 24, 073116 (2017)] to persistently over-inject a vacuum electron diode so that it self-oscillates in the GHz-frequency range and releases a periodic train of electron bunches is described. We present simulations in which we account for the finite rise time of the voltage accelerating electrons from a finite radius cylindrical thermionic cathode and finite grid electrode transparency, which make the experiment different from the original theoretical scheme. It was found that these experimental conditions do affect the expected results but a self-oscillating behavior is still possible. Our experimental results so far indicate the presence of the self-oscillatory behavior, but because the cathode used emits from a few hot-spots rather than uniformly, the beam current modulations are different from those obtained in the simulations.

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.