Plasma dynamics are essential in high-power electron beam diodes, as they influence the current density and can even cause gap closure because of fast expansion velocity during operation. In this study, the formation and expansion of the cathode plasma in a high-power planar diode has been investigated by particle-in-cell simulations. The results indicate that the expansion velocity of the cathode plasma in the planar diode is ∼2.5 cm/μs operating with a 340 kV peak voltage and 1.5 kA current, which possesses a maximum pressure of 1 Torr pressure and a gas desorption rate of 38 molecules per electron. Moreover, the enhanced emission on the edge causes a faster growth rate of the gas pressure and formation of plasma, which possesses a higher plasma density than other regions. A higher gas desorption rate and total amount of outgoing gas can cause a larger velocity of plasma expansion, and the expansion velocity is proportional to the logarithm of the rising speed of the diode voltage, while the amplitude of diode voltage did not show a clear correlation with plasma velocity. Finally, a combined dependence of the plasma velocity on the gas desorption rate, total gas volume, rising speed of the diode voltage, and diode voltage is concluded. This work provides new insights into the dynamics of cathode plasma in high-power diodes and may be helpful for engineering design.

An experimental platform based on a coaxial cylinder magnetically insulated transmission line (MITL) was established on the basis of Flash-II accelerator. An experimental study was conducted on the current loss characteristics of the transition region between the vacuum transmission line and the MITL under different impedance match conditions by adjusting the loading gap of an e-beam diode. The results showed that under shorted load, the whole vacuum transmission region suffers no current loss. Furthermore, the voltage waveform continuously decreases when approaching the load region. When the load impedance is smaller than the working impedance of the transmission line, the MITL becomes undermatched, and the current transmission efficiency of the transition region reaches 98% without the evident current loss. When the load impedance increases to make MITL overmatched, an MITL operates under a self-limited flow accompanied with huge current loss in the transition region. The current transmission efficiency decreases to 87%, and the current loss process significantly affects the working process of the probe on an anodic differential coil. On the basis of the working mechanism of an MITL and the wave reflection process, a qualitative analysis was conducted, and the experimental results were explained.

Photoconductive semiconductor switches (PCSSs) are noted for the rapid turn-on time and optically isolated triggering, making them suitable for high-power applications. Gallium oxide (Ga₂O₃), as an ultra-wide bandgap semiconductor, is promising for use in PCSSs with ultrahigh breakdown voltage and power density. This study investigates the avalanche process of Ga₂O₃ PCSSs triggered by pulsed optical sources through computational simulations, focusing on the electric field distribution, carrier dynamics, and optoelectronic response. The responsivity of Ga₂O₃ PCSS exceeds 100 mA/W when the optical power ranges from $10^{4}$ to $10^{6}$ W/cm². The rise time of the switch significantly decreases from 6.5 to 2 ns with an electric field of 6 MV/cm. Results reveal that a high-density avalanche domain forms and propagates within the device with peak electric fields ranging from 6 to 8 MV/cm. Furthermore, we demonstrate that the switching time of Ga₂O₃ PCSS can be shorter than 500 ps. This research provides design guidance and crucial insights into the carrier dynamics of an ultrafast Ga₂O₃ PCSS.

Accurate adjustment of long magnetic insulation transmission lines (MITLs) is crucial in high-power pulsed systems, which must guarantee insulation and high transmission efficiency of current under complicated electromagnetic environment. In this study, a spiral inductance-supported insulation structure in a coaxial MITL was designed, which achieved high efficiency of current transmission up to 90%. Moreover, the current transmission efficiency of the MITL under various working states and specific mechanism of the current loss was investigated by combining circuit with particle-in-cell (PIC) simulations. Results show that the current loss of the inductance-supported MITL in the under-matched state is caused by the inductance current on the support structure, as the electron charge layer is strongly constrained by magnetic field. However, the charge constraint among MITL electrodes is relatively weak in self-limited flow. The electrons bombard the supporting rod, which results in charge accumulation and unneglectable current loss in self-limited flow. This work uncovers the mechanism of current loss in the spiral inductance-supported MITL, which provides guidance for engineering design.

Magnetically insulated transmission lines (MITLs) are essential for transmitting high-power pulses in accelerators for high-energy-density physics. However, the off-centered effect of electrodes in long MITLs of large-scale pulsed power accelerators is nonnegligible, which can generate nonuniform electromagnetic fields and charged particle flow. In this study, the 1-D magnetically insulated flow theory is extended to a long MITL with an off-centered cross section based on the pressure balance equation. Solutions relating operating voltage V to the anode current I $_{\textit{a}}$ and cathode current I $_{\textit{c}}$ , are derived, and 3-D particle-in-cell (PIC) simulations are performed to verify and rescale the extended model. The results show that the azimuthal magnetic field of an off-centered MITL is approximately the cosine function of the azimuthal angle. Based on the agreement between the extended model and the PIC simulation, the dependence of the charge uniformity coefficient on the operating voltage and off-centered coefficient is summarized. The charge uniformity of an off-centered magnetically insulated flow increases with the operating voltage and decreases with the rising off-centered coefficient. Furthermore, the results illustrate that a 5% off-center displacement of the center conductor in an MITL gives about 6.5% non-uniformity in azimuthal magnetic field and about 0.13% increase in self-limited current to ensure power transmission.