This paper presents the effects of cathode materials on the oscillation mode of a virtual cathode oscillator (vircator). In the case of the stainless steel cathode, an oscillation mode hopping appeared with two separate frequencies. Interestingly, the vircator using the carbon fiber cathode exhibited an almost unchanged microwave frequency throughout the microwave pulse. To understand this phenomenon, several parameters are compared, including the diode voltage, accelerating gap, emitting area, and beam uniformity. It was found that a flat-top voltage and a relatively stable gap will provide a possibility of generating a constant microwave frequency. Further, the cathode operated in a regime where the beam current was between the space-charge limited current determined by Child–Langmuir law and the bipolar flow. On the cathode surface, the electron emission is initiated from discrete plasma spots and next from a continuing area, while there is a liberation process of multilayer gases on the anode surface. The changes in the emitting area of carbon fiber cathode showed a self-quenching process, which is not observed in the case of stainless steel cathode. The two-dimensional effect of microwave frequency is introduced, and the obtained results supported the experimental observations on the oscillation mode. By examining the cross section of electron beam, the electron beam for carbon fiber cathode was significantly centralized, while the discrete beam spots appeared for stainless steel cathode. These results show that the slowed diode closure, high emission uniformity, and stable microwave frequency tend to be closely tied.

In this work, design and performance of an annular carbon fiber cathode are presented. Measurements on this cathode were performed in a single high-voltage pulse generator (600 kV, 50 ns, and 50 Omega) and a repetitive one (350 kV, <10 ns, 180 Omega, and 100 Hz), respectively. In a single pulse regime, emphasis was placed on the uniformity of electron beam extracted from this cathode. It was found that this cathode could deliver uniform electron beams with current densities exceeding kA/cm(2). Cesium iodide (CsI) coating eliminated hot spots on the cathode surface, significantly improving the uniformity of electron emission. Under repetitively pulsed operation, this cathode exhibited a good shot-to-shot reproducibility at the pressure of 1.5x10(-4) Torr, suggesting an ability of surviving even in poor vacuum. However, once the base pressure rose up to 3.76x10(-4) Torr, the cathode performance gradually degraded as the pulse shot proceeded. Besides, some possible explanations for these experimental results are presented. These results show that given proper diode design, carbon fiber with CsI coating has great promise as electron emitter producing high-current electron beams.

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The expansion of cathode plasma has long been recognized as a limiting factor in the impedance lifetime of high-current electron beam diode. Realistic modeling of such plasma is of great necessity in order to discuss the dynamics of cathode plasma. Using the method of particle-in-cell, the expansion of cathode plasma is simulated in this paper by a scaled-down diode model. It is found that the formation of cathode plasma increases the current density in the diode. This consequently leads to the decrease of the potential at plasma front. Once the current density has been increased to a certain value, the potential at plasma front would then be equal to or lower than the plasma potential. Then the ions would move towards the anode, and the expansion of cathode plasma is thereby formed. Different factors affecting the plasma expansion velocity are discussed in this paper. It is shown that the decrease of proton genatation rate has the benefit of reducing the plasma expansion velocity.

This paper presents a method for moderate pulsed X rays produced by a series diode, which can be driven by high voltage pulse to generate intense large-area uniform sub-100-keV X rays. A two stage series diode was designed for Flash-II accelerator and experimentally investigated. A compact support system of floating converter/cathode was invented, the extra cathode is floating electrically and mechanically, by withdrawing three support pins several milliseconds before a diode electrical pulse. A double ring cathode was developed to improve the surface electric field and emission stability. The cathode radii and diode separation gap were optimized to enhance the uniformity of X rays and coincidence of the two diode voltages based on the simulation and theoretical calculation. The experimental results show that the two stage series diode can work stably under 700 kV and 300 kA, the average energy of X rays is 86 keV, and the dose is about 296 rad(Si) over 615 cm area with uniformity 2:1 at 5 cm from the last converter. Compared with the single diode, the average X rays' energy reduces from 132 keV to 88 keV, and the proportion of sub-100-keV photons increases from 39% to 69%.