Velvet Yin Strong Flow Of Multi-pulse Emission Characteristics

As a convenient and cheap cathode, velvet can emit intensive electron beam and is widely used in high power electron devices. However velvet's emission characteristics under multi-pulse mode are not systematically investigated. In the paper, a multi-pulse electron source is built. The voltage pulse duration is 90ns(FWHM) and pulse to pulse duration is 500ns. According to the requirement, the device can work under one pulse(2MV), two pulse(lMV), three pulse and four pulse mode(500kV). The electron source generates intensive electron beams with vacuum diode using velvet as cathode. Some efforts were made on the device to inspect velvet's emission characteristics under multi-pulse mode and some conclusions are made.Same as working under single pulse mode, velvet can emit intensive electron beams too under multi-pulse mode and electron emission density is up to 100A/cm~2. In the experiment, phenomena of emitting non-uniform and instability are observed. Based on the pre-work and experimental phenomena, the author proposes an opinion on velvet's emitting electron mechanism. Intensified electric field induces the velvet tips to emit electrons and the emitted electrons heats the tips and the thin intensive plasma layer is formed generated by electric field induced flashover along the fiber to the tip. Electron beams are extracted from the plasma layer by electric field.Cathode plasma determines the vacuum diode's state. Since electron beams don't interacted with anode, anode plasma is neglected. Cathode plasma moves ahead to the anode and changes diode effective gap. Plasma's expanding velocity is determined by electric field, density gradient and temperature gradient. In the experiment, plasma expanding velocity is usually between 1cm/μs~2cm/μs. A series of formula are used to describe the diode running state under the effect of cathode plasma. Based on the formula, simulation is adopted to identify that plasma expands in the way of column-like mode.The pepper-pot method is used to detect beams emittance. With the beams' emittance and current, beams' brightness is calculated and the beams' integral brightness is better than 5×10~7A/(m·rad)~2. Diamagnetic loop is also used to detect the beams' envelope variation. The experimental result indicates that beams' envelope radius changes litter during pulses process.At the same time, some challenging cathodes such as nano-diamand film, nano-carbon tube, carbon fiber and dispenser cathode are investigated too in the paper and some results are given.