Yonggui Liu

Plasma-induced evolution behavior of space-charge-limited current for multiple-needle cathodes

Properties of the plasma and beam flow produced by tufted carbon fiber cathodes in a diode powered by a ~500u2009kV, ~400u2009ns pulse are investigated. Under electric fields of 230–260u2009kVu2009cm−1, the electron current density was in the range 210–280u2009Au2009cm−2, and particularly at the diode gap of 20u2009mm, a maximum beam power density of about 120u2009MWu2009cm−2 was obtained. It was found that space-charge-limited current exhibited an evolution behavior as the accelerating pulse proceeded. There exists a direct relation between the movement of plasma within the diode and the evolution of space-charge-limited current. Initially in the accelerating pulse, the application of strong electric fields caused the emission sites to explode, forming cathode flares or plasma spots, and in this stage the space-charge-limited current was approximately described by a multiple-needle cathode model. As the pulse proceeded, these plasma spots merged and expanded towards the anode, thus increasing the emission area and shortening the diode gap, and the corresponding space-charge-limited current followed a planar cathode model. Finally, the space-charge-limited current is developed from a unipolar flow into a bipolar flow as a result of the appearance of anode plasma. In spite of the nonuniform distribution of cathode plasma, the cross-sectional uniformity of the extracted electron beam is satisfactory. The plasma expansion within the diode is found to be a major factor in the diode perveance growth and instability. These results show that these types of cathodes can offer promising applications for high-power microwave tubes.

An intense-current electron beam source with low-level plasma formation

The generation of high-current electron beams is accompanied by strong plasma formation on the cathode surface. The cathode plasma sheath expands towards the anode, which limits the pulse length. In this paper the experiments were performed using a high-voltage pulse generator with 400u2009kV output voltage and 300u2009ns pulse duration. We used electrical and optical diagnostics to study the process of plasma formation. This paper presents data on a caesium–iodide (CsI) coated carbon fibre cathode capable of operating with the lack of strong plasma formation. The addition of CsI caused an increase in the voltage pulse duration and reduced the turn-on electric field for electron emission, thus resulting in the fast-rise current. In particular, the CsI coating effectively inhibited the plasma expansion and, as a result, the diode gap remained almost unchanged within approximately 200u2009ns. These results show that CsI-coated carbon fibre cathodes are promising electron emitters for generating long pulse high-current electron beams.

Effects of CsI Coating of Carbon Fiber Cathodes on the Microwave Emission From a Triode Virtual Cathode Oscillator

We discuss the effects of cesium iodide (CsI) coating of carbon fiber cathodes on the microwave emission from a triode virtual cathode oscillator. As compared with the uncoated cathode, the CsI-coated carbon fiber cathode significantly improved the diode performance and, most notably, lengthened the microwave pulse, from 150 to 200 ns. The light emission from the diode, the diode perveance, and the diode gap change were introduced to explain the observed extension of microwave pulse. After CsI coating, the carbon fiber cathode exhibited the absence of strong plasma, a slow plasma expansion velocity, and an almost unchanged diode gap during the main voltage pulse. It was found that heavy plasma ions, slow plasma expansion velocities, and long microwave pulses tend to be closely tied. These results show that, given a proper diode design, the carbon fiber cathodes with CsI coating have great promise for generating long-pulse microwave radiation.

Relativistic electron-beam generation in a gas-loaded foil-less diode

Relativistic electron beams were generated in a gas-loaded foil-less magnetized diode. The helium gas was employed to fill the diode, and the experimental results have shown that the diode operated normally without the closure problem when the gas pressure was below a certain value (typically, 100 mTorr in the presence of an externally applied guide magnetic field). It has been observed that the current of the electron beam could increase with increasing the gas pressure, and that the presence of helium gas could reduce the operating voltage of the diode producing an electron beam with lower kinetic energy, which implies a decrease in the diode impedance. The externally applied magnetic field along the axial direction also exerted an influence on the electron-beam generation in the gas-loaded diode.

Monte Carlo Study of the Single-Surface Multipactor Electron Discharge on a Dielectric

This paper presents a Monte Carlo model to investigate the single-surface multipactor discharge on a dielectric surface in the presence of RF and dc electric fields. By employing a novel method in the numerical implementation of the secondary-electron emission, the susceptibility diagram is constructed, and beam loading and its power absorption by the multipactor discharge are examined. Meanwhile, the temporal evolution of the multipactor is also studied. The simulation results show clearly that a steady-state multipactor discharge can be built up from a very low density initial electron distribution, and an oscillatory steady state can be achieved when the positive charge, which is left by the emission of secondary electrons, is capable to build a large-enough dc electric field. During the saturation state, the normal electric field and the number of electrons in flight oscillate at twice the RF frequency. The average power absorbed by the multipactor, strongly depending on material parameters, is on the order of 1% incident power or less. Based on this model, several useful guidelines to prevent or extinguish the multipactor are presented.

High-power microwave generation by a plasma-loaded backward-wave oscillator

High-power microwaves were measured for a frequency range from 9.5 GHz to 13.4 GHz in a plasma-loaded backward-wave oscillator employing an annular relativistic electron beam with kinetic energy varying from 400 keV to 500 keV and current ranging from 1 kA to 3 kA. The relativistic electron beam, which was produced from a helium gas-loaded foil-less diode, was injected into the helium gas-loaded rippled-wall waveguide of the backward-wave oscillator, generating the plasma in the waveguide and then the high-power microwave. The gas-loaded diode operated stably without encountering the closure problem in the He gas pressure range (less than 100 mTorr) which was of interest to the operation of the backward-wave oscillator. An enhancement in the total microwave power emission over the vacuum case by a factor of up to 7 was found in the experiment, and the highest efficiency of electron beam to microwave radiation was estimated to be about 29%.