Jianchun Wen

Efficiency Enhancement of Reflex Triode Virtual Cathode Oscillator Using the Carbon Fiber Cathode

This paper presents the investigation on the reflex triode (RT) virtual cathode oscillator (vircator) using a carbon fiber cathode. Experimental results show that the carbon fiber cathode can obviously improve the electron beam quality and dramatically enhance the beam-to-microwave efficiency of the RT vircator. It was found that the beam-to-microwave efficiency increased from about 4%-6% in the case of the stainless steel cathode to over 10% in the case of the carbon fiber cathode. In order to understand the phenomenon of efficiency enhancement, experimental studies on the plasma formation on the surface of carbon fiber cathode were also done using the facility of the RT vircator. Physically, the electron emission of the carbon fiber cathode is attributed to the plasma formation due to the surface flashover along the whole surface of carbon fiber. This mechanism is different from the conventional explosive emission of metal cathode. Experimental results clearly show that the electrons emitted not only from the top of the carbon fiber but also from the side surface. Particularly, the plasma is more uniform for carbon fiber cathode due to the side surface flashover of the carbon fibers, and plasma expansion velocity is slower than that for conventional metal cathode, which significantly improves the electron beam quality of the carbon fiber cathode

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 ~500 kV, ~400 ns pulse are investigated. Under electric fields of 230–260 kV cm−1, the electron current density was in the range 210–280 A cm−2, and particularly at the diode gap of 20 mm, a maximum beam power density of about 120 MW cm−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.

Carbon fiber-based cathodes for magnetically insulated transmission line oscillator operation

We report experimental results of carbon fiber cathodes for the magnetically insulated transmission line oscillator (MILO). The fabrication process of carbon fiber cathodes is presented. In experiments employing a 500kV, 100ns high-voltage pulsed accelerator, microwave generation in the C band was obtained with ∼900MW peak power and ∼20ns pulse width at about 4.5% efficiency. The MILO operation exhibited high stability by using carbon fiber cathodes. These results show that carbon fiber cathodes can offer promising applications for high performance MILOs.

The dependence of vircator oscillation mode on cathode material

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....

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 400 kV output voltage and 300 ns 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 200 ns. 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.

Role of the rise rate of beam current in the microwave radiation of vircator

In this paper, the effect of the rise rate of beam current on the microwave radiation of a virtual cathode oscillator (vircator) is presented. Interestingly, it was observed that the rise rate of the beam current increased as the pulse shot proceeded, which is accompanied by the decrease in microwave power. By comparing the experimental results of two cathode materials (carbon fiber and stainless steel), it was found that the above behavior is independent of the cathode materials. The ion flow, induced by the repetitive action of beam electrons with the anode grid, directly affects the development of beam current. A twice-increased process of ion flow was observed, and there are two factors involved in this process, namely, the reflection of electrons between the cathode and virtual cathode and the effect of one-time bombardment of electron beam. After the irradiation of pulsed electron beam, some microprotrusions toward the cathode appeared on the anode surface, with a quasiperiodic structure. The appear...

A series of tufted carbon fiber cathodes designed for different high power microwave sources

Summary form only given. We report the fabrication technique of tufted carbon fiber cathodes for different microwave sources. Three carbon fiber cathodes were constructed, including a planar cathode, an annular cathode, and a cylindrical cathode for radial emission. Experimental investigations on these cathodes were performed in a reflex triode virtual cathode oscillator vircator, a backward wave oscillator BWO, and a magnetically insulated transmission line oscillator MILO, respectively. The pulse duration of microwave emission from the reflex triode vircator was lengthened by using the planar carbon fiber cathode. In the BWO with the annular carbon fiber cathode, the uniform electron beam with a 2kA/cm2 current density was observed. In addition, carbon fiber has great promise as field emitter for MILOs. These results show that the carbon fiber cathodes can be utilized for electron emission in high power diodes with different structures.

Dynamics of virtual cathode oscillation analyzed by impedance changes in high-power diodes

Investigations on the dynamics of virtual cathode oscillation are performed with a reflex triode virtual cathode oscillator powered by a 450 kV, 400 ns voltage pulse. Based on the diode impedance changes, the dynamics of virtual cathode oscillation are discussed. Four ensuing processes, corresponding to different stages of microwave emission, exist in the temporal behavior of diode impedance changes. When the turning point of diode impedance forms, the beam current develops into the parapotential flow. The process of microwave emission can be described by diode impedance changes. To confirm this mechanism, self-pinching of the beam current and current-voltage characteristics are introduced. This mechanism is independent of the cathode material and structure. When the experiments were carried out with a carbon fiber cathode, the 300 MW, 180 ns microwave emission was obtained, the beam current density reached kA/cm2 level, and the plasma expansion rate was 1.3 cm/μs. These results show that the carbon fiber...

Robust, easily shaped, and epoxy-free carbon-fiber-aluminum cathodes for generating high-current electron beams.

This paper presents the construction of carbon-fiber-aluminum (CFA) cathode by squeezing casting and its applications for generating high-current electron beams to drive high-power microwave sources. The fabrication process avoided using epoxy, a volatile deteriorating the vacuum system. These cathodes had a higher hardness than conventional aluminum, facilitating machining. After surface treatment, carbon fibers became the dominator determining emission property. A multineedle CFA cathode was utilized in a triode virtual cathode oscillator (vircator), powered by a approximately 450 kV, approximately 400 ns pulse. It was found that 300-400 MW, approximately 250 ns microwave was radiated at a dominant frequency of 2.6 GHz. Further, this cathode can endure high-current-density emission without detectable degradation in performance as the pulse shot proceeded, showing the robust nature of carbon fibers as explosive emitters. Overall, this new class of cold cathodes offers a potential prospect of developing high-current electron beam sources.