Lie Liu

Asymmetric-mode competition in a relativistic backward wave oscillator with a coaxial slow-wave structure

The initial experimental results of an L-band relativistic backward wave oscillator with a coaxial slow-wave structure are presented. The asymmetric-mode-competition mechanism in the device is investigated theoretically and experimentally. It is shown that the diode voltage, guiding-magnetic field, and concentricity play a key role in the suppression and excitation of the asymmetric-mode (coaxial quasi-TE11 mode). In the experiments, the asymmetric-mode with a frequency of 2.05 GHz is suppressed and excited, which is in good agreement with the theoretical results.

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

Transversal and longitudinal mode selections in double-corrugation coaxial slow-wave devices

To reduce the dimensions of relativistic backward wave oscillators (RBWOs) operating in the low frequency regime of less than 2 GHz, the theory of transversal and longitudinal mode selections are introduced in this paper. The transversal mode selection is achieved using the property of “surface wave” of the coaxial slow-wave structure (SWS) to excite the quasi transverse electromagnetic (quasi-TEM) mode without the higher transverse magnetic (TM) modes and it is proved that the coaxial SWS may decrease the transversal dimension of the SWS sections. In addition, the S-parameter method is employed to investigate the longitudinal resonant characteristic of the finite-length SWS, and the scheme of longitudinal mode selection is put forward. It is proposed that the introduction of a well-designed coaxial extractor to slow-wave devices can help to achieve the longitudinal mode selection and reduce the period number of the SWS, which not only can make the devices more compact, but also can avoid the destructive ...

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.

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...

Characterization of plasma expansion dynamics in a high power diode with a carbon-fiber-aluminum cathode

Thermal plasma expansion is characterised during the operation of a high power diode with an explosive emission carbon-fiber-aluminum cathode driven by a 250 kV, 150 ns accelerating pulse. It is found that a quasi-stationary state of plasma expansion is obtained during the main part of the accelerating pulse and the whole plasma expansion exhibits an “U”-shape velocity evolution. A theoretical model describing the dynamics of plasma expansion is developed, which indicates that the plasma expansion velocity is determined by equilibrium between the diode current density and plasma thermal electron current density.

T-junction waveguide-based combining high power microwave beams

Waveguide-based combining microwave beams is an attractive technique for enhancing the output capacities of narrow-band high power microwave devices. A specific T-junction combiner is designed for combining the X/X band microwave beams, and the detailed combining method and experimental results are presented. In the experiments, two microwave sources which can generate gigawatt level microwaves are driven by a single accelerator simultaneously, and their operation frequencies are 9.41 and 9.60 GHz, respectively. The two microwave beams with durations of about 35 ns have been successfully combined, and no breakdown phenomenon occurs.