Coaxial Virtual Cathode Oscillator Research Articles

Simulation study of coaxial virtual cathode oscillator as a THz source

In this paper, a coaxial virtual cathode oscillator (vircator) is simulated by the particle-in-cell method for the terahertz (THz) band. THz waves have many applications in human life, and many researchers work on related technologies, especially on THz sources like vacuum electronic devices (VEDs). A vircator is a kind of VED that is usually operated at a few GHz, but in this study, a coaxial vircator is proposed as a THz source for the first time. By adding a reflector and center rod to the base coaxial configuration and optimizing the parameters, an output power of about 29 W is achieved at 0.25 THz with the dominant mode of TM01. Working voltage and current are 26 kV and 100 mA, respectively. In addition, to indicate the scalability of the device, two other designs for higher and lower frequencies are simulated. Simplicity is one of the important advantages of the proposed THz coaxial vircator in comparison to other THz sources.

Operation Features of a Coaxial Virtual Cathode Oscillator Emitting Electrons in the Outer Radial Direction

The operation features of the coaxial virtual cathode oscillator emitting electrons in the outer radial direction were investigated through simulations and experiments. A coaxial vircator was compared with an axial vircator when the anode to cathode distance of both vircators was 6 mm. The proposed coaxial vircator was operated when the anode to cathode distance was 5 mm, 6 mm, and 7 mm. The peak power and frequency of the microwave generated from the proposed coaxial vircator when the anode to cathode distance was 6 mm were 20.18 MW and 6.17 GHz, respectively. The simulations and experiments show that the proposed coaxial vircator generates 80% more microwave power than the axial vircator with the same anode to cathode distance. According to the simulations and experiments, the proposed coaxial vircator tends to generate a higher power average when the anode to cathode distance was larger than 5 mm. The frequency of the proposed coaxial vircator when the anode to cathode distance was 5 mm and 7 mm was approximately 8 GHz and 5 GHz, respectively. The geometric factor of the proposed coaxial vircator was considered to be the reason for the greater microwave power generation than the axial vircator. The frequency of the proposed coaxial vircator decreases inversely proportional with the anode to cathode distance as observed in the axial and basic coaxial vircators.

Minimum breakdown parameters through H2, He, N2 and Ar gases

The gas discharge plasma has attracted the attention of scientists and technologists because it is a source of electrons and ions beams which are needed and used in several applications and modern instruments. Minimum breakdown voltage of the cylindrical coaxial virtual cathode oscillator is investigated on Nitrogen (N2), Argon (Ar), Helium (He) and Hydrogen (H2) through different anode transparencies. The study includes the effect of secondary electron emission on the minimum breakdown voltage (Vb)min and the product of gas pressure P and inter-electrode distance d, Pdmin. The changes in the minimum breakdown voltage with the gas pressure are studied where pressure is the main factor of electric discharge. Paschen curve is studied through electric discharge for Ar and H2 by using different anode transparencies. This study aims to understand the parameters that can affect the minimum voltage (Vb)min at which the gas transfers from a non-conducting state to a conducting state. The study is carried out on a cylindrical coaxial vircator for different gases and different anode transparencies.

TM<sub>01</sub> dominant mode coaxial virtual cathode oscillator with feedback construction

Virtual cathode oscillator, as a kind of space-charge high-power microwave source, has an output microwave mode that is generally an admixture of TM<sub>01</sub> mode and TE<sub>11</sub> mode. The analysis of the resonator in the anode mesh shows that when the transmission of the anode mesh is high, it is easy to produce strong reflected electron beam, forming a conical quasi-resonator structure, thus enhances the output of TE<sub>11</sub> mode. When the transmission of the anode mesh is low, the beam intensity of the reflected electron beam can be weakened due to the absorption of the metal mesh, and the TE<sub>11</sub> mode can be suppressed, so the output mode is mainly TM<sub>01</sub> mode. In this paper, a feedback coaxial virtual cathode oscillator is investigated with the use of numerical simulation and experimental data analysis. The feedback coaxial virtual cathode oscillator is formed by closing the end of the anode mesh through a metal plate and changing the path of the reflected electron beam from the metal mesh to the gap between cathode and anode. The particle in cell method is used in the numerical simulation of the virtual cathode oscillator, and the impedance of the 400 kV diode is about 13 Ω under a voltage of 400 kV. After the optimal design by numerical simulation, the average output microwave power from the virtual cathode oscillator is 1.5 GW, and the frequency of the microwave is about 4.2 GHz, which is basically consistent with the theoretical calculation results. In this new kind of virtual cathode oscillator, the distribution of reflected electrons is modified by the feedback sheet on the anode mesh, the output high power microwave pattern is demonstrated to be dominated by TM<sub>01</sub> mode. The microwave power obtained in the experiment is measured by the array antenna power density integration method. For axisymmetric mode, a receiving antenna array is formed by placing multiple receiving antennas on one side of the axis of the antenna pattern. The power densities of different angles on the horizontal circumference with the phase center of the transmitting antenna are measured, the average power density of two adjacent points is multiplied by the area of the spherical belt between these two points, and then the resulting power is added by the power between the adjacent two points, thereby obtaining the total radiation power. With this method, the microwave power is 850 MW with frequency 4.1 GHz and pulse width 30 ns under slaving voltage 400 kV.

A one-dimensional model illustrating virtual-cathode formation in a novel coaxial virtual-cathode oscillator

A one-dimensional electrostatic sheet model of a coaxial geometry Virtual Cathode Oscillator (VCO) is presented. The cathode is centrally located and connects to a peripherally located plate electrode to form a resonant cavity, and is thus considered to be a novel design. Charge is modelled as concentric sheets about the cathode whose absolute position and velocity are determined as a function of time by solving the relativistic equations of motion. The model predicts the formation of a virtual cathode between the grid and plate electrodes for the case of a space-charge limited current. Setting the electron reflexing frequency (as a function of the grid potential) comparable with the cavity resonant frequency is predicted to improve the efficiency of microwave emission.

Influence of Electron-Beam Diode Voltage and Current on Coaxial Vircator

The KALI-5000 pulse power system was utilized to generate high-power microwaves using a coaxial virtual cathode oscillator. The typical electron-beam parameters were 290 kV, 23 kA, and 100 ns, with a few hundreds of amperes per centimeter square current density. Experiments were conducted using a graphite cathode for two different radial anode-cathode gap distances to investigate the dependence of microwave output magnetic field and frequency on diode voltage and current waveform. It was found that the coaxial vircator always has more than one frequency component. It was shown that the microwave B-field and the frequency do not depend on the time-varying impedance and perveance of the diode. Electron-beam emission area, uniformity, and the anode-cathode gap play a major role in deciding the power density and frequency at the point of measurement. The highest microwave power is emitted when all the power is delivered into a single frequency at least for a certain time of the accelerating pulse.

Sensitive Dependence of Efficiency on Cathode-Wall Position in a Coaxial Vircator—Numerical Studies

A simple coaxial virtual cathode oscillator is studied in this paper by varying the wall position of the cathode holder. It is shown that power surges occur at discrete reflector (wall) positions. Efficiency levels above 10% are observed for tens of nanoseconds.

Study of a compact external magnetic field radial split-cavity oscillator

With the experiment result analyses of a coaxial virtual cathode oscillator (CVCO), a new kind of compact radial split cavity oscillator (RSCO) is presented in this paper. On the oscillator, a low resistance tube is formed by using the diode structure of a CVCO, and a radial split-cavity structure is formed by several meshes that cause the electronic beam to transmit. Calculating all kinds of parameter, at the input parameter 350 kV, 27 kA, the numerical simulation results show that the average output microwave power is about 4.0 GW, the microwave frequency is 1.37 GHz, and the electronic efficiency is 42.3%.

Three-dimensional particle-in-cell simulation studies on a new radial three-cavity coaxial virtual cathode oscillator

A new radial three-cavity structure of the coaxial virtual cathode oscillator is proposed and studied numerically in this paper. Using the radial three-cavity structure, the beam-wave conversion efficiency is enhanced by modulating the electric field in the beam-wave interaction area, while the resonator composed of the radial three-cavity configuration and the mesh anode helps restrain mode competition effectively . And then the coaxial extraction structure benefits the energy extraction, and it can also absorb the used electrons entering into the drifting tube. Therefore, this new kind of virtual cathode oscillator can achieve a high output power. With an electron beam of 50 kA at 400 kV, a peak power of about 6 GW is achieved by simulation at 4.5 GHz. The mean power reaches 3.1 GW and the beam-wave conversion efficiency is about 15%.

Microwave Power Enhancement in the Simulation of a Resonant Coaxial Vircator

Cavity resonance effect is a useful concept in beam-wave interaction for enhancing the conversion efficiency. It is shown that an interplay between the growth of field energy inside the cavity and its extraction can be used judiciously to extract a larger power after the initial transients. Cavity parameters in a coaxial virtual-cathode oscillator have thus been optimized to allow field energy inside the cavity to grow initially. This growth is limited by the energy that leaves the cavity through the extraction region so that the field strength inside the cavity saturates optimally at a maximum value. The energy lost by the beam to the field can then be directly extracted at a high conversion efficiency. The peak cycle-averaged power efficiency obtained thus in particle-in-cell simulation exceeds 9%, while the efficiency over several cycles is found to be 7.5%.

High power microwave generation from coaxial virtual cathode oscillator using graphite and velvet cathodes

High power microwave (HPM) generation studies were carried out in KALI-5000 pulse power system. The intense relativistic electron beam was utilized to generate HPMs using a coaxial virtual cathode oscillator. The typical electron beam parameters were 350 kV, 25 kA, and 100 ns, with a few hundreds of ampere per centimeter square current density. Microwaves were generated with graphite and polymer velvet cathode at various diode voltage, current, and accelerating gaps. A horn antenna setup with diode detector and attenuators was used to measure the microwave power. It was observed that the microwave power increases with the diode voltage and current and reduces with the accelerating gap. It was found that both the peak power and width of the microwave pulse is larger for the velvet cathode compared to the graphite cathode. In a coaxial vircator, velvet cathode is superior to the graphite cathode due to its shorter turn on time and better electron beam uniformity.

Output characteristics of the high-power microwave generated from a coaxial vircator with a bar reflector in a drift region

Numerical simulation studies of a coaxial virtual cathode oscillator with and without a bar reflector have been performed by the three-dimensional particle-in-cell (PIC) code, MAGIC. Also, the experimental studies are performed in our high-power microwave pulser, "Chundoong" to compare with numerical results. It consists of a cylindrical anode-mesh, a coaxial type cathode, and a bar reflector. Results show that the output microwave mode without a reflector is shown to be a mixture of TE and TM modes and the output microwave frequency has a narrow bandwidth and a main frequency is about 3.38 GHz from a coaxial vircator without a bar reflector. But the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode with its frequency of 3.34 GHz is shown to be dominant for the coaxial vircator with a bar reflector. It is found in this experiment that the output microwave power sensitively depends upon the position of a bar reflector

Two-dimensional theoretical analysis of the dominant frequency in the inward-emitting coaxial vircator

Two-dimensional steady-state analysis for the electron flow in an inward-emitting coaxial virtual cathode oscillator is presented. Based on the relativistic-fluid-Maxwell equations, the relativistic plasma frequency characteristics and its dependence on the geometric parameters are derived. The resulting plasma frequency solution scales linearly with the diode current. The exact dominant frequency is solved numerically, which gives the theoretical support for the simulation result that the microwave frequency in a coaxial vircator is similar to that produced by a planar vircator with the same small anode-cathode gap distance. Numerical result shows that higher frequency can be achieved in the coaxial vircator for higher applied voltage and smaller anode-cathode gap distance. The numerical solutions of the dominant frequencies predict the measured and simulated values within 10%

Resonance effect on a coaxial vircator

Some experiments at Institute of Applied Electronics of CAEP showed that the interaction between the virtual cathode oscillator and the cavity is a key factor in determining the output microwave power and propagating modes. Particularly, we observed that the E-beam plays an important role in the cavity formation. The experiments have been carried out with 350 kV diode voltage and 23 kA diode current, where pulsed microwaves of 500 MW in peak power and 3.3 GHz in frequency have been obtained. The energy conversion efficiency from the electron beam to microwave is about 6.2%, and the output microwave modes are mainly the TM01 mode and TE11 mode. The cavity resonance effect on a coaxial virtual cathode oscillator has been investigated in detail.

Influence of anode-cathode gap distance on output characteristics of high-power microwave from coaxial virtual cathode oscillator

We have investigated the influence of anode-cathode (A-K) gap distance on the output characteristics of high-power microwave from coaxial virtual cathode oscillator (vircator). This high-power microwave generator is powered by the "Chundoong" (Max 600 kV, 88 kA, 60 ns), which is an intense relativistic electron beam-pulse generator. In this experiment, it was observed that the microwave power has been strongly dependent upon the A-K gap distance, where the A-K gap has been varied from 4 to 8 mm. The maximum microwave power is observed to be 244 MW, whose efficiency is about 2.75% at the A-K gap distance 4 mm. Also the microwave frequencies are measured to be 5.1/spl les/f/spl les/5.9 GHz from the combination of a coupled line band pass filter with a 1-GHz bandwidth and dispersive delay line method. It is noted that measured output microwave frequency is in good agreement with that of simulation. It is also found that the dominant mode of output microwave is a TM/sub 01/ in this simulation result.

Microwave Frequency Determination Mechanisms in a Coaxial Vircator

Traditionally, the radiated microwave frequency in a coaxial vircator is considered to be determined primarily by the virtual cathode oscillation frequency and the electron reflection frequency. In this paper, some experiments showing different results are reported. In particular, the E-beam is observed to play an important role in the cavity formation. Some possible explanations, including a virtual cavity concept, are proposed. The cavity resonance effect on a coaxial virtual cathode oscillator with different geometries has been investigated in detail. Investigation of the E-beam performance will improve understanding of the interaction between the E-beam and microwaves, which is a key for determining the microwave frequency. These results are helpful in optimizing the design of a cylindrical diode to avoid microwave frequency shifting and mode competition.

Physics of the Interaction Process in a Typical Coaxial Virtual Cathode Oscillator Based on Computer Modeling Using MAGIC

Physical basis of the interaction process in a typical coaxial vircator has been studied using MAGIC, a three-dimensional PIC computer code. Detailed investigations of the complex dynamics of electrons in and near the interaction space have revealed that it takes /spl sim/6 ns for the virtual cathode to be formed and oscillations to begin. Once the oscillations have started three groups of electrons can be distinguished: those that pass through the semipermeable anode and form a virtual cathode, those that travel parallel to the cylindrical anode, or even around its inner edge in small circles, and finally those called "stray" electrons which are ejected from the cylindrical anode and land on the walls of the output waveguide. We have then shown that by reducing the influence of stray electrons the output power of the vircator can be increased by as much as 52%. As a test of the accuracy of the simulation model the input impedance of the device and its frequency of oscillations have been reproduced, respectively, to within 5% and 2.5% of their measured values.

Characteristics of Diode Perveance and Output Microwave Power in Coaxial Virtual Cathode Oscillator

We have investigated numerically and experimentallycharacteristics of diode perveance and output microwave power incoaxial virtual cathode oscillator (vircator). In this experiment, the diode perveance characteristics could be described using thediode voltages and currents. The sheath plasma expansion speed isto be about 7cm/μs under A-K gap distance 4 mm. We have alsoinvestigated an influence of anode-cathode (A-K) gap distance onoutput characteristics of high power microwave from coaxialvircator. In this experiment, the maximum microwave power isobserved to be 244 MW, whose efficiency is about 2.75 %, at theA-K gap distance of 4 mm. The microwave frequencies are alsomeasured to be 5.1 ≤ f ≥ 5.9 GHz from combination ofcoupled line band pass filter with 1 GHz band gap and dispersivedelay line. With microwave reflector, the maximum output microwavepower is observed to be about 400 MW, whose efficiency is about7.1 %, at the A-K gap distance 8 mm and microwave reflectorinner-hole diameter of 7.6 cm. The microwave frequencies are alsomeasured to be 4.8 ≤ f ≥ 5.2 GHz.

High power microwave generation by a coaxial virtual cathode oscillator

A new type of virtual cathode oscillator, the coaxial vircator, was studied analytically and experimentally. A one-dimensional analytical model was used to describe the steady-state behavior of the electron beam and the virtual cathode, from which the diode current, the space-charge limited current, the virtual cathode position, and the estimated oscillation frequency were obtained. The experiments were carried out with typical electron-beam parameters of 500 kV, 40 kA, and 30 ns, where pulsed microwaves of 400 MW in peak power and 2 GHz in frequency have been obtained. The energy efficiency from the electron beam to microwaves was /spl sim/2%. This efficiency is expected to be improved by increasing the microwave field strength around the vircator.

Efficiency enhancement of a coaxial virtual cathode oscillator

The microwave field intensity around the virtual cathode oscillator was enhanced by using a microwave reflector in the output waveguide. The experimental results show that the microwave output power strongly depends on the position and geometry of the microwave reflector. The maximum microwave efficiency obtained was twice as large as that without field enhancement by the microwave reflector.