Meiqin Liu

rf mode switching in a relativistic magnetron with diffraction output

The relativistic magnetron with diffraction output (RMDO) has demonstrated nearly 70% efficiency in recent simulations. This letter reports a rapid mode switching technique in the RMDO using a low power, short-pulse, external single frequency signal. The MAGIC electromagnetic finite-difference-time-domain particle-in-cell code used in simulations demonstrated that an input signal of 300 kW is sufficient to switch neighboring modes in a gigawatt output power A6 RMDO with a transparent cathode, whereas for the original A6 magnetron configuration with radial extraction driven by a transparent cathode 30 MW is required. This frequency agility adds additional versatility to this high power microwave source.

Frequency Switching in a 12-Cavity Relativistic Magnetron With Axial Extraction of Radiation

The possibility of mode switching from one pulse to another in a 6-cavity gigawatt magnetron with axial extraction of radiation through a horn antenna (such a magnetron is known as the MDO, i.e., magnetron with diffraction output) using a weak (200-300 kW), short (15-ns), and single-frequency RF signal was demonstrated using particle-in-cell simulations in our earlier work. This mode switching exploits the symmetric nature of the MDO that facilitates the use of any eigenmode as the operating mode. All scenarios of mode switching were considered using common properties of dynamical systems with two stable states separated by an unstable saddle point. In this paper, we continue to study the problem of mode switching, but this time for a 12-cavity MDO, for which we found splitting of the radiation frequency for each eigenmode owing to its different longitudinal distributions. Since splitting manifests as a bifurcation of frequency for definite values of the applied axial magnetic field, scenarios of frequency switching for this 12-cavity magnetron are considered.

Operation Characteristics of 12-Cavity Relativistic Magnetron With Single-Stepped Cavities

The possibility of using single-stepped cavities to replace the common tapered cavities was studied using particle-in-cell simulations in an A6 magnetron with diffraction output (MDO). The replacing of the tapered cavities by the single-stepped cavities in a 12-cavity MDO increases the interaction space where the charged particles interact with the induced RF waves. The electronic efficiency of the 12-cavity MDO with single-stepped cavities driven by the transparent cathode [2] of GW output power level can be as high as 73% for α = 18.2°, 74% for α = 17.5°, and 72% for α = 12.5° at β = 32°, where α is the angle between the outer wall and z-axis, and β is the angle between the inner wall and z-axis. The depth of single-stepped cavities is changed when α is changed, which results in different frequency range of magnetron operating modes. When a 400-kV voltage pulse of 10-ns duration is applied to a transparent cathode or a solid cathode, the output power can be as high as 1 GW. Without loss of generality, for α = 12.5° at β = 32°, the peak efficiency around 70% of 12-cavity MDO with single-stepped cavities design occurs at the voltage (V ~ 400 ± 50 kV). The results presented in this paper provide references for relativistic magnetron mode selection or mode switching experiments when choosing the input parameters (magnetic field and accelerating voltage) allowing the magnetron to operate in the desired operation mode.

Operation Characteristics of A6 Relativistic Magnetron Using Single-Stepped Cavities With Axial Extraction

We report the performance of an A6 relativistic magnetron with single-stepped cavities output. Particle-in-cell simulation shows the electronic efficiency of an A6 relativistic magnetron with an anode block made of six single-stepped cavities can be up to 78% with an output power of one gigawatt for an applied voltage of 400 kV. When the A6 relativistic magnetron with axial extraction using six single-stepped cavities is applied with a voltage of V ~ 400 kV ± 50 kV the electronic efficiency can be as high as 60%, while the output power can be as high as 1 GW. And, when a 10-ns voltage pulse of 400 kV is applied on this magnetrons with diffraction output using single-stepped cavities, the output power pulse can be as high 1.2 GW. The results in this paper will provide a reference for choosing a cavity that is easily manufactured, so that a physical experiment is more feasible.

Frequency switching in a relativistic magnetron with diffraction output

The possibility of mode switching from one pulse to the next in a 6-cavity gigawatt magnetron with diffraction output (MDO) using a weak (200–300 kW), short (about 10 ns), single frequency RF signal has been demonstrated using particle-in-cell (PIC) computer simulations [1]. This mode switching is possible because of the symmetric nature of the MDO that facilitates the use of any eigenmode as the operating mode. All scenarios of mode switching were considered using common properties of dynamical systems with two stable states separated by an unstable saddle point. In this paper we continue to study the problem of mode switching for a 12-cavity MDO for which we found a splitting of the radiated frequency for each eigenmode owing to its different longitudinal distribution. Since the splitting is realized as a bifurcation from a higher frequency to a lower one as the applied magnetic field is increased, scenarios of frequency switching are considered as in [1] for the TE41-mode whose radiated power is maximal for the MDO under consideration.

Investigation of the operating characteristics of a 12-cavity rising-sun relativistic magnetron with diffraction output using particle-in-cell simulations

We report on the performance of a 12-cavity rising-sun relativistic magnetron with diffraction output (12-cavity rising-sun RMDO). Particle-in-cell simulations show an electronic efficiency of 70% for a gigawatt output power 12-cavity rising-sun RMDO with a transparent cathode operating in the π mode for an applied voltage of U∼400 kV  ±50 kV. When the RMDO is driven by the “F” transparent cathode, which is a coaxial transparent cathode, the axial leakage current can be reduced by about 50%.

Operation Characteristics of a 12-Cavity Relativistic Magnetron When Considering Secondary and Backscattered Electrons’ Emission

The particle-in-cell (PIC) simulation of an A6 magnetron with diffraction output (MDO) in our earlier work shows the peak output power (especially the short power pulse with several nanoseconds duration) is lowered when electrons with high energy strike the anode block, which leads to the emission of secondary and backscattered electrons. In addition, the PIC simulation for a 12-cavity MDO also shows when secondary electron emission occurs, the boundary for neighboring modes becomes more complicated than before. Replacing tapered cavities by single-stepped cavities in a relativistic MDO increases the interaction space where the charged particles interact with the induced RF waves. The electronic efficiency of the A6 relativistic MDO as well as the 12-cavity MDO with single-stepped cavities driven by the transparent cathode can be over 70% with gigawatts output power level. And with single-stepped cavities, the distance between the cathode and the wall of relativistic MDO is increased such that the possibility for electrons with high energy striking on the wall is decreased, which results in less emission of secondary and backscattered electrons under the same applied voltage. The PIC simulation demonstrated that in the 12-cavity MDO with single-stepped cavities driven by the transparent cathode with 400-kV applied voltage, the electrons with above 500 eV from cathode under the crossed electromagnetic field can strike on the anode block of MDO, resulting in a secondary and backscattered electron current that can cause different modes to be prevalent or stop the designed mechanism from working. Especially when applied with short voltage pulse with 10-ns duration, the output power pulse is shorted with peak power lowered. The simulation results presented in this paper possibly provide references for multicavity resonator when considering mode selection or mode switching experiments.

Compensated Monte Carlo Collision Model for Particle-in-Cell Simulation in High-Pressure Plasmas

The Monte Carlo collision (MCC) model is widely adopted to simulate discharge plasmas using the particle-in-cell (PIC) method; however, it has low efficiency in high-pressure plasmas because of the small time steps required due to the constraint of high collision frequency. To relax this time step constraint, a compensated Monte Carlo collision model (CMCC) is proposed which considers multiple collisions in a time step as a series of single collisions to compensate for the neglected collisions. The electron motion in a high-pressure He gas for various reduced electric fields E/N and the streamer formation process in a laser-triggered spark gap were simulated using the CMCC model. Simulation results showed that the CMCC model with long time step obtained reasonable electron velocity distribution, temperature, drift velocity, plasma density, and space-charge field. It was demonstrated that the CMCC model had high accuracy and high efficiency, particularly for PIC simulation in high-pressure plasmas.

Optimizing the Parameters of a 12-Cavity Rising-Sun Relativistic Magnetron With Single-Stepped Cavities for

We investigated the use of single-stepped cavities instead of typical tapered cavities in an A6 magnetron with diffraction output (MDO) and a 12-cavity MDO through particlein-cell (PIC) simulations using MAGIC [1] and UNIPIC [2] in our earlier publications. The interaction space, where the charged particles interact with the induced RF waves, was increased by replacing tapered cavities with single-stepped cavities in a 12-cavity rising-sun relativistic magnetron with diffraction output (RMDO) that the electronic efficiency of π-mode generation in a 12-cavity rising-sun RMDO with singlestepped cavities driven by a transparent cathode [6] with gigawatt output power level can be as high as 83% for α = 12.5°, 79% for α = 17.5°, and 82% for α = 18.2° at β = 32°, where α is the angle between the inner wall and the z-axis, and β is the angle between the outer wall and the z-axis. When α, the angle between the inner wall and the z-axis, is changed, the depth of single-stepped cavities was changed. This would lead to the coupling between deep cavities and shallow cavities in the 12-cavity rising-sun MDO changing too, and which finally results in different frequencies of magnetron operation in the π-mode. As we know, gigawatt nanosecond output power pulse is useful for the nanoradar system, and our PIC simulations found that when a 400-kV voltage pulse of 10-ns duration is applied to a 12-cavity rising-sun MDO driven by a transparent cathode or a solid cathode, the output power of the π-mode can be as high as 1.5 GW. Without loss of generality, for α = 12.5° and β = 32°, the peak efficiency of π-mode generation in a 12-cavity risingsun MDO with single-stepped cavities design is around 82% and occurs for voltage V ~ 400 kV ± 50 kV. This paper describes optimizing the parameters in a 12-cavity rising-sun RMDO for the desired π-mode operation.

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The possibility of using an external RF signal to rapidly switch the operating mode in relativistic magnetrons is studied using the PIC code MAGIC1 for the A6 magnetron2 operating at 360 kV with anode block radius Ra =3D 2.11 cm. In the A6 magnetron with asymmetric extraction of radiation only nondegenerate modes (π-mode and the 2π-mode) can be used as the operating mode. Mode hopping to any neighboring mode leads to magnetron operation with an unloaded mode, resulting in overheating and erosion of its electrodes.