Choosing optimum method for the efficient design of a relativistic magnetron with diffraction output

The relativistic magnetron with axial extraction, also known as magnetron with diffraction output (MDO), is an important device for the miniaturization and compaetification of magnetron. It is also promising to be one of the most compact narrow band high power microwave sources. Based on the manners and traits of the MDO directly radiating the quasi-TE11 mode in axial direction, the investigation of the power combination of double MDOs with a horn antenna is carried out. After discussing the combination ways and results, a modified configuration is proposed for improving the radiation directivity. Finally, the results show that when the adjacent double MDOs have an open angle of 7 and the common horn antenna has the parameters of h = 600 mm and r = 340 mm, the mode reflection coefficient of the power combination system is 0.31, the maximum gain is 21.5 dB, the radiation mode is TE11.

The A6 relativistic magnetron with diffraction output (MDO) using a single-step cavity is described in this paper. The efficiency of this device can be as high as 78% with GW output power. The UNIPIC code was used to demonstrate that when the relativistic magnetron was driven using a 400 kV applied voltage, the energetic electrons emitted from the cathode strike the relativistic magnetron's anode block, emitting secondary electrons and backscattered electrons. The resultant secondary electron current lowers the output peak power, especially when the relativistic magnetron was driven using a short applied voltage pulse. This paper shows that in this design of the A6 relativistic MDO, the secondary electrons' influence on the operating characteristics is less than the secondary electrons' influence for the usual A6 relativistic MDO and the A6 magnetron with radial output. The A6 relativistic MDO with single-step cavity is more advantageous for mode switching and with more stable frequency. The result of this paper will provide reference for A6 relativistic magnetron mode switching experiments when selecting the boundary between neighboring modes.

A previous computational study of a relativistic magnetron with diffractive output at UNM using MAGIC [1] indicated an electronic efficiency of 70% [2]. The original anode design in this study was tested in experiment. Some subtle changes in the source design were necessary, such as introduction of a Helmholtz-like magnetic coil pair and use of a polished stainless steel endcap on the downstream end of the transparent cathode. The purpose of these changes was to protect the dielectric output window from electron bombardment, an approach that is separate from the work of others to do the same with modified magnetic fields [3]. An ethanol calorimeter and L- and S-band waveguide detectors placed in the near-field were used to obtain RF pulse shape, frequency, and to calculate peak power. A self-integrating Rogowski coil and D-dot probe, which measured total current and voltage respectively, were then used to calculate a total efficiency of 40.4% for the π-mode. This lower efficiency is suggestive of a specific mechanism by which the spherical endcap has poor efficacy. Finally, images of radiated field pattern on a neon-bulb grid and of air breakdown outside of the 21 cm diameter output window were captured.

A 12-cavity rising-sun relativistic magnetron with diffraction output (MDO) driven by transparent cathode is investigated with three-dimensional particle-in-cell simulation. It is shown that its electronic efficiency can be as high as 78% with gigawatt level of radiation power when applied voltage is 400 kV. The peak efficiency around 70% of the 12-cavity rising-sun relativistic MDO occurs at the voltage (V≈400kV±50kV). In simulations all deep cavities are continued axially along the wall of a conical horn antenna whose cross section exceeds cutoff for the radiated wave and all shallow cavities are tapered to the output. When choosing the depth ratio of the cavities as 0.84, an output power of 1.8 GW for 12-cavity rising-sun relativistic magnetron MDO with 400 kV applied voltage can be obtained. Also, when increasing the transparent azimuthal thickness and optimizing the length of transparent cathode, the axial leakage current can be reduced by 50%.

Particle-In-Cell (PIC) simulations were performed to study the leakage current of the A6 Magnetron with Diffraction Output (MDO). Due to the time lag for the high voltage pulse to travel through the cathode, phase space diagrams show explosive field emission generated electrons have higher energy at the front end than the late end; the repelling force from the space charges pushes the late electrons towards to output and form the forward “leakage current”, which is intrinsic in this type of device.

Particle-In-Cell (PIC) simulations were performed to study the resonance condition of A6 Magnetron with Diffraction Output (MDO) by conducting a magnetic field strength sweep and no resonance condition are found which results in lower than expectation output power. The cause of this is conjectured to be the impedance of the pulsed power system. Low pulsed power impedance results in more explosive field emission electrons, and they require higher than usual magnetic field to confine within the anode-cathode space to form resonance.

A compact high power microwave (HPM) generation system is described in this paper. The main parts of the HPM system are a Marx generator with a pulse forming line and a magnetron with diffraction output. The total weight and length of the system are 250 kg and 120 cm, respectively. The output microwave power of the HPM system at 550 kV of applied voltage and 0.33 T of magnetic field reaches 1 GW at 2.32 GHz of central frequency with 38 ns of pulse duration, 23% of power conversion efficiency, and Gaussian radiation pattern. In the bursts operation, both time and amplitude jitters are less than 4 ns and lower than 1.5 dB, respectively.

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.

We recently proposed replacing a physical cathode in a relativistic magnetron with diffraction output with a virtual cathode. An electron beam is injected into the relativistic magnetron from a cathode located external to the interaction space. The geometry of both the electron beam injector and the magnetron interaction space were optimized to produce a virtual cathode from which electron spoke formation originates. This paper discusses how the length of such a virtual cathode can be optimized.

We present a design of a photonic crystal fiber for high power laser and amplifier applications. Our fiber comprises a core with a diameter larger than 60 μm and exhibits single mode operation when the fiber is bent around a 10 cm radius at a wavelength of 1064 nm. Single mode guidance is enforced by the high loss of higher order modes which exceeds 80 dB/m whereas the loss of the fundamental mode (FM) is lower than 0.03 dB/m. The fiber can therefore be considered as an active medium for compact high power fiber lasers and amplifiers with a nearly diffraction limited beam output. We also analyze our fiber in terms of tolerance to manufacturing imperfections. To do so we employ a statistical design methodology. This analysis reveals those crucial parameters of the fiber that have to be controlled precisely during the fabrication process not to deteriorate the fiber performance. Finally we show that the fiber can be fabricated according to our design and we present experimental results that confirm the expected fiber performance.

The characteristics of plasma waves propagating in the Earth's plasmasphere and magnetosphere are important in terms of understanding the plasma environment in which these waves are excited and propagate. The propagation direction is a critical in propagation analysis of ELF/VLF band waves. The Means, eigenvector, amplitude, and SVD methods have been proposed previously to estimate the direction of arrival of a single wave, and the wave distribution function (WDF) method, which solves the ill‐posed problem, has been proposed to estimate the propagation direction of multiple arriving waves. In this paper, we propose a stochastically extended WDF method. Conventional WDF methods are based on maximum posterior probability estimation; however, the proposed method uses a Markov chain Monte Carlo method. In the proposed method, the estimated solution is obtained based on the sampling approximation by obtaining multiple samples following the posterior distribution. In addition, the WDF is discretized and quantized to improve calculation stability and efficiency. From the sampled WDF, the mean value of the WDF and 95% credible interval can be calculated without arbitrary parameters. The proposed method is more objective than conventional methods and can be used effectively as a basis for performance evaluation of the WDF method.

Summary form only given. This paper describes the numerical study and experimental results of an axial vircator with axial extraction. The microwave source is driven by a Marx generator and excludes any type of pulse forming device between the two subsystems. The main reason for this special arrangement is the final goal to build a compact and robust high power microwave system. The drawback is usually a lower efficiency of the microwave source. The vircator operates with the applied voltages between 200 and 400 kV and the impedance is around 50 Ohms during the process when microwave radiation is generated. The microwave pulse duration is for most cases between 50 to 100 ns. For the experiments described in this paper focus has been on different cathode geometries. Results that are given include microwave power, spectral content and mode characteristics. In an attempt to better understand and explain the experimental results particle-in-cell simulations have been carried out in MAGIC. These numerical results support to some extent the experimental results.

A compact high voltage power supply for the LNL recoil mass spectrometer

The realization technique of fast linear transformer driver on the compact modularized high power pulse generator was expounded. The influence on Modular amount and the output quality of fast linear transformer driver (FLTD) was analyzed. The limitation on FLTD primary inductance was reduced by optioned feeding structure. The waveform distortion was weakened by designed back swing coil. Through engineering experiments, electron beam closed 0.8MeV with repetition frequency of 25 Hz and top time of ~80 ns were obtained. So this paper gave a new idea for research on the modularization compact high power pulse generator with higher repetition rate.

Er 3+ and Yb 3+ doped active media for ‘eye safe’ laser systems