In this paper, we have proposed a metamaterial (MTM) which is suitable for the compact high-power vacuum electron devices. For example, an S-band slow-wave structure (SWS) based on the all-metal MTMs has been studied by both simulation and experiment. The results show that this MTM SWS is very helpful to miniaturize the high-power vacuum electron devices and largely improve the output power and the electronic efficiency. The simulation model of an S-band MTM backward wave oscillator (BWO) is built, and the particle-in-cell simulated results are presented here: a 2.454 GHz signal is generated and its peak output power is 4.0 MW with a higher electronic efficiency of 31.5% relative to the conventional BWOs.

All-metal metamaterial slow-wave structure for high-power sources with high efficiency

High power microwave (HPM) sources based on the backward-wave oscillator (BWO) have been investigated for the past two decades primarily because of their potential for very high efficiency (15 to 40%) operation Several different effects have been proposed to explain this high efficiency compared to conventional BWOs One of the major contributors to the high efficiency of the plasma-filled Pasotron HPM BWO source is the presence of optimally phased end reflections The Pasotron uses a long-pulse (/spl ges/100 /spl mu/s) plasma-cathode electron-gun and plasma filled slow-wave structure to produce microwave pulses in the range of 1 to 10 MW without the use of externally produced magnetic fields The efficiency of the Pasotron can be enhanced by up to a factor of two when the device is configured as a standing-wave oscillator in which properly phased reflections from the downstream collector end of the finite length SWS constructively interfere with the fundamental backward-wave modes and improve the coupling of the beam to the circuit Operation in this configuration increases the efficiency up to 30% but causes the frequency to vary in discrete steps and the output power to change strongly with beam parameters and oscillation frequency

Efficiency enhancement in high power backward-wave oscillators

A high efficiency relativistic backward wave oscillator working at a low guiding magnetic field is designed and simulated. A trapezoidal resonant reflector is used to reduce the modulation field in the resonant reflector to avoid overmodulation of the electron beam which will lead to a large momentum spread and then low conversion efficiency. The envelope of the inner radius of the slow wave structure (SWS) increases stepwise to keep conformal to the trajectory of the electron beam which will alleviate the bombardment of the electron on the surface of the SWS. The length of period of the SWS is reduced gradually to make a better match between phase velocity and electron beam, which decelerates continually and improves the RF current distribution. Meanwhile the modulation field is reduced by the introduction of nonuniform SWS also. The particle in cell simulation results reveal that a microwave with a power of 1.8 GW and a frequency of 14.7 GHz is generated with an efficiency of 47% when the diode voltage is 620 kV, the beam current 6.1 kA, and the guiding magnetic field 0.95 T.

Design of a high efficiency relativistic backward wave oscillator with low guiding magnetic field

Conventional microwave sources utilize a strong axial magnetic field to guide an electron beam through an interaction region. A plasma-assisted slow wave microwave oscillator (Pasotron) can operate without an external magnetic field because the presence of ions neutralizes the space charge in the beam, permits the self-pinch forces to provide beam propagation, and allows for the radial motion of electrons under the action of transverse fields of the wave. While the inherent efficiency of conventional microwave sources with 1D electron flow is limited to 15%–20%, it is shown in this work that both the calculated and measured inherent efficiency of devices with 2D electron flow can be higher than 50%. Both in situ diagnostics and analysis confirmed that the enhanced efficiency is due to the fact that rf forces dominate the beam dynamics.

Realization of high efficiency in a plasma-assisted microwave source with two-dimensional electron motion

This paper presents a three-dimensional particle-in-cell (PIC) simulation of a Ka-band relativistic Cherenkov source with a slow wave structure (SWS) consisting of metal photonic band gap (PBG) structures. In the simulation, a perfect match layer boundary is employed to absorb passing band modes supported by the PBG lattice with an artificial metal boundary. The simulated axial field distributions in the cross section and surface of the SWS demonstrate that the device operates in the vicinity of the π point of a TM01-like mode. The Fourier transformation spectra of the axial fields as functions of time and space show that only a single frequency appears at 36.27 GHz, which is in good agreement with that of the intersection of the dispersion curve with the slow space charge wave generated on the beam. The simulation results demonstrate that the SWS has good mode selectivity.

Three-dimensional simulation of a Ka-band relativistic Cherenkov source with metal photonic-band-gap structures

Backward-wave oscillators (BWO) have long been used as voltage-tunable signal sources, and more recently as high-power microwave (HPM) sources. The frequency at which a BWO oscillates is commonly viewed as being determined by the circuit geometry and the electron beam parameters. However, several techniques used to improve the efficiency and reduce the size of HPM BWOs, such as the Pasotron, cause the frequency to vary in discrete steps with changes in the beam parameters, and a plasma in these device can cause the frequency to chirp (increase significantly) during the pulse. End reflections in low-loss circuit designs and excessive plasma generation in these new source geometries produce nonclassical frequency and power behavior.

Frequency and power response of high-power plasma-filled backward-wave oscillators

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Efficiency enhancement in high power backward-wave oscillators

Frequency and power response of high-power plasma-filled backward-wave oscillators