Plasma expansion and impedance collapse in a foil-less diode for a klystronlike relativistic backward wave oscillator

Abstract

Klystronlike relativistic backward wave oscillator (RBWO) can produce microwave power exceeding 5 GW with a high efficiency larger than 40%. In the experiment of klystronlike RBWO, for about 1 MV peak diode voltages, increasing magnetic field from 1.43 to 1.89 T slowed the impedance collapse until it was suppressed completely. The introduction of a stainless steel obstructing ring aggravated the impedance collapse, whereas replacing the stainless steel obstructing ring with a flat stainless steel provided a more stable impedance variation during the pulse duration. These impedance collapses did not affect microwave generation seriously and may be attributed to the radial expansion of cathode plasma initialing from the cathode shank so that part of reverse currents were collected at the anode wall, contributing to the measured diode current. On the other hand, it was found that microwave generation shot-to-shot reproducibility was closely related to the diode impedance variation. When there was no or very low microwave measured, diode impedance collapse appeared at the latter of the pulse. The microwave generation shot-to-shot reproducibility was improved greatly after the electron collector was enlarged on radius with 1 mm. A possible explanation is that the anode plasma produced from electron collector expands axially and enters the diode region at a very high velocity of several mm/ns. The movement of the anode plasma in the beam-wave interaction region affects the microwave generation, which reduces the microwave power during the whole pulse duration significantly.

Investigation of 0.38 THz backward-wave oscillator based on slotted sine waveguide and pencil electron beam

“…In the relativistic terahertz devices, a Bragg reflector is used to reflect the signal. 21 By using this mode, a reflector which takes the place of the previous tapered section is adopted to shorten the distance from the electron gun to the main SWS. In our design, a section of the slotted sine waveguide is used as the novel reflector to reflect the signal.…”

An Axial Foilless Diode Guided by Composite Magnetic Field for the Production of Relativistic Electron Beams

“…erefore, foilless diodes are well capable of high-power density, long pulsed, and repetitive operated applications, such as high-power microwave devices and microsecond pulse electron accelerators [9]. e axial foilless diodes immersed in solenoid magnetic field are widely used in O-type high-power microwave devices [15][16][17][18], as shown in Figure 1. e advantage of the structure is that the diode cathode is completely immersed in the uniform strong magnetic field of solenoid coil, which is conducive to the generation of high-quality annular electron beams.…”

“…In the development of high-power microwave system, low power consumption and miniaturization are important conditions for the practical application [19][20][21]. However, the traditional foilless diode structure has the following disadvantages [17,[22][23][24]: (a) in order to avoid excessive radial electric field, the radius of drift tube is usually large, which makes the size of inner wall of solenoid coil much larger than that of microwave device and drift tube, resulting in waste of excitation space; (b) if the diode is to be further miniaturized, it is necessary to reduce the distance between diode cathode and drift tube wall, and this change will lead to the enhancement of the electric field intensity on the cathode emitter, which is very easy to cause the radial emission of electrons on the cathode, thus causing the loss of electron energy. Some electrons even form reverse motion, bombarding the insulator and damaging the insulation system [22]; (c) moreover, the volume, weight, and energy consumption of solenoid coil and auxiliary power equipment are large, which is not conducive to the application on mobile platforms.…”

High efficiency coaxial klystron-like relativistic backward wave oscillator with a premodulation cavity

“…[1][2][3][4][5][6][7][8][9][10][11] To further increase the beam-wave interaction efficiency and output power, in our earlier work we introduced a klystron-like RBWO combining transition radiation with Cerenkov radiation. [12][13][14] In that device, a resonant reflector was used before the slow wave structure (SWS) to reflect backward wave and premodulate the electron beam; a modulation cavity separated the SWS to decrease the energy spread of the modulated beam electrons; and an extraction cavity was added to recover energy from the beam. The beam dump was located very close to the extraction cavity to provide more kinetic energy for extraction.…”