Search

An annular cathode is generally fixed on a cylindrical cathode holder in a magnetically insulated coaxial foilless diode. Electrons emitted from cathode plasmas or the cathode holder may easily move backward under the action of the quasi-static electric field and axial magnetic field, leading to backward current loss, which will seriously reduce the efficiency of high power microwave sources, especially under the condition of a low guiding magnetic field. A novel storage electrode immersed in the uniform magnetic field is proposed to suppress the backward current. Reverse electrons will be decelerated and blocked by the storage electrode. Under the guiding magnetic field of 0.66 T, over 66% of the backward current is suppressed (from 3.6 kA to 1.2 kA) when the storage electrode is used, and furthermore, the waveforms of the diode voltage and current are improved significantly. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

This paper reports the space-charge limited current (SLC) and virtual cathode behaviors in one-dimensional grounded drift space. A simple general analytical solution and an approximate solution for the planar diode are given. Through a semi-analytical method, a general solution for SLC in one-dimensional drift space is obtained. The behaviors of virtual cathode in the drift space, including dominant frequency, electron transit time, position, and transmitted current, are yielded analytically. The relationship between the frequency of the virtual cathode oscillation and the injected current presented may explain previously reported numerical works. Results are significant in facilitating estimations and further analytical studies. [ABSTRACT FROM AUTHOR]