Abstract
The reverse current tends to occur in the transition region of the guiding magnetic field in a magnetically insulated coaxial diode (MICD). Influence of the guiding magnetic field on characteristics of the MICD especially on the reverse current is studied by the particle-in-cell (PIC) simulation in this paper. The reverse current is confirmed to be irrelevant with the guiding magnetic field strength. However, the reverse current is clarified quantitatively to depend on the electric and magnetic field distribution in the upstream of the cathode tip. As the MICD has been widely employed in microwave tubes, a simple approach to suppress the reverse current on the premise of little change of the original diode is valuable and thus proposed. The optimum matching point between the cathode and the magnetic field is selected in consideration of the entrance depth tolerance, the diode impedance discrepancy and the reverse current coefficient.
Highlights
An external axial magnetic field is essential for a magnetically insulated coaxial diode (MICD) to confine and guide an intense annular electron beam especially for O-type high power microwave (HPM) generators
The reverse electrons phenomenon is unavoidable for MICDs in the transition region before the uniform axial magnetic field, namely, a portion of electrons which stem from the stray emission on the cathode move in cycloidal orbits following a magnetic force line in a direction opposite to the downstream primary electron flow.[1,2,3,4]
Even though the reverse current loss is avoided when the reverse electrons strike on the shield hoop in the diode, the appearance of reverse current that shunts the useful forward current is especially critical in cases where the beam is used for in-phase excitation of several HPM oscillators.[8,9,10,11]
Summary
An external axial magnetic field is essential for a magnetically insulated coaxial diode (MICD) to confine and guide an intense annular electron beam especially for O-type high power microwave (HPM) generators. Even though the reverse current loss is avoided when the reverse electrons strike on the shield hoop in the diode, the appearance of reverse current that shunts the useful forward current is especially critical in cases where the beam is used for in-phase excitation of several HPM oscillators.[8,9,10,11] the MICD model generally does not take the cathode shank emission into account[12,13,14] and ignores the reverse current. References 1,2 discussed the reverse current in a weak guiding magnetic field when the electron beam is dispersed, which is inconclusive.
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