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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]

The breakdown problem in the relativistic backward wave oscillator (RBWO) limits the microwave pulse energy and should be solved. The slow wave structure (SWS) is the most key component of the RBWO. The breakdown of SWS may be triggered by field-induced electron emission and electron bombardment on the wall. This paper demonstrates that the space charge field of the relativistic electron beam (REB) can enhance the electric field forbidding field-induced electron emission and decrease the electric field igniting the RF breakdown, and therefore, the RF breakdown can be significantly suppressed. However, the enhanced electric field will enlarge the radial displacements of electrons, especially the high-energy electrons. When the RBWO operates with a low guiding magnetic field or a long-pulse REB, this situation will worsen and some electrons may possess large enough radial displacements and bombard on the SWS surface with the help of radial oscillation of REB and radial expansion of cathode plasmas. This may aggravate the breakdown of SWS. [ABSTRACT FROM AUTHOR]

We efficiently enhance the power capacity of klystron-like relativistic backward wave oscillators (RBWOs) with a TM021 mode extraction cavity. Theoretical analysis and simulation results are shown in detail. Theoretically, an electron cluster will be steadily decelerated in the whole region of the TM021 extraction cavity if the transiting time in the cavity approximates the period of microwaves. As a result, the energy of the electron beam will be efficiently extracted by the TM021 mode field. Due to the larger size, the electric field strength in a TM021 cavity can be much smaller than that in a TM020 cavity. In addition, once the extraction cavity breaks down, the emission electron will absorb less energy from the TM021 mode field than the TM020 mode field. In PIC simulations, for an X-band klystron-like RBWO with a frequency of 9.6 GHz and an output power of 2.3 GW, the maximum longitudinal electric field (Ez) in the extraction cavity decreases from 610 kV/cm to 320 kV/cm, when the TM020 extraction cavity is replaced with a TM021 extraction cavity. PIC simulation results agree well with the theoretical analysis. [ABSTRACT FROM AUTHOR]

This paper provides a fresh insight into the effect of non-uniform slow wave structure (SWS) used in a relativistic backward wave oscillator (RBWO) with a resonant reflector. Compared with the uniform SWS, the reflection coefficient of the non-uniform SWS is higher, leading to a lower modulating electric field in the resonant reflector and a larger distance to maximize the modulation current. Moreover, for both types of RBWOs, stronger standing-wave field takes place at the rear part of the SWS. In addition, besides Cerenkov effects, the energy conversion process in the RBWO strongly depends on transit time effects. Thus, the matching condition between the distributions of harmonic current and standing wave field provides a profound influence on the beam-wave interaction. In the non-uniform RBWO, the region with a stronger standing wave field corresponds to a higher fundamental harmonic current distribution. Particle-in-cell simulations show that with a diode voltage of 1.02 MV and beam current of 13.2 kA, a microwave power of 4 GW has been obtained, compared to that of 3 GW in the uniform RBWO. [ABSTRACT FROM AUTHOR]