Abstract
In this paper, we study an X-band klystron-like relativistic backward wave oscillator (RBWO) with a pre-modulation cavity, modulation ridge, drift tube in the slow wave structure (SWS) and extraction cavity in both theoretical and experiment. PIC simulations show that the conversion efficiency can be up to 62% with output power of 3.0 GW. In experiment, when the diode voltage is 660 kV, and the diode current is 6.4 kA, the output power is 1.9 GW, and the conversion efficiency is 45%(±5%). And we find that the conversion efficiency mainly suffers from the breakdown in the extraction cavity and the plasma from the cylindrical collector. To suppress the breakdown and plasma effect, the breakdown electric field threshold in RF structures(especially in the extraction cavity) had better be larger than 700 kV/cm. Moreover, the distance between the position where the electrons are collected and the end of extraction cavity should be larger than 20 mm in case of the cylindrical collector.
Highlights
The relativistic backward wave oscillator (RBWO) is one of promising high power microwave (HPM) generators because of its high power, high efficiency, high repetitive rate and good tunability.[1,2,3,4,5,6] The early experiment about RBWO was carried out in the 1970s,7–9 but the notion of RBWO was not clear at that time
In PIC simulations, we find that the conversion efficiency is quite sensitive to the distance Lec(as marked in Fig. 1), and Fig. 8 shows the curve of conversion efficiency versus Lec, where Lec is the distance between the position where the electrons are collected and the end of extraction cavity
We study an X-band klystron-like RBWO with a pre-modulation cavity, modulation ridge, drift tube in slow wave structure (SWS) and extraction cavity in simulation and experiment
Summary
The relativistic backward wave oscillator (RBWO) is one of promising high power microwave (HPM) generators because of its high power, high efficiency, high repetitive rate and good tunability.[1,2,3,4,5,6] The early experiment about RBWO was carried out in the 1970s,7–9 but the notion of RBWO was not clear at that time. Reported an X-band two-wave Cherenkov oscillator with output power of 3.2 GW and efficiency of about 45%.23. The conversion efficiency of RBWOs in recent experiments is in level of 40% with output power of multi-GWs, there are still some intractable problems to solve, such as the modulation of the relativistic electron beam (REB), the restriction of power capacity, the bad effect of the plasma from the electron collector, and so on. To obtain higher efficiency(say, 50%), new structures should be explored to suppress the RF breakdown and bad effect of the plasma, and following conditions should be satisfied, 1) the electron beam should be bunched well in space, for example, few refluent electrons, small energy spread, good match between electron bunches and the longitudinal electric field, and so on.
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