Experimental observation of the effect of aftercavity interaction in a depressed collector gyrotron oscillator

Abstract

This paper presents the experimental observation of the effect of an aftercavity interaction ͑ACI͒ in a depressed collector gyrotron oscillator. The gyrotron generates an output power of 1.5 MW at 110 GHz in 3 s pulses with a 96 kV and 40 A electron beam and has a single-stage depressed collector. The ACI arises from an unintended cyclotron resonant interaction between the microwave beam traveling out from the cavity and the gyrating electron beam. The interaction occurs in the uptaper of the launcher, immediately downstream from the cavity, where the magnetic field is slightly lower than its value in the cavity region. The ACI results in a reduction in efficiency since the electron beam tends to extract power from the wave. There is also a broadening of the spent beam energy profile, which reduces the effectiveness of the depressed collector and in turn limits the overall efficiency of a gyrotron. Measurements of the maximum depression voltage of the collector vs beam current at 96 kV are compared with simulations from the MAGY code ͓M. Botton et al., IEEE Trans. Plasma Sci. 26, 882 ͑1998͔͒. Excellent agreement is obtained between theory and experiment but only if the ACI is included. In the present experiment, it is estimated that the observed efficiency of 50% would have been about 60% in the absence of the ACI. These results verify the role of the ACI in reducing the efficiency of the gyrotron interaction.

Effects of after cavity interaction in a 1.5 MW, 110 GHz gyrotron with a depressed collector

“…The detailed experimental setup for the results reported in this section is provided in [3]. Figure 2 shows the relationship between the beam current required to excite the TE 22,6 mode and the collector depression voltage, with the cathode-anode voltage and the beam compression ratio fixed at 96 kV and 23.6, respectively.…”

“…Generally, without either altering the accelerating voltage or the magnetic field during the pulse, the efficiency of a MW-class gyrotron is usually below 50%, which is much lower than the theoretically predicted value. Our recent experimental study found a possible cause for this underperformance, namely, the after cavity interaction (ACI) [2,3]. In this paper, we describe this phenomenon and its effects on the performance of a gyrotron using MAGY simulations [4].…”

A Study on 94 GHz Low-Voltage, Low-Current Gyrotron

“…In practice, however, the transverse-to-axial velocity ratio does not reach a value of about 2 at present. In order to recover the kinetic energy related to the parallel momentum of the electrons, a depressed collector should be used [14]. The gyrotron oscillator should employ a cavity with a highquality factor.…”

After cavity interaction in gyrotrons: On the influence of different models for non-uniform magnetic fields

“…FTER cavity interaction (ACI) in gyrotron oscillators, namely in the uptaper section, has been discussed recently as a major factor influencing the overall efficiency, both with and without depressed collector 1,2,3 . The crucial point is that in the uptaper section, at increased waveguide radius, regions exist where the decreasing magnetic field allows for undesired gyrotron interaction.…”