Aftercavity interaction in gyrotrons

Abstract

The theory describing the cyclotron resonance interaction between electrons in a spent beam exiting the cavity and traveling electromagnetic waves propagating from the cavity exit into the output part of a gyrotron is developed. At present it is recognized [1, 2] that this parasitic effect not only reduces the gyrotron interaction efficiency but, more importantly, spoils the energy distribution of spent electrons [2], thus reducing the possibility to efficiently utilize depressed collectors. Our study shows that a proper consistent optimization of a profile of the waveguide wall in the region of the uptapered output waveguide and quasioptical launcher and axial distribution of the magnetic field of a solenoid may result in significant improvement of situation. First, the analytical theory was developed which allows one to establish the relation between the parameter characterizing the parabolic profile of the solenoidal magnetic field near its flat top and the parameter characterizing the wall uptapering leading to zeroing the beam to wave coupling in the region of after-cavity interaction (ACI). Then, the ACI effects were numerically studied in a 110 GHz, 1.3 MW CPI gyrotron [3] with the use of a self-consistent code MAGY. It was found that there are two regions of ACI. The first one starts right after the exit from the straight part of a waveguide playing a role of a cavity; in this region the ACI increases the efficiency. The second region is typically located in the beginning of the launcher. In this region, the efficiency degrades and the energy distribution spoils, thus reducing the effectiveness of using depressed collectors. Our calculations show that a proper optimization of the magnetic field profile in this region can significantly improve the situation. The interaction efficiency can be increased from 40% to 44% due to ACI, although the use of a single-stage depressed collector allows one to realize 60.5% efficiency in the presence of ACI while when the ACI is absent it can be as high as 64.8%. Main results of this work are published in Ref. 4.