Investigations of after cavity interaction in gyrotrons including the effect of non-uniform magnetic field

The increasing demand for powerful, reliable and efficient gyrotron oscillators as millimeter-wave sources for Electron Cyclotron Resonance Heating (ECRH) and non-inductive current drive in fusion plasma experiments calls for a close look on all possible parasitic oscillations in gyrotrons. New effects are observed where it has been experimentally found that some spurious oscillations are also excited by the electron beam in the region after the gyrotron interaction cavity. Therefore these effects are called After Cavity Interaction (ACI). In most publications, ACI is described as a stationary reduction in the overall efficiency caused by further energy exchange after the cavity between electron beam and RF field which here is named static ACI. In this study we considered dynamic ACI, which behaves differently: the interaction after the cavity, either with the main mode or with different modes at lower frequency, leads to a dynamic modulation of the output power. The self-consistent KIT multi-mode code SELFT has been modified in order to extend the simulation domain into the radius uptaper section of the gyrotron cavity where the probability of occurrence of dynamic ACI exists and the strength of the longitudinal magnetic field is no longer constant. This modified KIT SELFT code has been applied to dynamic ACI studies using four different gyrotron configurations (two conventional cylindrical cavity gyrotrons and two coaxial cavity gyrotrons). The study confirms that undesired interactions in the uptaper region can result in additional parasitic oscillations with power in the 1% region. Due to the implementation of a dBR(z)/dz ≠ 0 term and the significant reduction of the magnetic field towards the uptaper output, some spurious oscillations have been observed in the RF spectrum of the output signal. Theory and measurements are in good agreement verifying the modifications performed on the SELFT code. The results of this study confirm that uptaper radius contour and magnetic field profile have to be optimized in order to minimize ACI.