Space charge induced velocity spread in a gyrotron MIG

Summary form only given. Generally speaking, there are a variety of sources of velocity spread, including the initial thermal velocity spread, spread due to roughness of the surface on the cathode and spread due to the effect of beam space charge. The velocity spread caused by the first two items is difficult to eliminate in principle, and related problems have been studied. We focus on the third item, the velocity spread due to the beam space charge field, and regard the effects of temperature and surface roughness as initial conditions in our simulation. It has been shown that a change in velocity spread begins during the beam´s transition from an initially laminar state to a highly nonlaminar mixed state. Our model aimed at investigating this effect. A simple model is introduced to describe the mixing process. A periodically varying potential caused by the nonuniform space charge is found to be associated with spiraling electrons, and breaks the adiabatic constancy of the magnetic moment. This space charge potential does nor vanish until electrons are completely mixed. Consequently, a spread in perpendicular velocity is introduced in the mixing from the originally in phase state to the randomly mixed state. Our study suggests that the velocity spread due to temperature or cathode roughness is taken into account and could be less if a larger spread in guiding center radii can be tolerated. This new model provides a more accurate and efficient approach for analyzing the velocity spread due to mixing in MIG´s. Simulations are performed by using the design parameters of the MIT 170 GHz MIG. The velocity spread obtained from this model is a little larger than that predicted by the EGUN finite-element calculation, and the possible.