Influence of the quasi-elastically reflected electrons on the collector properties of powerful gyrotrons

Summary form only given. Collectors of powerful gyrotrons are exposed to a very strong electron bombardment and thermal loading. Therefore they are one of the most critical elements of the gyrotron design. The residual power of the electron beam deposited on the collector surface may reach 2-3 MW in CW operation regime in the tubes for ITER project. Till recently, numerical simulation of the collectors was performed in most cases on the basis of a model, which does not take into account the electrons reflected from its surface. In gyrotrons the angle of the magnetic field to the collector is rather small. Therefore the electrons are quasi-elastically reflected from its surface with the reflection coefficient more than 0.5-0.8 at least and then again are returned to its surface at the distance close to the Larmour step, and this process recurs again and again in weaker magnetic field up to the moment when the magnetic confinement of the beam fails. Such electrons have the directional pattern shaped as an ellipse with the big axis coinciding with the direction of elastic scattering of the particles, while their energies are only a little bit smaller than the energies of the primary particles. Number of reflections can be big enough. That is why the reflected (secondary) electrons may change the power density distribution along the collector essentially. The first works devoted to the influence of the secondary emission on the calculation of collector properties (A. Singh, L. Ives and J. Neilson, S. Illy and B. Pioszyk) were performed for some of the actual schemes of collectors only, which did not cover all possible versions of the gyrotron collectors. Besides, the physical model used in some codes is based on the utilization of the energy and angular distributions, which are valid for normal incidence of the particles to the surface, while for gyrotrons the grazing incidence of the electrons to the surface is typical. In the report the physical model taking into acco- - unt the above-mentioned characteristic features of the energy and angular distributions of the reflected electrons, as well as the specific features of the electron motion in the collector region is briefly described. The typical electron trajectories, as well as the power density distributions in different kinds of gyrotron collector systems, are investigated. It is shown that the secondary electrons decrease the maximum value of the power density and expand the operating area of the collector essentially.