Novel multistage depressed collector for high power fusion gyrotrons based on an E×B drift concept

A novel concept of a two-stage depressed collector for high-power, fusion gyrotrons using helix electrodes is investigated. The helix electrodes create an azimuthal component of the electric field, which causes a radial drift of the annular electron beam. This drift sorts the electrons according to their initial velocities. Particle-in-Cell simulations show that this concept is extremely robust against the negative effect of secondary electrons. The collector efficiency is reduced by only 1 %. Considering the space charge (including that of secondary electrons) a collector efficiency of 77 % can be achieved with only two stages. Furthermore, electron beam sweeping may be possible at the last stage, where the most kinetic energy of electrons is absorbed.