Secondary Electron Simulations of a Gyrotron Collector with Magnetic Sweeping and Voltage Depression

Megawatt-class gyrotrons are designed to distribute the residual electron beam energy across a large collecting surface, to keep power densities low enough to be dissipated without threatening long-term vacuum integrity. Because the incident beam is very narrow, various techniques are used to lower the instantaneous and time-averaged power densities on the collector surface, while keeping the size of the collector within the limits of current fabrication capabilities. Gyrotron collector design typically focuses on optimizing the power deposition of the incident (“primary”) beam. It is often assumed that the effects of secondary electron emission from the collector surface (whether due to reflection of primaries, or true secondary emission) will tend to further spread the power density profile. Such additional spreading can be beneficial if it lowers peak power densities, but can be detrimental if it deposits power in undesired locations or sends particles back toward the gyrotron’s interaction region. Here, we simulate the effects of secondary/reflected electrons in the VGT-8115, a 110 GHz, 1.2 MW, 10-second gyrotron used for electron cyclotron heating and current drive in the DIII-D tokamak. We examine the ramifications of secondary emission under various operating conditions, such as variations in collector sweeping parameters and collector depression voltage, comparing power densities and particle trajectories with and without secondaries.