Examination of parasitic after-cavity oscillations in the W7-X series gyrotron SN4R

At high-power operation points, in the improved W7-X SN4R series gyrotron spurious parasitic oscillations could be detected experimentally. This paper describes high-dynamic RF spectrum measurements in short and long pulse operation and corresponding simulations with a self-consistent gyrotron interaction code. Preliminary simulation results show good agreement with the measured spectra and suggest that the frequencies observed originate from so-called after-cavity interactions (ACI) in the uptaper/launcher region of the tube.