Study of RF output window ARC event in a megawatt class 140 GHz gyrotron

Summary form only given. The stellarator Wendelstein 7-X, currently in its final construction phase at Greifswald, Germany, will employ a 10 MW, 140 GHz, continuous wave (CW) gyrotron-based electron-cyclotron resonance heating (ECRH) and-current drive (ECCD) system. To fulfill this demand for CW millimeter-wave power, Karlsruhe Institute of Technology (KIT) and Thales Electron Devices (TED, France) have been maintaining a successful series production of reliable megawatt-class 140 GHz gyrotrons throughout the last years, providing numerous opportunities for experimetal gyrotron research during the ongoing factory and site acceptance tests of the various tubes. Recently, a novel time-based pulse spectrum analysis system was developed at KIT, capable of recording time-dependent unambiguous frequency spectra with multiple GHz of instantaneous bandwidth in the millimeter-wave domain. Using this spectrum analysis system, the highly transient event of an RF arc at the dielectric microwave output window of a gyrotron in coincidence with a cavity mode switch was documented. It happened during experimental parameter-space exploration of W7-X series gyrotron SN5 in the KIT gyrotron test facility. In this report, co-simulations using the self-consistent slow-variables gyrotron interaction simulation code SELFT3 are presented, which aim at providing a deeper understanding of the mechanisms involved in the undesired event. Major focus is also placed on the similarities as well as differences between measured and simulated behavior, investigating the overall challenges and limitations of the undertaking.