Operating the KIT 170 GHz 2 MW Coaxial-Cavity Gyrotron at 204 GHz: Performance Expectations and First Cold Test of the Quasi-Optical System

The KIT 170 GHz $\mathbf{TE}_{\pmb{34,19}}$ -mode coaxial-cavity gyrotron has been studied for an upgrade towards a dual- or even triple frequency operation at 170/204/(238) GHz. For this reason, the electron magnetron injection gun (MIG), the cavity and the launcher are simulated for operation with the $\mathbf{TE}_{\pmb{40,23}}$ -mode and $\mathbf{TE}_{\pmb{48,26}}$ -mode at 204 GHz and 238 GHz, respectively. A modification of the coaxial-cavity midsection length leads to an increase of the theoretical RF output power from 1.6 MW to around 2.1 MW at 204 GHz. Further, first experimental cold tests using a mode generator setup, show a successful excitation of the $\mathbf{TE}_{\pmb{40,23}}$ -mode, which is the highest-order mode ever excited.