Beam Quality Measurements at the ASDEX Upgrade ECRH system

An Electron Cyclotron Resonance Heating (ECRH) system employing 8 gyrotrons is in routine operation at the ASDEX Upgrade tokamak. The gyrotrons are of two-frequency
type operating at 105 and 140 GHz with a maximum output power of up to 1 MW and 10 s pulse length. The gyrotron output beams are coupled to 8 waveguide transmission lines via quasi-optical Matching Optics Units (MOUs). The oversized corrugated HE11 waveguides with a diameter of 87 mm are operated at atmospheric pressure with overall lengths between 65 and 102 meters. The number of quasi-optical miter bends per line is between 6 and 8. High mode purity in the transmission lines is critical with respect
to both, losses and atmospheric breakdowns. Beam measurements at low power have been performed along the transmission lines and are compared to high power measurements.