Experimental comparison of millimetre wave power monitor designs

The ASDEX Upgrade electron cyclotron resonance heating system [1] uses multi-hole waveguide couplers integrated into the matching mirrors ‘M2’ for power monitoring. While the full Gyrotron output power (105 or 140 GHz, typ. 1 MW per unit) is essentially optically reflected at the M2 mirror, a small fraction of the order of several milliwatt is diverted by the coupler and transmitted via fundamental mode waveguide towards a beam lead Schottky diode, which converts the millimetre wave electric field amplitude into a voltage signal. The system is calibrated by a calorimetric measurement, where short pulses of the full Gyrotron power are used to heat water. The calibration of power monitor designs from the years 2009 until 2012 showed variations up to 25% on different experiment days, which is clearly above the uncertainty of the calorimetric calibration procedure. Using the latest power monitor design, these variations seem to be significantly reduced, even on the timescale of eight years, where hardware was unchanged. In this latest design, both the waveguide coupler and the low power transmission and detection scheme including flange interfaces were revised. The improved longevity seems to be mainly caused by improvements in the mechanical precision, stability and rigidity, which is backed by laboratory measurements.