Validation of 500-kW/1000-s operation of 5-GHz klystron for KSTAR LHCD system

•Validation of 5-GHz 500-kW CW operation of the klystron which is relevant to the ITER LHCD system.•The weakest part was the RF window damage due to a thermal stress by the temperature gradient.•The accurate temperature of the test RF window was monitored using IR camera with a calibration.•The klystron operation at 500 kW for 1000s was successfully.•The klystron characteristics with reflective load.