In this study, a klystron for the KSTAR lower hybrid current drive (LHCD) system was developed as a prototype for a 5 GHz, 500 kW CW operation that is aimed to meet the requirement of the ITER LHCD system as well. Before the 2012 KSTAR campaign, the prototype klystron was validated only for 350 kW CW operation. The weakest part of the klystron for 500 kW CW operation was the RF output window failure due to a thermal stress caused by the temperature gradient. The klystron was equipped with two RF windows, each of which should transmit 250 kW power. Prior to the full performance test of the klystron, the performance of the test RF windows that were identical to those mounted on the klystron was tested with half of the klystron’s full power. The temperature increase in the test RF windows was monitored using an IR camera. The temperature difference, ΔTce, between the center and edge of the test window at 250-kW 1000-s transmitting power was predicted to be 23 °C, which is smaller than the safe ΔTce, which is 50 °C according to the manufacturer. Based on the result of the test window measurement, 500-kW 1000-s operation of the klystron was conducted successfully. The klystron output power characteristics depending on the phase of load reflection VSWR 1.4:1 were investigated. The klystron generated stable power on a load VSWR of 1.4:1 at various phases. The output power at the worst phase was 380 kW. Details of the IR-imaging setup are discussed and the results obtained are presented.
