High Power CW Testing of 3.7-GHz Klystron for SST1 LHCD System

A 3.7-GHz lower hybrid current drive (LHCD) system has been designed and installed for driving noninductive plasma current for steady-state operation of steady-state super-conducting tokamak-1 machine. Currently, its capability has been enhanced up to 2 MW by adding two additional klystrons, each capable of providing 500 kW, continuous wave (CW) power, to LHCD system. After performing factory acceptance test of these klystrons, they are installed and commissioned at site, at rated power, for more than 1000 s, before connecting them to main LHCD system. The auxiliary systems like supporting power supply system (magnet, filament, ion pump, etc.), active heat management system, protection system, transmission line pressurization system, low power RF drive system, and so on are interconnected with klystron system through versa module europa (VME)-based data acquisition and control system for remote CW operation of klystron at rated power. The klystron is protected by fast interlock protection in an event of arc or an electrical parameters excursion beyond a set window. The slow interlock is invoked in case of cooling failures, pressurization of waveguides, and so on. The klystron has two output arms and a recombiner is used to recombine power coming out from both the arms. The output power from the recombiner of the klystron is split into two arms, employing 3-dB power divider. This reduces the high power requirement of water dummy loads (DLs) to 250-kW CW. A directional coupler is included in both the arms for measuring forward and reflected power. The reflected power signal is fed to low power RF drive control, which removes the RF drive once reflected power beyond a threshold power is detected within 5 μs. The forward power signal is used for monitoring RF power being dumped into the water loads. The calorimetric measurements, employing Pt-100 sensors, are also carried out on water dummy loads. Our measurements suggest that the maximum RF power ( ~500-kW CW) extracted from klystron is dissipated on water cooled DLs. The unspent dc power ( ~800-kW CW) is dissipated in the collector that is heavily cooled with water flowing at ~1300 L/min. The power loss in the klystron body remained within 15 kW. The cavity temperature, measured using J-type thermocouple, remained below 150 °C. The output RF power, sampled through directional couplers and measured by RF detectors shows good agreement with calorimetric measurements. A detailed description of the klystron test setup and the test results obtained during its commissioning is presented in this paper.