High power CW klystrons for fusion experiments

On TORE SUPRA (TS) located in Cadarache, the Lower Hybrid Current Drive (LHCD) 3.7 GHz system is being upgraded* in the frame of the CIMES project. Since the end of 2001, several technological components have been under manufacturing or have been installed. Particularly, a new high power CW klystron has been successfully developed and tested at Thales Electron Devices (TED) for CEA Cadarache. The main required performances are to deliver at 3.7 GHz, 700kW minimum output power on a matched load with a 44% efficiency, and 620kW minimum output power on a 1.4:1 VSWR all phases with a 38% efficiency, this latter case corresponding to the shot mode on plasma. The klystron has been factory tested up to 700 kW in CW operation on a matched load, without any difficulty. This operating point has been achieved at 73.1 kV and 20.7 A, with an efficiency of 47% and a gain of 52 dB. Power variation is less than 0.15 dB over the specified 5 MHz bandwidth. No instabilities were observed during tests. The power dissipated in the two klystron body cooling circuits is about 16.5 kW. The overall RF losses are estimated at 15 kW, which means that the beam losses are quite low, at 1.5 kW to be compared to the 1500 kW beam power. As a result, the temperatures of the output cavity noses are kept below 110 degC at 700 kW. Ten 1000s 700 kW shots were applied to the klystron with remarkably stable thermal and RF performances. On a mismatched load (1.4:1 VSWR), the tube has delivered 620 kW min, any phase, with an efficiency of 40%. As expected, a slight temperature rise of the output cavity noses (about 20degC) was observed, when compared to operation on a matched load at the same output power. In the diode mode, at full beam power, the measured temperatures of the collector inner wall are lower than 220degC. These results demonstrate that this new 3.7 GHz 700kW CW klystron has been designed with large thermal safely margins, which will ensure high operational reliability on the TORE SUPRA facility, but also good manufacturing yield during the serial production recently started at TED.