Development of a 3.7-GHz 750-kW CW Klystron for Tore Supra

A 3.7-GHz 750-kW klystron in continuous-wave (CW) operation is under development to upgrade the lower hybrid RF power of Tore Supra up to 12 MW. Four different operating modes are required: diode regime (1680-kW dc of beam power in the collector), 750-kW RF on matched load, 700-kW min all phases on voltage standing wave ratio (VSWR) 1.4:1 (plasma conditions), and around 400-kW pulsed mode on VSWR 3.0:1 (for antenna conditioning phase). The klystron is equipped with a diode type electron gun, a five-cavity RF structure with a highly efficient cooling system for the output cavity, a single tapered waveguide, a single BeO disc RF window, and a large diameter hypervapotron collector. Two breadboard models have been built and tested. At 50% duty cycle (with 5-ms pulse length), an RF output power of 770 kW has been measured at a beam voltage of 75 kV and a beam current of 22 A. The efficiency is 46.6% and the gain is 54 dB. On VSWR 1.4, a minimum output power of 660 kW has been obtained and the klystron is stable at all phases. In CW operation, the klystron has produced an output power of 680 kW. The limitation is due to excessive body power. The final prototype with an improved beam optics and thermal design will be ready in the second half of the year 2004.