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 M 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.