5 MW 805 MHz SNS RF System Experience

The RF system for the 805 MHz normal conducting linac of the Spallation Neutron Source (SNS) accelerator was designed, procured and tested at Los Alamos National Laboratory (LANL), and then installed and commissioned at Oak Ridge National Laboratory (ORNL). The RF power for the room temperature coupled cavity linac (CCL) of the SNS accelerator is generated by four, pulsed 5 MW peak-power klystrons operating with a pulse width of 1.25 ms and a 60 Hz repetition frequency. The RF power from each klystron is divided and delivered to the CCL through two separate RF windows. The 5 MW RF system advanced the state of the art for simultaneous peak and average power. This paper summarizes the problems encountered, lessons learned, and results of the high power testing at LANL of the 5 MW klystrons, 5 MW circulators, 5 MW loads, and 2.5 MW windows.

HIGH POWER TEST RESULTS

Twelve circulators were ordered from AFT and five were high power tested. The 805 MHz circulator is a three port “Y” junction circulator rated for 5.0 MW forward peak power into a short at any phase. The insertion loss is specified to be less than 0.07 dB, the isolation greater than or equal to 20 dB, and the return loss less than -20 dB at the center frequency of the circulator. The circulator is pressurized with sulfur hexaflouride (SF₆) to 15 Pisa to prevent RF breakdown. The SF₆ system is confined by three Kapton windows, one on each port. The system was vacuum leak checked at a pressure less than 100 mTorr to a leak rate of less than 30 mTorr per hour. A digital Temperature Compensating Unit (TCU), also supplied by AFT, controls the circulator. The circulator is equipped with an arc detector port, and an additional arc detector port was used on the adjoining waveguide to protect the Kapton window on the output RF power port (Port 2).