THE SNS FRONT END LLRF SYSTEM

LBNL has built the Front End for the Spallation Neutron Source (SNS) project,** an H - injector that is designed to deliver 52 mA of beam current at 2.5-MeV energy in 1ms batches of 650-ns pulses at 60 Hz, for a macro beam duty factor of 6%. The Front End is comprised of an ion source, an electrostatic Low Energy Beam Transport line (LEBT), an RFQ, and a Medium Energy Beam Transport line (MEBT). The RFQ accelerates the beam from 65 keV to 2.5 MeV. The MEBT includes four bunching cavities that are designed to preserve the longitudinal bunch length during transport down the 3.7 m long beam line. An FPGA-based Low Level RF (LLRF) control system has been built and is now used to control cavity amplitudes to better than 1% and relative phases within 1 degree. The signal-processing chain that includes ADC, DSP in FPGA, and DAC, adds an analog delay of about 250 ns. The fast digital processing is networked to the EPICS global controls system with a commercial miniature embedded Linux computer (nanoEngine).