The Spallation Neutron Source (sns) Linac Rf System*

The SNS is a spallation neutron research facility being built at Oak Ridge National Laboratory in Tennessee [1]. The Linac portion of the SNS (with the exception of the superconducting cavities) is the responsibility of the Los Alamos National Laboratory (LANL), and this responsibility includes the RF system for the entire linac. The linac accelerates an average beam current of 2 mA to an energy of 968 MeV. The linac is pulsed at 60 Hz with an H beam pulse of 1 ms. The first 185 Mev of the linac uses normal conducting cavities, and the remaining length of the linac uses superconducting cavities [2]. The linac operates at 402.5 MHz up to 87 MeV and then changes to 805 MHz for the remainder. This paper gives an overview of the Linac RF system. The overview includes a description and configuration of the high power RF components, the HV converter/modulator, and the RF controls. Issues and tradeoffs in the RF system will be discussed, especially with regards to the use of pulsed superconducting cavities. Figure 1: SNS Linac configuration. 1 OVERVIEW OF SNS LINAC The SNS Linac is a nominal 968 MeV, 2 mA average, H accelerator (Figure 1). The system provides 52 mA peak at 1 ms pulse width and a 60 Hz repetition rate. The beam is chopped with a 68% chopping factor, for an average beam during the pulse of 36 mA. The accelerator begins with an injector and RFQ (from Berkeley), then a Drift Tube Linac (DTL) to 86.8 MeV, followed by a Coupled-Cavity Linac (CCL) to 186 MeV. Los Alamos is responsible for the DTL and CCL structures and all of the linac RF systems. The remaining part of the accelerator uses superconducting cavities (from Jefferson Laboratory), using ß=0.61 cavities to 375 MeV, then ß=0.81 cavities to 968 MeV. The RFQ and DTL operate at 402.5 MHz, and the CCL and superconducting cavities operate at 805 MHz. The different structures and frequencies require that 3 different types of klystrons be used. The RFQ and DTL will use 2.5 MW peak klystrons at 402.5 MHz. The CCL *Work supported by the US Department of Energy will use 5 MW peak klystrons at 805 MHz, and the superconducting cavities will use 550 kW klystrons at 805 MHz. The types and quantities and applications are listed in Table 1. Table 1: Types and quantities of klystrons and HV systems for the SNS Linac H Energy 968 MeV Average beam during pulse 36 mA Pulse Width 1 ms Rep Rate 60 Hz Klystrons 402.5 MHz, 2.5 MW pk (includes 1 for RFQ , 6 for DTL) 805 MHz, 5 MW pk (includes 4 for CCL, 2 for HEBT) 805 MHz, 0.55 MW pk, SC 7