The Spallation neutron source accelerator low level RF control system

The Spallation Neutron Source low level RF team includes members from Lawrence Berkeley, Los Alamos, and Oak Ridge national laboratories. The team is responsible for the development, fabrication and commissioning of 98 low level RF (LLRF) control systems for maintaining RF amplitude and phase control in the front end (FE), linac and high energy beam transport (HEBT) sections of the SNS accelerator, a 1 GeV, 1.4 MW proton source. The RF structures include a radio frequency quadrupole (RFQ), rebuncher cavities, and a drift tube linac (DTL), all operating at 402.5 MHz, and a coupled-cavity linac (CCL), superconducting linac (SCL), energy spreader, and energy corrector, all operating at 805 MHz. The RF power sources vary from 20 kW tetrode amplifiers to 5 MW klystrons. A single control system design that can be used throughout the accelerator is under development and will begin deployment in February 2004. This design expands on the initial control systems that are currently deployed on the RFQ, rebuncher and DTL cavities. An overview of the SNS LLRF control system is presented along with recent test results and new developments

Introduction

The SNS LLRF Control System is comprised of three main components: the RF control chassis, the High Power Protection Module (HPM), and the reference system. The RF control chassis is a digital feedback controller that uses a Field Programmable Gate Array (FPGA) for fast data processing. Three generations of control chassis are planned for supporting the near- and long-term goals of the SNS project: the 1st generation control chassis (Fig. 1) was designed at LBNL for use with the 402.5 MHz rebuncher cavities in the Medium Energy Beam Transport (MEBT) beamline downstream of the RFQ [1]. The 2nd generation control chassis (Fig. 2) is a refinement of the MEBT control chassis and will serve the RFQ and DTL sections of the linac through DTL commissioning [2]. The 3rd generation control chassis is under development and will begin deployment in the CCL, SCL and HEBT in early 2004. It is planned to eventually retrofit the RFQ and DTL with the 3rd generation control chassis. The HPM provides for fast shutoff of the RF drive to the klystron in case of RF over power, arc detection, and vacuum system interlocks [3]. The reference system provides for distribution of the phase-synchronous RF signals necessary to operate the linac and includes cavity and reference signal transport and downconversion (Fig. 3). Figure 1.

The 1st generation rf control chassis that is used for the mebt rebuncher cavities.

Figure 2.

The 2nd generation rf control chassis that is used for the rfq and dtl.