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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 operatingat 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.

The Superconducting Linac at SNS has been operating with beam for almost two years. As the first operational pulsed superconducting linac, many of the aspects of its performance were unknown and unpredictable. A lot of experience has been gathered during the commissioning of its components, during the beam turn on and during operation at increasingly higher beam power. Some cryomodules have been cold for well over two years and have been extensively tested. The operation has been consistently conducted at 4.4 K and 10 and 15 pulses per second, with some cryomodules tested at 30 and 60 Hz and some tests performed at 2 K. Careful balance between safe operational limits and the study of conditions, parameters and components that create physical limits has been achieved.

Spallation Neutron Source has undergone some recent hardware changes. The intended Field and Resonance Control Module (FRCM) design has been re-vamped to minimize functionality and ease implementation. This effort spans a variety of disciplines, and requires parallel development with distinct interface controls. This paper will discuss the platform chosen, the design requirements that will be met, and the parallel development efforts ongoing.The low-level RF (LLRF) control system for the OVERVIEW OF THE NEW FCMPerformance specifications were eased late last year for the LLRF control system for SNS. Physics simulations indicated that the field control requirement could be reduced from from HS%, iA).5" to kl.O%, k1.0". In addition required functions were changed such that onboard processing for resonance control was moved to the crate controller (IOC), as well as feedforward table update and access; fewer RF channels were required (no need to provide for possible beam information via a beam diagnostics channel); no real time data link needed between the control system and the operators; and milch less memory per channel was deemed appropriate. By moving the iterative learning controls onto the IOC as well, we completely eliminated the need for DSPs on the module. A collaborative effort was established between Lawrence Berkely, Los Alamos, and Oak Ridge national labs in order to develop a control system that meets these specifications [I].Due to these relaxed system performance specifications, and a new staged approach to implementation and integration of control system functions, the original complicated, meet-all-performance-specifications-at-once FRCM de-sign has been greatly simplified. The new Field Control Module (FCM) is still a basic VXIbusbased module with multiple daughter cards. The infrastructure for a VXIbus-based RF Control System (RFCS) was already in place at the SNS: VXIbus crates had been purchased and other modules within the system are VXIbus, so it only made sense to remain consistent. However the new FCM has three new daughter cards which minimize the functionality of the FCM; and the implementation of the firmware is in a staged approach where progressive phases will be achieved as the accelerator grows from one cavity to many and functional requirements of the RFCS increase [l]. A block diagram of the module hardware is given in figure I . PRlNTED CIRCUIT CARDS'rhe imis for thc rc-designed FCM is the successful SNS Diagnostics PCI-based clcctronics, specifically the Beam Position Monitors (BPMs) 121. In that vein, we attempted to make use of a'i tnuch of the foundation provided by the l3i;ignosticc HPMs 3s possible. The BPMs have an Analog Front End daughter card nianufacturecl by 'Bergm Instrumentation of France. Likewise, we specified a very similar I L R F AFE for use on the FCM. The BPMs havc a Digital Front End (DFE) proccssing card. We, too, utilize a DFE for providing the analog-to-digital conversion of the input RF and IF signals, as well as for providing an interface to the...

RF Test Facility (RFTF) has been constructed to support present and future needs in testing, processing and conditioning of various high power RF components of normal conducting and superconducting systems at the SNS. The facility is expected to have additional subsystems that are needed for complete superconducting RF (SRF) testing and processing. A full capacity high voltage converter modulator (HVCM) with 11 MW peak power at 8% duty cycle has been installed for driving one or two klystron RF amplifiers. The waveguides are completed in WR-2100 and WR-1150 for the 402.5 MHz and 805 MHz klystrons being used in the SNS. The 805 MHz system has been used for RF processing the coaxial fundamental power couplers (FPCs) for the SNS superconducting linac (SCL) [1]. The high power RF system can be reconfigured or modified for various tests and conditioning processes along with the neighboring cryo-plant.