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The SNS linac RF control system (RFCS) is currently in development. A system is being installed in a superconducting test stand at Jefferson Laboratory presently. Two systems will soon be installed at Oak Ridge National Laboratory (ORNL) and more are due to be installed early next year. The RF control system provides field control for the entire SNS linac, including an RFQ and 6 DTL cavities at 402.5 MHz as well as three different types of cavities at of 805 MHz: 4 CCL cavities, 36 medium beta superconducting (SRF) cavities, and 45 high beta superconducting cavities. In addition to field control, it provides cavity resonance control, and incorporates high power protect functions. This paper will discuss the RFCS design to date, with emphasis on the challenges of providing a universal digital system for use on each of the individual cavity types. The RF control system hardware has been designed to minimize the amount of changes for all of the applications. Through software/ firmware modification and changing a couple of frequency-dependent filters, the same control system design can be used for all five cavity types. The SNS is the first to utilize SRF cavities for a pulsed highcurrent proton accelerator, thereby making RF control especially challenging. Figure 1 shows a block diagram of a typical RFCS. The dotted lines represent different VXIbus modules that perform the functions described above. The overall design of the SNS relies on a variety of cavity types as well as RF sources [1]. Due to physical layout constraints, a single VXIbus crates houses a single RFCS for the RFQ, and each DTL and CCL cavity. However, we take advantage of the smaller klystrons and power supply configuration for the SRF sections to increase the number of systems in each crate. For the medium-beta sections, there are typically three RF control systems housed in a single crate, while for the high-beta, two systems per crate are the norm. The VXIbus architecture takes advantage of the backplane in order to stream-line data-acquisition, system-monitoring and real-time-event-processing tasks. The VXIbus serves as an interface for the global experimental physics and industrial control system (EPICS) to send and receive information about the way the RFCS controls the in-phase and quadrature (I/Q), rather than amplitude and phase, components of the accelerator cavity RF field. SYSTEM IMPLEMENTATION DIGITAL TECHNOLOGY EMPHASISThe core module of the system that perform the field and resonance control functions (FRCM) uses digital feedback-and feedforward-controls to regulate the field parameters of an RF cavity of the SNS linac. The FRCM relies on Complex Programmable Logic Devices (CPLDs) and two digital signal processors optimized for real-time signal processing. The FRCM performs feedback-and feedforward-control algorithms on the field intermediate frequency (IF), resulting in baseband-control in-phase and quadrature (I/Q) outputs, which are then upconverted to the appropriate carrier frequency prior to amplification by the klyst...

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.

This paper addresses the modeling problem of the linear accelerator RF system in SNS. Klystrons are modeled as linear parameter varying systems. The effect of the high voltage power supply ripple on the klystron output voltage and the output phase is modeled as an additive disturbance. The cavity is modeled as a linear system and the beam current is modeled as the exogenous disturbance. The output uncertainty of the low level RF system which results from the uncertainties in the RF components and cabling is modeled as multiplicative uncertainty. Also, the feedback loop uncertainty and digital signal processing signal conditioning subsystem uncertainties are lumped together and are modeled as multiplicative uncertainty. Finally, the time delays in the loop are modeled as a lumped time delay. For the perturbed open loop system, the closed loop system performance, and stability are analyzed with the PI feedback controller.

Objective. Recent SiPM developments and improved front-end electronics have opened new doors in TOF-PET with a focus on prompt photon detection. For instance, the relatively high Cherenkov yield of Bismuth-Germanate (BGO) upon 511 keV gamma interaction has triggered a lot of interest, especially for its use in total body PET scanners due to the crystal’s relatively low material and production costs. However, the electronic readout and timing optimization of the SiPMs still poses many questions. Lab experiments have shown the prospect of Cherenkov detection, with coincidence time resolutions (CTRs) of 200 ps FWHM achieved with small pixels, but lack system integration due to an unacceptable high power uptake of the used amplifiers. Approach. Following recent studies the most practical circuits with lower power uptake (<30 mW) have been implemented and the CTR performance with BGO of newly developed SiPMs from Fondazione Bruno Kessler (FBK) tested. These novel SiPMs are optimized for highest single photon time resolution&#xD;(SPTR). Main results. We achieved a best CTR FWHM of 123 ps for 2 × 2 × 3 mm³ and 243 ps for 3 × 3 × 20 mm³ BGO crystals. We further show that with these devices a CTR of 106 ps is possible using commercially available 3 × 3 × 20 mm³ LYSO:Ce,Mg crystals. To give an insight in the timing properties of these SiPMs, we measured the SPTR with black coated PbF2 of 2 × 2 × 3 mm³ size. We&#xD;confirmed an SPTR of 68 ps FWHM published in literature for standard devices and show that the optimized SiPMs can improve this value to 42 ps. Pushing the SiPM bias and using 1 x 1 mm² area devices we measured an SPTR of 28 ps FWHM. Significance. We have shown that advancements in readout electronics and SiPMs can lead to improved CTR with Cherenkov emitting crystals. Enabling TOF with BGO will trigger a high interest for its use in low-cost and total-body PET scanners. Furthermore, owing to the prompt nature of Cherenkov emission, future CTR improvements are conceivable, for which a low-power electronic implementation is indispensable. In an extended discussion we will give a roadmap to best timing with prompt photons.