M. Champion

Overview of the Spallation Neutron Source Linac Low-Level RF Control System

The design and production of the Spallation Neutron Source Linac Low-Level RF control system is complete, and installation will be finished in Spring 2005. The warm linac beam commissioning run in Fall 2004 was the most extensive test to date of the LLRF control system, with fourteen (of an eventual 96) systems operating simultaneously. In this paper we present an overview of the LLRF control system, the experience in designing, building and installing the system, and operational results.

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

High Power RF Test Facility at the SNS

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.

Operational performance of the SNS LLRF interim system

A new approach has been taken to develop and build the Low-Level RF Control System for the SNS Front End and Linear Accelerators, as reported in a separate paper in this conference. An interim version, based on the proven LBNL MEBT design, was constructed to support short-term goals and early commissioning of the Front End RFQ and DTL accelerators, while the final system is under development. Additional units of the interim system are in use at JLab and LANL for concept testing, code development, and commissioning of SNS SRF cryomodules. The conceptual design of the MEBT system had already been presented elsewhere, and this paper will address operational experiences and performance measurements with the existing interim system hardware, including commissioning results at the SNS site for the Front End and DTL Tank 3 together with operational results from the JLab test stand.

SNS Low-Level RF Control System: Design and Performance

A full digital RF field control module (FCM) has been developed for SNS LINAC. The digital hardware for all the control and DSP functionalities, including the final vector modulation as well as IF output synthesis, is implemented on a single high-density FPGA. Two of its HDL models have been written in VHDL and Verilog respectively, and both have being used to support the testing and commissioning of the LINAC to the date. The control algorithm used in the HDL produces a latency as low as 150nS. During the commissioning, the flexibility and capacity for needed precise controls that only digital design can provide has proved to be a necessity for meeting the great challenge of a high-power pulsed SCL.

Newly designed field control module for the SNS

The low-level RF (LLRF) control system for the 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

4.2 K Operation of the SNS Cryomodules

The Spallation Neutron Source being built at Oak Ridge National Laboratory employs eighty one 805 MHz superconducting cavities operated at 2.1 K to accelerate the H-beam from 187 MeV to about 1 GeV. The superconducting cavities and cryomodules with two different values of beta (. 61 and .81) have been designed and constructed at Jefferson Lab for operation at 2.1 K with unloaded Q’s in excess of 5×109. To gain experience in testing cryomodules in the SNS tunnel before the final commissioning of the 2.1 K Central Helium Liquefier, integration tests are being conducted on the cryomodules at 4.2 K. This is the first time that a superconducting cavity system specifically designed for 2.1 K operation has been extensively tested at 4.2 K without superfluid helium.

The Spallation Neutron Source RF Reference System

The Spallation Neutron Source (SNS) RF Reference System includes the master oscillator (MO), local oscillator(LO) distribution, and Reference RF distribution systems. Coherent low noise Reference RF signals provide the ability to control the phase relationships between the fields in the front-end and linear accelerator (linac) RF cavity structures. The SNS RF Reference System requirements, implementation details, and performance are discussed.

Operational Experience with the Spallation Neutron Source High Power Protection Module

The Spallation Neutron Source (SNS) High Power Protection Module provides protection for the High Power RF Klystron and Distribution System and interfaces with the Low-Level Radio-Frequency (LLRF) Field Control Module (FCM). The fault detection logic is implemented in a single FPGA allowing modifications and upgrades to the logic as we gain operational experience with the RF LINAC systems. This paper describes the integration and upgrade issues we have encountered during the initial operations of the SNS systems.

Design and high power processing of RFQ input power couplers

A RF power coupling system has been developed for future upgrade of input coupling of the RFQ in the SNS linac. The design employs two coaxial loop couplers for 402.5 MHz operation. Each loop is fed through a coaxial ceramic window that is connected to an output of a magic-T waveguide hybrid through a coaxial to waveguide transition. The coaxial loop couplers are designed, manufactured, and high power processed. Two couplers will be used in parallel to power the accelerating structure with up to total 800 kW peak power at 6% duty cycle. RF and mechanical properties of the couplers are discussed. Result of high power RF conditioning that is performed in the RF test facility of the SNS is presented.