Mark Crofford

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.

Status of the SNS cryomodule test

The cryomodule tests are on going to have better understandings of physics as a whole and eventually to provide safe and reliable operation for neutron production. Some features are revealed to be interesting issues and need more attentions than expected, such as operating condition, collective effects between cavities, HOM coupler issues, end-group stability, cavity-coupler interactions, and vacuum/gas physics, waiting for more investigations. Up to now SNS cryomodules were mainly tested at 2.1 K/4.4 K, 10 pulse per second (pps) and 30 pps/60 pps tests are under progress. This paper presents briefly the experiences and the observations during tests of cryomodules.

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.

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.

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.

Adaptive Feed Forward Beam-Loading Compensation Experience at the Spallation Neutron Source Linac

When initial beam studies at the Spallation Neutron Source (SNS) indicated a need for better compensation of the effects of beam-loading, a succession of rapid-prototyping and experimentation lead to the development of a simple yet successful adaptive feed forward (AFF) technique within a few weeks. We describe the process and first results.

The spallation neutron source accumulator ring RF system

The spallation neutron source (SNS) accumulator ring is a fixed-frequency proton storage ring located at the output of the SNS linear accelerator (Linac). Its purpose is to redistribute the 1 millisecond long H-beam pulses from the SNS Linac into high-intensity 695 nanosecond long pulses of protons for delivery to the neutron target. The RF bunching system controls longitudinal beam distribution during the accumulation process and maintains a 250+ nanosecond gap required for beam extraction. The RF system consists of three stations which operate at the beam revolution frequency of 1.05 MHz and a fourth station providing a second harmonic component at 2.1 MHz. The beam pulse at extraction consists of 1.6e14 protons representing a peak beam current of 52 amperes. The system utilizes four 600 kW tetrodes to provide the RF current necessary to produce the 40 kV peak fundamental frequency bunching voltage and to control phase and amplitude at high beam current. A 20 kV peak second harmonic voltage is intended to control longitudinal beam distribution to control the peak circulating current. In this paper we review the design concepts incorporated into this heavily beam-loaded RF system and discuss its commissioning status.

Status and Performance of ORNL Spallation Neutron Source Accelerator Systems

The Spallation Neutron Source (SNS) accelerator systems have been performing continuously and progressively since commissioning in 2006 to deliver the neutrons to beamlines. The 1.4 MW design beam power has been demonstrated during 24/7 operation while developments and investigations for system improvements are still ongoing to achieve the full design beam power and availability. Numerous difficulties that impeded reaching the full performance of the SNS accelerator systems have been identified and are being eliminated through repairs, upgrades, and developments. In this report, operational performance and developments of the accelerator systems are presented along with the efforts for future upgrades of the SNS.