Hengjie Ma

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

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.

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.

Transient beam-loading detection in an SNS cavity- simulations and initial measurements

Beam phase measurement based on the detection of a transient beam-loading signal in a SC cavity has the potential to become a very fast linac tune-up technique, especially for an electron linac, as it may precisely set up the cavitys synchronous phase. A large signal-to-noise ratio is critical in the development of a transient detector technique. This paper discusses the study of a transient detector in the Spallation Neutron Source proton linac and the major challenge for this method: stochastic noise in the rf system, which reduces the precision and increases the time needed for accurate phase measurement. Using simulation studies with a beam-cavity model, signal measurement with the Low-Level RF system, and the experiment with prototype detectors, we analyzed the influence of noise on phase measurement.