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.
The low-level rf control system currently commissioned throughout the Spallation Neutron Source (SNS) LINAC evolved from three design iterations over 1 yr intensive research and development. Its digital hardware implementation is efficient, and has succeeded in achieving a minimum latency of less than 150 ns which is the key for accomplishing an all-digital feedback control for the full bandwidth. The control bandwidth is analyzed in frequency domain and characterized by testing its transient response. The hardware implementation also includes the provision of a time-shared input channel for a superior phase differential measurement between the cavity field and the reference. A companion cosimulation system for the digital hardware was developed to ensure a reliable long-term supportability. A large effort has also been made in the operation software development for the practical issues such as the process automations, cavity filling, beam loading compensation, and the cavity mechanical resonance suppression.
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 [1]. An interim version, based on the proven LBNL MEBT design, was constructed to support shortterm goals and early commissioning of the Front End RFQ and DTL accelerators, while the final system[2] 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 [3], 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.
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