Abstract
Maintaining stable phasing in a linear accelerator is crucial for maintaining optimal performance. If phasing is incorrect, the beam will in general have an energy error and increased energy spread. While an energy error can be readily detected and corrected using position readings from beam position monitors at dispersion locations, this method is not useful for correcting energy spread in a system with many possible phase errors. While energy spread can be corrected by looking at beam size at a dispersive location, this typically involves a beam-intercepting diagnostic and is not compatible with top-up operation. Uncorrected energy spread results in poor capture efficiency in downstream accelerators, such as the Advanced Photon Source (APS) particle accumulator ring or booster synchrotron. To address this issue, APS has implemented beam-to-rf phase detectors in the linac, along with software for automatic correction of phase errors. We discuss the design, implementation, and performance of these detectors, as well as their use in feedback to automatically correct linac phase errors during top-up operation.
Highlights
The Advanced Photon Source (APS), at Argonne National Laboratory is a high-brightness, third-generation synchrotron light source that operates in top-up mode 75% of the time to maintain a storage ring current of 102 mA to 1% tolerance
We will cover the scheme for arranging the phase detectors in the linac to maximize their usefulness, show how the phase detectors are interfaced to the beam position monitors (BPMs) and linac rf systems, describe related electronics, and discuss the software used to maintain hands-free linacbeam phasing for APS top-up operation
Phase setpoint in each linear range, respectively. This is somewhat different in absolute value than the L2 SLED phase calibration factor of 36:6=volt due to the nonlinearity of the phase detector response apparent in the figure
Summary
The Advanced Photon Source (APS), at Argonne National Laboratory is a high-brightness, third-generation synchrotron light source that operates in top-up mode 75% of the time to maintain a storage ring current of 102 mA to 1% tolerance. This paper will describe the linac-beam phase-control system, which is one of many automated tools used at APS This system measures and corrects the phase of the beam relative to each rf system that provides acceleration in the linac. The APS linac was originally designed to provide a 400 – 450 MeV positron beam to the particle accumulator ring (PAR). This beam had a relatively large energy spread and emittance which the PAR was designed to accept. The phase detectors presented here are applied to the problem of long-term phase drift of the beam relative to the linac accelerating structures since the shot-to-shot jitter of the electron beam is not a limitation on PAR accumulation efficiency. At APS, energy spread increase due to shot-to-shot phase noise of 0.25 is within the PAR energy acceptance of 0.8% [2]
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