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 cavity's 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.

INTRODUCTION

The transient beam-loading signal in an SC cavity for high-intensity pulsed beams is utilized to measure beam phase in an electron linac at DESY [1] because the beam signal phase is exactly 180º to the RF crest. It provides an additional synchronous phase measurement for SC cavities based on beam-cavity interaction techniques. Two other methods are (1) introducing RF modulations into an SC cavity for phase measurement [2], which is limited to CW beams or long-pulse operations, and (2) measurement of drifting beams, which is applied to short pulse beams. The cavity needs to be turned off and on during the drifting beam measurement [3], so it is time consuming. A precise and fast linac tune-up technique is needed for an SC linac with multiple independently phased cavities. A transient detector has such potential, but it has to properly handle the stochastic noise in the RF systems.