Abstract
Wakefields are an important factor in accelerator design, and are a real concern when preserving the low beam emittance in modern machines. Charge dependent beam size growth has been observed at the Accelerator Test Facility (ATF2), a test accelerator for future linear collider beam delivery systems. Part of the explanation of this beam size growth is wakefields. In this paper we present numerical calculations of the wakefields produced by several types of geometrical discontinuities in the beam line as well as tracking simulations to estimate the induced effects. We also discuss precision beam kick measurements performed with the ATF2 cavity beam position monitor system for a test wakefield source in a movable section of the vacuum chamber. Using an improved model independent method we measured a wakefield kick for this movable section of about $0.49\text{ }\text{ }\mathrm{V}/\mathrm{pC}/\mathrm{mm}$, which, compared to the calculated value from electromagnetic simulations of $0.41\text{ }\text{ }\mathrm{V}/\mathrm{pC}/\mathrm{mm}$, is within the systematic error.
Highlights
Wakefields are increasingly important in the context of precision beam operation and control
To study the beam distortion and orbit change induced by the wakefields and to investigate the possibility of compensation of the wakefields generated by other sources, a movable test section has been installed in the Accelerator Test Facility 2 (ATF2) beam line
The measured response appears to be 20% higher than the prediction, so that the measurement corresponds to an average wakefield potential of about 0.49 V=pC=mm compared to the estimate of 0.41 V=pC=mm based on numerical calculations
Summary
Wakefields are increasingly important in the context of precision beam operation and control. The problem of wakefield measurement and simulation has been a standing item in the design of circular machines for decades It is important for single pass accelerators due to stringent requirements imposed by the luminosity (colliders) or brightness (free electron lasers) specifications. We demonstrate an improved method of measuring wakefields in single pass beam lines using model independent methods for jitter subtraction that do not require prior knowledge of the reference orbit or the beam optics. Using this method a large pulse-to-pulse orbit jitter can be effectively subtracted. (CBPM) system, the orbit reconstruction in a beam test carried out at the Accelerator Test Facility 2 (ATF2) reached submicrometer precision, allowing us to characterize relatively weak wakefield sources
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