Abstract
When a beam travels near collimator jaws, it gets an energy loss and a transverse kick due to the backreaction of the beam field diffracted from the jaws. The effect becomes very important for an intense short bunch when a tight collimation of the background beam halo is required. In the Linac Coherent Light Source at SLAC a collimation system is used to protect the undulators from radiation due to particles in the beam halo. The halo is most likely formed from gun dark current or dark current in some of the accelerating sections. However, collimators are also responsible for the generation of wake fields. The wake field effect from the collimators not only brings an additional energy jitter and change in the trajectory of the beam, but it also rotates the beam on the phase plane, which consequently leads to a degradation of the performance of the Free Electron Laser at the Linac Coherent Light Source. In this paper, we describe a model of the wake field radiation in the SLAC linac collimators. We use the results of a numerical simulation to illustrate the model. Based on the model, we derive simple formulas for the bunch energy loss andmore » the average kick. In addition, we also present results from experimental measurements that confirm our model.« less
Highlights
The electron beam collimators in the LCLS must remove the halo particles in the beam before they affect and eventually degrade the very precise fields of the permanent magnet undulators [1]
The wake field effect of collimators with small apertures on the transverse beam dynamics was observed during the operation of the Stanford Linear Collider (SLC) [2]
We suggest that because of the rectangular shape of the collimator jaw, a geometrical diffraction model may be used to calculate the longitudinal and transverse wake fields excited by a short bunch
Summary
The electron beam collimators in the LCLS must remove the halo particles in the beam before they affect and eventually degrade the very precise fields of the permanent magnet undulators [1]. Special measurements have been made for longitudinally tapered collimators, designed for the applications in future linear colliders. The “head” of the bunch is not deflected at all, but the “tail” gets the maximum deflection force This kind of kick leads to the bunch being geometrically tilted. We suggest that because of the rectangular shape of the collimator jaw, a geometrical diffraction model may be used to calculate the longitudinal and transverse wake fields excited by a short bunch. Because it is difficult to find this reference, we will briefly explain the model following the description of the SLAC linac collimator below
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