Abstract
This article presents the design and operation of a longitudinally tapered collimator in a single-pass $S$-band linac driving a high brightness electron beam. Measurements were done for the transverse emittance growth induced by the collimator wakefield as a function of the lateral displacement of the beam inside the collimator and the energy acceptance provided by an identical collimator installed in a dispersive region. The measurements demonstrate that: (i) the proposed design allows very precise and reproducible motion down to the micron level of the compact, four-hole collimator; (ii) the collimator does not degrade the beam emittance in the presence of standard trajectory control; (iii) the measured kick factor and energy acceptance are in agreement with the theoretical expectations. These measurements were made using 500 pC, 2.4 ps long bunches at the FERMI@Elettra free electron laser facility.
Highlights
In high-energy linear colliders, collimators are generally metallic blocks that can be used to protect detectors from being directly struck by a beam pulse that is not properly controlled in optics or energy and to minimize the detector background from halo particles while transmitting core particles to the collision point
The energy acceptance of the FERMI collimation system was determined by measuring the charge transmitted through the 30 m long transfer line as a function of the beam trajectory offset in the collimator
A high brightness, 500 pC electron beam was used for the measurement of the projected emittance growth induced by the transverse wakefield of the longitudinally tapered collimator in the FERMI@Elettra free electron laser (FEL)
Summary
In high-energy linear colliders, collimators are generally metallic blocks that can be used to protect detectors from being directly struck by a beam pulse that is not properly controlled in optics or energy and to minimize the detector background from halo particles while transmitting core particles to the collision point. In linac-driven light sources, collimators prevent halo particles from hitting the vacuum chamber and creating electromagnetic showers that can destroy electronics and/or demagnetize permanent magnet blocks in undulators In all these cases, the collimators are designed to restrict the vacuum chamber physical aperture without affecting the main beam. Depending on the bunch charge, the bunch length, and the collimator geometry, a small aperture could generate significant transverse wakefields that impart a kick to the beam. The kick factor was measured by determining the beam deflection angle induced by the collimator as a function of the beam lateral displacement These studies, did not examine the collimator wakefield effects upon the beam emittance nor the energy acceptance of the line.
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