Abstract
Synchrotron radiation produced as an electron beam passes through a bending magnet is a significant source of background in many experiments. Using modeling, we show that simple modifications of the magnet geometry can reduce this background by orders of magnitude in some circumstances. Specifically, we examine possible modifications of the four dipole magnets used in Jefferson Lab’s Hall A Compton polarimeter chicane. This Compton polarimeter has been a crucial part of experiments with polarized beams and the next generation of experiments will utilize increased beam energies, up to 11 GeV, requiring a corresponding increase in Compton dipole field to 1.5 T. In consequence, the synchrotron radiation (SR) from the dipole chicane will be greatly increased. Three possible modifications of the chicane dipoles are studied; each design moves about 2% of the integrated bending field to provide a gentle bend in critical regions along the beam trajectory which, in turn, greatly reduces the synchrotron radiation within the acceptance of the Compton polarimeter photon detector. Each of the modifications studied also softens the SR energy spectrum at the detector sufficiently to allow shielding with 5 mm of lead. Simulations show that these designs are each capable of reducing the background signal duemore » to SR by three orders of magnitude. The three designs considered vary in their need for vacuum vessel changes and in their effectiveness.« less
Highlights
The total energy of the synchrotron radiation produced by an electron following an arc of constant radius r as it is bent through an angle Δθ is
The synchrotron radiation has an angular dependence given by a leading term of the form 1⁄21 þ ðγθÞ2−3, where γ 1⁄4 E=ðmc2Þ is the Lorentz factor determined by the electron beam energy E and electron mass m
Because of the high relativistic γ, the simulated synchrotron radiation photons were modeled as being produced at zero degrees with respect to the electron trajectory
Summary
The total energy of the synchrotron radiation produced by an electron following an arc of constant radius r as it is bent through an angle Δθ is (see, for example, Ref. [1]). Planned Electron-Ion Collider, the magnet used to direct the electron beam towards the intersection point In some cases, such as the Compton polarimeter discussed below, the entrance to the first steering magnet after the target generates an additional source of synchrotron background. That factor combines with the additional factor of γ in (2) to give a γ4 dependence to the total synchrotron radiation power as expressed in (1) This can be mitigated by increasing the bending radius r by implementing a design which features a reduced local magnetic field in critical regions, as discussed below. While suitable low-field regions could, in some situations, be achieved by introducing additional dipoles, the implementation of the designs discussed below are relatively inexpensive, require no additional power supplies, fit into contained spaces, are customized to work around obstacles, and are selfscaling with the dipole field
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