Abstract
Coherent synchrotron radiation (CSR) is a collective effect that mainly occurs when the trajectory of an electron beam is bent in a dipole magnet. It affects the electron beam by distorting the phase space along its slice distribution, which leads to emittance growth. Therefore, CSR should be suppressed to transport electron beams without further degradation of the emittance. In linear optics, CSR-induced emittance can be suppressed by controlling the Twiss parameters along the electron-beam transfer line. However, owing to some physical constraints, transfer-line optics may be governed by higher-order terms in the transfer map, and the use of a sextupole magnet to suppress these terms would be very challenging for low-energy-spread and low-emittance beams. Therefore, without using a sextupole magnet, we estimate the region of the Twiss parameters where the first-order terms are dominant along the transfer line by introducing chromatic amplitude. In this region, we can apply the suppression condition that is valid in a linear matrix system. This minimization of the emittance growth becomes even more important when the electron-beam transfer line is used for external injection into a plasma wakefield because mismatched beam conditions could induce an additional increase in the emittance during the acceleration. In this paper, we discuss a method of emittance-growth minimization driven by the CSR effect along the transfer line, which is particularly used for electron-beam injection into plasma wakefields. In addition, using the particle-in-cell simulation, we investigate the evolution of electron beam parameters during the acceleration through plasma wakefields in the presence of the CSR effect on the electron beam. We confirm that the beam emittance growth is minimized when the CSR effect is properly controlled. Otherwise, it is found that 11%--32% emittance growths by the CSR effect along the transfer line lead to additional 20%--40% increase of the maximum slice emittance.
Highlights
A low-emittance, low-energy-spread, short bunchlength, and high-peak-current electron beam is essential for high-quality x-ray generation [1,2] and future highenergy physics facilities such as electron–positron colliders [3,4]
IV, we introduce the chromatic amplitude and concatenation of the transfer map to find the source of the emittance growth from both higher-order terms and the coherent synchrotron radiation (CSR) effect
V, considering various conditions for an electron beam extracted from the achromatic dog-leg transfer line, we present the evolution of the emittance, slice distribution and longitudinal phase space during the acceleration through plasma wakefields in the presence of the CSR effect on the electron beam
Summary
A low-emittance, low-energy-spread, short bunchlength, and high-peak-current electron beam is essential for high-quality x-ray generation [1,2] and future highenergy physics facilities such as electron–positron colliders [3,4]. It was shown that the shielding of the radiation by installing metallic plates inside the dipole magnet or by controlling the longitudinal bunch shape can reduce the emittance growth driven by the CSR effect [15,16,17] Both measurements and particle tracking simulations have shown that the emittance growth is well suppressed by controlling the phase advance (optics balance) or by matching the linear Twiss parameters between dipole magnets (envelope matching) [18,19,20,21,22,23,24,25]. VI that the emittance growth can be minimized once the CSR effect is well suppressed before injection
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