Cancellation of Coherent Synchrotron Radiation Kicks with Optics Balance

TLDR

Minimizing transverse emittance is essential in linear accelerators designed to deliver very high‑brightness electron beams, and coherent synchrotron radiation (CSR) is a key contributor to emittance degradation. The study proposes canceling CSR‑induced emittance growth by imposing symmetric conditions on the electron transport system. The authors relate beam Courant‑Snyder parameters to emittance growth and present a general CSR suppression scheme using asymmetrically balanced optics along the line. Experimental results at FERMI@Elettra show that the proposed optics balance preserves the transverse emittance of a 500 pC, sub‑picosecond beam through an achromatic transfer line, with growth occurring only when the balance is broken, and the data agree with the theoretical model, indicating the feasibility of compact dispersive lines with large bending angles.

Abstract

Minimizing transverse emittance is essential in linear accelerators designed to deliver very high brightness electron beams. Emission of coherent synchrotron radiation (CSR), as a contributing factor to emittance degradation, is an important phenomenon to this respect. A manner in which to cancel this perturbation by imposing certain symmetric conditions on the electron transport system has been suggested.We first expand on this idea by quantitatively relating the beam Courant-Snyder parameters to the emittance growth and by providing a general scheme of CSR suppression with asymmetric optics, provided it is properly balanced along the line. We present the first experimental evidence of this cancellation with the resultant optics balance of multiple CSR kicks: the transverse emittance of a 500 pC, sub-picosecond, high brightness electron beam is being preserved after the passage through the achromatic transfer line of the FERMI@Elettra free electron laser, and emittance growth is observed when the optics balance is intentionally broken. We finally show the agreement between the theoretical model and the experimental results. This study holds the promise of compact dispersive lines with relatively large bending angles, thus reducing costs for future electron facilities.