CSR effect and mitigation

When a high-brightness electron beam bends through a magnetic field, it can emit coherent synchrotron radiation, or CSR. This radiation can disturb the beam itself, increasing energy spread and degrading the beam quality.

CSR is especially important in beamlines that compress or strongly bend short, high-charge beams. Understanding and controlling this effect is necessary for preserving beam brightness in advanced accelerators.

This research studies CSR through theory, simulation, and experiment, with emphasis on mitigation methods such as parallel-plate shielding and tailored beam shaping. The goal is to reduce CSR-induced beam degradation while maintaining the beam structures needed for accelerator applications.

To study CSR and its mitigation, we use a reversed-chicane beamline equipped with dipole chambers that have tunable shielding gaps. When a short, high-charge electron beam bends through the dipoles, it emits coherent synchrotron radiation. The adjustable gap allows us to control how strongly the surrounding conducting plates shield the CSR field.

The key idea is to measure how the CSR wake modifies the beam itself. As the beam travels through the bend, the CSR field generated by the head of the bunch can interact with particles behind it. This interaction changes the beam’s longitudinal phase space, especially its energy distribution along the bunch. By measuring this final longitudinal phase space, we can infer the strength and structure of the CSR wakefield.

For this purpose, we use a longitudinal phase-space diagnostic based on a transverse deflecting cavity and a spectrometer. The transverse deflecting cavity maps time onto transverse position, while the spectrometer maps energy onto another transverse direction. Together, they provide a two-dimensional image of the beam’s longitudinal phase space after CSR interaction.

The measured phase-space images show how the beam changes for different shielding gaps. By comparing cases such as 3 cm, 2 cm, and 1 cm gaps, we can directly study how shielding modifies the CSR-induced energy modulation. These measurements provide experimental information on CSR suppression and help benchmark simulation tools used to design high-brightness accelerator beamlines.