Provision for Timing Mode Users at Multi-Bend Achromat Synchrotron Light Sources

A step-change in the design of electron storage-rings for synchrotron light sources is currently underway. The latest structures consist of cells of magnets with multiple bending magnets which reduce the electron beam emittance by an order of magnitude and provide a corresponding increase in the source brightness. One consequence of these designs is a substantial reduction in the electron beam lifetime. This is typically compensated for by using harmonic RF cavities to stretch the electron bunches and reduce the particle density. One drawback however is that the emitted x-ray pulses become longer than in previous designs, and the longitudinal dynamics of the circulating electron bunches become more complex. Within this context, the aim of the project is to study how best to meet the needs of timing-mode users, taking the Diamond-II storage ring as an example.

Three potential methods are proposed for study. The first is to use a hybrid filling pattern, in which the ring is filled with one long train of electron bunches to provide the light for the majority of users, and a single electron bunch in a gap for those studying time-dependent phenomenon. The second would be to use 'pulse-picking by resonant excitation', in which fast magnets are used to excite vertical oscillations in a single electron bunch. The light from this bunch can then be spatially separated from the remainder for use in timing experiments. The final method would be to use the harmonic cavity to compress the electron bunches rather than stretch them. This would reduce the x-ray pulse length, improving the temporal resolution for users. For each method, particle tracking studies are required to investigate how the electrons react to the applied conditions in order to determine the equilibrium bunch properties and maximum charge before it becomes unstable.