Abstract
A general architecture of an emittance exchanger (EEX) is considered, where the horizontal and longitudinal phase spaces are exchanged. A family of designs is described which can lead to extremely short final longitudinal lengths, even subfemtosecond. Using higher-order particle simulations, a preferred configuration is found, which has better compression capability and less emittance growth than the standard EEX design at high beam energy. An alternative design is also found which eliminates any final energy-phase coupling. Features of compression using an EEX are significantly different than with a chicane because the final longitudinal phase space is decoupled from the initial longitudinal phase space. Advantages of using an EEX for compression include less susceptibility to the coherent synchrotron radiation (CSR) microbunch instability, less susceptibility to bunch length broadening from CSR effects, and elimination of the initial energy-phase correlation that is needed for compression using a chicane as well as any residual energy-phase correlation after compression. A key disadvantage of using an EEX is that the final horizontal emittance tends to strongly depend on the initial bunch length and beam energy.
Highlights
The emittance exchanger (EEX) is a remarkable example of the conservation of eigenemittances [1]
The lowest emittance EEX architecture has some modest final longitudinal correlation, but even that can be made to vanish by using an alternative geometry with only minor degradation in other performance
One disadvantage of using an EEX for compression is that the final bunch slice transverse emittances have grown to the size of the final rms emittance, in direct contrast to compression in a chicane where the slice emittance is nearly preserved through the compression process
Summary
The emittance exchanger (EEX) is a remarkable example of the conservation of eigenemittances [1]. All this work employed the form of the EEX described in [4] which includes two standard doglegs in the same orientation It was recognized in [4] that second-order dispersion can lead to growth of the final horizontal emittance, and that can be minimized by employing an initial energy chirp to the beam (which is used to minimize the effect of the thickness of the rf cavity). Because of the exchange between the horizontal and longitudinal phase spaces, the final compression only depends on the beam’s initial transverse parameters, in a linear sense This is a significant and very attractive difference, because (1) the final bunch length can be tailored with just transverse optics before the EEX without needing rf cavities, and (2) the final particle axial positions and energies can be made uncorrelated, making compression at higher energy. We consider optics that are not constrained by overall length
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