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abstract:
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We propose a mechanism for the generation of ultrashort (sub-ps to a few ps) relativistic electron bunches having a ramp-shaped current profile that rises gradually from the head to the tail, followed by a sharp cutoff. This type of current distribution approximates the idealized profile predicted by linear 1D and nonlinear 2D calculations to be optimal for driving large-amplitude wakefields in a plasma wakefield accelerator (PWFA). The proposed scheme utilizes a dogleg beamline (or dispersionless translating section) as a bunch compressor to impart a linear negative longitudinal dispersion transformation on the longitudinal phase space of a beam that is initially chirped in energy versus longitudinal position within the bunch. A theory to describe this mechanism is derived using first and second-order transport matrix theory. The theoretical results, combined with simulations using the particle transport codes ELEGANT and PARMELA, indicate that sextupole corrector magnets are required in order to cancel out nonlinear chromatic effects which tend to otherwise disrupt the ramped shape of the current distribution.
To provide a venue for a proof-of-principle experiment to test these predictions, a dogleg beamline was designed and built at the UCLA Neptune accelerator laboratory. A standing wave deflecting cavity was designed and built as a temporal diagnostic for measuring the current profiles of the electron bunches after passing through the dogleg. Second-order horizontal dispersion measurements and coherent transition radiation bunch length measurements of the electron beam after passing through the dogleg show good agreement with the predictions of theory and simulation.
Deflecting cavity measurements were conducted to directly measure the current profiles of both compressed and uncompressed electron bunches, with and without sextupole correction. The uncompressed bunch is found to have an asymmetrical (non-Gaussian) shape, presumably inherited from the pulse shape of the photoinjector drive laser. Results for an initially chirped beam show that ramp-shaped bunches can be produced by the proposed method, although due to the asymmetric initial (i.e. pre-compression) current profile of the electron bunches produced by the Neptune photoinjector, it is found to be necessary to overcompensate with the sextupole magnets in order to achieve a ramp-shaped profile.
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