An electron beam experiences chromatic emittance growth in a plasma-based accelerator if it is not matched to the focusing force in the plasma wake. A ramped plasma density profile at the entrance and exit of the plasma source can control the focusing of the beam into and out of the plasma accelerator, limiting emittance growth. Here, we present a comprehensive, analytic theory to describe the transverse beam dynamics and emittance growth in a nearly arbitrary plasma ramp profile. For a given incoming beam, this theory can be used to determine the length of the ramp required to correctly focus the electron beam, the optimal location of the beam's vacuum focus, and the chromatic emittance growth in the ramp. In addition, the theory can be used to determine the effect that errors in the beam focusing and plasma profile have on the emittance of the beam. We illustrate two example ramps to demonstrate the theory: one that provides very fast focusing for beam matching, and one that is robust to errors in the plasma density profile.
We present a laser-ionized, beam-driven, passive thin plasma lens that operates in the nonlinear blowout regime. This thin plasma lens provides axisymmetric focusing for relativistic electron beams at strengths unobtainable by magnetic devices. It is tunable, compact, and it imparts little to no spherical aberrations. The combination of these features make it more attractive than other types of plasma lenses for highly divergent beams. A case study is built on beam matching into a plasma wakefield accelerator at SLAC National Accelerator Laboratory's FACET-II facility. Detailed simulations show that a thin plasma lens formed by laser ionization of a gas jet reduces the electron beam's waist beta function to half of the minimum value achievable by the FACET-II final focus magnets alone.
A current challenge that is facing the plasma wakefield accelerator (PWFA) community is transverse beam emittance preservation. This can be achieved by balancing the natural divergence of the beam against the strong focusing force provided by the PWFA plasma source in a scheme referred to as beam matching. One method to accomplish beam matching is through the gradual focusing of a beam with a plasma density ramp leading into the bulk plasma. Here, the beam dynamics in a Gaussian plasma density ramp are considered, and an empirical formula is identified that gives the ramp length and beam vacuum waist location needed to achieve near-perfect matching. The method uses only the beam vacuum waist beta function as an input. Numerical studies show that the Gaussian ramp focusing formula is robust for beta function demagnification factors spanning more than an order of magnitude with experimentally favourable tolerances for future PWFA research facilities.
This article is part of the Theo Murphy meeting issue ‘Directions in particle beam-driven plasma wakefield acceleration’.
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