Beam emittance preservation using Gaussian density ramps in a beam-driven plasma wakefield accelerator

Abstract

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’.

First emittance measurement of the beam-driven plasma wakefield accelerated electron beam

“…We also can see a clear oscillations of the witness envelope along the propagation distance as well as of its emittance. The growth of the emittance by approximately 37% is mainly due to an mismatched witness spot size [51]. In conclusion, in this work we presented the results of a beam driven PWFA experiments where the high-quality preformed witness beam was injected inside of the plasma and accelerated.…”

Chromatic Dynamics of an Electron Beam in a Plasma Based Accelerator

“…where η = n e /n eu , n eu is the uniform density in the bulk, E 1 η/∆ξ is the slope of the wakefield in the blowout regime [31,32] and E 0 √ η − 2η describes the variation in longitudinal phase and maximum wake amplitude with plasma density [42]. The fields can be approximated from first principles as…”

Laser-ionized, beam-driven, underdense, passive thin plasma lens

“…To demonstrate that a plasma lens can focus a beam for beam matching, we consider a PWFA with a 10 cm-long uniform density flat-top region of 3 × 10 16 cm −3 and Gaussian density ramps on either side with a half-width at half-maximum of σ hw ¼ 2.54 cm. For a 10 GeV electron beam to be matched into this plasma source, it must have a vacuum waist beta function of β Ã ¼ 2.5 cm at a position 4.55 cm before the start of the flattop [16].…”

“…Energy gain in the PWFA is modeled according to Ref. [16], and an increase of 1.72 GeV for the witness beam slightly diminishes the focusing strength of the exit lens with respect to its counterpart at the PWFA entrance. This can be compensated for by simply adjusting the thickness or position of the exit lens to achieve the desired value of β Ã f .…”

“…These magnets typically suffer the drawbacks of smaller apertures and fixed magnet strength; however, progress has been made in developing PMQs with variable focusing strength [11]. Another focusing scheme that is often discussed in the literature is the use of gradual plasma density ramps at the entrance and exit of the PWFA plasma source [8,9,[12][13][14][15][16][17]. This method should work in principle, but precise control over the plasma density profile is challenging, and beam matching has yet to be demonstrated using this technique.…”