X-2: Plasma Booster

Principle Investigator:
Carl Andreas Lindstroem

Contact in absentia:
Sarah Schröder

Description and goals:
In order to utilise beam-driven plasma-wakefield acceleration for high-energy applications,
an external injection scheme must be used. This scheme, in contrast to internal
injection, requires two bunches to be injected into plasma: the front bunch (driver) creates
the wakefield, while the trailing bunch (witness) is accelerated by this wakefield. The
energy-depleted driver is dumped after it exits the plasma, while the witness beam
propagates further.

Witness beam quality preservation in the external injection scheme is the key
requirement for usability in practical applications. The main goal of the FLASHForward X-2
experiment is therefore to demonstrate emittance and energy spread preservation, in a
stable and reproducible way, in the external injection scheme.

For this experiment, electron beams from the FLASH free-electron laser with 1-3 μm
emittance and ~1% energy spread are split with a metallic mask (scraper) in a dispersive
section of FLASHForward to form a driver-witness pair. Care has to be taken to generate
the suitable length of the bunches and a separation between them to match the plasma
density. Transverse phase-space optimisation is important for emittance preservation
(matching), while tuning the longitudinal phase-space of the witness beam is important for
energy spread preservation through beam-loading.

Key milestones

  • Generation of driver-witness pairs using a mask in the dispersive section (scraper commissioning)
  • Acceleration of the witness beam and depletion of the drive beam
  • Preservation of the witness beam emittance
  • Preservation of the witness beam energy-spread
  • Optimisation of the energy gain (transformer-ratio, overlaps with X-10 Experiment)