X-13: Longitudinal Phase-Space Manipulation

Principle Investigator:
Richard D'Arcy

Contact in absentia:
Stephan Wesch

Description and goals:
The large negative energy chirps characteristic of witness bunches accelerated using selfgenerated
longitudinal wakefields in plasma are potentially harmful for beam quality preservation.
For example, chromaticity effects when focussing/defocussing in quadrupoles with large chirps
would lead to emittance blowup, limiting the potential impact these types of bunches could have
on future applications e.g. in photon science for free-electron laser (FEL) operation. It is therefore
essential to mitigate these chirp using a method with sufficiently high gradients.

The longitudinal electric field component seen by a short drive beam is decelerating and of the
opposite sign as the accelerating field in which a witness beam may be placed. This offers the
possibility to mitigate the accumulated energy chirp of a witness beam by making the witness
beam itself drive a plasma wave. The resulting wakefields force the tail of the wake-driving
witness beam to lose energy, while the head remains unaffected. In the case that the witness
beam is much shorter than the drive beam and of sufficient current density to drive a non-linear
wake, the plasma density can be adjusted at the end of the witness acceleration section to bring
the drive beam out of resonance and allow the witness beam to drive a wake for self-dechirping.

The FLASHForward X-13 experiment intends to demonstrate this process of dechirping in plasma
by a) generating a pseudo-witness bunch from the FLASH linac manipulated by the accelerating
cavities and bunch compressors to have a percentage-level energy chirp, then b) generating a
witness bunch in plasma and manufacturing a plasma cell with a high density section at the
downstream end to de-chirp this internally injected bunch.