Controlled plasma generation for beam driven plasma wakefield accelerators

Plasma-based wakefield acceleration is a promising approach in shrinking the size and cost of future particle accelerators and free-electron lasers. In the FLASHForward project, the ionisation of hydrogen with a laser creates a plasma inside which a wakefield is generated by an electron bunch of the FLASH accelerator. Disentangling these two processes enables improved control over plasma density profiles and therefore the structure of the wakefields. This has a crucial effect on the accelerated beams. In particular, the transverse focusing fields experienced by the beams are responsible for emittance evolution during acceleration. Simulations have shown that this focusing can be controlled by decoupling transverse and longitudinal fields using a hollow-core plasma channel. To precisely control the plasma generation we compute over-barrier strong-field ionization depending on the temporal evolution of the laser-intensity profile from a measured beam intensity and phase distribution. Various optics can be introduced into the beam path to affect laser-beam propagation. We plan to implement these concepts and realise different plasma shapes in proof of concept experiments.