Final focus system for injection into a laser plasma accelerator

ARES is an electron linear accelerator at the SINBAD facility at DESY. It aims to deliver reliable high-brightness beams with an energy in the range of 100 to 150 MeV with fs to sub-fs bunch lengths. This is ideal for injection into novel high-gradient acceleration devices, such as dielectric laser accelerators and laser-plasma accelerators (LPAs), which feature fields with fs to ps period. Here, we report the conceptual design of a final focus system for injecting into an LPA experiment at ARES. The design includes permanent magnetic quadrupoles (PMQ), sufficient longitudinal space for collinear laser and electron transport, space for required diagnostics and an LPA setup. The performance of the design is evaluated by means of start-to-end simulations of the linac, focusing system, and injection into the LPA, including sensitivity studies to relevant error sources.

Figure 1

Schematic of the ARES beam line including injection into an LPA (not to scale).

Figure 2

Evolution of the laser beam along the direction of beam propagation. The origin here is set at the focal point of the laser beam which is the entrance position of the plasma cell. A Gaussian laser beam is assumed.

Figure 3

The Twiss parameter ${\beta }_{x,y}$ and ${\alpha }_{x,y}$ and dispersion ${\eta }_x$ evolution is illustrated along the length of the linac. The BC terminates at $z\sim 31.5\mathrm{m}$, the plasma is anticipated to be located at $z\sim 35\mathrm{m}$.

Figure 4

Laser and electron beam propagation through the PMQ triplet. The laser and electron beam is plotted from the position of laser in-coupling mirror (2.8 m behind the plasma cell) until the entrance of the plasma cell. To the scale dimension and position of optimized quadrupole triplet are also shown.

Figure 5

Evolution of the electron beam parameters, simulated in astra, along the beam line from the exit of the BC until the entrance of the plasma cell. The origin here is set at the exit of the last magnet of the BC. The parameters shown are the Twiss parameters in (a) ${\beta }_{x,y}$ and in (b) ${\alpha }_{x,y}$, in (c) the normalized transverse emittance ${ϵ}_{x,y}$, in (d) the rms transverse beam sizes ${\sigma }_{x,y}$, in (e) the energy spread ${\sigma }_E$ and in (f) the bunch length ${\sigma }_z$.

Figure 6

Evolution of the transverse and longitudinal phase spaces, simulated in astra, at (a) the BC exit and matched beam at (b) the entrance of the plasma cell. Color scales indicate normalized electron density.

Figure 7

Variation in observed beam parameters at the plasma entrance for the two error scenarios defined in the text (mismatch cases 1 and 2). 10,000 random cases are simulated for each case.

Figure 8

Simulated evolution of beam parameters through an LPA plasma cell, using fbpic code, in the ARES linac for the matched beam and the mismatched case 1 beam considered in this study.

Figure 9

Phase spaces at the exit of the plasma cell for (a) simulated beam matched to the plasma entrance (reference case from Sec. 5) (b) perturbed beam, according to mismatch case 1 of Fig. 7. The beam is tracked through the plasma cell using fbpic code.