Design of a prototype laser-plasma injector for an electron synchrotron

The present state of progress in laser wakefield acceleration encourages considering it as a practical alternative to conventional particle accelerators. A promising application would be to use a laser-plasma accelerator as an injector for a synchrotron light source. Yet, the energy spread and jitter of the laser-plasma beam pose a significant difficulty for an efficient injection. In this paper, we propose a design of a prototype injector to deliver 500 MeV low-intensity electron bunches to the DESY-II electron synchrotron. The design utilizes presently available conventional accelerator technology, such as a chicane and an X-band radio frequency cavity, to reduce the energy spread and jitter of the electron beam down to a sub-per-mille level.

Figure 1

Schematic of the laser-plasma injector prototype (a): the laser-plasma accelerator produces a 500 MeV beam with $\sim 1\%$ energy spread; a quadrupole triplet (red) captures the beam and matches it to a dispersive section (chicane), made up of dipoles (blue) and sextupoles (green), that corrects the chromatic emittance growth. The beam line continues with a chicane that induces a large longitudinal decompression and a linear energy-time correlation over the beam. This energy correlation is canceled out by a short X-band rf cavity (orange), compressing the energy output of the beam by a factor 160. Electron bunch at the plasma cell exit, simulated with the particle-in-cell code FBPIC (b) and after the beam line, simulated with particle tracking code Ocelot (c): longitudinal phase-space distribution (top); current profile, slice energy spread, and normalized emittances (bottom). $\zeta =z-ct$ is the comoving coordinate of the bunch. The energy spread of the beam is computed via the median absolute deviation (see Sec. 2).

Figure 2

Configuration of the plasma target providing optimized electrons beams at 500 MeV. The longitudinal distribution of the different gaseous species forming the target and the resulting plasma density are shown together with the laser spot size evolution in vacuum.

Figure 3

Beam line optics function (top) and rms beam sizes of a Gaussian beam (bottom). Dipoles are shown in blue, quadrupoles—red, sextupoles—green, X-band rf—orange, BPMs—black, a section for laser beam removal and diagnostics—pink. The last magnet, a Y-chamber dipole of the present electron beam line, is turned off during the injection from the LPA. ${\sigma }_{x,y}$ denote the beam sizes in the limit ${\sigma }_{\gamma }\to 0$.

Figure 4

Using sextupoles allows significantly reducing the unwanted chromatic emittance increase in the horizontal plane. Bunch phase space densities at the exit of the beam line with the chromatic sextupoles off and on.

Figure 5

Horizontal emittance after chromatic correction as a function of the initial energy spread (c). The S-chicane (b) while slightly longer preserves the emittance better than the C-chicane (a).

Figure 6

The active energy compensation scheme with an X-band dechirper efficiently reduces the energy spread of the LPA beam in the presence of energy jitter and chirp. Examples of initial longitudinal distributions when varying the central energy (a). Panel (b) shows the distribution of the bunch charge at the exit of the injector for 1000 simulated LPA beams with and rms central energy jitter of 1%; the initial bunch charge of 83 pC is shown in red. (c)—energy spectra at the exit of the plasma cell (top) and at the exit of the injector, after the energy compression (bottom); panels on the right show the average beam spectra. The dashed lines represent $\pm 1\%$ deviation from the design energy of 500 MeV. Panels (d,e,f) show the same results for a Gaussian spread of initial energy chirp. All simulation results include a Gaussian timing jitter between the LPA and the rf with an rms spread of 100 fs.