Front-to-end simulations of the design of a laser wakefield accelerator with external injection

Abstract

We report the design of a laser wakefield accelerator (LWA) with external injection by a rf photogun and acceleration by a linear wakefield in a capillary discharge channel. The design process is complex due to the large number of intricately coupled free parameters. To alleviate this problem, we performed front-to-end simulations of the complete system. The tool we used was the general particle-tracking code, extended with a module representing the linear wakefield by a two-dimensional traveling wave with appropriate wavelength and amplitude. Given the limitations of existing technology for the longest discharge plasma wavelength (∼50μm) and shortest electron bunch length (∼100μm), we studied the regime in which the wakefield acts as slicer and buncher, while rejecting a large fraction of the injected bunch. The optimized parameters for the injected bunch are 10pC, 300fs at 6.7MeV, to be injected into a 70mm long channel at a plasma density of 7×1023m−3. A linear wakefield is generated by a 2TW laser focused to 30μm. The simulations predict an accelerated output of 0.6pC, 10fs bunches at 90MeV, with energy spread below 10%. The design is currently being implemented. The design process also led to an important conclusion: output specifications directly comparable to those reported recently from “laser-into-gas jet” experiments are feasible, provided the performance of the rf photogun is considerably enhanced. The paper outlines a photogun design providing such a performance level.

Parameter study of acceleration of externally injected electrons in the linear laser wakefield regime

“…This allows the use of nonidealized electron bunches and evaluation of the effect of the injection beamline on the final electron bunch, as was shown in Ref. 7.…”

“…22,23 It used to be thought that such bunches would be necessary for the generation of ͑quasi-͒monoenergetic electrons bunches. However, recently, simulations and theory have been published 7,11,24 showing that bunches significantly longer than the plasma wavelength can still lead to the generation of electrons with low energy spread and a bunch length on the order of 10 fs. In this scenario, the plasma wave acts as a slicer and as a compressor in addition to an accelerator, as can be seen in Fig.…”

“…In reality the emittance of the bunch and the space-charge forces inside the bunch will limit the minimum size to which the electrons can be focused. Based on previous investigations of realistic beamlines, 7,24 it can be shown that the electrons can be focused down to at least 30 m. That is the value will be assumed for the full width at half maximum ͑FWHM͒ of the Gaussian radial distribution so that both laser and electron bunch have the same spot size. The results are representative for other electron beam radii as long as the laser spot size and laser power are adjusted to maintain the same laser intensity and relative spot sizes compared to the ones presented here.…”

“…Simulations predict that this length is on the order of a few to ten femtoseconds, 3,[5][6][7] below the current electron bunch length measurement lower limit of 50 fs. 8 The generation of electron bunches with such short duration opens up new applications in fields such as coherent THz generation 9 and compact, femtosecond freeelectron lasers.…”

“…A previous article 7 has already focused on the specific design and optimization of an injector beamline for injecting electrons into a low amplitude wakefield. This article first describes the framework and simulation methods used to obtain the results.…”