The laser–plasma wakefield accelerator (LWFA)[1] produces high quality relativistic electron beams with energies currently ranging from 100's of MeV to several GeV by exploiting the large electric field gradients generated by intense laser pulses interacting with plasma [2–7]. Potential applications of LWFAs include drivers of compact synchrotron sources [8, 9], which have been demonstrated first in the visible [10] and then in the vacuum-ultraviolet [11] and at gamma ray energies [12]. The accelerating structure of the LWFA operating in the nonlinear blowout regime, consists of a string of ion 'bubbles' of evacuated regions of plasma created by the combination of the ponderomotive force of an intense, ultra-short, laser pulse and the electrostatic restoring force of background ions acting on plasma electrons. Electrons can be injected into the LWFA structure, if their velocity exceeds that of the bubble, from either an external injector, from the background plasma, or from further ionized gas. High brightness beams with narrow energy spreads % [13], geometric transverse emittance of mm mrad [14–16] and ultra-short duration fs [17] have recently been demonstrated. The highest quality beams with bunch charges of 1–20 pC are produced close to threshold for injection [13, 18]. These attractive parameters make the LWFA a suitable candidate for driving a compact free-electron laser (FEL) [8, 9]. FELs require beams of low emittance, narrow energy spread and high peak current electron bunches [19], which are possible when the bunch duration is short.
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