Abstract
Short period, high field undulators can enable short wavelength free electron lasers (FELs) at low beam energy, with decreased gain length, thus allowing much more compact and less costly FEL systems. We describe an ongoing initiative to develop such an undulator based on an approach that utilizes novel cryogenic materials. While this effort was begun in the context of extending the photon energy regime of a laser-plasma accelerator based electron source, we consider here implications of its application to sub-fs scenarios in which more conventional injectors are employed. The use of such low-charge, ultrashort beams, which has recently been proposed as a method of obtaining single-spike performance in x-ray FELs, is seen in simulation to give unprecedented beam brightness. This brightness, when considered in tandem with short wavelength, high field undulators, enables extremely high performance FELs. Two examples discussed in this paper illustrate this point well. The first is the use of the SPARX injector at 2.1 GeV with 1 pC of charge to give 8 GW peak power in a single spike at 6.5 \AA{} with a photon beam peak brightness greater than ${10}^{35}\text{ }\text{ }\mathrm{photons}/(\mathrm{s}\text{ }{\mathrm{mm}}^{2}\text{ }{\mathrm{mrad}}^{2}\text{ }\text{ }0.1%\text{ }\text{ }\mathrm{BW})$, which will also reach LCLS wavelengths on the 5th harmonic. The second is the exploitation of the LCLS injector with 0.25 pC, 150 as pulses to lase at 1.5 \AA{} using only 4.5 GeV energy; beyond this possibility, we present start-to-end simulations of lasing at unprecedented short wavelength, 0.15 \AA{}, using 13.65 GeV LCLS design energy.
Highlights
The x-ray free electron laser based on self-amplified spontaneous emission (SASE free electron lasers (FELs) [1]) is a state-of-theart instrument for scientific research with enormous potential and an impressive footprint, both in physical scale and cost
The cost and complexity of such extensive scientific infrastructure limits the diffusion of this revolutionary 4th generation light source, which promises to allow fundamental investigations of matter at the length and time scale of Angstroms and femtoseconds
The lengths of the LCLS and X-FEL undulators are well over 100 meters [2,3]; the multi-GeV electron injectors are measured in km
Summary
The x-ray free electron laser based on self-amplified spontaneous emission (SASE FEL [1]) is a state-of-theart instrument for scientific research with enormous potential and an impressive footprint, both in physical scale and cost. In one example presented in this paper, we show promising operation of a SASE FEL using the LCLS injector in low-charge mode that yields lasing at 0.15 A This represents a leap of 1 order of magnitude in photon energy to 83 keV, a coherent x-ray source of unparalleled capabilities in probing dense, high-Z materials. It should be noted that this design was stimulated first by application to a laser wakefield accelerator (LWFA)-driven FEL at MPQGarching [11] This initiative, in which one is aggressively applying the principle of making the injector compact ( $ cm), one is presently limited in beam energy to $1 GeV [12].
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