Demonstration of the Echo-Enabled Harmonic Generation Technique for Short-Wavelength Seeded Free Electron Lasers

Abstract

We report the first experimental demonstration of the echo-enabled harmonic generation (EEHG) technique which holds great promise for generation of high power, fully coherent short-wavelength radiation. In this experiment, coherent radiation at the 3rd and 4th harmonic of the second seed laser is generated from the so-called beam echo effect. The experiment confirms the physics behind this technique and paves the way for applying the EEHG technique for seeded x-ray free electron lasers.PACS numbers: 41.60.CrFree electron lasers (FELs) can provide high-intensity coherent short-wavelength radiation which is essential for the studies of molecular and atomic dynamics. In the xray wavelength, the two leading concepts are the selfamplified spontaneous emission (SASE) configuration [1,2] and the high-gain harmonic generation (HGHG) scheme [3,4]. One FEL working in the SASE mode has been successfully operated at hard x-ray wavelengths [5]. While the radiation from a SASE FEL has excellent transverse coherence, it typically has rather limited temporal coherence because a SASE FEL starts from electron beam shot noise. Alternatively, the HGHG scheme allows the generation of fully coherent radiation by upconverting the frequency of a high-power seed laser. However, due to the relatively low up-frequency conversion efficiency, multiple stages of HGHG FELs are needed [6] in order to generate coherent x-rays from a UV laser.The up-frequency conversion efficiency can be greatly improved with the recently proposed echo-enabled harmonic generation (EEHG) technique [7,8]. In the EEHG scheme, the beam is energy modulated by a laser with wave number k 1 in the first modulator and then sent through a chicane with strong dispersion after which the modulation obtained in the first modulator is macroscopically smeared. Simultaneously, complicated fine structures are introduced into the phase space of the beam. A second laser with wave number k 2 is used to further modulate the beam energy in the second modulator. After passing through a second chicane the echo signal then occurs at the wave number k E = nk 1 + mk 2 as a recoherence effect, where n and m are integers. The main advantage of EEHG is that the bunching factor is a very slowly decaying function of the harmonic number, thus allowing the generation of coherent soft x-ray radiation directly from a UV seed laser in a single stage.The remarkable up-frequency conversion efficiency of the EEHG technique has stimulated world-wide interest in using EEHG to achieve fully coherent radiation in the x-ray wavelength from UV seed lasers [9][10][11]. While significantly relaxing the requirements on laser power and beam slice energy spread as compared to the HGHG scheme, EEHG requires more challenging control of the beam dynamics as the beam goes through the undulators and chicanes, because it involves a long-term memory of the beam phase space correlations.In this Letter we report the first experimental demonstration of the EEHG technique at the Next Linear Collider Test Accelerator (NLCTA)...

Coherent optical vortices from relativistic electron beams

“…Accelerator (NLCTA) [24,25]. The initially unmodulated 120 MeV e-beam (γ = 235, 0.5 picosecond fwhm duration) interacts with the λ =800 nm transversely Gaussian laser pulse in the helical undulator (modulator) with periodicity λ u = 5.26 cm and strength K = 1.52.…”

Subradiant spontaneous undulator emission through collective suppression of shot noise

“…Experimental work was carried out at the Next Linear Collider Test Accelerator (NLCTA) beam line [18,30] using the parameters of Table I. Figure 1 shows a schematic of the experiment.…”

“…Following recent shortwavelength seeding demonstrations [14] there is increasing interest in pushing seeding to the x-ray regime. Promising seeding schemes including high gain harmonic generation [15,16], echo-enabled harmonic generation [17,18], and direct seeding with high harmonic generation [19] are all limited by the need for a high-power, short-wavelength seed that can outcompete the self-amplified spontaneous emission (SASE) [20] process in the startup of an FEL [21]. However, subradiance has the potential to extend the shortwavelength limit; spontaneous undulator radiation competes with the external seed laser to kick-start the FEL process, so decreasing the spontaneous radiation reduces the required seed power and opens the possibility of seeding at shorter wavelengths.…”

Much Ado about Microbunching: Coherent Bunching in High Brightness Electron Beams

“…Multiple-stage HGHG can reach shorter wavelengths, but is more technically challenging. A recent and promising seeding scheme is echo enabled harmonic generation (EEHG), which manipulates hidden structure in phase space to produce high harmonic seeding [29,30].…”