PSI, Villigen
The SwissFEL requires an emittance in the range from 0.18 to 0.43 mm mrad. To achieve this ambitious goal we are developing a wavelength-tunable laser system providing powerful UV pulses with arbitrary temporal shape in the range of 3-10 ps. The system should allow exact matching of the photon energy to the work function of the cathode material and consequently the reduction of the thermal emittance. Transverse and longitudinal laser pulse shaping is foreseen to minimize nonlinear space charge forces in the electron bunch to maintain lowest emittance during acceleration. In this paper we present the design and concept of this novel laser system and show first experimental results.
LOA, Palaiseau
GoLP, Lisbon
PSI, Villigen
Free-electron lasers have been recently evolving very fast in the extreme-ultraviolet to soft X-ray region. Once seeded with high harmonics, these schemes are considered as next generation soft X-ray light sources delivering ultrashort pulses with high temporal and spatial coherence. Here we present a detailed experimental study of a kHz two-colour (fundamental + second harmonic) high harmonic generation and investigate its potential as a suitable evolution of the actual seeding sources. It turns out that this source (both odd and even harmonics) is highly tuneable, and delivers intense radiations with only one order of magnitude difference in the photon yield from 65 nm to 13 nm. We also observed an astonishing aberration-free character of these harmonics (aberration below λ/17 rms at 44 nm). Finally, the variable linear polarization of the harmonics was revealed to be easily controllable with the generation conditions. Then, the implementation of this technique on seeded FELs would allow amplifications, with perfect beam quality, to be achieved at wavelengths shorter than previously accessible.
PSI, Villigen
In the frame of the SwissFEL project, an electron gun based on diode acceleration followed by a two cell RF cavity is under test at PSI. The diode consists of a photocathode / anode assembly and is driven with a voltage pulse of 500 kV maximum in 200 ns FWHM . The metal photocathode is illuminated by a Nd:YLF laser operating at 262 nm wavelength with a pulselength of 10 to 35 ps (FWHM) producing electron bunches of up to 200 pC. The distance from cathode to anode can be varied from 0 to 30 millimeters with a typical cathode field of 50 MV/m during the commissioning phase. Electrons leave the diode through an anode aperture and enter a two cells RF Cavity (1.5 GHz), which accelerates the beam to a maximum energy of 5 MeV. Beam characteristic measurements are presented and compared with simulations.