The COXINEL line has been designed at Synchrotron SOLEIL for electron beam manipulation in view of a seeded free electron laser using Laser plasma acceleration (LPA). After first studies on electron beam transport and undulator radiation in the spontaneous emission regime using LPA from Laboratoire d’Optique Appliquée (Ecole Polytechnique, France), the line has been moved to the HZDR, Dresden, Germany, for high quality LPA electrons driven by the DRACO laser. We report here on the demonstration of a seeded FEL at 275 nm driven by the HZDR LPA.

We report on the operation of the DRACO Laser Driven electron source for stable multi-day operation for FEL applications. The nC-class accelerator delivers charge densities around 10 pC/MeV , <1 mrad rms divergence at energies up to 0.5 GeV and peak currents of over 10 kA [1]. Precise characterisation is paramount for controlled operation, including: spectrally resolved charge diagnostic, coherent optical transition radiation (TR) to resolve microbunch beam structures [2] and TR-based multioctave high-dynamic range spectrometry for sub-fs resolved characterisation of the 10 fs rms electron bunches [3]. Achieved stability allows for systematic exploration of demanding applications, resulting in the recent demonstration of the first LWFA based Beam-driven Plasma Wakefield Accelerator [4]. Fulfilling the high demands required for FEL operation, the COXINEL manipulation line developed at Synchotron SOLEIL has recently been installed at our facility. Based on successful beam transport of over 13000 shots within 9 experimental days during commissioning, we were able to demonstrate the very first operation of a seeded FEL driven by a laser plasma accelerator [5].

We report the first lasing of a seeded FEL fully driven by a laser plasma accelerator. The experiment was performed at HZDR (Germany), coupling the high quality electron beams of the HZDR laser plasma accelerator with the versatile COXINEL beam manipulation line. Using an external seed at 270 nm, the FEL signal was observed at 275 nm. We explain how this slight red-shift confirms previous predictions [1], show the precise control over the FEL wavelength and give evidence of the longitudinal coherence of the emitted pulses. All experimental results are strongly supported by analytic modeling and Genesis numerical simulations. Our results substantiate the continuous progress of LPA technology to enable FEL operation and finally bring temporal coherence to those compact promising sources. [1] M Labat et al 2020 New J. Phys. 22 013051.