Electron and photon diagnostics for plasma acceleration-based FELs

Abstract

It is now well established that laser plasma acceleration (LPA) is an innovative and good candidate in the beam acceleration field. Relativistic beams are indeed produced up to several GeV but their quality remains to be demonstrated in the highly demanding case of Free Electron Lasers (FELs). Several experiments already showed the feasibility of synchrotron radiation delivery based on a LPA but free electron lasing has still to be achieved. Since the quality of the LPA beam inside the undulator is the critical issue, any LPA based FEL experiment requires a refined characterization of the beam properties along the transport line and of the photon beam at the undulator exit. This characterization relies on diagnostics which must be adapted to the LPA specificities. We will review the electron and photon diagnostics already used on LPAs and required for LPA based FELs, and illustrate the critical points using recent experiments performed around the world.

Determining the duration of an ultra-intense laser pulse directly in its focus

“…One possible solution is quantifying the emission patterns of electrons scattered from the laser focus, giving access to a direct determination of its carrier-envelope phase 35,36 and intensity 18 , with the latter already implemented experimentally 37 . This latter experiment is in line with a series of recent experiments on laser-electron accelerators 38–40 and all-optical radiation sources 4,5,41–43 , for which there exists an abundance of refined detectors for both the electrons and emitted radiation 44 . These schemes are based on the interaction of electrons (mass and charge m and e < 0, respectively) with initial momenta , where c is the speed of light, with an ultra-intense laser pulse of peak electric field E 0 and central frequency ω 0 , corresponding to a wavelength .…”

Control of laser plasma accelerated electrons for light sources

“…The second set of quadrupoles matches the beam inside an in-vacuum undulator that creates a periodic magnetic field (period λ u = 18 mm, period number N u = 107). Several scintillator screens can be inserted along the line to image the electron beam in the transverse plane 45 . Transfer line components and LWFA laser are aligned within ±100 μm on the same reference axis using a laser tracker.…”

“…For beam charge measurements, the line is equipped with two Turbo Integrating Current Transformers (T-ICT) from Bergoz (specified for 10 fC noise), one after the electron generation chamber and a second one at the undulator exit 45 , and with two cavity beam position monitors (from SwissFEL 58 ) on both sides of the undulator, operating in charge mode. The screen of the imager located after the undulator has been calibrated with the measurement performed with the second ICT, which has been calibrated by the supplier BERGOZ, leading to a conversion of 1.5 × 10 7 counts pC −1 45 . The agreement with the absolute calibration 59 – 61 , is found within a factor 2.…”

Free electron lasers driven by plasma accelerators: status and near-term prospects

“…The COXINEL line has been designed with baseline reference parameters (1 ·mm·mrad total normalized RMS emittance, 1 mrad RMS divergence, RMS relative energy spread with a 1 RMS bunch length, 34 pC charge, and 4 kA peak current for 180–400 MeV). In the seeded configuration, it can lead to spectral interference between the coherently emitted radiation and the input seed, allowing for a full temporal reconstruction of the FEL pulse amplitude and phase distributions [ 50 ] . Presently, the main limitation for the FEL demonstration comes from the achieved electron beam parameters, as confirmed by sensitivity studies [ 51 ] .…”

“…Undulator radiation is generated in the UV and VUV bands thanks to cryogenic permanent magnet undulators (the 2 m long CPMU18 operated at room temperature and the 3 m long CPMU15) [ 41 – 43 ] . The beamline is fully equipped with diagnostics such as current transformers, cavity beam position monitors, and imaging scintillator screens [ 44 ] . The laser-plasma accelerator (LPA) operated in the robust ionization injection regime [ 45 , 46 ] with a supersonic jet of He-N gas mixture, providing electrons with energies up to 250 MeV, 0.5 pC/MeV charge density, and few millirad divergence (1.2–2 mrad root mean square (RMS)).…”