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Register a new userTunable High Gradient Quadrupoles For A Laser Plasma Acceleration Based FEL
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Laser Plasma Acceleration (LPA) is capable of producing a GeV beam within a cm accelerating distance, but with a rather high initial divergence and large energy spread. COXINEL aims to demonstrate a compact Free Electron Laser using such a source, where a specific transport line with adequate elements is used, such as tunable high gradient quadrupoles for handling the divergence. An innovative permanent magnet based quadrupole (QUAPEVA) made of two quadrupoles superimposed capable of generating a gradient of 200 T/m is presented. The first quadrupole consists of magnets shaped as a ring and attaining a constant gradient of 155 T/m, and the second one made of four cylindrical magnets surrounding the ring and capable of rotating around their axis to achieve a gradient tunability of $pm$ 46 T/m. Each tuning magnet is connected to a motor and controlled independently, enabling the gradient to be tuned with a rather good magnetic center stability ($pm$10 $mu$m) and without any field asymmetry. The measurements and field optimization of seven quadrupoles with different magnetic lengths are reported. A set of QUAPEVA triplet, installed at COXINEL, achieved good focusing and enabled beam based alignment.
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Laser Plasma Accelerators (LPA) can sustain GeV/m accelerating fields offering outstanding new possibilities for compact applications. Despite the impressive recent developments, the LPA beam quality is still significantly lower than in the conventional radio-frequency accelerators, which is an issue in the cases of demanding applications such as Free Electron Lasers (FELs). If the electron beam duration is below few tens of femtosecond keeping pC charges, the mrad level divergence and few percent energy spread are particularly limiting. Several concepts of transfer line were proposed to mitigate those intrinsic properties targetting undulator radiation applications. We study here the robustness of the chromatic matching strategy for FEL amplification at 200~nm in a dedicated transport line, and analyze its sensitivity to several parameters. We consider not only the possible LPA source jitters, but also various realistic defaults of the equipment such as magnetic elements misalignements or focussing strength errors, unperfect undulator fields, etc...
We are proposing a facility based on high gradient acceleration via x-band RF structures and plasma acceleration. We plan to reach an electron energy of the order of 1 GeV, suitable to drive a Free Electron Laser for applications in the so called water window (2 - 4 nm). A conceptual design of the beamline, from the photon beam from the undulators to the user experimental chamber, mainly focusing on diagnostic, manipulation and transport of the radiation is presented and discussed. We also briefly outline a user end station for coherent imaging, laser ablation and pump-probe experiments.
We present methods and preliminary observations of two pulse Direct Laser Acceleration in a Laser-Driven Plasma Accelerator. This acceleration mechanism uses a second co-propagating laser pulse to overlap and further accelerate electrons in a wakefield bubble, increasing energy at the cost of emittance when compared to traditional laser wakefield acceleration (LWFA). To this end, we introduce a method of femtosecond scale control of time delay between two co-propagating pulses. We show energy enhancement when the separation between the two pulses approaches the bubble radius.
Plasma-based electron and positron wakefield acceleration has made great strides in the past decade. However one major challenge for its applications to coherent light sources and colliders is the relatively large energy spread of the accelerated beams, currently at a few percent level. This energy spread is usually correlated with particle position in the beam arising from the longitudinal chirp of the wakefield amplitude. Therefore a dechirper is highly desirable for reducing this spread down to $sim0.1%$ level, while at the same time for maintaining the emittance of the accelerated beam. Here we propose that a low-density hollow channel plasma can act as a near-ideal dechirper for both electrons and positrons. We demonstrate the concept through large-scale three-dimensional particle-in-cell simulations. We show that the initial positive correlated energy spread (chirp) on the beam exiting a plasma accelerator can be compensated by the nearly linear self-wake induced by the beam in the hollow channel from few percent level down to $leq 0.1%$. Meanwhile, the beam emittance can be preserved due to the negligible transverse field inside the channel. This passive method may significantly improve the beam quality of plasma-based accelerators, paving the way for their applications to future compact free electron lasers and colliders.
We show using particle-in-cell (PIC) simulations and theoretical analysis that a high-quality electron beam whose density is modulated at angstrom scales can be generated directly using density downramp injection in a periodically modulated density in nonlinear plasma wave wakefields. The density modulation turns on and off the injection of electrons at the period of the modulation. Due to the unique longitudinal mapping between the electrons initial positions and their final trapped positions inside the wake, this results in an electron beam with density modulation at a wavelength orders of magnitude shorter than the plasma density modulation. The ponderomotive force of two counter propagating lasers of the same frequency can generate a density modulation at half the laser wavelength. Assuming a laser wavelength of $0.8micrometer$, fully self-consistent OSIRIS PIC simulations show that this scheme can generate high quality beams modulated at wavelengths between 10s and 100 angstroms. Such beams could produce fully coherent, stable, hundreds of GW X-rays by going through a resonant undulator.
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