Abstract
The use of a short-pulse petawatt (PW) laser (τL<200 fs, wavelength ≈1 μm) enables experimental realization of a self-guided, multi-centimetre-long multi-GeV laser wakefield electron accelerator. A comprehensive set of numerical simulations showed that a 150 fs, 1.33 PW pulse is self-guided over 10 cm of a static filling gaseous plasma of density 1–3×1017 cm−3 and is stable against relativistic filamentation. A fully broken electromagnetic wake (electron density 'bubble') is excited over the entire interaction length. Variations of bubble size and shape associated with nonlinear evolution of the driving pulse result in self-injection of background plasma electrons. Self-injection begins immediately after the first nonlinear laser focus, where pulse de-focusing forces the bubble to grow. Injection continues without interruption while the bubble expands, and ceases when the laser becomes self-guided and bubble evolution stabilizes. Self-injected electrons are accelerated to ∼7 GeV with less than 10% energy spread and ∼1.3 nC charge. Numerical modelling of the laser pulse dynamics over the entire plasma length is carried out using a time-averaged, fully relativistic, quasi-static three-dimensional (3D) axi-symmetric particle-in-cell (PIC) code, WAKE. The process of electron self-injection is explored by means of both test-particle modelling (WAKE) and 3D PIC simulations using the recently developed CALDER-Circ code in quasi-cylindrical geometry.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Modern laser-plasma accelerators (LPAs) routinely produce sub-GeV monoenergetic, collimated electron beams out of a few mm-length plasmas. To achieve beam quality comparable to conventional accelerators, the laser pulse has to create a 'bubble' completely devoid of electrons in its immediate wake, which captures electrons from the surrounding plasma and accelerates them in an exceptionally uniform way. New petawatt lasers with pulse duration <200 fs are expected to push the energy frontier of the LPA towards many GeV, and beam currents to several tens kA. In view of planned experiments, a robust and simple experimental strategy is needed to achieve the exceptionally high beam quality demanded by applications (e.g. compact x-ray sources).
Main results. A comprehensive numerical study of a broad parameter range accessible to modern petawatt facilities is presented. It establishes the regime where the laser pulse stably self-guides and drives the electron bubble over 10-cm long plasma without a pre-formed plasma channel. Evolution of the accelerating bucket (electron density bubble), which is directly caused by the nonlinear focusing and defocusing of the driving laser pulse, results in formation of a high-quality electron beam (as indicated in the figure). Electron self-injection can be controlled, and beam quality optimized, by means of nonlinear laser focusing in a specially designed multi-layered plasma target. Predicted electron energy (~ 7 GeV with a spread of a few per cent) is an order of magnitude higher than presently achieved in LPAs.
Wider implications. The reported results help optimize and scale compact accelerators for future applications in high-energy physics, radiotherapy and molecular biology.
Figure. Electron self-injection and acceleration in the expanding plasma bubble driven by the defocusing laser pulse (red contour: the pulse propagates to the right)—3D particle-in-cell simulation. Background electron density is 1017 cm−3, laser power 1.33 PW and duration 150 fs. As the driver defocuses, the bubble grows in size by 50 per cent over 4 mm of propagation (from top to bottom row) and traps ~1010 electrons.