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.
Highlights
The use of a short-pulse petawatt (PW) laser enables experimental realization of a self-guided, multicentimetre-long multi-GeV laser wakefield electron accelerator
The Texas Petawatt (TPW) laser is strongly overcritical, 8 < P/Pcr < 25, and its normalized length belongs to the interval 2.7 < ωpeτL < 4.2
It is found that at low plasma density electron self-injection critically depends on the nonlinear evolution (i.e. relativistic selffocusing (RSF) and refraction) of the driving pulse
Summary
A specific choice of the driving laser dictates the particular experimental configuration. Numerical modelling shows that the range of densities 1–3×1017 cm−3 (corresponding to 2–6 Torr doubly ionized helium) is optimal from the standpoint of laser guiding, stability and electron self-injection. In this density range, the TPW laser is strongly overcritical, 8 < P/Pcr < 25 (where Pcr = 16.2ω02/ωp2e GW is the critical power for the RSF [23]), and its normalized length belongs to the interval 2.7 < ωpeτL < 4.2. The pulse is long enough to avoid vacuum-like diffraction [6] (i.e. ωpeτL > 1), and too short (i.e. shorter than a plasma period) to experience catastrophic RSF [29] Such a pulse is expected to self-guide over 10 cm, which promises up to 6 GeV electron energy gain. This is favourable for visualization by frequency-domain holography [36, 37] with green probe and reference pulses split from the main pulse upstream of the amplifier chain
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