Relativistic nonlinear optics of plasmas

A train of few-laser-cycle relativistically intense radiation spikes with a terahertz repetition rate can be organized self-consistently in plasma from two frequency detuned co-propagating laser beams of low intensity. Large frequency bandwidth for the compression of spikes is produced via laser-induced periodic modulation of the plasma refractive index. The beat-wave-driven electron plasma wave downshifted from the plasma frequency creates a moving index grating thus inducing a periodic phase modulation of the driving laser
in spectral terms, electromagnetic cascading. The group velocity dispersion compresses the chirped laser beat notes to a few-cycle duration and relativistic intensity either concurrently in the same, or sequentially in different plasmas. Particle-in-cell simulations indicate that the effect persists in a realistic three-dimensional axisymmetric geometry.

The use of a short-pulse petawatt (PW) laser (T_L < 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 1–3×10^17 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.

Laser wakefield acceleration experiments with the 150fs Texas Petawatt laser will be carried out in the unique regime with the shortest pulse duration among available petawatt facilities. Simulations via the time-averaged, fully relativistic, quasi-static 3D axi-symmetric particle-in-cell (PIC) code WAKE show that combination of nonlinear plasma wave focusing, relativistic self-focusing, and nonlinear temporal compression guide a 1.33 PW pulse over