C. Benedetti

Multistage coupling of independent laser-plasma accelerators

Laser-plasma accelerators (LPAs) are capable of accelerating charged particles to very high energies in very compact structures. In theory, therefore, they offer advantages over conventional, large-scale particle accelerators. However, the energy gain in a single-stage LPA can be limited by laser diffraction, dephasing, electron-beam loading and laser-energy depletion. The problem of laser diffraction can be addressed by using laser-pulse guiding and preformed plasma waveguides to maintain the required laser intensity over distances of many Rayleigh lengths; dephasing can be mitigated by longitudinal tailoring of the plasma density; and beam loading can be controlled by proper shaping of the electron beam. To increase the beam energy further, it is necessary to tackle the problem of the depletion of laser energy, by sequencing the accelerator into stages, each powered by a separate laser pulse. Here, we present results from an experiment that demonstrates such staging. Two LPA stages were coupled over a short distance (as is needed to preserve the average acceleration gradient) by a plasma mirror. Stable electron beams from a first LPA were focused to a twenty-micrometre radius—by a discharge capillary-based active plasma lens—into a second LPA, such that the beams interacted with the wakefield excited by a separate laser. Staged acceleration by the wakefield of the second stage is detected via an energy gain of 100 megaelectronvolts for a subset of the electron beam. Changing the arrival time of the electron beam with respect to the second-stage laser pulse allowed us to reconstruct the temporal wakefield structure and to determine the plasma density. Our results indicate that the fundamental limitation to energy gain presented by laser depletion can be overcome by using staged acceleration, suggesting a way of reaching the electron energies required for collider applications.

Active Plasma Lensing for Relativistic Laser-Plasma-Accelerated Electron Beams

Compact, tunable, radially symmetric focusing of electrons is critical to laser-plasma accelerator (LPA) applications. Experiments are presented demonstrating the use of a discharge-capillary active plasma lens to focus 100-MeV-level LPA beams. The lens can provide tunable field gradients in excess of 3000 T/m, enabling cm-scale focal lengths for GeV-level beam energies and allowing LPA-based electron beams and light sources to maintain their compact footprint. For a range of lens strengths, excellent agreement with simulation was obtained.

Numerical investigation of electron self-injection in the nonlinear bubble regime

The process of electron self-injection in the nonlinear bubble wake generated by a short and intense laser pulse propagating in a uniform underdense plasma is studied by means of fully self-consistent particle-in-cell simulations and test-particle simulations. We consider a wake generated by a non-evolving laser driver traveling with a prescribed velocity, which then sets the structure and the velocity of the wake, so the injection dynamics is decoupled from driver evolution, but a realistic structure for the wakefield is retained. We show that a threshold for self-injection into a non-evolving bubble wake exists, and we characterize the dependence of the self-injection threshold on laser intensity, wake velocity, and plasma temperature for a range of parameters of interest for current and future laser-plasma accelerators.

HiPACE: a quasi-static particle-in-cell code

We introduce the Highly efficient Plasma Accelerator Emulation (HiPACE) code. It is a relativistic, electromagnetic, three-dimensional and fully parallelized particle-in-cell (PIC) code and uses the quasi-static approximation to efficiently simulate a variety of beam-driven plasma-wakefield acceleration scenarios. HiPACE exploits the disparity of time scales in the interaction of highly relativistic particle beams with plasma to decouple beam and plasma evolution. This enables time steps which are many times greater than those used in full PIC codes. Comparisons to the fully explicit PIC code OSIRIS show the capability of the quasi-static PIC code to consistently simulate problems in beam-driven plasma acceleration while reducing the required number of core hours by orders of magnitude. This work outlines the physical basis, describes the numerical implementation and assesses the parallel performance of the code which in combination lead to high computational efficiency.

Generation and pointing stabilization of multi-GeV electron beams from a laser plasma accelerator driven in a pre-formed plasma waveguidea)

Laser pulses with peak power 0.3 PW were used to generate electron beams with energy >4 GeV within a 9 cm-long capillary discharge waveguide operated with a plasma density of ≈7×1017 cm−3. Simulations showed that the super-Gaussian near-field laser profile that is typical of high-power femtosecond laser systems reduces the efficacy of guiding in parabolic plasma channels compared with the Gaussian laser pulses that are typically simulated. In the experiments, this was mitigated by increasing the plasma density and hence the contribution of self-guiding. This allowed for the generation of multi-GeV electron beams, but these had angular fluctuation ≳2 mrad rms. Mitigation of capillary damage and more accurate alignment allowed for stable beams to be produced with energy 2.7±0.1 GeV. The pointing fluctuation was 0.6 mrad rms, which was less than the beam divergence of ≲1 mrad full-width-half-maximum.

Quasi-matched propagation of ultra-short, intense laser pulses in plasma channels

The propagation of an ultrashort and relativistically intense laser pulse in a preformed plasma channel is investigated. The nonlinear paraxial wave equation describing the laser propagation in the plasma is solved both analytically and numerically. For any arbitrary temporal laser pulse profile with a given power (less then the critical power for self-focusing) and any prescribed transverse density profile in the channel, we determine the laser intensity distribution along the pulse ensuring quasi-matched propagation, neglecting non-paraxial effects. For the case of a Gaussian laser with an initially uniform spot throughout the pulse, we determine the optimal channel depth that minimizes laser evolution (e.g., minimizes spot size oscillations). The analytical and semi-analytical results obtained for both cases in the weakly relativistic regime are presented and validated through comparison with numerical simulations.

Plasma wakefields driven by an incoherent combination of laser pulses: a path towards high-average power laser-plasma accelerators

The wakefield generated in a plasma by incoherently combining a large number of low energy laser pulses (i.e., without constraining the pulse phases) is studied analytically and by means of fully self-consistent particle-in-cell simulations. The structure of the wakefield has been characterized and its amplitude compared with the amplitude of the wake generated by a single (coherent) laser pulse. We show that, in spite of the incoherent nature of the wakefield within the volume occupied by the laser pulses, behind this region, the structure of the wakefield can be regular with an amplitude comparable or equal to that obtained from a single pulse with the same energy. Wake generation requires that the incoherent structures in the laser energy density produced by the combined pulses exist on a time scale short compared to the plasma period. Incoherent combination of multiple laser pulses may enable a technologically simpler path to high-repetition rate, high-average power laser-plasma accelerators, and associated applications.

Laser-heater assisted plasma channel formation in capillary discharge waveguides

A method of creating plasma channels with controllable depth and transverse profile for the guiding of short, high power laser pulses for efficient electron acceleration is proposed. The plasma channel produced by the hydrogen-filled capillary discharge waveguide is modified by a ns-scale laser pulse, which heats the electrons near the capillary axis. This interaction creates a deeper plasma channel within the capillary discharge that evolves on a ns-time scale, allowing laser beams with smaller spot sizes than would otherwise be possible in the unmodified capillary discharge.

Group velocity and pulse lengthening of mismatched laser pulses in plasma channels

Analytic solutions are presented to the non-paraxial wave equation describing an ultra-short, low-power, laser pulse propagating in a plasma channel. Expressions for the laser pulse centroid motion and laser group velocity are derived, valid for matched and mismatched propagation in a parabolic plasma channel, as well as in vacuum, for an arbitrary Laguerre-Gaussian laser mode. The group velocity of a mismatched laser pulse, for which the laser spot size is strongly oscillating, is found to be independent of propagation distance and significantly less than that of a matched pulse. Laser pulse lengthening of a mismatched pulse owing to laser mode slippage is examined and found to dominate over that due to dispersive pulse spreading for sufficiently long pulses. Analytic results are shown to be in excellent agreement with numerical solutions of the full Maxwell equations coupled to the plasma response. Implications for plasma channel diagnostics are discussed.

Laser red shifting based characterization of wakefield excitation in a laser-plasma accelerator

Optical spectra of a drive laser exiting a channel guided laser-plasma accelerator (LPA) are analyzed through experiments and simulations to infer the magnitude of the excited wakefields. The experiments are performed at sufficiently low intensity levels and plasma densities to avoid electron beam generation via self-trapping. Spectral redshifting of the laser light is studied as an indicator of the efficiency of laser energy transfer into the plasma through the generation of coherent plasma wakefields. Influences of input laser energy, plasma density, temporal and spatial laser profiles, and laser focal location in a plasma channel are analyzed. Energy transfer is found to be sensitive to details of laser pulse shape and focal location. The experimental conditions for these critical parameters are modeled and included in particle-in-cell simulations. Simulations reproduce the redshift of the laser within uncertainties of the experiments and produce an estimate of the wake amplitudes in the experiments as...