Guiding of relativistically intense laser pulses with peak power of 0.85 PW over 15 diffraction lengths was demonstrated by increasing the focusing strength of a capillary discharge waveguide using laser inverse bremsstrahlung heating. This allowed for the production of electron beams with quasimonoenergetic peaks up to 7.8 GeV, double the energy that was previously demonstrated. Charge was 5 pC at 7.8 GeV and up to 62 pC in 6 GeV peaks, and typical beam divergence was 0.2 mrad.

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Petawatt laser guiding and electron beam acceleration to 8 GeV in laser-heated capillary discharge waveguides

An alternative configuration of the laser wake-field accelerator is proposed in which enhanced acceleration is achieved via resonant self-modulation of the laser pulse. This requires laser power in excess of the critical power for relativistic guiding and a plasma wavelength short compared to the laser pulse length. Relativistic and density wake effects strongly modulate the laser pulse at the plasma wavelength, resonantly exciting the plasma wave and leading to enhanced acceleration.

Enhanced acceleration in a self-modulated-laser wakefield accelerator

Ultrashort laser pulses interact with matter in unique, nonselective ways, due to electronic direct excitation in an ultrafast time scale. Additionally, these pulses can cover peak powers from kW to PW, allowing its use to induce from nonlinear optics up to relativistic phenomena. Here we present a quick overview of the important processes induced in matter by high-intensity ultrashort pulses, and describe results obtained in our laboratory, including our most recent goal of accelerating electrons with ultrashort pulses for practical applications.

High Intensity ultrashort laser pulses and their applications at IPEN

In this work, particle-in-cell simulations were carried out to investigate the role of the down-ramp length of a H2 gas jet in accelerating electrons ionized by the laser pulse. The laser and plasma density were chosen so that the system is operating in the self-modulated regime. Preliminary results show how the down-ramp length can control the injection of electrons in the first bubble induced in the plasma by the laser pulse.

Simulation Study of Laser Wakefield Acceleration Varying the Down-Ramp Length of a Gas Jet

Side-view density profiles of a laser-induced plasma were measured by a home-built, time-resolved, Mach-Zehnder-like interferometer. Due to the pump-probe femtosecond resolution of the measurements, the plasma dynamics was observed, along with the pump pulse propagation. The effects of impact ionization and recombination were evidenced during the plasma evolution up to hundreds of picoseconds. This measurement system will integrate our laboratory infrastructure as a key tool for diagnosing gas targets and laser-target interaction in laser wakefield acceleration experiments.

Femtosecond laser-plasma dynamics study by a time-resolved Mach-Zehnder-like interferometer.

This work presents a study on laser wakefield electron acceleration in the self-modulated regime (SM-LWFA) using 50-fs laser pulses with energy on the mJ scale, at λ = 0.8 µm, impinging on a thin H2 gas jet. Particle-in-cell simulations were performed using laser peak powers ranging from sub-terawatt to a few terawatts and plasma densities varying from the relativistic self-focusing threshold up to values close to the critical density. The differences in the obtained acceleration processes are discussed. Results show that bunched electron beams with full charge on the nC scale and kinetic energy in the MeV range can be produced and configurations with peak density in the range 0.5–5 × 1020 atoms/cm3 generate electrons with maximum energies. In this range, some simulations generated quasimonoenergetic bunches with ∼0.5% of the total accelerated charge and we show that the beam characteristics, process dynamics, and operational parameters are close to those expected for the blowout regime. The configurations that led to quasimonoenergetic bunches from the sub-TW SM-LWFA regime allow the use of laser systems with repetition rates in the kHz range, which can be beneficial for practical applications.

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

We review the fundamentals of electron beam generation in self-modulated laser wakefield acceleration (SM-LWFA) using laser pulses with peak powers on the TW and sub-TW scale. The results of several new particle-in-cell simulations for λ = 0.8 µm and a thin H2 gas target are shown. By scanning the peak gas density and peak power values of the incident laser, we discuss the resulting acceleration processes, as well as the characteristics of the electron beams generated for different conditions. Our approach provides a method to optimize these beams with lasers operating at kHz repetition rates.

Electron beam properties in self-modulated laser wakefield acceleration using TW and sub-TW pulses

In this work, the current status on the development of a laser plasma accelerator at the Nuclear and Energy Research Institute (Instituto de Pesquisas Nucleares e Energéticas, IPEN/CNEN), in São Paulo, Brazil, is presented. Short pulses to be produced by an under-development near-TW, kHz laser system will be used to ionize a gas jet, with a density profile designed to optimize the self-injection of plasma electrons. The same laser pulse will also drive a plasma wakefield, which will allow for electron acceleration in the self-modulated regime. The current milestone is to develop the experimental setup, including electron beam and plasma diagnostics, required to produce electron bunches with energies of a few MeV. Once this has been achieved, the next milestone is to produce beams with energies higher than 50 MeV. Besides kickstarting the laser wakefield accelerator (LWFA) technology in Brazil, this project aims to pave the way for conducting research on the production of radioisotopes by photonuclear reactions, triggered by LWFAaccelerated beams.

On the Development of a Low Peak-Power, High Repetition-Rate Laser Plasma Accelerator at IPEN

In this work, particle-in-cell simulations were carried out to investigate the dynamics of a laser pulse propagating along a H2 gas jet. The laser-driven wakefield and the density of ionized electrons are analyzed during the pulse propagation through the gas jet. The laser and plasma quantities were chosen in order to have the system operating in the self-modulated regime. Results show how the selfmodulation fragments the laser pulse, originating higheramplitude pulses that can induce bubble formation with wave-breaking and particle injection.

Alexandre Bonatto

Papers

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

Electron beam properties in self-modulated laser wakefield acceleration using TW and sub-TW pulses

On the Development of a Low Peak-Power, High Repetition-Rate Laser Plasma Accelerator at IPEN

Simulation Study of Laser Wakefield Acceleration Varying the Down-Ramp Length of a Gas Jet

Laser Pulse Dynamics in the Self-Modulated Regime