Chirped pulse Raman amplification in warm plasma: towards controlling saturation

Stimulated Raman backscattering in plasma is potentially an efficient method of amplifying laser pulses to reach exawatt powers because plasma is fully broken down and withstands extremely high electric fields. Plasma also has unique nonlinear optical properties that allow simultaneous compression of optical pulses to ultra-short durations. However, current measured efficiencies are limited to several percent. Here we investigate Raman amplification of short duration seed pulses with different chirp rates using a chirped pump pulse in a preformed plasma waveguide. We identify electron trapping and wavebreaking as the main saturation mechanisms, which lead to spectral broadening and gain saturation when the seed reaches several millijoules for durations of 10's - 100's fs for 250 ps, 800 nm chirped pump pulses. We show that this prevents access to the nonlinear regime and limits the efficiency, and interpret the experimental results using slowly-varying-amplitude, current-averaged particle-in-cell simulations. We also propose methods for achieving higher efficiencies.close0

Tapered capillaries applied in laser wakefield acceleration

This paper presents realisation of linearly tapered capillary discharge waveguides (CDWs), manufactured using a femtosecond laser micromachining technique. Waveguiding of a low power, 50 fs duration laser pulse is demonstrated and, despite a slight mismatch of the laser focal spot size with respect to the capillary entrance size, efficient guiding of the Gaussian-shaped laser pulse is obtained. Energy transmission of 80% is obtained for optimal delay of the laser pulse arrival time with respect to the discharge current pulse

The role of the gas/plasma plume and self-focusing in a gas-filled capillary discharge waveguide for high-power laser-plasma applications

The role of the gas/plasma plume at the entrance of a gas-filled capillary discharge plasma waveguide in increasing the laser intensity has been investigated. Distinction is made between neutral gas and hot plasma plumes that, respectively, develop before and after discharge breakdown. Time-averaged measurements show that the on-axis plasma density of a fully expanded plasma plume over this region is similar to that inside the waveguide. Above the critical power, relativistic and ponderomotive self-focusing lead to an increase in the intensity, which can be nearly a factor of 2 compared with the case without a plume. When used as a laser plasma wakefield accelerator, the enhancement of intensity can lead to prompt electron injection very close to the entrance of the waveguide. Self-focusing occurs within two Rayleigh lengths of the waveguide entrance plane in the region, where the laser beam is converging. Analytical theory and numerical simulations show that, for a density of 3.0 × 1018 cm−3, the peak normalized laser vector potential, a 0, increases from 1.0 to 1.85 close to the entrance plane of the capillary compared with a 0 = 1.41 when the plume is neglected

Electron injection into a capillary laser wakefield accelerator

The role played by self-focusing of a high-intensity femtosecond laser in the entrance plume of a gas-filled capillary discharge waveguide is investigated for laser-plasma wakefield acceleration of electrons