Prepulse controlled electron acceleration from solids by a femtosecond laser pulse in the slightly relativistic regime

Abstract

We present results from the experimental and numerical study of electron heating and acceleration under the action of a 50 fs high contrast laser pulse [intensities ∼(1–4) × 1018 W/cm2] with a controlled preplasma that was created by a 6 ns laser “prepulse” with intensity ∼1012 W/cm2. A substantial increase both in the gamma yield and “temperature” was obtained by the proper adjustment of the time delay between the two pulses (0–5 ns), while the gamma yield dropped to almost zero values if the nanosecond pulse came 10–20 ns in advance of the femtosecond one. Comprehensive optical diagnostics (shadowgraphy, interferometry, and angular resolved self-emission measurements) data allowed us to estimate the electron density profile. The latter profile was used for making numerical Particle-in-cell simulations which describe the gamma yield enhancement well. We also illustrate how the observed drop in the gamma yield within a certain range of delays was due to ionization defocusing of the femtosecond beam in an expanding long-scale (L/λ > 1) preplasma.