Abstract
Experiments were performed to study electron acceleration by intense sub-picosecond laser pulses propagating in sub-mm long plasmas of near critical electron density (NCD). Low density foam layers of 300–500 μm thickness were used as targets. In foams, the NCD-plasma was produced by a mechanism of super-sonic ionization when a well-defined separate ns-pulse was sent onto the foam-target forerunning the relativistic main pulse. The application of sub-mm thick low density foam layers provided a substantial increase of the electron acceleration path in a NCD-plasma compared to the case of freely expanding plasmas created in the interaction of the ns-laser pulse with solid foils. The performed experiments on the electron heating by a 100 J, 750 fs short laser pulse of 2–5 × 1019 W cm−2 intensity demonstrated that the effective temperature of supra-thermal electrons increased from 1.5–2 MeV in the case of the relativistic laser interaction with a metallic foil at high laser contrast up to 13 MeV for the laser shots onto the pre-ionized foam. The observed tendency towards a strong increase of the mean electron energy and the number of ultra-relativistic laser-accelerated electrons is reinforced by the results of gamma-yield measurements that showed a 1000-fold increase of the measured doses. The experiment was supported by 3D-PIC and FLUKA simulations, which considered the laser parameters and the geometry of the experimental set-up. Both, measurements and simulations showed a high directionality of the acceleration process, since the strongest increase in the electron energy, charge and corresponding gamma-yield was observed close to the direction of the laser pulse propagation. The charge of super-ponderomotive electrons with energy above 30 MeV reached a very high value of 78 nC.
Highlights
The experimental investigation of high energy density (HED) matter states created with intense laser [1, 2] and heavy ion beams [3, 4] requires active backlighting with highly penetrating gamma-rays and energetic particles that provide important diagnostic tools to access plasma parameters and structural information from inside the high areal density samples
In [30, 31], it was shown that the interaction of relativistic laser pulses with foam targets of near critical electron density (NCD) provides an efficient conversion of the laser energy into relativistic electron beams with energies far above those predicted by Wilks
The results of electron spectra measured by two electron spectrometers ES1 and ES2 and the thermo-luminescence dosimetry (TLD)-dose caused by the bremsstrahlung radiation were very stable from shot to shot for every type of laser pulse and target combination
Summary
L P Pugachev, D Khaghani, F Horst, N G Borisenko, G Sklizkov and V G Pimenov. L P Pugachev, D Khaghani, F Horst, N G Borisenko, G Sklizkov and V G Pimenov8 Original content from this 5 Institute of Optics and Quantum Electronics, Abbe Center of Photonics, Friedrich Schiller University Jena, Max-Wien-Platz 1, D-07745 work may be used under the terms of the Creative. Commons Attribution 3.0 6 THM University of Applied Sciences, Wiesen Str. 14, D-35390 Giessen, Germany licence.
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