Abstract
The dependence of the mean kinetic energy of laser-accelerated relativistic electrons (REs) on the laser intensity, so-called ponderomotive scaling, explains well the experimental results to date; however, this scaling is no longer applicable to multi-picosecond (multi-ps) laser experiments. Here, the production of REs was experimentally investigated via multi-ps relativistic laser–plasma-interaction (LPI). The lower slope temperature shows little dependence on the pulse duration and is close to the ponderomotive scaling value, while the higher slope temperature appears to be affected by the pulse duration. The higher slope temperature is far beyond the ponderomotive scaling value, which indicates super-ponderomotive REs (SP-REs). Simulation and experimental evidence are provided to indicate that the SP-REs are produced by LPI in an under-critical plasma, where a large quasi-static electromagnetic field grows rapidly after a threshold timing during multi-ps LPI.
Highlights
Background levelElectron energy (MeV)dN/dE (MeV–1) dN/dE (MeV–1) b (Exp.)1.2 ps with 1.0 × 1019 W cm–2 (Exp.) 1.2 ps with 3.4 × 1018 W cm–2 (PIC) 1.2 ps with 1.36 × 1019 W cm–2 (PIC)The energy distribution of the low-energy relativistic electrons (REs) was evaluated from high-energy X-ray spectrometer (HEXS) signals by coupling with a Monte Carlo simulation that handles radiation-particle-matter interactions, such as the GEANT4 code[19]
We have experimentally investigated the dependence of RE energy distributions on the pulse durations under conditions free from pre-plasma formation
The LFEX laser consists of four beams, where the spot diameter of the spatially overlapped LFEX beams on a target was 70 μm of the full width at half maximum (FWHM), and 30% of the laser energy was contained in this spot
Summary
Background levelElectron energy (MeV)dN/dE (MeV–1) dN/dE (MeV–1) b (Exp.)1.2 ps with 1.0 × 1019 W cm–2 (Exp.) 1.2 ps with 3.4 × 1018 W cm–2 (PIC) 1.2 ps with 1.36 × 1019 W cm–2 (PIC)The energy distribution of the low-energy REs was evaluated from HEXS signals by coupling with a Monte Carlo simulation that handles radiation-particle-matter interactions, such as the GEANT4 code[19]. 1.2 ps with 1.0 × 1019 W cm–2 (Exp.) 1.2 ps with 3.4 × 1018 W cm–2 (PIC) 1.2 ps with 1.36 × 1019 W cm–2 (PIC). The energy distribution of the low-energy REs was evaluated from HEXS signals by coupling with a Monte Carlo simulation that handles radiation-particle-matter interactions, such as the GEANT4 code[19]. The REs are converted into Bremsstrahlung X-rays in the gold cube. A spectrum of the Bremsstrahlung X-rays reflects the energy distribution of REs moving in the gold cube. The energy distribution of REs (f(E)) was approximated using Maxwell–Boltzmann distribution functions with two different slope temperatures (T1, T2, and T1 < T2) and a relative coefficient (A), i.e., f(E) = A exp (−E/T1) + (1 − A) exp (−E/T2).
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