Laser absorption and hot electron temperature scalings in laser–plasma interactions

Laser absorption in the interaction between ultra-intense femtosecond laser and solid density plasma is studied integratedly for the intensity range Iλ2 ≃ 1014–1020 W cm−2 µm2 by particle-in-cell simulations with collision modulus included. The collisional effect is found to be significant when the incident laser intensity is less than 1016 W cm−2 µm2, which tends to enhance the resonance absorption and reduce the vacuum heating under different plasma parameters. At higher intensities, various collisionless absorption mechanisms dominate with a large number of hot electrons produced. The scaling of hot electron temperatures is found to depend upon the dominant absorption mechanisms. At moderate intensity around 1017 W cm−2, the scaling law is Thot∝(Iλ2)1/3 when the incident angle matches the optimized angle of resonance absorption; otherwise, Thot ∝ (Iλ2)α with α > 1/3, which changes with laser incident angles and preplasma scale lengths; in the case of vacuum heating, usually α > 1. At laser intensity above 1018 W cm−2 µm2 when the absorption mechanism is dominated by ponderomotive acceleration, the scaling becomes Thot ∝ (Iλ2)1/2. The angular distributions of hot electrons are also shown to be dependent upon the absorption mechanisms.