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 I lambda(2) similar or equal to 10(14)-10(20) W cm(-2) mu m(2) 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 10(16) W cm(-2) mu m(2), 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 10(17) W cm(-2), the scaling law is T-hot proportional to (I lambda(2))(1/3) when the incident angle matches the optimized angle of resonance absorption; otherwise, T-hot proportional to (I lambda(2))(alpha) with alpha > 1/3, which changes with laser incident angles and preplasma scale lengths; in the case of vacuum heating, usually alpha > 1. At laser intensity above 10(18) W cm(-2) mu m(2) when the absorption mechanism is dominated by ponderomotive acceleration, the scaling becomes T-hot proportional to (I lambda(2))(1/2). The angular distributions of hot electrons are also shown to be dependent upon the absorption mechanisms.