Absorption of high-contrast 12-ps uv laser pulses by solid targets

Anomalous collisional absorption of laser pulses in underdense plasma at low temperature

In a previous paper [M. Kundu, Phys. Plasmas 21, 013302 (2014)], fractional collisional absorption (α) of laser light in underdense plasma was studied by using a classical scattering model of electron-ion collision frequency ν(ei), where total velocity v=√[v(th)(2)+v(0)(2)] (with v(th) and v(0) as the thermal and the ponderomotive velocity of an electron) dependent Coulomb logarithm lnΛ(v) was shown to be responsible for the anomalous (unconventional) increase of ν(ei) and α(∝ν(ei)) with the laser intensity I(0) up to a maximum value about an intensity I(c) in the low temperature (T(e)<15eV) regime and a conventional ≈I(0)(-3/2) decrease when I(0)≫I(c). One may object that the anomalous increase in ν(ei) and α were partly due to the artifact introduced in lnΛ through the maximum cutoff distance b(max)∝v. In this work, we show similar anomalous increase in ν(ei) and α versus I(0) (in the low temperature and underdense density regime) with more accurate quantum and classical kinetic models of ν(ei) without using lnΛ, but with a proper choice of the total velocity dependent inverse cutoff length k(max)∝v(2) (classical) or k(max)∝v (quantum). For a given I(0)<5×10(14)Wcm(-2), ν(ei) versus T(e) also exhibits so far unnoticed identical anomalous increase as ν(ei) versus I(0), even if the conventional k(max)∝v(th)(2) or k(max)∝v(th) (without v(0)) is chosen. The total velocity dependent k(max) in the kinetic models, as proposed here, is found to explain the anomalous increase of α with I(0) measured in some earlier laser-plasma experiments.

Soft-x-ray emission dynamics in picosecond laser-produced plasmas

Plasmas were generated by prepulse-free picosecond laser irradiation of solid targets consisting of five different fluorine salts. Picosecond-time-resolved x-ray spectroscopy of K-shell emission from H-like and He-like fluorine was performed to study the temporal evolution of plasma parameters including electron density and temperature. Measurements show that the fluorine line emission intensity reaches its maximum in a time comparable with the rise time of the laser pulse. This peak is then followed by a decay whose rate depends upon the atomic number of the alkali component of the target. Our measurements show clear evidence of radiation cooling effects in higher-Z targets.

Absorption of subpicosecond UV laser pulses during interaction with solid targets

The absorption of subpicosecond uv laser pulses has been measured at intensities above 10(17) W/cm(2). High levels of absorption were observed, up to 55% for s polarization and 65% for p polarization. The behavior with angle of incidence and polarization can be interpreted as due to a combination of resonance and collisional absorption, taking place in a plasma with a scale length of the order of a fraction of the laser wavelength.