Generation of hot solid-density plasmas by laser radiation pressure confinement

The need for speed

This paper reviews the challenges posed by the physics of the interaction of high-peak power femtosecond lasers with ultrathin foil targets. Initially designed to produce warm solid-density plasmas through the isochoric heating of solid matter, the interaction of an ultrashort pulse with ultrathin foils is becoming more and more complex as the laser intensity is increased. The dream of achieving very hot solid density matter with extreme specific energy density faces several bottlenecks discussed here as related to the laser technology, to the complexity of the physical processes, and to the limits of our current time-resolved instrumentations.

Direct observation of the ponderomotive force effects in short-scale-length laser plasmas by frequency-domain interferometry

We report on single-shot frequency-domain interferometric measurements showing space- and time-resolved ponderomotive electron density profile steepening of a short-scale-length ultraintense laser-produced plasma. The density gradient scale length is varied by applying a time-delayed laser prepulse. The measured absolute position of the critical density surface is found to be in agreement with one-dimensional hydrodynamic simulations for the range of scale lengths studied.

Time-resolved x-ray K-shell spectra from high density plasmas generated by ultrashort laser pulses

We present time-resolved x-ray spectra of C, F, Na, and Al, generated by focusing ultrashort frequency doubled Ti:sapphire laser pulses on solid plane targets. Using a high resolution x-ray streak camera in combination with a laser triggered accumulation system, we achieved a time resolution of 1.7 ps when adding the x-ray signal of many thousands of laser shots. K-shell resonance line emission with a duration in the range of 2-4 ps is observed. Ly alpha emission is generally observed to be faster than He alpha emission and the x-ray pulse duration is observed to decrease with increasing atomic number. A hydrodynamic code in combination with an atomic kinetics code is applied for simulation of time-resolved plasma emission, showing good agreement with experimental data.

Hydrodynamic simulation of subpicosecond laser interaction with solid-density matter

The interaction of ultrashort subpicosecond laser pulses with initially cold and solid matter is investigated in a wide intensity range (10(11) to 10(17) W/cm(2)) by means of the hydrodynamic code MULTI-FS, which is an extension of the long pulse version of MULTI [R. Ramis, R. Schmalz, and J. Meyer-ter-Vehn, Comput. Phys. Commun. 49, 475 (1988)]. Essential modifications for the treatment of ultrashort pulses are the solution of Maxwell's equations in a steep gradient plasma, consideration of the nonequilibrium between electrons and ions, and a model for the electrical and thermal conductivity covering the wide range from the solid state to the high temperature plasma. The simulations are compared with several absorption measurements performed with aluminum targets at normal and oblique incidence. Good agreement is obtained by an appropriate choice of the electron-ion energy exchange time (characterized by 10 to 20 ps in cold solid Al). In addition we discuss the intensity scaling of the temperature, of the pressure, and of the density, where the laser energy is deposited in the expanding plasma, as well as the propagation of the heat wave and the shock wave into the solid. For laser pulse durations >/=150 fs considered in this paper the amount of isochorically heated matter at solid density is determined by the depth of the electron heat wave in the whole intensity range.

Analysis of the M-shell spectra emitted by a short-pulse laser-created tantalum plasma

The spectrum of tantalum emitted by a subpicosecond laser-created plasma, was recorded in the regions of the 3d-5f, 3d-4f, and 3d-4p transitions. The main difference with a nanosecond laser-created plasma spectrum is a broad understructure appearing under the 3d-5f transitions. An interpretation of this feature as a density effect is proposed. The supertransition array model is used for interpreting the spectrum, assuming local thermodynamic equilibrium (LTE) at some effective temperature. An interpretation of the 3d-4f spectrum using the more detailed unresolved transition array formalism, which does not assume LTE, is also proposed. Fitted contributions of the different ionic species differ slightly from the LTE-predicted values.

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.