We present the design and performance of the Texas Petawatt Laser, which produces a 186 J167 fs pulse based on the combination of optical parametric chirped pulse amplification (OPCPA) and mixed Nd:glass amplification. OPCPA provides the majority of the gain and is used to broaden and shape the seed spectrum, while amplification in Nd:glass accounts for >99% of the final pulse energy. Compression is achieved with highly efficient multilayer dielectric gratings.

Demonstration of a 1.1 petawatt laser based on a hybrid optical parametric chirped pulse amplification/mixed Nd:glass amplifier

A scheme for collimating fast electrons in a specially engineered solid target is proposed. Unlike previous approaches, the collimation is achieved by generating an azimuthal magnetic field as opposed to a radial electric field. The target is engineered such that it consists of a fiber surrounded by material of a lower resistivity than that of the fiber. The fast electrons are collimated along the fiber. Hybrid Vlasov-Fokker-Planck simulations supported by analytic calculations show that this concept is viable.

/pdf/magnetic-collimation-of-fast-electrons-produced-by-52sw1462h2.pdf

Magnetic collimation of fast electrons produced by ultraintense laser irradiation by structuring the target composition

Proton beams laser accelerated from thin foils are studied for various plasma gradients on the foil rear surface. The beam maximum energy and spectral slope reduce with the gradient scale length, in good agreement with numerical simulations. The results also show that the jxB mechanism determines the temperature of the electrons driving the ion expansion. Future ion-driven fast ignition of fusion targets will use multikilojoule petawatt laser pulses, the leading part of which will induce target preheat. Estimates based on the data show that this modifies by less than 10% the ion beam parameters.

Laser-foil acceleration of high-energy protons in small-scale plasma gradients.

Laser absorption is reviewed in the context of fast ignition inertial fusion. This leads us to consider laser absorption by overdense plasmas for values of intensity times wavelength squared (Iλ2) greater than 1010 W (1018 W cm−2 µm2), which corresponds to the onset of relativistic electron motion in the laser fields. A collection of published absorption values obtained in laser–solid experiments and in numerical modelling of laser–plasma interactions is presented.

https://iopscience.iop.org/article/10.1088/0741-3335/51/1/014006/pdf

Laser absorption by overdense plasmas in the relativistic regime

The heating of plane solid targets by the Vulcan petawatt laser at powers of 0.32–0.73 PW and intensities of up to 4×1020 W cm−2 has been diagnosed with a temporal resolution of 17 ps and a spatial resolution of 30 μm, by measuring optical emission from the opposite side of the target to the laser with a streak camera. Second harmonic emission was filtered out and the target viewed at an angle to eliminate optical transition radiation. Spatial resolution was obtained by imaging the emission onto a bundle of fibre optics, arranged into a one-dimensional array at the camera entrance. The results show that a region 160 μm in diameter can be heated to a temperature of ~107 K (kT/e~ keV) in solid targets from 10 to 20 μm thick and that this temperature is maintained for at least 20 ps, confirming the utility of PW lasers in the study of high energy density physics. Hybrid code modelling shows that magnetic field generation prevents increased target heating by electron refluxing above a certain target thickness and that the absorption of laser energy into electrons entering the solid target was between 15–30%, and tends to increase with laser energy.

https://iopscience.iop.org/article/10.1088/1367-2630/10/4/043046/pdf

Space and time resolved measurements of the heating of solids to ten million kelvin by a petawatt laser

Time-resolved x-ray spectra from solid targets irradiated by the VULCAN Petawatt laser focused to 1020Wcm−2 show that material at solid density is heated to temperatures above 500 eV to a depth of about 15 μm and for a duration of more than 30 ps. Modeling with the implicit hybrid plasma code LSP shows that the heating is sensitive to the laser prepulse through resistive inhibition of the laser accelerated electrons in the blow off layer.

S.F. James

Papers

Rapid heating of solid density material by a petawatt laser