We characterized the plasma emission produced by the interaction of multi-millijoule, 40 ns duration, mid-infrared laser pulses with a silicon surface. The laser pulses were produced by a Q-switched Ho:YLF master oscillator power amplifier system. Using spectral measurements and a framing camera, we observed a spatial separation of the plasma plume, increased emission signal with low white-light generation, and a drop in the time- and space-averaged apparent plasma density with increasing pump energy. Our results can be explained by continuous heating of the plasma by the pump pulse due to the more efficient inverse bremsstrahlung absorption at longer wavelengths.

When an ultrashort laser pulse is tightly focused to a size approaching its central wavelength, the properties of the focused spot diverge from the diffraction-limited case. Here, we report on this change in behavior of a tightly focused petawatt-class laser beam by an f/1 off-axis parabolic mirror (OAPM). Considering the effects of residual aberration, the spatial profile of the near-field, and pointing error, we estimate the deviation in peak intensities of the focused spot from the ideal case. We verify that the estimated peak intensity values are within an acceptable error range of the measured values. With the added uncertainties in target alignment, we extend the estimation to infer on-target peak intensities of >= 10(22) W/cm(2) for a target at the focal plane of this f/1 OAPM. (C) 2019 Optical Society of America

Calculations on the dynamics of ions and electrons in near-infrared laser fields at intensities up to 3 x 10(23) W/cm(2) are presented. We explore the acceleration of ions in a laser focus by conservation of canonical momentum during ionization events and by the ponderomotive force in the f/1 focal geometry required to reach such intensity. At intensities exceeding 10(23) W/cm(2), highly charged ions are expelled from the laser focus before they can interact with the laser pulse at peak intensity, decreasing the predicted ionization yields of deeply bound states. We consider the interaction of a tightly focused, f/1 laser pulse with krypton at an intensity of 3 x 10(23) W/cm(2) and a pulse duration of 140 fs. We find that the ions and electrons are accelerated to energies in excess of 2 MeV/nucleon and 1.4 GeV, respectively. Ponderomotive expulsion of the parent ions decreases the total number of ultrarelativistic above-threshold ionization electrons produced by tunneling ionization from the K-shell states of krypton but does not change their energy spectrum.

We present the design, construction, and first use of a magnetic electron-positron spectrometer at the Texas Petawatt Laser facility. The Global Spectrometer for Positron and Electron Characterization (GSPEC) is capable of detecting electrons and positrons over a large energy range from 3-150 MeV and has been designed to diagnose the electrons and positrons accelerated by high-intensity laser interactions with over-critical targets.

Interest in studying power flow dynamics has grown in recent years, with new power flow diagnostics being developed at Sandia National Laboratories for the Z Pulsed Power Facility. Presently, the only power flow loads that have been studied are cylindrical static or imploding loads that are driven by synchronous short pulse (100 ns rise time). Presented is a design that utilizes the dynamic materials properties program's stripline geometry in a high voltage pulsed shaped (asymmetric asynchronous) driving mode. This design has exhibited repeatable current loss with a large time-varying inductance that is well matched to the machine at pulse initialization but which triples to high inductance in 800 ns. Evidence is presented that plasma not captured in the magnetohydrodynamic approximation and ill represented by any of our existing predictive pulsed power codes is adversely affecting load current delivery. The authors believe this design could be of great interest to the experimental and modeling communities for studying power flow dynamics.

We discuss and numerically test a method for direct and unambiguous measurement of ultrahigh laser intensities exceeding 10(20) W/cm(2). The method is based on the use of multiple sequential tunneling ionization of heavy atoms with sufficiently high ionization potentials. We show that, due to a highly nonlinear dependence of tunneling ionization rates on the electromagnetic field strength, an offset in the charge distribution of ions appears sufficiently sensitive to the peak value of intensity in the laser focus. A simple analytic theory is presented which helps in estimating the maximal charge state produced at a given intensity via the tunnel-ionization mechanism. The theory also allows for calculating qualitatively a distribution in charge states generated in different parts of the laser focus. These qualitative predictions are supported by numerical simulations of the tunneling cascades developed in the interaction of a short intense laser pulse with a low-density target consisting of noble gases including argon, krypton, and xenon. Results of these simulations show that, using this technique, intensities in the range 10(20)-10(24) W/cm(2) can be measured with sufficient reliability. The method could be extremely useful and of high demand in view of the expected commissioning of several new laser facilities capable of delivering ultrapowerful light pulses in this domain of intensities.

Calibrated diagnostics for energetic particle detection allow for the systematic study of charged particle sources. The Fujifilm BAS-TR imaging plate (IP) is a reusable phosphorescent detector for radiation applications such as x-ray and particle beam detection. The BAS-TR IP has been absolutely calibrated to many low-Z (low proton number) ions, and extending these calibrations to the mid-Z regime is beneficial for the study of laser-driven ion sources. The Texas Petawatt Laser was used to generate energetic ions from a 100 nm titanium foil, and charge states Ti10+ through Ti12+, ranging from 6 to 27 MeV, were analyzed for calibration. A plastic detector of CR-39 with evenly placed slots was mounted in front of the IP to count the number of ions that correspond with the IP levels of photo-stimulated luminescence (PSL). A response curve was fitted to the data, yielding a model of the PSL signal vs ion energy. Comparisons to other published response curves are also presented, illustrating the trend of PSL/nucleon decreasing with increasing ion mass. Published under license by AIP Publishing.

We report on the status of the re-commissioning of a high energy OPCPA laser system with programmable spectrum that serves as a frontend for a 10 PW laser at ELI-Beamlines. The OPCPA chain was developed by a consortium of National Energetics and Ekspla along with scientists of ELI-Beamlines.(1) The laser system, consisting of three picosecond OPCPA stages, pulse cleaner, Offner stretcher, and 5 nanosecond OPCPA stages pumped by Nd:YAG lasers with programmable pulse shape (NL944, Ekspla), allows for precise spectral shaping while achieving high nonlinear conversion efficiency. Employing a subsequent Nd:glass power amplifiers (PA), the system was demonstrated to yield > 1 kJ of energy, while maintaining broad spectrum of > 13 nm (FWHM). After recommissioning the OPCPA frontend in Dolni Brezany, an output energy of 4.3 J, flat beam-profile and good far-field quality has been demonstrated. The spectral shape has been optimized to support > 15 nm bandwidth and > 1.5 kJ, consistent with 10 PW operation of the fully integrated laser system after compression.

The limited aperture and damage threshold of the compressor gratings remains one of the bottlenecks in reaching higher peak powers for the current state-of-the-art laser systems. Object-image-grating self-tiling method provides a way how to double the effective aperture of compressor gratings by phasing them with perpendicularly positioned mirrors. This method is planned to be used in the main compressor for the L4 beamline in ELI Beamlines. A subaperture version of the main compressor was designed to test the feasibility of the object-image-grating self-tiling method and to measure the temporal profile of the pulse throughout the amplification stages during the operation. The subaperture compressor was successfully implemented and temporal profile of the amplified pulse close to its transform limit was retrieved. The grating-mirror alignment was secured through the online measurement using an in-house developed Fizeau interferometer.