Focal cone high harmonic generation driven by a 400 TW laser.

The generation of self-focusing beams of extreme ultraviolet (XUV) radiation using the focal cone high harmonic generation (FCHHG) technique is examined for high energy lasers. The FCHHG geometry is created by passing a focusing laser beam through a gas sheet prior to reaching focus and thus creating a converging beam of high harmonic radiation. This leads to a larger interaction area that increases the total area of XUV emission while not exceeding the saturation intensity of the target atoms or increasing the density of the atoms. Such a method allows for scaling of HHG to any incident laser power. An experiment was conducted demonstrating such scaling to incident 400 TW pulses, showing both the expected spectral signature of HHG and the converging cone of XUV radiation. It was found that this technique is very sensitive to spatial non-uniformity in the driving laser, which has become more prevalent in high energy laser systems.

Multichannel Doppler fiber-imager spectrometer for spatiotemporal characterization of high-intensity laser-driven plasmas.

We demonstrate a flexible multichannel fiber-based imaging Doppler spectrometer to characterize plasmas in high intensity (≥1 × 1018 W/cm2) laser-plasma experiments at high repetition rates. This instrument collects data from ×21 different plasma locations combining optical fibers and a single imaging spectrometer. This diagnostic maps the plasma velocity evolution as a function of time with sub-pico-second resolution. Experimental results showing 2D velocity measurements of plasma with 20 µm spatial resolution are presented. Intensities of the order of 1018 W/cm2 were used to generate a plasma, while a much less intense, frequency doubled (400 nm), probe beam (1011 W/cm2) was used to measure the Doppler shift from the plasma critical surface. The instrument can be scaled to a larger number of channels (e.g., 100) still using a single spectrometer.

Flexible tape-drive target system for secondary high-intensity laser-driven sources.

We present the development of a flexible tape-drive target system to generate and control secondary high-intensity laser-plasma sources. Its adjustable design permits the generation of relativistic MeV particles and x rays at high-intensity (i.e., ≥1 × 1018 W cm-2) laser facilities, at high repetition rates (>1 Hz). The compact and robust structure shows good mechanical stability and a high target placement accuracy (<4 µm RMS). Its compact and flexible design allows for mounting in both the horizontal and vertical planes, which makes it practical for use in cluttered laser-plasma experimental setups. The design permits ∼170° of access on the laser-driver side and 120° of diagnostic access at the rear. A range of adapted apertures have been designed and tested to be easily implemented to the targetry system. The design and performance testing of the tape-drive system in the context of two experiments performed at the COMET laser facility at the Lawrence Livermore National Laboratory and at the Advanced Lasers and Extreme Photonics (ALEPH) facility at Colorado State University are discussed. Experimental data showing that the designed prototype is also able to both generate and focus high-intensity laser-driven protons at high repetition rates are also presented.

Investigation on the origin of hot electrons in laser plasma interaction at shock ignition intensities.

Shock Ignition is a two-step scheme to reach Inertial Confinement Fusion, where the precompressed fuel capsule is ignited by a strong shock driven by a laser pulse at an intensity in the order of [Formula: see text] W/cm[Formula: see text]. In this report we describe the results of an experiment carried out at PALS laser facility designed to investigate the origin of hot electrons in laser-plasma interaction at intensities and plasma temperatures expected for Shock Ignition. A detailed time- and spectrally-resolved characterization of Stimulated Raman Scattering and Two Plasmon Decay instabilities, as well as of the generated hot electrons, suggest that Stimulated Raman Scattering is the dominant source of hot electrons via the damping of daughter plasma waves. The temperature dependence of laser plasma instabilities was also investigated, enabled by the use of different ablator materials, suggesting that Two Plasmon Decay is damped at earlier times for higher plasma temperatures, accompanied by an earlier ignition of SRS. The identification of the predominant hot electron source and the effect of plasma temperature on laser plasma interaction, here investigated, are extremely useful for developing the mitigation strategies for reducing the impact of hot electrons on the fuel ignition.

A flexible proton beam imaging energy spectrometer (PROBIES) for high repetition rate or single-shot high energy density (HED) experiments (invited).

The PROBIES diagnostic is a new, highly flexible, imaging and energy spectrometer designed for laser-accelerated protons. The diagnostic can detect low-mode spatial variations in the proton beam profile while resolving multiple energies on a single detector or more. When a radiochromic film stack is employed for "single-shot mode," the energy resolution of the stack can be greatly increased while reducing the need for large numbers of films; for example, a recently deployed version allowed for 180 unique energy measurements spanning ∼3 to 75 MeV with <0.4 MeV resolution using just 20 films vs 180 for a comparable traditional film and filter stack. When utilized with a scintillator, the diagnostic can be run in high-rep-rate (>Hz rate) mode to recover nine proton energy bins. We also demonstrate a deep learning-based method to analyze data from synthetic PROBIES images with greater than 95% accuracy on sub-millisecond timescales and retrained with experimental data to analyze real-world images on sub-millisecond time-scales with comparable accuracy.

Compact high repetition rate Thomson parabola ion spectrometer.

We present the development of a compact Thomson parabola ion spectrometer capable of characterizing the energy spectra of various ion species of multi-MeV ion beams from >1020W/cm2 laser produced plasmas at rates commensurate with the highest available from any of the current and near-future PW-class laser facilities. This diagnostic makes use of a polyvinyl toluene based fast plastic scintillator (EJ-260), and the emitted light is collected using an optical imaging system coupled to a thermoelectrically cooled scientific complementary metal-oxide-semiconductor camera. This offers a robust solution for data acquisition at a high repetition rate, while avoiding the added complications and nonlinearities of micro-channel plate based systems. Different ion energy ranges can be probed using a modular magnet setup, a variable electric field, and a varying drift-distance. We have demonstrated operation and data collection with this system at up to 0.2 Hz from plasmas created by irradiating a solid target, limited only by the targeting system. With the appropriate software, on-the-fly ion spectral analysis will be possible, enabling real-time experimental control at multi-Hz repetition rates.

Ultra-compact x-ray spectrometer for high-repetition-rate laser-plasma experiments.

We present in this work the development of an ultra-compact, multi-channel x-ray spectrometer (UCXS). This diagnostic has been specially built and adapted to perform at high-repetition-rate (>1 Hz) for high-intensity, short-pulse laser plasma experiments. X-ray filters of varying materials and thicknesses are chosen to provide spectral resolution up to ΔE ≈ 1 keV over the x-ray energy range of 1-30 keV. These filters are distributed over a total of 25 channels, where each x-ray filter is coupled to a single scintillator. The UCXS is designed to detect and resolve a large variety of laser-driven x-ray sources such as low energy bremsstrahlung emission, fluorescence, and betatron radiation (up to 30 keV). Preliminary results from commissioning experiments at the ABL laser facility at Colorado State University are provided.

Single-shot measurements of pulse-front tilt in intense ps laser pulses and its effect on accelerated electron and ion beam characteristics (invited).

We report recent single-shot spatiotemporal measurements of laser pulses, including pulse-front tilt (PFT) and spatial chirp, taken at the Compact Multipulse Terawatt laser at the Jupiter Laser Facility in Livermore, CA. STRIPED FISH, a device that measures the complete 3D electric field of fs to ps laser pulses on a single shot, was adapted to near infrared for these measurements. We present the design of the instrument used for these experiments, the on-shot measurements of systematic high-order PFT, and shot-to-shot variations in the measurements of spatiotemporal couplings. Finally, we simulate the effect of PFT in target normal sheath acceleration experiments. These simulations showed that pulse front tilt can steer hot electrons, shape the distribution of the accelerating sheath field, and increase the variability of cutoff energy in the resulting proton spectra. While these effects may be detrimental to experimental accuracy if the pulse front tilt is left unmeasured, hot electron steering shows promise for precision manipulation of the particle source for a range of applications, including irradiation of secondary targets for opacity measurements, radiography, or neutron generation.

Applications of machine learning to a compact magnetic spectrometer for high repetition rate, laser-driven particle acceleration.

Accurately and rapidly diagnosing laser-plasma interactions is often difficult due to the time-intensive nature of the analysis and will only become more so with the rise of high repetition rate lasers and the desire to implement feedback on a commensurate timescale. Diagnostic analysis employing machine learning techniques can help address this problem while maintaining a high degree of accuracy. We report on the application of machine learning to the analysis of a scintillator-based electron spectrometer for experiments on high intensity, laser-plasma interactions at the Colorado State University Advanced Lasers and Extreme Photonics facility. Our approach utilizes a neural network trained on synthetic data and tested on experiments to extract the accelerated electron temperature. By leveraging transfer learning, we demonstrate an improvement in the neural network accuracy, decreasing the network error by 50%.

Integrated quantitative PIXE analysis and EDX spectroscopy using a laser-driven particle source.

Among the existing elemental characterization techniques, particle-induced x-ray emission (PIXE) and energy-dispersive x-ray (EDX) spectroscopy are two of the most widely used in different scientific and technological fields. Here, we present the first quantitative laser-driven PIXE and laser-driven EDX experimental investigation performed at the Centro de Láseres Pulsados in Salamanca. Thanks to their potential for compactness and portability, laser-driven particle sources are very appealing for materials science applications, especially for materials analysis techniques. We demonstrate the possibility to exploit the x-ray signal produced by the co-irradiation with both electrons and protons to identify the elements in the sample. We show that, using the proton beam only, we can successfully obtain quantitative information about the sample structure through laser-driven PIXE analysis. These results pave the way toward the development of a compact and multifunctional apparatus for the elemental analysis of materials based on a laser-driven particle source.

Author Correction: Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Quantitative phase contrast imaging of a shock-wave with a laser-plasma based X-ray source.

X-ray phase contrast imaging (XPCI) is more sensitive to density variations than X-ray absorption radiography, which is a crucial advantage when imaging weakly-absorbing, low-Z materials, or steep density gradients in matter under extreme conditions. Here, we describe the application of a polychromatic X-ray laser-plasma source (duration ~0.5 ps, photon energy >1 keV) to the study of a laser-driven shock travelling in plastic material. The XPCI technique allows for a clear identification of the shock front as well as of small-scale features present during the interaction. Quantitative analysis of the compressed object is achieved using a density map reconstructed from the experimental data.

Towards an in situ, full-power gauge of the focal-volume intensity of petawatt-class lasers.

About 50 years ago, Sarachick and Schappert [Phys. Rev. D. 1, 2738-2752 (1970)] showed that relativistic Thomson scattering leads to wavelength shifts that are proportional to the laser intensity. About 28 years later, Chen et al. [Nature 396, 653-655 (1998)] used these shifts to estimate their laser intensity near 1018 W/cm 2. More recently, there have been several theoretical studies aimed at exploiting nonlinear Thomson scattering as a tool for direct measurement of intensities well into the relativistic regime. We present the first quantitative study of this approach for intensities between 1018 and 1019 W/cm 2. We show that the spectral shifts are in reasonable agreement with estimates of the peak intensity extracted from images of the focal area obtained at reduced power. Finally, we discuss the viability of the approach, its range of usefulness and how it might be extended to gauge intensities well in excess of 1019 W/cm 2.

Development of an adjustable Kirkpatrick-Baez microscope for laser driven x-ray sources.

A prototype of a highly adjustable Kirkpatrick-Baez (KB) microscope has been designed, built, and tested in a number of laser driven x-ray experiments using the high power (200 TW) VEGA-2 laser system of the Spanish Centre for Pulsed Lasers (CLPU). The presented KB version consists of two, perpendicularly mounted, 500 µm thick silicon wafers, coated with a layer of platinum, a few tens of nanometers thick. Unlike the usual millimeter thick glass substrate, this design allows for a larger bending flexibility and large adjustment range. According to simulations, this KB microscope offers broadband multikiloelectron volt reflection spectra (1 eV-20 keV), allowing more spectral tunability than conventional Bragg crystals. In addition to be vacuum compatible, this prototype is characterized by a relatively small size (21 cm × 31 cm × 27 cm) and permits remote control and modification both of the radii of curvature (down to 10 m) and of the grazing incidence angle (up to 60 mrad). A few examples of focusing performance tests and experimental results are discussed.

Emergence of a Higher Energy Structure in Strong Field Ionization with Inhomogeneous Electric Fields.

Studies of strong field ionization have historically relied on the strong field approximation, which neglects all spatial dependence in the forces experienced by the electron after ionization. More recently, the small spatial inhomogeneity introduced by the long-range Coulomb potential has been linked to a number of important features in the photoelectron spectrum, such as Coulomb asymmetry, Coulomb focusing, and the low energy structure. Here, we demonstrate using midinfrared laser wavelength that a time-varying spatial dependence in the laser electric field, such as that produced in the vicinity of a nanostructure, creates a prominent higher energy peak. This higher energy structure (HES) originates from direct electrons ionized near the peak of a single half-cycle of the laser pulse. The HES is separated from all other ionization events, with its location and width highly dependent on the strength of spatial inhomogeneity. Hence, the HES can be used as a sensitive tool for near-field characterization in the "intermediate regime," where the electron's quiver amplitude is comparable to the field decay length. Moreover, the large accumulation of electrons with tuneable energy suggests a promising method for creating a localized source of electron pulses of attosecond duration using tabletop laser technology.