Session YO4: AB: Intense Short Pulse Laser-Plasma Interactions

Enhanced Electron Acceleration in Aligned Nanowire Arrays Irradiated at Relativistic Intensities

Electron acceleration by the irradiation of solid density targets with short-pulse ultra-high intensity lasers follows Beg's scaling [1]. When irradiated at relativistic intensities, nanowire arrays have been shown to greatly enhance energy deposition compared to traditional slab targets [2]. Such targets are of interest for enhanced electron acceleration for applications such as ion acceleration and hard x-ray production. Here we compare measured spectra of electrons from nanowire and slab targets irradiated at intensities \textgreater 10$^{\mathrm{21}}$ Wcm$^{\mathrm{-2}}$. We observe a significant increase in both cut off electron energy and total flux from nanowire targets due to the acceleration of electrons beyond Beg's scaling values. Detailed 3D particle in cell simulations will be discussed along with measurements of electron spectra. Supported by AFOSR grant FA9550-17-1-0278 and DOE grant DE-SC0014610 at Facilities supported by LaserNetUS grant DE-SC0019076. 1. F. N. Beg et al, PoP, 4, 447 (1997) 2. M.A. Purvis et al, Nat. Photonics 7, 796 (2013)   

Stochastic acceleration of electrons in colliding laser beams.

The electron dynamics in counter-propagating laser beams has attracted a great deal of interest and the stochastic acceleration was thought the reason for high energized electron tails observed in laser-plasma interaction. However, due to the multidimensional spatiotemporal characteristics of the laser waves and strong nonlinearity of relativistic electron in these waves, the analytic investigation of stochastic electron acceleration in the colliding laser waves is rather limited or complicated. The numerical simulations could also provide criterion for stochasticity but the lack of theoretic analysis limits its universal applicability. In this work, we investigate the mechanism of stochastic electron acceleration in colliding laser waves by employing proper canonical variables and effective time, such that the new Hamiltonian becomes time independent when the perturbative (weaker) laser wave is absent. The analysis clearly reveals the physical picture of stochastic electron dynamics. It shows that when the amplitude of the perturbative laser field exceeds some critical value, stochastic electron acceleration occurs within some electron energy range. It demonstrates that the essential role of the perturbation is to change the dephasing rate (new Hamiltonian) between the electron and dominant laser such that the maximum electron kinetic energy, which could be gained under stochastic acceleration, can significantly exceed the ponderomotive scaling for the dominant laser. The numerical simulations integrating the Hamiltonian equations are in a very good agreement with the findings from our analytic theory.   

Energy deposition of highly relativistic laser pulses into solid and near solid density high Z plasmas

The irradiation of near solid density, high Z nanostructure arrays at moderately relativistic intensities (a$_{\mathrm{o}}=$ 1) offers nearly complete absorption of the laser light, which penetrates deep into the array [1]. As the intensity increases, however at highly relativistic intensities (a$_{\mathrm{o}}=$ 20), the wires explode before the peak of the laser pulse forming a plasma exceeding even the relativistically corrected electron density, on the order of 10$^{\mathrm{23}}$ electrons/cc. Despite the formation of this supercritical density surface, the laser energy is still deposited deep into the nanowires by accelerated high energy electrons that ionize gold up to Au$^{\mathrm{+69}}$ (Ne-like Au). Ionized K shell spectroscopy of buried Ni tracers reveals the heat penetration depth in solid density Au slab targets and near solid density Au nanowire plasmas is \textgreater 1 micron and \textgreater 5 micron, respectively. These experimental results are in agreement with fully relativistic three dimensional particle in cell simulations. [1] M.A. Purvis et al Nature Photonics 7, 796 (2013)   

Self-consistent PIC Simulations of Neutron Generation from Intense Laser-plasma Interactions

High intensity laser-plasma interactions have the potential to produce bright, compact neutron sources. Recent experiments using a deuterium jet have demonstrated the generation of up to $10^{10}$ neutrons per shot, but the details of the laser-plasma interactions and deuteron heating are not yet understood. We have followed a Direct Simulation Monte Carlo (DSMC) approach to model fusion reactions self-constentily in PIC codes, inspired by the methods used for Coulomb collisions. This module has been implemented and tested in the OSIRIS PIC code. We will discuss the implementation options and validity tests and will present results from novel simulations of neutron generation in the interaction of intense lasers with deuterium jets.   

Non-stochastic electron acceleration in relativistic multipicosecond laser-solid interaction

Understanding the mechanisms by which relativistic multipicosecond laser pulses incident on solid density targets produce high energy electrons is of fundamental importance for applications of picosecond laser-plasma interactions. We elucidate a potential route to non-stochastic electron acceleration in the underdense ion shelf and opaque plasma wall preplasma profile formed by such pulses. In 1D particle-in-cell simulations, electrons gain energy on a single-bounce trajectory consisting of backward acceleration along the shelf, bounce past the shelf edge, and forward acceleration into the wall. We find that the direct laser acceleration of electrons during forward propagation corresponds to a high energy, non-stochastic regime of electron acceleration in counter-propagating laser pulses made accessible by the pre-acceleration of electrons during backward propagation. Backward energy gain is dominated not by the evolving electrostatic potential, as previously proposed, but by direct laser acceleration in the reflected laser pulse. Furthermore, the observed pre-acceleration is in principle sufficient to enable the final electron energy to exceed the maximum energy attainable from stochastic heating. [arXiv:1906.11975]   

Enhanced electron acceleration from intense multi-picosecond laser pulses interacting with focusing cone targets

NIF's Advanced Radiography Capability (ARC) laser has been used to create a high flux short-pulse driven proton source via target normal sheath acceleration (TNSA). A maximum proton energy of five times over that predicted by conventional scalings has been observed at quasirelativistic laser intensities (\sim 10$^{18}$ W/cm$^{2}$), indicative of super-ponderomotive electron acceleration. Laser coupling into relativistic electrons is further enhanced by the use of a focusing compound parabolic concentrator (CPC) cone on the target front surface, which serves to geometrically focus incident rays and confine the expanding plasma during the pulse interaction. Correspondingly, electron spectra observed from targets with a CPC cone show increased slope temperatures over flat targets. To investigate the CPC cone's effect on the laser plasma interaction, 2D PIC simulations are presented studying the absorption of the incident laser on the cone walls. PIC simulations of the focused laser interacting with the confined plasma at the cone tip and the resultant hot electron generation will also be discussed.   

A plasma solenoid driven by a OAM laser beam

The generation of a quasi-static long time standing magnetic field in the laser plasma interaction is a subject of many theoretical and experimental studies. In most of the studies, the magnetization of the plasma originates from the inverse faraday effect, where the spin angular momentum of a laser beam is transfered to the electrons due to dissipation processes such as collisions, ionization or radiation friction. Here, we present a novel setup in which he laser to electron angular momentum transfer does not require any dissipative process, but takes place within a purely optical process. It is based on the irradiation of a low density electron plasma with a strongly focused polarized laser beam carrying an orbital angular momentum. Three dimensional “Particle In Cell” (PIC) simulations show an irreversibly transfer of angular momentum from the laser to the electrons, and the generation of a quasi-static axial magnetic field over long time. These numerical results are confirmed with theoretical analysis. The PIC simulations coupled to the analytical theory show that the B field amplitude and direction may be tuned by controlling the laser beam characteristics such as for example the orbital angular momentum and/or the pulse duration.   

Radiation Hydrodynamic Simulations of Laser Plasma Interactions inside Parabolic Cone Targets

A novel target [1] for long focal length, multi-picosecond, intense short laser pulses was recently designed and fielded on the ARC short pulse laser at the NIF facility. Several diagnostics indicated that the hot electron temperatures reached in these new cone targets far exceeded (by a factor of 5) the values predicted by the usual sub-picosecond (ponderomotive) scaling with intensity. We investigate the laser plasma interaction involved and find that two major factors caused these increases in performance: first, a focusing effect (due to the cone geometry) and second, a laser-plasma effect (where the cone fills with a near-critical plasma resulting in an increase in the amount of direct-laser-acceleration[2].) Simulations of the laser plasma interaction inside the cone will be presented that help elucidate the relative importance of each factor. [1] A. MacPhee et al., ``Parabolic Concentrator Targets for Increasing Intensity of Long focal Length Lasers'', submitted Optics Express, (2019) [2] A. Krygier et al, Physics of Plasmas 21, 023112 (2014)   

Experimental and theoretical study of wavelength dependence of plasma dynamics in laser filamentation in solids.

We experimentally and theoretically investigate plasma dynamics in laser filamentation in fused silica by varying the driver wavelength from 1.2 to 2.3 $\mu $m covering the near-zero to the anomalous group-velocity dispersion regimes. First, we perform femtosecond time-resolved interferometry to measure plasma densities in filaments, which unexpectedly reveals that plasma densities are not monotonically decreasing with increasing wavelength. This result is in sharp contrast to recent theoretical work in filamentation in air/gases [1,2] as well as our own numerical simulations in fused silica in which the electron collision time is assumed to be constant for all the wavelengths. Therefore, to investigate further, we also perform time-resolved shadowgraphy which, combined with interferometry, enables us to determine the electron collision time in plasma [3]. We find out that the electron collision time is not a constant for different wavelengths, which can change the plasma dynamics in filamentation significantly.[1]L. Berg\'{e} et al., Phys. Rev. A 88, 023816 (2013).[2]Y. E. Geints et al., Appl. Opt. 56, 1397--1404 (2017).[3]A. Couairon et al., Eur. Phys. J-Spec. Top. 199, 76 (2011).[4]Q. Sun et al., Opt. Lett. 30, 3 (2005).   

Presence and future of X-ray scattering techniques for the understanding of ultra-short pulse laser matter interactions

The development of next generation laser plasma sources for novel applications ranging from astro-physics, fusion research to particle acceleration and tumor therapy requires methods to study the dynamics and heating of dense plasmas on nanometer and femtosecond scales simultaneously. FELs are identified as a new tool to achieve this goal since they combine short bunches, high photon numbers with small bandwidth and high penetration power. We review our recent advances in theory and experiments for transferring scattering techniques into the short-pulse laser domain. Besides the future potentials of the small angle scattering technique we will focus on the possible impact of resonant scattering for opacity measurements.   

Structured targets for generation of Megatesla-level magnetic fields and their detection through Faraday rotation of XFEL beams

Laser-driven Megatesla-level magnetic fields have been identified as the key feature of laser-plasma interactions that enables effective direct laser acceleration of electrons to GeV-level energies and efficient generation of directed gamma-ray beams. Experimental detection of these fields is essential. However, the combination of the unprecedented field strength and high plasma density rules out conventional optical and charged particle probing techniques. As an alternative, we have examined the feasibility of utilizing an XFEL beam as a magnetic field detection tool, based on the magnetic field inducing a polarization rotation due to the Faraday effect [Phys. Plasmas 26, 013105 (2019)]. Our PIC simulations and post-processing show that structured targets with a pre-filled channel are necessary to achieve rotations that exceed 0.1 mrad. The polarization impurity of an XFEL beam with $5\times {10}^{12}$ photons must not exceed ${10}^{-8}$.   

On the control of electron heating for optimal radiation pressure ion acceleration

Intense laser-plasma interactions offer the possibility of producing short high-energy ion beams for a wide range of applications. However, it is not yet well understood how the details of the laser-plasma interaction impact the spectral quality of the accelerated ions. We have performed 2D and 3D particle-in-cell simulations for a large set of laser and plasma parameters to explore how electron heating impacts the quality of ions produced in the radiation pressure acceleration regime. We show how the electron heating, stability of the target surface, and ion acceleration depend on the laser polarization, profile, and angle of incidence, as well as on the plasma density. Based on this study, we have developed a model for optimal radiation pressure acceleration and validate it against 3D PIC simulations. These results are expected to help optimize the beam quality in future ion acceleration experiments.   

Development Considerations for High-Repetition-Rate HEDP

A new generation of high-repetition-rate lasers is being proposed and developed for use in the field of High Energy Density Physics (HEDP). While the highest-energy facilities may continue to operate at low shot-rates, higher-repetition-rate facilities with lower energies but comparable intensities can now complement them. Experiments designed to take advantage of a shot rate of $>$1/minute have the opportunity to expand HEDP into a computationally-intensive and rich landscape of real-time feedback and "big data" statistical analysis. However, success in this area will require more than lasers and scientific questions: high-repetition-rate operation requires a broad range of technical development and a significant shift in experimentalist thinking. We discuss experimental techniques and considerations associated with the transition from operation at several-shots-per-day towards operation at $>$1/minute, and we apply these to recent particle-acceleration datasets taken at high shot-rate. We focus on the digitization of HEDP particle detectors, the automation of data acquisition/analysis, and approaches to experimentation unique to high repetition-rate.   

Excitation of magnetosonic solitons with high power, pulsed CO2 laser in an overdense gas-jet target

The recent availability of long wavelength ($10\mu m$), pulsed $CO_2$ laser has enabled us to use newer targets like gas jet target for overcritical laser plasma interaction. It may simplify high repetition rate operation compared to a solid target since the latter needs to be mechanically replaced or displaced in a very short time. The excitation of magneto-sonic solitons by carrying out the 2D Particle-in-Cell simulation under OSIRIS-4.0 framework in over-dense gas jet targets has been shown with a p-polarised, pulsed $CO_2$ laser with an intensity $I=7 \times 10^{17}W/cm^2$, incident normally to the gas jet target in presence of Kilo-Tesla order of an external magnetic field. Furthermore, it has been shown that such an excitation is independent of the polarisation of the $CO_2$ laser. The solitary structures generated henceforth, are stable for several thousands of plasma periods.The interaction between two magneto-sonic solitons has also been studied by the generation of solitary structures with two $CO_2$ lasers at both ends of the system.