Experiments using the OMEGA EP laser system were performed to study collisionless shock acceleration of ions driven by the interaction of a relativistically intense laser pulse with underdense plasma. The energy spectrum of accelerated ions in the direction transverse to laser propagation is measured to have several narrow-band peaks which are quasi-monoenergetic with a typical energy bandwidth of 3%. In deuterium plasmas, these ions generate a significant number of fast fusion neutrons. Particle-in-cell simulations confirm that these ions were accelerated by the interaction of transverse shocks and that the appearance of quasi-monoenergetic spectral features depends on the growth of an ion-electron two-stream instability during the interaction.

The krypton-fluoride Nike laser delivers up to 2 KJ in 248 nm ultraviolet radiation to planar targets with the most uniform illumination of all existing high-energy lasers for inertial confinement fusion (ICF) research. That, combined with high-resolution monochromatic x-ray imaging pioneered at NRL in the 1990s and the recently added two-dimensional (2D) VISAR diagnostics, provides a unique platform for experimental studies of hydrodynamic phenomena important to the inertial confinement fusion (ICF). These include absolute experimental determination of the primary Hugoniots of low-density plastic foams in the Mbar pressure range. Although foam materials have many uses for the manufacturing of ICF targets ranging from laser fusion to Z-pinch-driven dynamic hohlraums, no experimental data on their shock compressibility in the ICF-relevant pressure range was available until our experiments. We describe the first systematic experimental study of the nonlinear, multi mode perturbation evolution triggered by localized target non-uniformities, such as straight grooves and dots, which emulate fill tubes, tents, and other target-mounting structures in laser capsules. The experimental data turned out to be counter-intuitive and challenging for simulations. Finally, the 2D VISAR snap-shot images of the shock velocity field made it possible to directly measure the roughness of a uniformly driven shock front propagating into the target, which is caused by ISI-smoothed laser imprint. Initial measurements for planar CH targets with and without thin high-Z layers added for the imprint mitigation have been successfully performed.

Rayleigh-Taylor (RT) perturbation growth in laser fusion capsules can be triggered by isolated target non-uniformities, such as fill tubes, mounting stalks, tents, etc. Recently, the first experimental study of the perturbation evolution caused by isolated non-uniformities, straight grooves characteristic sizes of < 1 to 10s of $\mu \mathrm{m}$ machined on planar plastic targets, has been carried out on the Nike KrF laser at NRL. Initial target defects are relatively large-amplitude, inherently multi-mode, and grow into the strongly nonlinear regime. Face-on monochromatic x-ray radiography used to diagnose the areal-mass perturbation development indicated an early-time growth of the localized cavity in the target in depth and width, followed by its apparent closure. The initial “scar” on the target appears to be self-healing, making the accelerated foil more uniform instead of being cut through.

Inherently multi-mode evolution of isolated defects, such as straight grooves and axisymmetric dots on planar laser targets, is studied theoretically. The development of perturbations is considered for a propagating rippled shock front, a material interface subject to the classical Richtmyer–Meshkov instability (RMI), a rippled rarefaction wave produced by the feedout process, an ablation front subject to the ablative RMI, and a thin fluid layer subject to the classical Rayleigh–Taylor instability (RTI). For the small-amplitude regime, we have established specific characteristics of the perturbation evolution initiated with such defects, scaling, and conservation laws governing it. The main features of the nonlinear growth of the classical and ablative RTI starting from isolated defects are the lateral expansion of the bubbles and the oblique with respect to the direction of the acceleration, ejection of spikes. It results in filling up the void left from the bubble growth by the laterally converging spike material. This effect, first discovered in simulations by Dahlburg et al. [Phys. Fluids B 5, 571 (1993)], and very recently observed by Zulick et al. [Phys. Rev. Lett. 125, 055001 (2020)], is captured by the appropriate modification of the Ott–Basko thin-layer classical RTI theory for arbitrary defect profiles. Predictions for novel hydrodynamic experiments on multi-mode hydrodynamic perturbation evolution are presented.

The Nike KrF laser facility was used to study the evolution of isolated defects with characteristic sizes of <1 to 10s of μm in laser-accelerated plastic foils. The experimental platform permitted, for the first time, the systematic study of localized perturbation growth, which is inherently multimode, through ablative Richtmyer-Meshkov and Rayleigh-Taylor stages and into the strongly nonlinear regime. Initial target defects were relatively large amplitude, but spatially localized, and emulated tent, fill-tube, and other nonuniformities that are present in inertial confinement fusion capsules. Face-on x-ray radiography indicated initial growth of the perturbation in both depth and width, followed by its apparent closure due to oblique spike growth. Hollow jetlike profiles of laterally expanding, rising, Rayleigh-Taylor bubbles were observed on the rear surface of the target from each isolated defect. Radiation hydrodynamic simulations provided insight into the mechanism of the closure and other features of the bubble and spike evolution specific to isolated defects.

The hydrodynamic growth of pre-imposed isolated defects has been studied with varied laser drive. Targets were machined at NRL by etching narrow isolated grooves into thin polystyrene (CH) foils using femtosecond laser ablation. Two laser pulse shapes were used to drive the foils with and without a thin high-Z overcoat which produced a hybrid indirect–direct drive. The growth rate and saturation time were observed to vary with different laser drives as well as the initial perturbation depth and width. Faster growth was observed with a low adiabat drive both experimentally and in simulations. Hybrid drive reduced the apparent “closure” of defects at late time. Rear-surface structures were observed from the isolated defects, with the largest structures observed from high adiabat square pulses and the smallest from hybrid drive.

Swept-wavelength Raman signatures have been measured for isotopic variants of polyethylene, acetic acid, and potassium sulfates. The swept-wavelength measurements produce two-dimensional Raman signatures which enable identification techniques based on changes in Raman peak amplitudes as a function of wavelength. In addition to the typical Raman peak energy shifts, which results from the change in isotope mass, three wavelength dependent mechanisms for isotope identification have been identified. Changes in the shape of the Raman signal, the presence and absence of Raman peaks over specific wavelength ranges, and changes in absorption of the Raman signal were observed as a result of isotopic substitution. These features provide additional specificity in the isotopic Raman signatures which suggests swept-wavelength Raman signatures will facilitate the identification of isotopes in complex and dirty mixtures. Measurements in the visible range suggest that the identification mechanisms are primarily evident in the ultraviolet, or resonance Raman, region.

A scaled experiment comprising a laser-driven explosion in a cavity is used to characterize the coupling of mechanical energy into the surrounding solid material. Experiments are performed using poly(methyl methacrylate) and dry Salem limestone as the explosion containment blocks materials, in which are milled scaled spherical cavities of various dimensions. Measurements of the coupled shock, taken with fiber optic probes at the cavity wall, show the critical radius where wall deformation transitions from plastic to elastic deformation. These measurements also provide a diagnostic of the air blast, which is validated against GEODYN simulation code. The measurement of the coupled shock amplitude taken farther from the wall in the linear region indicates increased coupling efficiency in small cavities over the range of scaled cavity radii from 6 to 20 m/kt1/3, a phenomenon not previously observed in experiments. A comparison of results taken in this experiment with a parallel experiment using high explosive (HE) as the source shows that coupled shocks generated with HE are characteristically different with much larger amplitude than those produced by a high energy density laser-driven source with the same yield. This experimental technique potentially provides a rapid and cost-effective method to analyze the consequences of a full-scale, low yield, buried explosion.

Measurements taken with new fiber optic shock wave diagnostics are used to identify plastic and elastic deformation of material under shock compression. This deformation is caused by a point like explosion in a spherical cavity within a containment material and is observed by monitoring cavity wall motion using a fiber tip sensor. Additionally, a new fiber optic loop diagnostic analogous to a Dremin loop is also demonstrated for the first time and the benefits are discussed.

Superponderomotive-energy electrons are observed experimentally from the interaction of an intense laser pulse with a relativistically transparent target. For a relativistically transparent target, kinetic modeling shows that the generation of energetic electrons is dominated by energy transfer within the main, classically overdense, plasma volume. The laser pulse produces a narrowing, funnel-like channel inside the plasma volume that generates a field structure responsible for the electron heating. The field structure combines a slowly evolving azimuthal magnetic field, generated by a strong laser-driven longitudinal electron current, and, unexpectedly, a strong propagating longitudinal electric field, generated by reflections off the walls of the funnel-like channel. The magnetic field assists electron heating by the transverse electric field of the laser pulse through deflections, whereas the longitudinal electric field directly accelerates the electrons in the forward direction. The longitudinal electric field produced by reflections is 30 times stronger than that in the incoming laser beam and the resulting direct laser acceleration contributes roughly one third of the energy transferred by the transverse electric field of the laser pulse to electrons of the super-ponderomotive tail.

Magnetic reconnection is a fundamental process occurring in many plasma systems. Magnetic field lines break and reconfigure into a lower energy state, converting released magnetic field energy into plasma kinetic energy. Around some of the universe's most energetic objects, such as y -ray burst or active galactic nuclei, where the magnetic field energy exceeds the plasma rest mass energy, the most extreme magnetic reconnection in the relativistic regime is theorized. The presented experiments and three-dimensional particle-in-cell modeling recreate in the laboratory the scaled plasma conditions necessary to access the relativistic electron regime and therefore approach conditions around these distant, inaccessible objects. High-power, ultrashort laser pulses focused to high intensity (I > 2.5 x 10(18) Wcm(-2)) on solid targets produces relativistic temperature electrons within the focal volume. The hot electrons are largely confined to the target surface and form a radial surface current that generates a huge, expanding azimuthal magnetic field. Focusing two laser pulses in close proximity on the target surface leads to oppositely directed magnetic fields being driven together. The fast electron motion due to the magnetic reconnection is inferred using an experimental x-ray imaging technique. The x-ray images enable the measurement of the reconnection layer dimensions and temporal duration. The reconnection rates implied from the aspect ratio of the reconnection layer, delta/L approximate to 0.3, was found to be consistent over a range of experimental pulse durations (40 fs-20 ps) and agreed with the modeling. Further experimental evidence for magnetic reconnection is the formation of a nonthermal electron population shown by the modeling to be accelerated in the reconnection layer.

Swept-wavelength Raman signatures have been measured for isotopic variants of polyethylene and acetic acid. Wavenumber shifts, peak amplitude variation, and wavelength dependent amplitudes have been identified as isotope identification mechanisms.

The reflectivity of a short-pulse laser at intensities of 2 × 10 21 Wcm − 2 with ultra-high contrast ( 10 − 15 ) on sub-micrometer silicon nitride foils was studied experimentally using varying polarizations and target thicknesses. The reflected intensity and beam quality were found to be relatively constant with respect to intensity for bulk targets. For submicron targets, the measured reflectivity drops substantially without a corresponding increase in transmission, indicating increased conversion of fundamental to other wavelengths and particle heating. Experimental results and trends observed in 3D particle-in-cell simulations emphasize the critical role of ion motion due to radiation pressure on the absorption process. Ion motion during ultra-short pulses enhances the electron heating, which subsequently transfers more energy to the ions.

Reduced surface area targets were studied using an ultra-high intensity femtosecond laser in order to determine the effect of electron sheath field confinement on electron dynamics. X-ray emission due to energetic electrons was imaged using a Ka imaging crystal. Electrons were observed to travel along the surface of wire targets, and were slowed mainly by the induced fields. Targets with reduced surface areas were correlated with increased hot electron densities and proton energies. Hybrid Vlasov-Fokker-Planck simulations demonstrated increased electric sheath field strength in reduced surface area targets.

We perform relativistic-intensity laser experiments using the Omega EP laser to investigate channeling phenomena and particle acceleration in underdense plasmas. A fundamental understanding of these processes is of importance to the hole-boring fast ignition scheme for inertial confinement fusion. Proton probing was used to image the electromagnetic fields formed as the Omega EP laser pulse generated a channel through underdense plasma. Filamentation of the channel was observed, followed by self-correction into a single channel. The channel radius as a function of time was found to be in reasonable agreement with momentum conserving snowplough models.