Research at the Naval Research Laboratory pursues advanced laser technologies that can mitigate laser plasma instabilities (LPI) in applications such as inertial confinement fusion. Increased laser bandwidth is currently being developed as a strategy to suppress LPI. In particular, stimulated rotational Raman scattering (SRRS) has been demonstrated using the Nike krypton fluoride (KrF) laser facility as a viable method for enhancing bandwidth. The technique is applicable to other large laser facilities. Through a combination of high-intensity propagation and optimization of the laser spectrum in the low-energy stages of the laser, the output spectrum of Nike has been broadened with SRRS from its standard operating value of 1 THz full width at half-maximum (FWHM) bandwidth to span a range greater than 4.5 THz. This amount of bandwidth has been shown in simulations to be effective for mitigation of slow-growth laser plasma instabilities, such as cross-beam energy transport (CBET).

It has long been recognized that broadband laser light has the potential to control parametric instabilities in inertial-confinement-fusion (ICF) plasmas. Here, we use results from laser-plasma-interaction simulations to estimate the bandwidth requirements for mitigating the three predominant classes of instabilities in direct-drive ICF implosions: cross-beam energy transfer (CBET), two-plasmon decay (TPD), and stimulated Raman scattering (SRS). We find that for frequency-tripled, Nd:glass laser light, a bandwidth of 8.5 THz can significantly increase laser absorption by suppressing CBET, while ∼13 THz is needed to mitigate absolute TPD and SRS on an ignition-scale platform. None of the glass lasers used in contemporary ICF experiments, however, possess a bandwidth greater than 1 THz and reaching larger values requires the use of an auxiliary broadening technique such as optical parametric amplification or stimulated-rotational-Raman scattering. An arguably superior approach is the adoption of an argon-fluoride (ArF) laser as an ICF driver. Besides having a broad bandwidth of ∼10 THz, the ArF laser also possesses the shortest wavelength (193 nm) that can scale to the high energy/power required for ICF—a feature that helps to mitigate parametric instabilities even further. We show that these native properties of ArF laser light are sufficient to eliminate nearly all CBET scattering in a direct-drive target and also raise absolute TPD and SRS thresholds well above those for broadband glass lasers. The effective control of parametric instabilities with broad bandwidth is potentially a “game changer” in ICF because it would enable higher laser intensities and ablation pressures in future target designs.

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.

The scattering of laser light due to cross-beam energy transfer (CBET) is an undesirable process in direct-drive inertial confinement fusion (ICF) that degrades both the compression and symmetry of the imploding target. Here, we present results from laser-plasma interaction simulations performed with the wave-based code LPSE that explore two techniques for suppressing CBET in frequency-tripled, Nd:glass laser beams crossing in a transonic plasma: a.) frequency detuning using two or three discrete "colors" of narrowband laser light; and b.) broad laser bandwidth. We find that for beams modeled with random speckle patterns, distributed phase plates and polarization smoothing, and for plasma conditions similar to those on the National Ignition Facility, the former method reduces CBET to an extent, but the degree of mitigation plateaus once the frequency separation greatly exceeds the resonance width of the CBET instability. Broadband lasers, on the other hand, are predicted to suppress CBET completely at a bandwidth of about 8 THz (Delta omega/omega(0) similar or equal to 1%, where Delta omega/2 pi and omega(0) are the laser bandwidth and angular frequency, respectively) for the same conditions. Although the Nd:glass lasers used for ICF research today have bandwidths far below this value, the spectra from such lasers could likely be broadened to multi-terahertz levels by utilizing stimulated rotational Raman scattering in a gaseous diatomic medium. Alternatively, the required bandwidth could be obtained with an excimer laser driver such as argon-fluoride, which has a native bandwidth in excess of 7 THz. Either of these two options would enable higher and more symmetric ablation pressures in future, direct-drive, ICF target designs.

Laser plasma instabilities are problematic for inertial confinement fusion because they can spoil illumination uniformity, reduce laser-target coupling, and create unwanted fast electrons. Recent experiments and simulations have shown that self-seeded stimulated rotational Raman scattering (SRRS) in air might achieve enough spectral broadening to mitigate these instabilities with only moderate unwanted broadening of the focal spot. The theoretical model for the simulations included chaotic broadband and spatially multimode light, but was a scalar formulation suitable for only linear polarization, where the SRRS gains and spectral broadening are limited by Stokes--anti-Stokes coupling. This paper derives a tensor formulation of SRRS theory suitable for modeling spectral broadening of arbitrarily polarized spatially and temporally incoherent light; it then describes the algorithms used to simulate the theory and provides some preliminary results that compare linear and elliptical polarizations. It begins with a paraxial wave equation for an arbitrarily polarized optical field envelope, which is phase modulated by a term proportional to a Raman driven molecular polarizability tensor. Treating the air molecules as rigid rotators, it uses a quantum treatment to derive a driven harmonic oscillator equation for that polarizability, then expresses these vector and tensor equations in terms of the field's right- and left-handed circular polarization components to derive the final coupled equations for arbitrary polarization. The formulation includes possible ac Stark shift contributions, but shows that they are negligible for intensities below $10\phantom{\rule{0.16em}{0ex}}\mathrm{GW}/{\mathrm{cm}}^{2}$. It then describes the algorithms used in the simulation code and the numerical model of the chaotic light, whose initial spectral bandwidth is broad enough to self-seed the SRRS. In this algorithm, the SRRS process accurately conserves the total energy at each axial plane along the propagation path. Finally, it compares simulations of power spectra and far-field profiles for elliptical vs linear polarization, which show that elliptically polarized light produces significantly more broadening of both profiles than linear polarization. For linear polarization, the SRRS process reduces the incident coherence time from 0.54 to 0.27 ps; for elliptical polarization, it reduces to 0.19 ps. The theory and simulation algorithms presented here provide a framework for evaluating techniques that combine beams of alternating circular polarizations with different spectra and angular divergences to improve SRRS spectral broadening without excessive focal spot broadening.

Argon fluoride (ArF) is currently the shortest wavelength laser that can credibly scale to the energy and power required for high gain inertial fusion. ArF's deep ultraviolet light and capability to provide much wider bandwidth than other contemporary inertial confinement fusion (ICF) laser drivers would drastically improve the laser target coupling efficiency and enable substantially higher pressures to drive an implosion. Our radiation hydrodynamics simulations indicate gains greater than 100 are feasible with a sub-megajoule ArF driver. Our laser kinetics simulations indicate that the electron beam-pumped ArF laser can have intrinsic efficiencies of more than 16%, versus about 12% for the next most efficient krypton fluoride excimer laser. We expect at least 10% ‘wall plug' efficiency for delivering ArF light to target should be achievable using solid-state pulsed power and efficient electron beam transport to the laser gas that was demonstrated with the U.S. Naval Research Laboratory's Electra facility. These advantages could enable the development of modest size and lower cost fusion power plant modules. This would drastically change the present view on inertial fusion energy as being too expensive and the power plant size too large. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)'.

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.

Absolute Hugoniot measurements for empty plastic foams at ~10% of solid polystyrene density and supporting rad-hydro simulation results are reported. Planar foam slabs, $\sim 400\mu \mathrm{m}$ thick and $\sim 500\mu \mathrm{m}$ wide, some of which were covered with a 10 $\mu \mathrm{m}$ solid plastic ablator, were directly driven with 4 ns long Nike krypton-fluoride 248 nm wavelength laser pulses that produced strong shock waves in the foam. The shock and mass velocities in our experiments were up to 104 km/s and 84 km/s, respectively, and the shock pressures up to ~9 Mbar. The motion of the shock and ablation fronts was recorded using side-on monochromatic x-ray imaging radiography. The steadiness of the observed shock and ablation fronts within $\sim$ 1 % has been verified. The Hugoniot data inferred from our velocity measurements agree with the predictions of the SESAME and CALEOS equation-of-state models near the highest pressure ~9 Mbar and density compression ratio ~5. In the lower pressure range 2 to 5 Mbar, a lower shock density compression is observed than that predicted by the models, see the Figure. Possible causes for this discrepancy are discussed.

Cross-beam energy transfer (CBET) is a significant energy-loss mechanism in directly driven inertial-confinement-fusion (ICF) targets. One strategy for mitigating CBET is to increase the bandwidth of the laser light, thereby disrupting the resonant three-wave interactions that underlie this nonlinear scattering process. Here, we report on numerical simulations performed with the wave-based code lpse that show a significant reduction in CBET for bandwidths of 2-5 THz (corresponding to a normalized bandwidth of 0.2%-0.6% at a laser wavelength of 351nm) under realistic plasma conditions. Such bandwidths are beyond those available with current high-energy lasers used for ICF, but could be achieved using stimulated rotation Raman scattering in diatomic gases like nitrogen.

The Nike KrF laser facility can deliver a few kJ of UV light $( \lambda =248\ \text{nm})$ to a target. This is sufficient to ionize a solid high-atomic-number target, such as molybdenum, producing HED plasma. The emitted molybdenum L-shell x-rays were measured using a high resolution spectrometer with a mica crystal (2d = l 19.84 Å). The recorded spectral lines can be used to identify the ion charge states (F-, Ne-, and Na-like) present and non-LTE modeling can be used to diagnose the temperature and density of the plasma. A 1-D non-LTE collisional-radiative kinetic model that solves the radiation transport equation frequency-by-frequency was used to produce a synthetic spectrum to match the recorded experimental spectrum. We report diagnostically useful emission lines that characterize the molybdenum L-shell plasma.

Stimulated rotational Raman scattering (SRRS) in the ultraviolet region (lambda = 248 nm) has been observed at the Nike laser over extended propagation paths in air during high power operation. Although this phenomenon is not significant for standard operating configurations at Nike, broadening of the laser spectrum and far-field focal profiles has been observed once the intensity-path length product exceeds a threshold of approximately 1 TW/cm. This paper presents experimental results and a new theoretical evaluation of these effects. The observations suggest that significantly broader spectra can be achieved with modest degradation of the final focal distribution. These results point to a possible path for enhanced laser-target coupling with the reduction of laser-plasma instabilities due to broad laser bandwidth produced by the SRRS.

Direct drive implosions of plastic capsules have been performed at the National Ignition Facility to provide a broad-spectrum (500-2000 eV) X-ray continuum source for X-ray transmission spectroscopy. The source was developed for the high-temperature plasma opacity experimental platform. Initial experiments using 2.0 mm diameter polyalpha-methyl styrene capsules with ∼20 μm thickness have been performed. X-ray yields of up to ∼1 kJ/sr have been measured using the Dante multichannel diode array. The backlighter source size was measured to be ∼100 μm FWHM, with ∼350 ps pulse duration during the peak emission stage. Results are used to simulate transmission spectra for a hypothetical iron opacity sample at 150 eV, enabling the derivation of photometrics requirements for future opacity experiments.

Consequences of an explosion inside an air-filled cavity under the earth's surface are partly duplicated in a laboratory experiment on spatial scales 1000 smaller. The experiment measures shock pressures coupled into a block of material by an explosion inside a gas-filled cavity therein. The explosion is generated by suddenly heating a thin foil that is located near the cavity center with a short laser pulse, which turns the foil into expanding plasma, most of whose energy drives a blast wave in the cavity gas. Variables in the experiment are the cavity radius and explosion energy. Measurements and GEODYN code simulations show that shock pressures measured in the block exhibit a weak dependence on scaled cavity radius up to ∼25 m/kt1/3, above which they decrease rapidly. Possible mechanisms giving rise to this behavior are described. The applicability of this work to validating codes used to simulate full-scale cavity explosions is discussed.