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).

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.

In inertial confinement (ICF) experiments at the NIKE laser facility, the high-power krypton fluoride (KrF) laser output beams propagate through long (similar to 75 m) air paths to achieve angular multiplexing, which is required because the KrF medium does not store energy for a sufficiently long time. Recent experiments and simulations have shown that, via stimulated rotational Raman scattering, this propagation can spectrally broaden the laser beam well beyond the similar to 1 THz laser linewidth normally achieved by the induced spatial incoherence (ISI) technique used in NIKE. These enhanced bandwidths may be enough to suppress the laser-plasma instabilities which limit the maximum intensity that can be incident on the ICF target. In this paper we investigate an alternative technique that achieves spectral broadening by self-phase modulation in Xe gas, which has a large, negative nonlinear refractive index similar to 248 nm, and thus completely avoids transverse filamentation issues. The collective, nonlinear atomic response to the chaotic, nonsteady state ISI light is modeled using a two-photon vector model, and the effect of near-resonant behavior on the spectral broadening is studied.

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.

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.

Laser fusion researchers have realized since the 1970s that the deep UV light from excimer lasers would be an advantage as a driver for robust high-performance capsule implosions for inertial confinement fusion (ICF). Most of this research has centered on the krypton-fluoride (KrF) laser. In this article we review the advantages of the KrF laser for direct-drive ICF, the history of high-energy KrF laser development, and the present state of the art and describe a development path to the performance needed for laser fusion and its energy application. We include descriptions of the architecture and performance of the multi-kilojoule Nike KrF laser-target facility and the 700 J Electra high-repetition-rate KrF laser that were developed at the U.S. Naval Research Laboratory. Nike and Electra are the most advanced KrF lasers for inertial fusion research and energy applications.

The laser fusion program at the Naval Research Laboratory has been exploring and developing several means to reduce the hydro and laser plasma instabilities that impede achievement of high performance laser fusion. If implemented on a MJ class implosion system, the krypton-fluoride (KrF) laser’s combination of (a) deeper UV light, (b) broad bandwidth ISI-smoothed beams and (c) capability for multi-stage zooming would significantly alleviate these instabilities. Here we discuss two instability mitigation techniques that are applicable to both frequency tripled Nd:glass and KrF lasers. Experiments on the Nike KrF facility and simulations at NRL have shown that the addition of a thin gold or palladium coating on the laser illuminated surface of plastic targets can dramatically reduce laser imprint that seeds subsequent hydrodynamic instability [1, 2]. Similar effects have been observed in OMEGA experiments [3]. This configuration creates an early time x-ray drive of the target from the high-Z layer. The separation distance between the laser absorption in the high-Z plasma layer and the x-ray driven substrate target prevents imprinting by the shorter spatial wavelength laser illumination non-uniformity. Eventually the laser burns through the layer and direct drive commences. Simulations indicate that use of still thicker high-Z layers delays this burn-through and thus extends the duration of the xray drive. This could further inhibit hydro-instability and might also delay onset laser-plasma instability near quarter critical density. Extreme laser bandwidth is predicted to inhibit laser plasma instability by theory and this suppression of instability by bandwidth has been observed in microwave-plasma interaction experiments [4]. We have been conducting plasma fluid and PIC simulations that are exploring what bandwidths would be needed to suppress laser-plasma instability near quarter critical density. The simulations indicate that bandwidths greater than 3 THz can significantly reduce the instability growth. This bandwidth is several times larger than that which can be directly obtained with current ICF lasers. We are investigating several means for obtaining such bandwidths on both KrF and Nd:glass laser systems.

The krypton-fluoride (KrF) laser is an attractive choice for inertial confinement fusion due to its combination of short wavelength (lambda = 248 nm), large bandwidth (up to 3 THz), and superior beam smoothing by induced spatial incoherence. These qualities improve the overall hydrodynamics of directly driven pellet implosions and should allow use of increased laser intensity due to higher thresholds for laser plasma instabilities when compared to frequency tripled Nd:glass lasers (lambda = 351 nm). Here, we report the first observations of the two-plasmon decay instability using a KrF laser. The experiments utilized the Nike laser facility to irradiate solid plastic planar targets over a range of pulse lengths (0.35 ns <= tau <= 1.25 ns) and intensities (up to 2 x 10(15) W/cm(2)). Variation of the laser pulse created different combinations of electron temperature and electron density scale length. The observed onset of instability growth was consistent with the expected scaling that KrF lasers have a higher intensity threshold for instabilities in the quarter critical density region. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4789379]

Recent designs for laser driven, direct drive inertial confinement fusion (ICF) indicate that substantial gains (G>100) might be achieved with lower total laser energy (E similar to 500 kJ) than previously considered possible. A leading contender is the shock ignition approach which compresses low aspect ratio pellets with high intensity laser pulses (10(15) W/cm(2)) before achieving ignition with a final higher intensity spike (10(16) W/cm(2)). Excimer laser systems based on a krypton-fluoride (KrF) medium are particularly well suited to these new ideas as they operate in the ultraviolet (248 nm), provide highly uniform illumination, possess large bandwidth (1-3 THz), and can easily exploit beam zooming to improve laser-target coupling for the final spike pulse. This paper will examine target physics advantages of KrF lasers in relation to the new implosion designs and the balancing of hydrodynamic instability and laser-plasma instabilities. Supporting experimental and theoretical studies of are being conducted by the Nike laser group at the U. S. Naval Research Laboratory. Recent experimental work has also shown that the high ablation pressures and smooth profiles obtained with the Nike laser can be used to accelerate planar targets to velocities consistent with the requirements of impact ignition.