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Focusing laser light onto a very small target can produce the conditions for laboratory-scale nuclear fusion of hydrogen isotopes. The lack of accurate predictive models, which are essential for the design of high-performance laser-fusion experiments, is a major obstacle to achieving thermonuclear ignition. Here we report a statistical approach that was used to design and quantitatively predict the results of implosions of solid deuterium–tritium targets carried out with the 30-kilojoule OMEGA laser system, leading to tripling of the fusion yield to its highest value so far for direct-drive laser fusion. When scaled to the laser energies of the National Ignition Facility (1.9 megajoules), these targets are predicted to produce a fusion energy output of about 500 kilojoules—several times larger than the fusion yields currently achieved at that facility. This approach could guide the exploration of the vast parameter space of thermonuclear ignition conditions and enhance our understanding of laser-fusion physics.

AbstractThe goal of the National Direct-Drive Program is to demonstrate and understand the physics of laser direct drive (LDD). Efforts are underway on OMEGA for the 100-Gbar Campaign to demonstrate and understand the physics for hot-spot conditions and formation relevant for ignition at the 1-MJ scale, and on the National Ignition Facility to develop an understanding of the direct-drive physics at long scale lengths for the MJ Direct-Drive Campaign. The strategy of the National Direct-Drive Program is described; the requirements for the deuterium-tritium cryogenic fill-tube target being developed for OMEGA are presented; and preliminary LDD implosion measurements of hydrodynamic mixing seeded by laser imprint, the target-mounting stalk, and microscopic surface debris are reported.

Laser-plasma interactions (LPI) have been studied experimentally in high-temperature, high-energy density plasmas. The studies have been performed using the Omega laser at the Laboratory for Laser Energetics (LLE), Rochester, NY. Up to 10 TW of power was incident upon reduced-scale hohlraums, distributed in three laser beam cones. The hot hohlraums fill quickly with plasma. Late in the laser pulse, most of the laser energy is deposited at the laser entrance hole, where most of the LPI takes place. Due to the high electron temperature, the stimulated Raman scattering (SRS) spectrum extends well beyond ${\rm \omega}_{0}$/2, due to the Bohm-Gross shift. This high-temperature, high-energy density regime provides a unique opportunity to study LPI beyond inertial confinement fusion (ICF) conditions.

The characterization of OMEGA cryogenic targets is based on shadowgraphs obtained from multiple angular views taken with the target in the layering sphere. The D2ice has been observed to re-layer during slow rotations, leading to procedural changes that avoid re-layering thus ensuring high-quality, spherical-harmonic, 3-D ice layer reconstructions. Shadowgrams taken inside the target chamber within 20 ms of shot time have verified that the ice layers remain preserved during the transport.

Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)], is currently being investigated on the OMEGA laser system by using 40 beams in six rings repointed to more uniformly illuminate the target [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005).]. The OMEGA experiments are being performed with standard, “warm” targets (865-μm-diam, 20-μm-thick, CH shells filled with 15-atm D$_{2})$with and without the use of an equatorial “Saturn-like” toroidally shaped CH ring [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 095002 (2005)] (nominal dimensions: 2.2-mm diam measured to ring center, 0.3-mm thick). For the Saturn case, the plasma formed around the ring refracts light toward the target equator as the ring plasma expands. The nominal laser drive is a 1-ns flat pulse, ${\sim}$400 J per beam, employing 1-THz, 2-D SSD with polarization smoothing. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core. The best results have been obtained with Saturn targets by varying the beam pointing and ring diameter, achieving ${\sim}$75% of the fusion yield from symmetrically illuminated targets with the same total energy (60 beams, 15.3 kJ).

An experimental campaign to maximize radiation drive in small-scale hohlraums has been carried out at the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA, USA) and at the OMEGA laser at the Laboratory for Laser Energetics (Rochester, NY, USA). The small-scale hohlraums, laser energy, laser pulse, and diagnostics were similar at both facilities but the geometries were very different. The NIF experiments used on-axis laser beams whereas the OMEGA experiments used 19 beams in three beam cones. In the cases when the lasers coupled well and produced similar radiation drive, images of x-ray burnthrough and laser deposition indicate the pattern of plasma filling is very different.

We experimentally demonstrate that application of laser smoothing schemes including smoothing by spectral dispersion (SSD) and polarization smoothing (PS) increases the intensity range for efficient coupling of frequency doubled (527 nm) laser light to a long scalelength plasma with ne/n$_{\rm cr} = 0.14$and T$_{\rm e} = 2$keV.

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D2implosions with neutron yields that are a significant fraction of the “clean” 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights some of the recent experimental and theoretical progress toward this validation.
The NIF will initially be configured for x-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the “polar-direct-drive” (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. 
LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA laser systems, will begin in FY08.

AbstractBacklit optical shadowgraphy is the primary diagnostic for hydrogenic ice-layer characterization in cryogenic targets at the Laboratory for Laser Energetics (LLE). Reflection and refraction of light passing through the ice layer produce characteristic rings on the image. The position of the most prominent of the shadowgraph rings, known as the bright ring, can be resolved to ~0.1-pixel rms, corresponding to less than 0.2 μm for typical target shadowgraphs. The LLE target characterization stations use two camera angles and target rotation to record target shadowgraphs from many different views (typically 48) and build a three-dimensional (3-D) topology of the ice layer. The standard method of bright-ring analysis using spherically symmetric ray-trace calculations to determine the ice surface is limited to mode numbers up to around [script l]max= 10 by gaps in the data and the effects of ice-layer asymmetries that invalidate the symmetric ray trace calculations. A 3-D ray-tracing model has been incorporated into the shadowgraph analysis. The result is a self-consistent determination of the hydrogen/vapor surface structure for cryogenic targets up to higher-mode numbers ([script l]max= 16). This reduces the standard deviation between the measured bright rings and those predicted for the 3-D ice surface (by 45% from 1.5 μm to 0.8 μm in the example shown).

Recent advances in hydrodynamics theory and experiments at the Laboratory for Laser Energetics are described. Particular emphasis is laid on improvements in the implosion stability achieved by shaping the ablator adiabat and on the newly developed designs for fast ignition fuel assembly. The results of two-dimensional simulations and a recent set of implosion experiments on OMEGA are presented to verify the role of adiabat shaping on the hydrodynamic stability of direct-drive implosions. Adiabat shaping laser pulses are also used to implode massive capsules on a low adiabat and low implosion velocity in order to assemble high density plasmas for fast ignition. The areal densities measured in implosion experiments of such targets on OMEGA are among the highest ever recorded in a laser-driven compression experiment. Slow low-adiabat implosions of massive wetted-foam DT capsules are used in the simulations to generate the fuel assemblies for different driver energies. Such dense cores are then ignited by a fast electron beam and the resulting thermonuclear yield is used to compute the target gain. It is shown that a 200?kJ UV laser can assemble fuel yielding about 18?MJ of energy when ignited by 15?kJ of 1-2?MeV electrons.

We experimentally demonstrate that application of laser smoothing schemes including smoothing by spectral dispersion (SSD) and polarization smoothing (PS) increases the intensity range for efficient coupling of frequency doubled (527 nm) laser light to a long scalelength plasma with ne/ncr = 0.14 and Te = 2?keV.

Laser-plasma interactions (LPI) have been studied experimentally in high-temperature, high-energy density plasmas. The studies have been performed using the Omega laser at the Laboratory for Laser Energetics (LLE), Rochester, NY. Up to 10 TW of power was incident upon reduced-scale hohlraums, distributed in three laser beam cones. The hot hohlraums fill quickly with plasma. Late in the laser pulse, most of the laser energy is deposited at the laser entrance hole, where most of the LPI takes place. Due to the high electron temperature, the stimulated Raman scattering (SRS) spectrum extends well beyond ?0/2, due?to the Bohm-Gross shift. This high-temperature, high-energy density regime provides a unique opportunity to?study LPI beyond inertial confinement fusion (ICF) conditions.

Polar direct drive (PDD), a promising ignition path for the NIF while the beams are in the indirect-drive configuration [S. Skupsky et al., Phys. Plasmas 11, 2763 (2004)], is currently being investigated on the OMEGA laser system by using 40 beams in six rings repointed to more uniformly illuminate the target [R. S. Craxton et al., Phys. Plasmas 12, 056304 (2005).]. The OMEGA experiments are being performed with standard, ?warm? targets (865-?m-diam, 20-?m-thick, CH shells filled with 15-atm D2) with and without the use of an equatorial ?Saturn-like? toroidally shaped CH ring [R. S. Craxton and D. W. Jacobs-Perkins, Phys. Rev. Lett. 94, 095002 (2005)] (nominal dimensions: 2.2-mm diam measured to ring center, 0.3-mm thick). For the Saturn case, the plasma formed around the ring refracts light toward the target equator as the ring plasma expands. The nominal laser drive is a 1-ns flat pulse, ?400?J per beam, employing 1-THz, 2-D SSD with polarization smoothing. Target implosion symmetry is diagnosed with framed x-ray backlighting using additional OMEGA beams and by time-integrated x-ray imaging of the stagnating core. The best results have been obtained with Saturn targets by varying the beam pointing and ring diameter, achieving ?75% of the fusion yield from symmetrically illuminated targets with the same total energy (60 beams, 15.3?kJ).

An experimental campaign to maximize radiation drive in small-scale hohlraums has been carried out at the National Ignition Facility (NIF) at the Lawerence Livermore National Laboratory (Livermore, CA, USA) and at the OMEGA laser at the Laboratory for Laser Energetics (Rochester, NY, USA). The small-scale hohlraums, laser energy, laser pulse, and diagnostics were similar at both facilities but the geometries were very different. The NIF experiments used on-axis laser beams whereas the OMEGA experiments used 19 beams in three beam cones. In the cases when the lasers coupled well and produced similar radiation drive, images of x-ray burnthrough and laser deposition indicate the pattern of plasma filling is very different.

The characterization of OMEGA cryogenic targets is based on shadowgraphs obtained from multiple angular views taken with the target in the layering sphere. The D2 ice has been observed to re-layer during slow rotations, leading to procedural changes that avoid re-layering thus ensuring high-quality, spherical-harmonic, 3-D ice layer reconstructions. Shadowgrams taken inside the target chamber within 20 ms of shot time have verified that the ice layers remain preserved during the transport.

Direct-drive inertial confinement fusion (ICF) is expected to demonstrate high gain on the National Ignition Facility (NIF) in the next decade and is a leading candidate for inertial fusion energy production. The demonstration of high areal densities in hydrodynamically scaled cryogenic DT or D2 implosions with neutron yields that are a significant fraction of the ?clean? 1-D predictions will validate the ignition-equivalent direct-drive target performance on the OMEGA laser at the Laboratory for Laser Energetics (LLE). This paper highlights some of the recent experimental and theoretical progress toward this validation.?The NIF will initially be configured for x-ray drive and with no beams placed at the target equator to provide a symmetric irradiation of a direct-drive capsule. LLE is developing the ?polar-direct-drive? (PDD) approach that repoints beams toward the target equator. Initial 2-D simulations have shown ignition. ?LLE is currently constructing the multibeam, 2.6-kJ/beam, petawatt laser system OMEGA EP. Integrated fast-ignition experiments, combining the OMEGA EP and OMEGA laser systems, will begin in FY08.

AbstractBacklit optical shadowgraphy is the primary diagnostic for D2ice layer characterization of cryogenic targets for the OMEGA Laser System at the Laboratory for Laser Energetics (LLE). Reflection and refraction of light passing through the ice layer produce characteristic rings. The position of the most prominent of the shadowgraph rings, known as the bright ring, can be resolved to ∼0.1-pixel rms, corresponding to about 0.12 μm for typical LLE target shadowgraphs. Measurement of the bright ring position in conjunction with ray-trace model predictions determines the ice layer thickness and the Fourier-mode spectrum of the ice roughness for that view. The LLE target characterization stations use two camera angles and target rotation to record target shadowgraphs from many different views. Combining these views allows construction of a 3-D ice layer representation, an estimation of the global surface roughness, and a determination of a Legendre-mode spectrum suitable for implosion modeling. The standard operating procedure is to construct a 3-D ice layer representation using the analysis of 48 separate shadowgraphic views. The 3-D ice surface is then decomposed in terms of spherical harmonics, allowing the determination of low-mode number (ℓ ≤ to 10) elements of a Legendre-mode power spectrum. Higher-mode number elements of the Legendre power spectrum are determined by mapping the Fourier-mode power spectrum averaged over all views

AbstractThe OMEGA cryogenic target handling system provides deuterium-filled cryogenic targets for direct-drive implosion experiments. The targets are 0.9 mm in diameter with a 3-μm-thick outer plastic ablator and an inner ice layer that ranges from 80 to 100 μm thick. The smoothest ice layer possessed an average root-mean-square (rms) roughness of 1.2 μm, although values ranging from 2 to 4 μm are more typical. Implosion experiments achieved a maximum yield of 2.11 × 1011primary neutrons (70% of the clean one-dimensional yield) with an average areal density of 50 mg/cm2with a 1-ns square, high-adiabat (α = 25) laser pulse. Lower yields (1 × 1010primary neutrons) and higher areal densities (88 mg/cm2) were obtained using a lower-adiabat (α = 4) laser pulse. Better performance is expected once smoother ice layers (better than 2-μm average rms roughness) are positioned within 10 μm of where the laser beams are pointed. Currently, the offset between the target’s location and where the laser beams are pointing at the moment of implosion is 14 to 60 μm.

Significant theoretical and experimental progress continues to be made at the University of Rochester's Laboratory for Laser Energetics (LLE), charting the path to direct-drive inertial confinement fusion (ICF) ignition. Direct drive offers the potential for higher-gain implosions than x-ray drive and is a leading candidate for an inertial fusion energy power plant. LLE's direct-drive ICF ignition target designs for the National Ignition Facility (NIF) are based on hot-spot ignition. A cryogenic target with a spherical DT-ice layer, within or without a foam matrix, enclosed by a thin plastic shell, will be directly irradiated with ∼1.5 MJ of laser energy. Cryogenic and plastic/foam (surrogate-cryogenic) targets that are hydrodynamically scaled from these ignition target designs are imploded on the 60-beam, 30 kJ, UV OMEGA laser system to validate the key target physics issues, including energy coupling, hydrodynamic instabilities and implosion symmetry. Prospects for direct-drive ignition on the NIF are extremely favourable, even while it is in its x-ray-drive irradiation configuration, with the development of the polar-direct-drive concept. A high-energy petawatt capability is being constructed at LLE next to the existing 60-beam OMEGA compression facility. This OMEGA EP (extended performance) laser will add two short-pulse, 2.6 kJ beams to the OMEGA laser system to backlight direct-drive ICF implosions and study fast-ignition physics with focused intensities up to 6 × 1020 W cm−2.

Smoothing, caused by the small-spatial-scale B integral, was measured on the OMEGA laser (a high-power, solid-state laser used for inertial confinement fusion research) without applied bandwidth. The intrinsic nonuniformity of laser irradiation [i.e., irradiation without smoothing by spectral dispersion] was determined from fluence distributions in equivalent-target-plane images of beams with phase plates. These data are compared with simulations that include both small-spatial-scale and whole-beam B integrals. The nonuniformity decreases with increasing average intensity. High-intensity beams can acquire bandwidth as a result of the intensity-dependent phase accumulated in the laser chain. The far-field speckle pattern produced by a phase plate can shift as the near-field phase front changes, which decreases the nonuniformity. The far-field power spectrum is affected mainly in the high spatial frequencies, where it is not expected to mitigate hydrodynamic instabilities.

A large number of diagnostics have been used to study the performance of gas-filled, plastic-shell direct-drive implosions on the 60-beam OMEGA laser system (Boehly T R et al 1997 Opt. Commun. 133 495). These diagnostics allow an estimate of the amount of fuel-pusher mixing in the compressed core. Implosions have primary neutron yields and fuel areal densities that are ~35% and ~100% of the predicted one-dimensional values, respectively. A highly constrained model of the core conditions and fuel-shell mix has been developed. It suggests that there is a `clean' fuel region surrounded by a mixed region, with the clean region accounting for ~70% of the fuel areal density.

Experiments to demonstrate the effects of various beam-smoothing techniques have been performed on the 60-beam, 30-kJ UV OMEGA laser system. These include direct measurements of the effect beam-smoothing techniques have on laser beam nonuniformity and on both planar and spherical targets. Demonstrated techniques include polarization smoothing and “dual-tripler” third-harmonic generation required for future broad bandwidth (~1 THz) smoothing by spectral dispersion (SSD). The effects of improvements in single-beam uniformity are clearly seen in the target-physics experiments, which also show the effect of the laser pulse shape on the efficacy of SSD smoothing. Saturation of the Rayleigh-Taylor (RT) growth of the broad-bandwidth features, in agreement with the Haan model (Haan, 1989), produced by laser imprinting has also been observed.

We report on an experimental characterization of Bessel beams with finite apertures. We show that real Bessel beams can be generated with intensity profiles that closely resemble the ideal Jo^2 transverse-intensity distribution of Bessel beams. We also show interferometrically that these beams have planar phase fronts with π-phase shifts from one Bessel ring to the next. We report tolerance conditions for Bessel beam generation and give an example of this generation that uses an unstable resonator as the light source.

A one-beam prototype of a large twenty-four-beam phosphate-glass laser system has been built and tested. Basic design characteristics include Nd-doped phosphate glass, rod amplifiers up to 90-mm diam, the propagation of a circularly polarized beam, extensive spatial filtering and imaging, and the use of large-aperture Pockels cells. The prototype system has demonstrated focusable power in excess of 750 GW/beam at 50-psec full width at half-maximum (FWHM) pulse duration and has delivered 165 J of focusable energy in a 500-psec (FWHM) pulse. Maximum beam brightness has been measured to be 4 × 10^19 W/cm^2 · sr at a firing rate of 2 shots/hr. Peak-to-background energy contrast of 10^8 has been achieved with this system. A detailed description of the system design and performance is presented.

The dual-tripler scheme for enhancing the bandwidth of third-harmonic generation proposed by Eimerl ??[Opt.??Lett.??22, 1208 (1997)] is experimentally demonstrated for the conversion of 1054-nm radiation to 351??nm. It is shown that the spacing between the triplers must be carefully controlled. The results are in excellent agreement with theory and indicate that fusion lasers can be frequency tripled with a threefold increase in bandwidth.