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We intend to carry out a series of plasma lens experiments at the Final Focus Test Beam facility at SLAC. These experiments will be the first to study the focusing of particle beams by plasma focusing devices in the parameter regime of interest for high energy colliders, and is expected to lead to plasma lens designs capable of unprecedented spot sizes. Plasma focusing of positron beams will be attempted for the first time. We will study the effects of lens aberrations due to various lens imperfections. Several approaches will be applied to create the plasma required including laser ionization and beam ionization of a working gas. At an increased bunch population of 2.5/spl times/10/sup 10/, tunneling ionization of a gas target by an electron beam-an effect which has never been observed before-should be significant. The compactness of our device should prove to be of interest for applications at the SLC and the next generation linear colliders. >

The requirement of a high brightness electron source for the proposed Linac Coherent Light Source (LCLS) project at SLAC has led to the development of the Gun Test Facility (GTF). The facility consists of a photocathode RF gun, emittance compensation solenoid, a single 3-meter SLAC S-band linac section, a single XK-5 klystron, low and high energy diagnostic sections and a cathode drive laser. In this paper, the specifications of the desired electron beam are discussed along with results of PARMELA simulations to determine the optimum laser pulse shape for minimum beam emittance. The diagnostics of the GTF are discussed and measurements are presented which were made at the SUNSHINE Facility at Stanford to develop the instrumentation to be used at the Gun Test Facility once it is operational. Future beam diagnostics are also discussed.

The proposal to perform a series of plasma lens experiments at the Final Focus Test Beam at SLAC has been described earlier. We report on our progress towards validation of concepts involved in the experiments, including the laser ionized plasma production test, development of the supersonic gas jet as the plasma source, and study on focused beam size measurement techniques. Most importantly, the effects of background events due to plasma lenses in future linear collider detectors, such as that in the NLC, are studied in details and are shown to be within detector tolerances.

Summary form only given, as follows. The achievement of high-density compressed cores using the 60-beam OMEGA laser system to implode cryogenic DT targets is an important milestone on the path to direct-drive ignition. The paper reviews the cryogenic designs for OMEGA and show the scaling to ignition designs for the NIF. Results from the first series of cryogenic implosion experiments and surrogate, gas-filled, plastic shell implosion experiments are presented. A wide range of nuclear and spectroscopic diagnostics have been applied to the surrogate implosions, which has resulted in the development of a highly constrained model of the core conditions and fuel-shell mix. The recent improvement in target performance with improved single-beam uniformity are described in detail. 1 THz- bandwidth, 2-D smoothing by spectral dispersion (SSD) and polarization smoothing (PS) using birefringent wedges have reduced the on-target nonuniformity to less than 1% when averaged over 300 ps. Targets imploded using this improved beam smoothing show significant performance improvements over targets imploded with a third of the bandwidth and no PS. These targets have similar acceleration-phase stability as ignition-scaled implosions proposed for the NIF. They produce /spl sim/40% of their predicted 1 D neutron yield and nearly 100% of their predicted fuel and shell areal densities.

The temporal history of the target areal-density near peak compression of direct-drive spherical target implosions has been inferred with 14.7-MeV deuterium-helium-3 ${\mathrm{D}}^{\mathrm{3}}\mathrm{H}\mathrm{e}$ proton spectroscopy of the 60-beam, 30-kJ UV OMEGA laser system. The target areal-density grows by a factor of $\ensuremath{\sim}8$ during the time of neutron-production ($\ensuremath{\sim}400\text{ }\mathrm{p}\mathrm{s}$) before reaching $123\ifmmode\pm\else\textpm\fi{}16\text{ }\mathrm{m}\mathrm{g}/\mathrm{c}{\mathrm{m}}^{\mathrm{2}}$ at peak compression in the implosion of a 950-$\ensuremath{\mu}\mathrm{m}$-diam, $20\mathrm{\text{\ensuremath{-}}}\ensuremath{\mu}\mathrm{m}$-thick plastic CH capsule filled with 4 atm of ${\mathrm{D}}^{\mathrm{3}}\mathrm{H}\mathrm{e}$ fuel.

Fuel-shell mix and implosion performance are studied for many capsule types in direct-drive experiments at OMEGA. The amount of mixing and the size of the mix region are inferred from charged-particle spectrometry data and confirmed with an experimentally constrained model. Measured yields and convergence ratios CR fall short of one-dimensional predictions, especially for low capsule fill pressures. CR is approximately 11 for pressures from 3 to 15 atm, in contrast to predictions of approximately 25 for 3 atm and approximately 12 for 15 atm. The performance shortfalls are likely to be caused by fuel-shell mix.

The Rayleigh-Taylor instability in its highly nonlinear, turbulent stage causes atomic-scale mixing of the shell material with the fuel in the compressed core of inertial-confinement fusion targets. The density of shell material mixed into the outer core of direct-drive plastic-shell spherical-target implosions on the 60-beam, OMEGA laser system is estimated to be 3.4(+/-1.2) g/cm(3) from time-resolved x-ray spectroscopy, charged-particle spectroscopy, and core x-ray images. The estimated fuel density, 3.6(+/-1) g/cm(3), accounts for only approximately 50% of the neutron-burn-averaged electron density, n(e)=2.2(+/-0.4)x10(24) cm(-3).

We report on multibeam, laser-plasma interaction experiments in plasmas relevant to future direct-drive-ignition experiments. Six interaction beams are incident on preformed plasmas containing critical density. Stimulated Brillouin scattering (SBS) shows strong evidence of electromagnetic wave seeding of side- and backscattering. The data are also consistent with shared ion waves driven by the six symmetrically arranged interaction beams. Early SBS quenching is observed and attributed to the hydrodynamics of the plasma.

Obtaining high performance from direct-drive inertial confinement fusion (ICF) targets requires careful control of the target irradiation. The temporal pulse shape and the on-target irradiation uniformity are of particular concern. A broad arsenal of techniques and diagnostics has been developed for the OMEGA laser1 to ensure that the on-target irradiation meets the requirements. Most of these directly diagnose the laser itself. However, x-rays emitted from the target are also used to confirm that the required irradiation-uniformity levels have been achieved. The improvements to laser uniformity made possible with these diagnostics, combined with improvements to individual-beam uniformity provided by beam-smoothing techniques, have resulted in significantly improved target performance and reduced shot-to-shot variability.

Understanding of the coupling of laser light to the plasma corona is essential for the success of laser-driven inertial confinement fusion (ICF). The most desirable way of imparting laser energy to the target plasma is via inverse bremsstrahlung absorption.1 In addition, at elevated intensities, the thresholds for various instabilities may be exceeded, channeling laser energy into scattered electromagnetic (EM) waves and various electrostatic waves. As a result, part of the incident laser energy may be unavailable for heating the plasma and the target may be irradiated less uniformly. Furthermore, some of the laser energy may ultimately be transferred to energetic electrons that can preheat the fuel and degrade target performance. Many of these processes, such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), two-plasmon-decay (TPD) instability, and filamentation, have been studied extensively.2 Presently, however, the understanding of these processes is not sufficient to predict their importance under realistic ICF plasma conditions. Therefore continued surveying of laser-plasma interaction processes is required to guard against unforeseen surprises. This paper surveys the plasma diagnostics used on the OMEGA laser facility at the University of Rochester’s Laboratory for Laser Energetics (LLE).

A wide variety of diagnostics have been deployed to study direct-drive spherical implosions of gas-filled, plastic shells on the 60-beam OMEGA laser system at the Laboratory for Laser Energetics. Neutron detectors using plastic scintillators measure primary yield, ion temperature, secondary neutron yield, and burn history. Charged- particle detectors record, on CR-39 samples, the secondary protons in pure D2 implosions and elastically scattered protons, deuterons, and tritons in DT implosions to measure the shell and fuel areal densities. Primary D3He protons from plastic shells with a CD layer at the shell-gas interface and a 3He fill provide a significant signal only if the shell and fuel regions are mixed. Shells with an Ar-doped fill are used to infer the emissivity- averaged core electron temperatures and densities from the measured time-dependent Ar K-shell spectra. The level of nonuniformity in the shell areal density at stagnation is measured using gated x-ray images of Ti-doped shells.

In inertial confinement fusion (ICF), a high temperature and high density plasma is produced by the spherical implosion of a small capsule1. A spherical target capsule is irradiated uniformly by a laser beam (direct irradiation) or x-rays from a high Z enclosure (hohlraum) that is irradiated by laser or ion beams (indirect irradiation). Then high- pressure ablation of the surface causes the fuel to be accelerated inward. Thermonuclear fusion reactions begin in the center region of the capsule as it is heated to sufficient temperature (10 keV) by the converging shocks (hot spot formation). During the stagnation of the imploded shell, the fuel in the shell region is compressed to high density (∼103 times solid density in fuel region). When these conditions are established, energy released by the initial nuclear reactions in center “hot-spot” region can heat up the cold “fuel” region and cause ignition.

Increasing the ablation pressure is a path to achieving cryogenic implosion performance on the OMEGA laser that will hydrodynamically scale to ignition on the National Ignition Facility. An increased ablation pressure will allow a more-massive shell (i.e. thicker and more hydrodynamically stable) and a higher adiabat to achieve ignition-relevant velocities (>3.5 × 107 cm s−1), areal densities (>300 mg cm−2) and hot-spot pressures (>100 Gbar). Two approaches have demonstrated increased ablation pressure: (1) a target design is shown that uses a Be ablator to increase the hydrodynamic efficiency, resulting in a ~10% increase in the ablation pressure, in comparison to a CH ablator; (2) reducing the beam size is shown to recover all of the ablation pressure lost to cross-beam energy transfer (CBET), i.e. the ablation pressure calculated without CBET, but the degraded illumination uniformity reduces the integrated target performance. The hydrodynamic efficiency is measured for the current cryogenic design, multiple ablator material design and CH capsule designs with various beam focal-spot sizes. In each case, an excellent agreement is observed with 1D hydrodynamic simulations that include CBET and nonlocal heat-transport models.

The effects of shock dynamics on compressibility of indirect-drive ignition-scale surrogate implosions, CH shells filled with D3He gas, have been studied using charged-particle spectroscopy. Spectral measurements of D3He protons produced at the shock-bang time probe the shock dynamics and in-flight characteristics of an implosion. The proton shock yield is found to vary by over an order of magnitude. A simple model relates the observed yield to incipient hot-spot adiabat, suggesting that implosions with rapid radiation-power increase during the main drive pulse may have a 2× higher hot-spot adiabat, potentially reducing compressibility. A self-consistent 1-D implosion model was used to infer the areal density (ρR) and the shell center-of-mass radius (Rcm) from the downshift of the shock-produced D3He protons. The observed ρR at shock-bang time is substantially higher for implosions, where the laser drive is on until near the compression bang time (“short-coast”), while longer-coasting implosions have lowe...

Reaching ignition in direct-drive (DD) inertial confinement fusion implosions requires achieving central pressures in excess of 100 Gbar. The OMEGA laser system [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] is used to study the physics of implosions that are hydrodynamically equivalent to the ignition designs on the National Ignition Facility (NIF) [J. A. Paisner et al., Laser Focus World 30, 75 (1994)]. It is shown that the highest hot-spot pressures (up to 40 Gbar) are achieved in target designs with a fuel adiabat of α ≃ 4, an implosion velocity of 3.8 × 107 cm/s, and a laser intensity of ∼1015 W/cm2. These moderate-adiabat implosions are well understood using two-dimensional hydrocode simulations. The performance of lower-adiabat implosions is significantly degraded relative to code predictions, a common feature between DD implosions on OMEGA and indirect-drive cryogenic implosions on the NIF. Simplified theoretical models are developed to gain physical understanding of the implosion dynamics that dictate the target performance. These models indicate that degradations in the shell density and integrity (caused by hydrodynamic instabilities during the target acceleration) coupled with hydrodynamics at stagnation are the main failure mechanisms in low-adiabat designs. To demonstrate ignition hydrodynamic equivalence in cryogenic implosions on OMEGA, the target-design robustness to hydrodynamic instability growth must be improved by reducing laser-coupling losses caused by cross beam energy transfer.

The theory of ignition for inertial confinement fusion capsules [R. Betti et al., Phys. Plasmas 17, 058102 (2010)] is used to assess the performance requirements for cryogenic implosion experiments on the Omega Laser Facility. The theory of hydrodynamic similarity is developed in both one and two dimensions and tested using multimode hydrodynamic simulations with the hydrocode DRACO [P. B. Radha et al., Phys. Plasmas 12, 032702 (2005)] of hydro-equivalent implosions (implosions with the same implosion velocity, adiabat, and laser intensity). The theory is used to scale the performance of direct-drive OMEGA implosions to the National Ignition Facility (NIF) energy scales and determine the requirements for demonstrating hydro-equivalent ignition on OMEGA. Hydro-equivalent ignition on OMEGA is represented by a cryogenic implosion that would scale to ignition on the NIF at 1.8 MJ of laser energy symmetrically illuminating the target. It is found that a reasonable combination of neutron yield and areal density for OMEGA hydro-equivalent ignition is 3 to 6 × 1013 and ∼0.3 g/cm2, respectively, depending on the level of laser imprinting. This performance has not yet been achieved on OMEGA.

Shock-driven implosions of thin-shell capsules, or “exploding pushers,” generate low-density, high-temperature plasmas in which hydrodynamic instability growth is negligible and kinetic effects can play an important role. Data from implosions of thin deuterated-plastic shells with hydroequivalent D3He gas fills ranging from pure deuterium to pure 3He [H. G. Rinderknecht et al., Phys. Rev. Lett. 112, 135001 (2014)] were obtained to evaluate non-hydrodynamic fuel-shell mix mechanisms. Simulations of the experiments including reduced ion kinetic models support ion diffusion as an explanation for these data. Several additional kinetic mechanisms are investigated and compared to the data to determine which are important in the experiments. Shock acceleration of shell deuterons is estimated to introduce mix less than or comparable to the amount required to explain the data. Beam-target mechanisms are found to produce yields at most an order of magnitude less than the observations.

We report on laser-driven, strong-shock generation and hot-electron production in planar targets in the presence of a pre-plasma at shock-ignition (SI) relevant laser and pre-plasma conditions. 2-D simulations reproduce the shock dynamics well, indicating ablator shocks of up to 75 Mbar have been generated. We observe hot-electron temperatures of ∼70 keV at intensities of 1.4 × 1015 W/cm2 with multiple overlapping beams driving the two-plasmon decay instability. When extrapolated to SI-relevant intensities of ∼1016 W/cm2, the hot electron temperature will likely exceed 100 keV, suggesting that tightly focused beams without overlap are better suited for launching the ignitor shock.

The shock heating of a laser-driven, direct-drive target can determine its stability by affecting Rayleigh-Taylor growth rates through target decompression and ablative stabilization. Measurements indicate that pulses that rise rapidly to 10(14) W/cm(2) produce shock-induced temperatures of approximately 25 eV, whereas more slowly rising pulses show less heating. Analysis of the observed target behavior produced by these two pulses demonstrates that shock heating improves hydrodynamic stability because ablative stabilization increases when the targets are preheated by shocks.

The evolution of shell modulations near peak compression of direct-drive spherical-target implosions has been measured using the 60-beam, 30-kJ UV OMEGA laser system. The spatial size and amplitude of shell-areal-density modulations decrease during the target compression, then increase during its decompression as expected. The shell uniformity at peak compression has been increased by reducing single-beam, laser-drive nonuniformity.

Using EXAFS to characterize metals shocked to -Mbar pressures by a laser has been studied. High-contrast modulations were obtained using a laser-imploded spherical target as the source. EXAFS visibility as function of shock pressure was calculated.

Summary form only given. For most direct-drive ICF experiments, it is important that the 0.5- to 4-ns laser pulses that irradiate targets on OMEGA have high optical contrast. Using a photodiode that is sensitive to intensities that are 10/sup -8/ of the main OMEGA pulse, the UV output of OMEGA was measured for a duration of /spl sim/20 ns before the arrival of the main pulse. By cross-calibrating this with the main pulse shape, we constructed the entire temporal pulse shape over 20 ns and over approximately eight orders of magnitude. To assess the tolerable level of contrast, we performed experiments that probe the Al barrier layers placed on most OMEGA targets. We also correlate the performance of imploding spherical targets with the measured optical contrast.

The Plasma Lens Collaboration proposed and has been approved to perform the Plasma Lens Experiment at the Final Focus Test Beam (SLAC E-150bis). The experiment is a revised and simplified proposition which reflects the pertinent scientific and technological advances since an earlier proposal (1) was submitted. The goals of the experiment are to study plasma focusing of high energy, high density particle beams: to investigate plasma beamsstrahlung suppression: and to develop technologies for plasma lens applications in present and future linear colliders.

In this paper we describe the first spatially-resolved measurements of the time-dependence of high-order harmonic radiation. We show that the harmonic source can become annular with a radius that increases in time during the laser pulse owing primarily to plasma formation and that under appropriate conditions this effect could be used to produce ultra-short harmonic pulses.

In 1957, it was shown that charged particles interacting with an oscillating electromagnetic field will experience a force proportional to the gradient of the field intensity in the direction of decreasing intensity.1 This so-called ponderomotive force results in the rapid expulsion of free electrons from a high-intensity Gaussian laser focus.2 If, however, the gradient of the intensity is made to point inwards, this force could be used to confine rather than repel. This was known for the case of RF-fields in 1958,3 and was extended to laser fields in 1966.4 Since that time, laser peak intensities have seen a dramatic rise.5 This has led to the observation of a vast array of non-linear light-matter interactions, but the accompanying increase in the magnitude of the ponderomotive force has made the observation of some effects difficult. In this paper, we re-visit the idea of ponderomotive trapping, but with a specific goal in mind: the confinement of electrons in a high-field region, with the expectation of observing second-harmonic generation from their relativistic oscillations.6 We present a novel scheme to generate a single-beam, three-dimensional trap, along with simulated electron trajectories that demonstrate confinement in a high-field region. Also, we present images of the trapping beam generated with a high-power laser. It is important to note that there exist other schemes of laser-based trapping 7,8 These more complicated methods would trap electrons in only two dimensions, while our single-beam trap offers three dimensions of confinement. To our knowledge, this is the only single-beam, three-dimensional trap for energetic electrons to be proposed, and the only trap of any kind to be generated with a high-power laser.

Frequency-domain interferometry has been used to find the optical axis of a uniaxial, birefringent crystal. The dependence of the interference fringe spacing on the incident angle and the direction of the optical axis were determined in a 4.6-cm-thick KDP crystal.

The results from a series of single-mode Rayleigh-Taylor (RT) instability growth experiments performed on the OMEGA laser system using planar targets are reported. Planar targets with imposed mass perturbations were accelerated using five to six 351-nm laser beams overlapped with total intensities up to 2.5×1014 W/cm2. Experiments were performed with both 3-ns ramp and 3-ns flat-topped temporal pulse shapes. The use of distributed phase plates and smoothing by spectral dispersion resulted in a laser-irradiation nonuniformity of 4%–7% over a 600-μm-diam region defined by the 90% intensity contour. The temporal growth of the modulation in optical depth was measured using through-foil radiography and was detected with an x-ray framing camera for CH targets with and without a foam buffer. The growth of both 31-μm and 60-μm wavelength perturbations was found to be in good agreement with ORCHID simulations when the experimental details, including noise, were included. The addition of a 30-mg/cc, 100-μm-thick po...

Electrons injected into a high intensity laser focus by tunneling ionization quickly escape due to the strong ponderomotive force. This limits the probability of observing harmonic radiation from free electrons oscillating in the field1 and other nonlinear effects. Two dimension(radial) optical confining schemes have been proposed to increase the life-time of the electron in the intense field.2,3 These do not confine the electron in the direction of the laser propagation which limits their effectiveness. In intense fields the electrons have significant momentum in this direction.4 To our knowledge, neither of these schemes have been demonstrated.

The energy and number distributions of photoelectrons observed outside the ionizing field can give insight into the ionization process in a strong laser field. The physics of this process can be considered to occur in a two-step process known as the “simpleman’s model.”1 First, the electron escapes the atom by tunneling, and second, it interacts with the external field. If the time it takes the electron to tunnel out of the atom is much shorter than the laser period, then ionization occurs in the adiabatic limit (the ionization potential Ip is greater than the photon energy ω), and static field calculations of the ionization rate are valid.1 In the experiments described below, the Keldysh adiabaticity parameter γ = (I p /2U p )1/2 ≪ 1,2 where Ip is the ionization potential of the atom and Up = F 2/2ω2 is the ponderomotive energy, indicating that ionization occurs in the tunneling regime. (Here the electric field F is proportional to the square root of the laser intensity.) Once liberated, the electron will gain a drift momentum which is determined by the phase of the laser field at the time of ionization. If the polarization is linear, then the drift momentum will be zero unless the external electric field is not at its peak value. The drift momentum will be nonzero and directed perpendicular to the polarization vector if the laser is elliptically polarized. If the laser is circularly polarized, the drift due to initial conditions will give the electron a directed kinetic energy of about 2Up when ponderomotive effects are included. We have used a 2-ps, 1-μm laser with intensities up to 1 × 1018 W/cm2 and two electron spectrometers to test the static tunneling models.

The acceleration-driven growth of perturbations initially seeded (or imprinted) by irradiation nonuniformities is a major concern for direct-drive ICF. We report on experiments where unperturbed, CH2 targets were irradiated with 3-ns square pulses at 2×1014 W/cm2 using five overlapped UV beams. The uniformity of these beams was varied and subsequent unstable growth of irradiation imprinting was observed using time-gated x-radiography. We demonstrate that increased irradiation uniformity reduces imprinting, and we show the beneficial effects of distributed polarization rotators—a new beam-smoothing device. In addition, we discuss experiments using a “perturbing beam” to study the time dependence of imprinting.

Though a vast number of studies have been conducted on the various properties of intense field high order harmonic radiation, there have been no studies of the actual spatial coherence of optical harmonics at short wavelengths (< 1000 Å). Measurements of harmonic far field profiles have suggested that the harmonics exhibited high coherence [1,2,3], but harmonic profile measurements alone are inadequate in determining the spatial coherence of the harmonic beam [4], To address this aspect of the harmonic generation, we have conducted a series of two-slit interference measurements on harmonics in the 270 Å to 480 Å range to determine unambiguously their spatial coherence properties.

The University of Rochester's Laboratory for Laser Energetics (LLE) performs direct-drive inertial confinement fusion (ICF) research. LLE's Omega Laser Facility is used to study direct-drive ICF ignition concepts, developing an understanding of the underlying physics that feeds into the design of ignition targets for the National Ignition Facility (NIF). The baseline symmetric-illumination, direct-drive?ignition target design consists of a 1.5?MJ multiple-picket laser pulse that generates four shock waves (similar to the NIF baseline indirect-drive design) and is predicted to produce a one-dimensional (1D) gain of 48. LLE has developed the polar-drive (PD) illumination concept (for NIF beams in the x-ray?drive configuration) to allow the pursuit of direct-drive ignition without significant reconfiguration of the beam paths on the NIF. Some less-invasive changes in the NIF infrastructure will be required, including new phase plates, polarization rotators, and a PD-specific beam-smoothing front end. A suite of PD ignition designs with implosion velocities from 3.5 to 4.3???107?cm?s?1 are predicted to have significant 2D gains (Collins et al 2012 Bull. Am. Phys. Soc. 57 155). Verification of the physics basis of these simulations is a major thrust of direct-drive implosion experiments on both OMEGA and the NIF. Many physics issues are being examined with symmetric beam irradiation on OMEGA, varying the implosion parameters over a wide region of design space. Cryogenic deuterium?tritium target experiments with symmetric irradiation have produced areal densities of ?0.3?g?cm?2, ion temperatures over 3?keV, and neutron yields in excess of 20% of the ?clean? 1D predicted value. The inferred Lawson criterion figure of merit (Betti R. et al 2010 Phys. Plasmas 17 058102) has increased from 1.7?atm?s (IAEA 2010) to 2.6?atm?s.

Cross-beam energy transfer (CBET) during OMEGA low-adiabat cryogenic experiments reduces the hydrodynamic efficiency by ∼35%, which lowers the calculated one-dimensional (1-D) yield by a factor of 7. CBET can be mitigated by reducing the diameter of the laser beams relative to the target diameter. Reducing the diameter of the laser beams by 30%, after a sufficient conduction zone has been generated (two-state zooming), is predicted to maintain low-mode uniformity while recovering 90% of the kinetic energy lost to CBET. A radially varying phase plate is proposed to implement two-state zooming on OMEGA. A beam propagating through the central half-diameter of the phase plate will produce a large spot, while a beam propagating through the outer annular region of the phase plate will produce a narrower spot. To generate the required two-state near-field laser-beam profile, a picket driver with smoothing by spectral dispersion (SSD) would pass through an apodizer, forming a beam of half the standard diameter. A second main-pulse driver would co-propagate without SSD through its own apodizer, forming a full-diameter annular beam. Hydrodynamic simulations, using the designed laser spots produced by the proposed zooming scheme on OMEGA, show that implementing zooming will increase the implosion velocity by 25% resulting in a 4.5× increase in the 1-D neutron yield. Demonstrating zooming on OMEGA would validate a viable direct-drive CBET mitigation scheme and help establish a pathway to hydrodynamically equivalent direct-drive–ignition implosions by increasing the ablation pressure (1.6×), which will allow for more stable implosions at ignition-relevant velocities.

Spherically symmetric, low-adiabat (adiabat α ≲ 3) cryogenic direct-drive-implosion experiments on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1995)] yield less than 10% of the neutrons predicted in one-dimensional hydrodynamic simulations. Two-dimensional hydrodynamic simulations suggest that this performance degradation can be explained assuming perturbations from isolated defects of submicron to tens-of-micron scale on the outer surface or inside the shell of implosion targets. These defects develop during the cryogenic filling process and typically number from several tens up to hundreds for each target covering from about 0.2% to 1% of its surface. The simulations predict that such defects can significantly perturb the implosion and result in the injection of about 1 to 2 μg of the hot ablator (carbon-deuterium) and fuel (deuterium-tritium) materials from the ablation surface into the targets. Both the hot mass injection and perturbations of the shell reduce the final shell convergen...

Rayleigh–Taylor (RT) growth experiments were performed on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] using planar SiO2 targets seeded with a single mode 60-μm wavelength perturbation driven at peak laser intensities up to 9 × 1014 W/cm2. These are the first RT measurements in SiO2 at conditions relevant to direct-drive inertial confinement fusion ignition. The measured average modulation growth rates agree with the 2-D hydrodynamics code DRACO, providing an important step in the development of target ablators that are robust to RT growth and hot- electron preheat considerations when driven at the intensities required to achieve thermonuclear ignition.

Proton radiography of the spatial structure and temporal evolution of plasma blowing off from a hohlraum wall reveals how the fill gas compresses the wall blow-off, inhibits plasma jet formation and impedes plasma stagnation in the hohlraum interior. The roles of spontaneously generated electric and magnetic fields in hohlraum dynamics and capsule implosions are demonstrated. The heat flux is shown to rapidly convect the magnetic field due to the Nernst effect, which is shown to be ?10 times faster than convection by the plasma fluid from expanded wall blow-off (vN???10v). This leads to inhibition of heat transfer from the gas region in the laser beam paths to the surrounding cold gas, resulting in a local plasma temperature increase. The experiments show that interpenetration of the two materials (gas and wall) occurs due to the classical Rayleigh?Taylor instability as the lighter, decelerating ionized fill gas pushes against the heavier, expanding gold wall blow-off. This experiment provides physics insight into the effects of fill gas on x-ray-driven implosions, and would impact the ongoing ignition experiments at the National Ignition Facility.

A flexible direct-drive target platform is used to implode cryogenic deuterium–tritium (DT) capsules on the OMEGA laser [Boehly et al., Opt. Commun. 133, 495 (1997)]. The goal of these experiments is to demonstrate ignition hydrodynamically equivalent performance where the laser drive intensity, the implosion velocity, the fuel adiabat, and the in-flight aspect ratio (IFAR) are the same as those for a 1.5-MJ target [Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)] designed to ignite on the National Ignition Facility [Hogan et al., Nucl. Fusion 41, 567 (2001)]. The results from a series of 29 cryogenic DT implosions are presented. The implosions were designed to span a broad region of design space to study target performance as a function of shell stability (adiabat) and implosion velocity. Ablation-front perturbation growth appears to limit target performance at high implosion velocities. Target outer-surface defects associated with contaminant gases in the DT fuel are identified as the dominant perturbation source at the ablation surface; performance degradation is confirmed by 2D hydrodynamic simulations that include these defects. A trend in the value of the Lawson criterion [Betti et al., Phys. Plasmas 17, 058102 (2010)] for each of the implosions in adiabat–IFAR space suggests the existence of a stability boundary that leads to ablator mixing into the hot spot for the most ignition-equivalent designs.

Filamentary electromagnetic fields previously observed in the coronae of laser-driven spherical targets [F. H. Seguin et al., Phys. Plasma. 19, 012701 (2012)] have been further investigated in laser-irradiated plastic foils. Face-on proton-radiography provides an axial view of these filaments and shows coherent cellular structure regardless of initial foil-surface conditions. The observed cellular fields are shown to have an approximately constant scale size of ∼210 μm throughout the plasma evolution. A discussion of possible field-generation mechanisms is provided and it is demonstrated that the likely source of the cellular field structure is the magnetothermal instability. Using predicted temperature and density profiles, the fastest growing modes of this instability were found to be slowly varying in time and consistent with the observed cellular size.

Polar-drive (PD) experiments on the OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser are described. Continuous pulse shapes, where a low-power foot is followed by a rise to the main pulse, and triple-picket pulse shapes, where three pickets precede the main pulse, are used to irradiate warm plastic shell capsules. Both of these pulse shapes set the target on a low, ignition-relevant adiabat of ∼3.5. The areal density is modeled very well in these implosions indicating that shock timing is well modeled in PD geometry. It is shown that the symmetry can be predictably varied by changing the beam pointings. Symmetry is also well reproduced across the two pulse shapes. Limitations of OMEGA experiments are discussed. Preliminary designs for PD implosion experiments on the NIF, with the goal of addressing ignition-relevant issues for PD, including symmetry are presented.

High-convergence polar-drive experiments are being conducted on OMEGA [T. R. Boehly et al., Opt. Commum. 133, 495 (1997)] using triple-picket laser pulses. The goal of OMEGA experiments is to validate modeling of oblique laser deposition, heat conduction in the presence of nonradial thermal gradients in the corona, and implosion energetics in the presence of laser–plasma interactions such as crossed-beam energy transfer. Simulated shock velocities near the equator, where the beams are obliquely incident, are within 5% of experimentally inferred values in warm plastic shells, well within the required accuracy for ignition. High, near-one-dimensional areal density is obtained in warm-plastic-shell implosions. Simulated backlit images of the compressing core are in good agreement with measured images. Outstanding questions that will be addressed in the future relate to the role of cross-beam transfer in polar drive irradiation and increasing the energy coupled into the target by decreasing beam obliquity.

The implementation of polar drive (PD) at the National Ignition Facility (NIF) will enable the execution of direct-drive implosions while the facility is configured for x-ray drive. The Laboratory for Laser Energetics (LLE), in collaboration with LLNL, LANL and GA, is implementing PD on the NIF. LLE has designed and participates in the use of PD implosions for diagnostic commissioning on the NIF. LLE has an active experimental campaign to develop PD in both warm and cryogenic target experiments on OMEGA. LLE and its partners are developing a Polar Drive Project Execution Plan, which will provide a detailed outline of the requirements, resources, and timetable leading to PD-ignition experiments on the NIF.

The first measurement of the emittance of intense laser-produced positron beams has been made. The emittance values were derived through measurements of positron beam divergence and source size for different peak positron energies under various laser conditions. For one of these laser conditions, we used a one dimensional pepper-pot technique to refine the emittance value. The laser-produced positrons have a geometric emittance between 100 and 500 mm·mrad, comparable to the positron sources used at existing accelerators. With 1010-1012 positrons per bunch, this low emittance beam, which is quasi-monoenergetic in the energy range of 5–20 MeV, may be useful as an alternative positron source for future accelerators.

We intend to carry out a series of plasma lens experiments at the Final Focus Test Beam facility at SLAC. These experiments will be the first to study the focusing of particle beams by plasma focusing devices in the parameter regime of interest for high energy colliders, and is expected to lead to plasma lens designs capable of unprecedented spot sizes. Plasma focusing of positron beams will be attempted for the first time. We will study the effects of lens aberrations due to various lens imperfections. Several approaches will be applied to create the plasma required including laser ionization and beam induced tunneling ionization of a working gas—the latter which has never been observed before. The compactness of our device should prove to be of interest for applications at the SLC and the next generation linear colliders.

Spherical shock-ignition experiments on OMEGA used a novel beam configuration that separates low-intensity compression beams and high-intensity spike beams. Significant improvements in the performance of plastic-shell, D2 implosions were observed with repointed beams. The analysis of the coupling of the high-intensity spike beam energy into the imploding capsule indicates that absorbed hot-electron energy contributes to the coupling. The backscattering of laser energy was measured to reach up to 36% at single-beam intensities of ∼8 × 1015 W/cm2. Hard x-ray measurements revealed a relatively low hot-electron temperature of ∼30 keV independent of intensity and timing. At the highest intensity, stimulated Brillouin scattering occurs near and above the quarter-critical density and the two-plasmon-decay instability is suppressed.

A magnetic recoil spectrometer (MRS) has been installed and extensively used on OMEGA and the National Ignition Facility (NIF) for measurements of the absolute neutron spectrum from inertial confinement fusion implosions. From the neutron spectrum measured with the MRS, many critical implosion parameters are determined including the primary DT neutron yield, the ion temperature, and the down-scattered neutron yield. As the MRS detection efficiency is determined from first principles, the absolute DT neutron yield is obtained without cross-calibration to other techniques. The MRS primary DT neutron measurements at OMEGA and the NIF are shown to be in excellent agreement with previously established yield diagnostics on OMEGA, and with the newly commissioned nuclear activation diagnostics on the NIF.

Measurements of the total energy carried by fast ablator-ions in direct-drive implosions on OMEGA have been conducted using magnetic and Thomson Parabola spectrometers. It is shown that the total laser energy lost to fast ablator-ions for plastic and glass targets is comparable and that it is a modest fraction of the incident laser energy (≲1%). These measurements have been used to infer a non-linear, voltage-dependent target capacitance (∼0.1 nF) associated with the space-charge that accelerates the fast-ions.

A Thomson parabola ion spectrometer, implemented on the Laboratory for Laser Energetics Multi-Terawatt laser facility, has been used to study heavy-ion acceleration from ultra-intense laser-plasma interactions. These studies were conducted using 20 μm-thick flat foil targets with on-target intensities of 4−5×1019 W-cm−2. Several charge states of energetic heavy ions were accelerated from the rear of Al and Cu foils in the presence of hydrocarbon contaminants. In contrast to previous work, we show that the mean and maximum energies of each heavy-ion species scale linearly with only the charge of the ion, and not the charge-to-mass ratio, implying that the space-charge is not readily depleted as ions expand. It has been shown previously and observed here that the maximum heavy-ion energies are lower than that of protons, though a fundamental explanation for this discrepancy has not been provided. We show how a two-temperature electron distribution, observed in these experiments, explains these observations.

Low-adiabat polar-drive (PD) [Skupsky et al., Phys. Plasmas 11, 2763 (2004)] implosion designs for the OMEGA [Boehly et al., Opt. Commun. 133, 495 (1997)] laser are described. These designs for cryogenic deuterium–tritium and warm plastic shells use a temporal laser pulse shape with three pickets followed by a main pulse [Goncharov et al., Phys. Rev. Lett. 104, 165001 (2010)]. The designs are at two different on-target laser intensities, with different in-flight aspect ratios (IFARs). These designs permit studies of implosion energetics and target performance closer to ignition-relevant intensities (∼7 × 1014 W/cm2 at the quarter-critical surface, where nonlocal heat conduction and laser–plasma interactions can play an important role) but at lower values of IFAR ∼ 22 or at lower intensity (∼3 × 1014 W/cm2) but at a higher IFAR (IFAR ∼ 32, where shell instability can play an important role). PD geometry requires repointing of laser beams to improve shell symmetry. The higher-intensity designs optimize target performance by repointing beams to a lesser extent, compensating for the reduced equatorial drive by increasing the energies of the repointed beams. They also use custom beam profiles that improve equatorial illumination at the expense of irradiation at higher latitudes. These latter designs will be studied when new phase plates for the OMEGA Laser System, corresponding to the custom beam profiles, are obtained.