NIF Laser Research Articles

Multi-keV x-ray radiator from titanium cylindrical cavity at the Shenguang-III prototype laser facility

Great efforts have been made to create a bright K-shell source using the thin metal cylindrical cavities in the past few decades. Several metal materials such as titanium (Ti), iron, and so on have been tested for high x-ray conversion efficiency mainly at the OMEGA and NIF laser facilities. Recently, x-ray sources in Ti K-shell transition energy range were investigated at the Shenguang-III prototype laser facility with ∼5 kJ laser energy. The experiments were aimed to reproduce the previous ones at OMEGA, but with an extraordinarily small volume of cylindrical cavity and detailed characterizations of the x-ray source. The cavities were 800 μm inner diameter, 800 μm length, and 30 μm thick plastic tubes supporting 1 μm thick Ti. Seven laser beams were focused to 200 μm diameter. The combination of the small cavity volume and the focused laser spots is intended to improve the electron temperature with limited laser energy since the electron temperature is a key issue for high x-ray conversion efficiency. Thomson scattering was adopted to experimentally probe the electron temperatures at special time and space zones as well as the average temperature obtained from the Ti K-shell spectrum. The evolutions of the electron temperature and density are predicted by the radiation hydrodynamic simulation. A top view and two photon energy bands of x-ray source images provide a way to directly observe the plasma movement toward the cavity axis and distinguish the different emission mechanisms between the Ti K-shell and lower energy x rays. Six Higher-energy x-ray detectors located at different angles were used to record the Ti K-shell x-ray emission and demonstrate its isotropic feature. The characteristics of the x-ray radiate intensity including the time evolution, the angular distribution, and the total yields for both the photon energy regions above and below 4 keV are compared between the cavity and planar targets. Obviously, different behaviors were found between the two photon energy regions and the two types of targets. The x-ray conversion efficiency of the Ti cylinder was determined to be ∼4% and ∼21% in 4π sr in the Ti K-shell (4–7 keV) and <4 keV range, respectively. The Ti K-shell conversion efficiency obtained in the present experiments is between the ones driven by 13.5 and 4.5 kJ laser energy at OMEGA.

Laser-plasma instabilities in long scale-length plasmas relevant to shock-ignition

We present 2D particle-in-cell simulations of laser-plasma instabilities (LPI) performed using conditions relevant to experiments on the OMEGA and NIF laser facilities. The laser intensity used is 2×1015 W cm−2, which allows comparison with previous experiments and simulations investigating shock ignition, though this is a relatively low intensity for the scheme. We find that the large convective gain of the instabilities leads to dynamics that are largely controlled by pump depletion and to scattering or absorption by LPIs predominantly occurring away from the quarter-critical density. The two-plasmon decay (TPD) instability is dominant for OMEGA-type conditions and remains important at the NIF-scale. In both cases, most absorption from TPD occurs near its Landau cutoff density, and we examine its nonlinear dynamics in this region, focusing on the ion-acoustic waves responsible for saturation. For NIF-type conditions, stimulated Raman scattering (SRS) and stimulated Brillouin scattering divert significant fractions of laser energy, and we find that SRS backscatter can occur with high gain at low densities due to kinetic effects. An in-depth analysis of hot-electron production is performed to identify the main sources and their characteristic electron temperature. For both simulations presented, the overall hot-electron temperature lies between 30 and 35 keV. This is relatively low and will likely increase shock pressure; however, the distributions also contain a significant number of high-energy (Ek>100 keV) electrons that would likely cause unacceptable preheat. We, therefore, suggest some strategies that may be utilized to minimize this high-energy component.

Hydro-scaling of direct-drive cylindrical implosions at the OMEGA and the National Ignition Facility

Deceleration-phase Rayleigh–Taylor instability (RTI) growth during inertial confinement fusion capsule implosions significantly affects the performance as it mixes cold ablator material into the fuel. Precise measurements of such instability growth are essential for both validating the existing simulation codes and improving our predictive capability. RTI measurements on the inner surface of a spherical shell are limited and are often inferred indirectly at limited convergence. In contrast, cylindrical implosions allow for direct diagnostic access to the converging interface by imaging down the cylinder axis while retaining the effects of convergence. We have performed direct-drive cylindrical implosion experiments at both the OMEGA and the NIF laser facilities using scaled targets. RTI growth is demonstrated to be scale-invariant between the cylindrical targets at OMEGA and similar targets at the NIF, which are scaled up by a factor of three in the radial dimension. Single-mode (m = 20) instability growth factors of ∼14 are measured at a convergence ratio (CR) ∼ 2.5 with nearly identical mode growth at both scales. The measurements are in agreement with xRAGE radiation-hydrodynamics simulations. In addition, we have developed the Bayesian-inference-engine method to account for the variations in the target alignment, magnification, and the parallax effect in the measurement, allowing a more precise comparison between the experimental data and the simulations.

A platform for thin-layer Richtmyer-Meshkov at OMEGA and the NIF

Imperfections at the interface between the ablator and fuel in an ICF capsule can give rise the Richtmyer-Meshkov instability (RMI). The effects of multiple shocks on this impulse-driven instability has been well studied in traditional, low energy density (LED) regimes, but work is limited in the high energy density (HED) regime. Instability and turbulent characteristics are difficult to diagnose in an ICF capsule, with its three dimensional and converging geometry. This paper highlights the platform development of a new planar HED experiment at the OMEGA and NIF laser facilities designed specifically to study the feedthrough of the RMI in a thin layer. Using a simple shock and then re-shock setup, Multi-shock (Mshock) has successfully redesigned the previous Reshock experiment to produce a well-defined mixing layer post-shock and reshock. Localized doping profiles have been developed to improve feature contrast, and reduce edge effects. A series of initial conditions have been precision machined and characterized for comparison, though the effects of preheat are distorting these profiles. First results quantifying the growth via the mixing layer width have been made with an 1D Eulerian code and an 1D RANS mix-model.

On the Problem of Thermonuclear Ignition and Burning in the NIF Laser: Ignition with Capsules Taking into Account the Influence of the Hohlraum Radiation on Implosion

One of the significant problems of modern physics is the creation and use of new energy sources. The decisive step in this direction is the realization of a positive yield in thermonuclear energy on a laboratory scale with the facilities that create and retain plasma. Between 2010 and 2017, a large series of experiments were performed at the National Ignition Facility laser at the Livermore Laboratory in the United States with the aim of achieving such a positive outcome. Until now, however, the ignition has not been reached. In this paper, for the first time the influence of the hohlraum radiation on capsule layers including a deuterium–tritium layer is discussed. It is shown that the negative influence of the ablator transparency can be compensated for by the selection of a capsule design with a greater optical thickness of the part of the ablator. This part of the ablator is not evaporated upon heating by an X-ray pulse. The design of such a capsule is proposed.

Comparison and analysis of the results of direct-driven targets implosion

The article presents calculation results, which were received for the implosion of the typical cryogenic thermonuclear direct-drive targets that are intended for use at the OMEGA facility, NIF and Russian laser facility. The compression and burning characteristics, which were obtained using various numerical codes of different scientific groups, are compared. The data indicate good agreement between the numerical results.Various sources of target irradiation inhomogeneity and their influence on the implosion parameters are considered. The nominal scales of these disturbances for various facilities are close to each other. The main negative effect on the efficiency of compression and burning is due to the accidental offset of the target from the center of the chamber.

Investigating Turbulent Mix in HEDLP Experiments

Mix is an important issue in High Energy Density Laboratory Plasmas (HEDLP), specifically Inertial Confinement Fusion (ICF) implosions. In ICF, shock waves traverse fuel capsule defects and material interfaces, and due to hydrodynamic instabilities transitioning into turbulence, these shocks can initiate mix between shell and fuel, degrading yield. To this end, a series of laser-driven mix experiments has been designed for the OMEGA and NIF laser facilities to investigate the turbulent mixing of materials proceeded by reshock and shear, which initiates Richtmyer-Meshkov and\\or Kelvin-Helmholtz instabilities on a tracer layer. The experiments are designed to understand if the Besnard-Harlow-Rauenzahn (BHR) mix model that has been implemented in LANL's RAGE hydrodynamics code has coefficients that are properly determined for an HEDLP environment.

Automation of NIF Target Characterization and Laser Ablation of Domes Using the 4pi System

Capsules for inertial confinement fusion require precise measurement of isolated features and domes on the capsule’s outer surface. Features that are too large must be removed. A 4pi capsule mapping and characterization system has been developed to map, identify, and measure domes using a Leica confocal microscope. An ultraviolet wavelength laser was integrated to laser-ablate the offending domes that exceed the allowable mix mass. Current process methods to remove domes require three different stations in different locations. The 4pi system achieves automated capsule handling, metrology, and laser polishing/ablation of domes on one device without losing track of the capsule’s orientation. The measurement technique and metrology accuracy are compared to patch atomic force microscopy scans and phase-shifting diffraction interferometer measurements with good correlation. The laser polishing method has demonstrated analogous results to the current process methods, but in an automated fashion. Additionally, the 4pi capsule-handling capability of the system has been used to laser-ablate purposeful engineered designs into specialty capsules.

Formation of nonlinear holographic images in a system of periodically located nonlinear mediums

The formation of nonlinear holographic images in a system of periodically located nonlinear mediums is studied. Analytical expressions which describe the magnitudes and locations of intensity maximums depending on the corresponding image number are derived. Comparison with numerical calculation results is presented. Dependence of a refraction index on intensity leads to beam self-focusing (1), transverse instability (2) and to formation of nonlinear holographic images (NHI) of obstacles in the optical path of powerful laser systems (3). In Ref. (4) it has been shown that at value of B-integral of only 0.2rad a round opaque obscuration increases the intensity peak which exceeds the average level by almost 1.5 times, and for a phase obstacle the peak excess is 2.5. One can present a disk amplifier as a system of periodically located (SPL) nonlinear mediums (NM). Formation of NHI in SPLNM differs from the NHI formation in continuous media with the same value of B-integral. For the first time this problem has been considered in Ref. (3). However, these authors (3) focused only on the analysis of the intensity dependence upon the size of an obscuration at a single point of space. Accidentally for SPLNM configuration considered in Ref. (3), the intensity in the chosen point is not a peak one. The locations of the NHI and their intensity were not explored. In Refs (5, 6 )t he results of peak intensity calculations for the various elements of the NIF laser chain with account for NHI formation were published. In Refs (7-9) the process of the NHI formation in a SPLNM was viewed in more details and it was shown that for KNM there are K NHI. However the chosen model parameters (number of NM, increment of B-integral for one NM, thickness of NM, size of the beam) are far from the parameters of typical disk amplifiers. In this paper, we are considering the simplified system of infinitely thin NM without energy gain and losses. According to expression (39) in Ref. (3), when the powerful plane wave and spherical scattered wave incident on the single NM, the new wave is generated. For the infinitely thin NM the complex amplitude A1 of this wave in the plane of NHI is equal to A1 =− iBA ∗ (1)

A Laser Technology Test Facility for Laser Inertial Fusion Energy (LIFE)

A LIFE laser driver needs to be designed and operated which meets the rigorous requirements of the NIF laser system while operating at high average power, and operate for a lifetime of >30 years. Ignition on NIF will serve to demonstrate laser driver functionality, operation of the Mercury laser system at LLNL demonstrates the ability of a diode-pumped solid-state laser to run at high average power, but the operational lifetime >30 yrs remains to be proven. A Laser Technology test Facility (LTF) has been designed to specifically address this issue. The LTF is a 100-Hz diode-pumped solid-state laser system intended for accelerated testing of the diodes, gain media, optics, frequency converters and final optics, providing system statistics for billion shot class tests. These statistics will be utilized for material and technology development as well as economic and reliability models for LIFE laser drivers.

NIF unconverted light and its influence on DANTE measurements

NIF laser facility produces 1053 nm light and a fundamental requirement for NIF is to give up to 1.8 MJ of 351 nm light for target physics experiments. The 351 nm light is provided by frequency tripling the 1053 nm light in nonlinear crystals in the final optics assembly, just before the laser light enters the target chamber. Since this tripling process is not 100% efficient, unconverted light from the conversion process also enters the chamber. This unconverted light does not directly hit the target but it can strike target support structures at average intensities of few TW/cm2 where it can generate unwanted, background soft x-rays that are measured by the soft x-ray diagnostic DANTE installed on the NIF target chamber. This diagnostic quantifies the x-radiation intensity inside the hohlraum by measuring the x-ray flux coming from the target's laser entrance hole. Due to its centimeter wide field of view, it integrates x-ray emission from both the flux exiting a hohlraum laser entrance hole and from the target support structure irradiated by residual 1omega and 2omega unconverted light. This work gives quantitative evaluations of the unconverted light for the first time and the effects on DANTE measurements for the future NIF tuning experiment called "Shock timing." Emission spectra are significantly modified leading to an overestimation of radiative temperature during the foot of the laser pulse since background x-rays are predominant in first two DANTE channel measurements. Mitigations of these effects by coating silicon paddle with plastic, using a smaller collimator to reduce DANTE field of view or eliminating DANTE channels in the analysis have been investigated.

NIF laser facility close to completion

NIF laser facility close to completion

Quasi-isentropic material property studies at extreme pressures: from Omega to NIF

We are developing an experimental platform that can compress materials quasi-isentropically to very high pressures at ultrahigh strain rates. This laser driven, ramped (shockless) drive is used to study material properties such as strength, equation of state, phase, and phase transition kinetics under extreme conditions. We have achieved a ramped, shockless drive up to 2 Mbar on the Omega laser using both direct laser illumination and indirect x-ray illumination. In order to probe high-Z materials under extreme pressures, we are also developing high energy x-ray backlighters, 17 to 100 keV, created by high intensity (>1018 W/cm2) short pulse lasers (1 to 50 ps) such as the Titan laser at LLNL. Using a micro-wire embedded in a low-Z substrate, we have obtained radiographs with better than 10 ?m spatial resolution. This paper will show designs of isentropic platforms that can reach >10 Mbar on the NIF laser, using both direct and indirect drive configurations.

Single-shot laser driven inertial confinement fusion based on nanosecond and picosecond laser pulses

Nuclear fusion gains of deuterium-tritium (DT) at volume ignition are discussed for the forthcoming conditions of the NIF or LMJ laser pulses in the range of nanosecond duration at direct drive. This turns out to be similar to the expected values known form spark ignition. These conditions should be reached also by single pulse interaction of picosecond pulses using the unique anomaly of nonlinear force driven plasma block acceleration with ps pulses for volume ignition. Total gains of 200 for MJ laser pulses are expected.

Response to comment on “The National Ignition Facility laser performance status”

We appreciate Stephen Bodner's continuing interest in the performance of the NIF laser system. However, we find it necessary to disagree with the conclusions he reached in his comments [Appl. Opt.47, XXX (2008)] on “National Ignition Facility Laser Performance Status” [Appl. Opt.46, 3276 (2007)]. In fact, repeated and ongoing tests of the NIF beamlines have demonstrated that NIF can be expected not only to meet or exceed its requirements as established in the mid-1990s in the document National Ignition Facility Functional Requirements and Primary Criteria [Revision 1.3, Report NIF-LLNL-93-058 (1994)], but also to have the flexibility that provides for successfully meeting an ever expanding range of mission goals, including those of ignition.

Hard X-ray and hot electron environment in vacuum hohlraums at NIF

Time resolved hard x-ray images (hv > 9 keV) and time integrated hard x-ray spectra (hv = 18-150 keV from vacuum hohlraums irradiated with four 351 nm wavelength NIF laser beams are presented as a function of hohlraum size and laser power and duration. The hard x-ray images and spectra provide insight into the time evolution of the hohlraum plasma filling and the production of hot electrons. The fraction of laser energy detected as hot electrons (f hot ) shows correlation with both laser intensity and with an analytic plasma filling model.

Polarization smoothing on the national ignition facility

We have recently implemented polarization smoothing (PS) on one quad of the NIF laser. Specially cut KDP and DKDP crystals at 420 x 420 mm sizes were used to scramble the incident 351 nm laser polarization over the beam aperture. The intensity contrast deduced from the measured focal spot images for one of the NIF beams is in very good agreement with the expected contrast. KDP and DKDP crystals are known to produce considerable amount of transverse stimulated Raman scattering (SRS) when irradiated with large beams at {approx}1-2 GW/cm2. In order to measure the transverse SRS, we attached optical fibers on the side of one of the PS crystals. The KDP PS crystal showed > 1 J/cm2 side scattered SRS at irradiances of 1.2 GW/cm2. The DKDP (70% deuteration level) PS crystal showed significantly less SRS. Detailed analysis of the SRS scattering in the PS crystal is in progress.

Thermoreflectance Measurements of the Temperature Distributions in Laser Diodes with Non Injected Facet

AbstractNon Injected Facet (NIF) lasers were developed to increase catastrophic optical damage level of the devices. In this work we present the analysis of the laser diodes (LD) front facet temperature distribution. Micro-thermoreflectance technique is used to perform a detailed temperature maps of the operating LDs. Such a technique gives us temperature maps with a spatial resolution better than 1mm and area of the maps covers active region and substrate crystal. We demonstrate micro thermoreflectance as a perfect tool to determine real temperature distribution of the operating laser diode front facet. Examined series of LDs under investigation have been made in two different ways. Major point of this work is comparison of the front facet temperature distribution of devices with and without injected facets. We compare temperature value and temperature distribution in the waveguide region in both types of the LDs. In our experiment NIF laser diodes exhibit facet temperatures increased by a factor of 1.4.

Calibration of initial measurements from the full aperture backscatter system on the National Ignition Facility

The full aperture backscatter system provides a measure of the spectral power, and integrated energy scattered by stimulated Brillouin (348–354 nm) and Raman (400–800 nm) scattering into the final focusing lens of the first four beams of the NIF laser. The system was designed to provide measurements at the highest expected fluences with: (1) spectral and temporal resolution, (2) beam aperture averaging, and (3) near-field imaging. This is accomplished with a strongly attenuating diffusive fiber coupler and streaked spectrometer and separate calibrated time integrated spectrometers, and imaging cameras. A new technique determines the wavelength dependent sensitivity of the complete system with a calibrated Xe lamp. Data from the calibration system are combined with scattering data from targets to produce the calibrated power and energy measurements that show significant corrections due to the broad band calibrations.

Full aperture backscatter diagnostic for the NIF laser facility (abstract)

The current schemes for achieving ignition on the National Ignition Facility require efficient coupling of energy from 192 laser beams to the deuterium–tritium fuel capsule. Each laser beam must propagate through a long scalelength plasma region before being converted to x rays (indirect drive) or being absorbed on the capsule (direct drive). Laser-plasma instabilities such as stimulated Brillouin and stimulated Raman scattering (SBS and SRS) will scatter a fraction of the incident laser energy out of the target leading to an overall reduction in the coupling efficiency. It is important to measure the character of this scattered light in order to understand it and to develop methods for reducing it to acceptable levels. We are designing a system called the full aperature backscatter diagnostic with the capability to measure the time-dependent amplitude and spectral content of the light which is backscattered through the incident beam focusing optic. The backscattered light will be collected over about 85% of the full beam aperture and separated into the SBS wavelength band (348–354 nm) and the SRS wavelength band (400–700 nm). Spectrometers coupled to streak cameras will provide time-resolved spectra for both scattered light components. The scattered light amplitude will be measured with fast and slow diodes. The entire system will be routinely calibrated. Analysis of the data will provide important information for reducing scattered power, achieving power balance, and finally achieving ignition.