Implosion Experiments Research Articles

Core condition analysis of radiation driven implosion for maximum compression

Core condition studies of radiation driven implosion for maximum compression time are the key contents of inertial confinement fusion research. Core conditions refer to the electron temperature and mass density in core region. The spatial distribution of core emission is calculated based on local thermal equilibrium by Multi one-dimensional simulation of core temperature and density. Assumption is made that the core temperature and density distributions each meet a Gauss distribution. Peak values and full widths at half maximum of temperature and density spatial distribution can be inferred by parameter optimization. The data-processing for implosion experiment on Sheng-GuangIII prototype facility indicates that the peak values of temperature and density are 1.7 keV and 1.2 g/cm3 respectively. The full widths at half maximum of temperature and density distribution are 20 μm and 18 μm respectively.

PHELIX: Design and Analysis of a Transformer-Driven Liner Implosion System

To provide substantial reduction in the size and energy of high-energy-density experiments, we have designed, built, and operated a liner implosion system that is driven by a multiturn-primary, single-turn-secondary, current step-up toroidal transformer. The Precision High Energy-density Liner Implosion eXperiment (PHELIX) pulsed-power driver, which is currently under development at Los Alamos National Laboratory, Los Alamos, NM, can provide >;400 kJ of capacitively stored energy and peak load currents of >;5 MA to implode centimeter-size liners in 10-20 μs, attaining speeds of 1-4 km/s. Diagnosis of scaled-down liner implosion experiments will be performed with the 800-MeV proton radiographic (pRad) system at Los Alamos Neutron Science Center (LANSCE); therefore, PHELIX is designed to be portable with a footprint of only 8 ×25 ft <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The multiframe, high-resolution imaging capability of pRad will be used to study hydrodynamic and material phenomena. Experiments with scaled-down electromagnetic railguns, pulsed high-field magnets, and magnetic flux compression are also under consideration. This paper discusses the overall PHELIX design concept and layout, and details of the electromechanical design needed to ensure repeatable operation.

Two dynamics modes in planar wire array Z pinch implosion

Two dynamics modes, named short ablation mode and long ablation mode, are observed in implosion experiments of planar wire array Z pinch on ‘QiangGuang-I’ facility utilizing an optical streak camera. The long ablation mode has a lagged trajectory compared with the short ablation mode. For shot 10035 in a short ablation mode, the initial time of K-shell radiation is consistent with the interaction time for ablation plasma arriving at the centre of wire array, while for shot 10038 in long ablation mode, the initial time of K-shell radiation is about 10 ns earlier. In the two modes, the partial ablation plasma could traverse the wire array plane and then collide in the centre to form a dense plasma column with a diameter of 2.2 mm for shot 10035 and 1.5 mm for shot 10038.

Diagnosing and controlling mix in National Ignition Facility implosion experiments

High mode number instability growth of “isolated defects” on the surfaces of National Ignition Facility [Moses et al., Phys. Plasmas 16, 041006 (2009)] capsules can be large enough for the perturbation to penetrate the imploding shell, and produce a jet of ablator material that enters the hot-spot. Since internal regions of the CH ablator are doped with Ge, mixing of this material into the hot-spot results in a clear signature of Ge K-shell emission. Evidence of jets entering the hot-spot has been recorded in x-ray images and spectra, consistent with simulation predictions [Hammel et al., High Energy Density Phys. 6, 171 (2010)]. Ignition targets have been designed to minimize instability growth, and capsule fabrication improvements are underway to reduce “isolated defects.” An experimental strategy has been developed where the final requirements for ignition targets can be adjusted through direct measurements of mix and experimental tuning.

X-ray ablation rates in inertial confinement fusion capsule materials

X-ray ablation rates have been measured in beryllium, copper-doped beryllium, germanium-doped plastic (Ge-doped CH), and diamondlike high density carbon (HDC) for radiation temperatures T in the range of 160–260 eV. In beryllium, the measured ablation rates range from 3 to 12 mg/cm2/ns; in Ge-doped CH, the ablation rates range from 2 to 6 mg/cm2/ns; and for HDC, the rates range from 2 to 9 mg/cm2/ns. The ablation rates follow an approximate T3 dependence and, for T below 230 eV, the beryllium ablation rates are significantly higher than HDC and Ge-doped CH. The corresponding implied ablation pressures are in the range of 20–160 Mbar, scaling as T3.5. The results are found to be well predicted by computational simulations using the physics packages and computational techniques employed in the design of indirect-drive inertial confinement fusion capsules. An iterative rocket model has been developed and used to compare the ablation rate data set to spherical indirect-drive capsule implosion experiments and to confirm the validity of some aspects of proposed full-scale National Ignition Facility ignition capsule designs.

Analysis of Ar line spectra from indirectly-driven implosion experiments on SGII facility

This paper reports on the indirectly-driven implosion experiments on SGII laser facility in which Ar emission spectrum from Ar-doped D-filled plastic capsule is recorded with the crystal spectrometer. Spectral features of Ar Heβ line and its associated satellites are analysed to extract the electron temperature and density of the implosion core. Non local thermal equilibrium (NLTE) collisional-radiative atomic kinetics and Strark broadening line shape are included in the present calculation. By comparing the calculated spectrum with the measured one, the core electron temperature and density are inferred to be 700 eV and 2.5 × 1023 cm−3 respectively. With these inferred values of electron temperature and density, neutron yield can be estimated to agree with the measured value in magnitude despite of the very simple model used for the estimation.

The Production and Characterization of Banded GDP Capsules for Defect Implosion Experiments on Omega

General Atomics has supported the Los Alamos National Laboratory Defect Implosion Experiment series on OMEGA with the process design and production of banded, gas-tight, glow discharge polymer (GDP) capsules. Production of a banded target is a multistep, multidisciplinary process requiring micromachining for the band, GDP coating for the capsule wall, and aluminum sputter coating to seal the capsules for subsequent gas fill. Challenges included applying a micromachining technique to create the channel that would result in the desired band after coating, and modification of the aluminum coating process to create a permeation barrier that would cover the banded region to allow for gas filling. Information describing the fabrication steps and characterization techniques employed to analyze the banded targets will be presented.

Hot Explosive Compaction of Zinc Sulfide Based Electroluminescent by Double Cylinder Implosion Technique

Zinc sulfide is inorganic material for electroluminescent applications. This work examines the possibility of obtaining good luminescent properties of the zinc sulfide mixture by double implosion experiment at elevated temperature.

Crossed-beam energy transfer in implosion experiments on OMEGA

Radiative hydrodynamic simulations of implosion experiments on the OMEGA laser system [Boehly et al., Opt. Commun. 133, 495 (1997)] show that energy transfer between crossing laser beams can reduce laser absorption by 10%–20%. A new quantitative model for the crossed-beam energy transfer has been developed, allowing one to simulate the coupling of multiple beams in the expanding corona of implosion targets. Scattered-light and bang-time measurements show good agreement with predictions of this model when nonlocal heat transport is employed. The laser absorption can be increased by narrowing laser beams and/or employing two-color light, which both reduce the crossed-beam energy transfer.

Radiation hardening of gated x-ray imagers for the National Ignition Facility (invited)

The National Ignition Facility will soon be producing x-ray flux and neutron yields higher than any produced in laser driven implosion experiments in the past. Even a non-igniting capsule will require x-ray imaging of near burning plasmas at 10(17) neutrons, requiring x-ray recording systems to work in more hostile conditions than we have encountered in past laser facilities. We will present modeling, experimental data and design concepts for x-ray imaging with electronic recording systems for this environment (ARIANE). A novel instrument, active readout in a nuclear environment, is described which uses the time-of-flight difference between the gated x-ray signal and the neutron which induces a background signal to increase the yield at which gated cameras can be used.

Diagnosing inertial confinement fusion gamma ray physics (invited)

The gamma reaction history (GRH) diagnostic is a multichannel, time-resolved, energy-thresholded γ-ray spectrometer that provides a high-bandwidth, direct-measurement of fusion reaction history in inertial confinement fusion implosion experiments. 16.75 MeV deuterium+tritium (DT) fusion γ-rays, with a branching ratio of the order of 10(-5)γ/(14 MeV n), are detected to determine fundamental burn parameters, such as nuclear bang time and burn width, critical to achieving ignition at the National Ignition Facility. During the tritium/hydrogen/deuterium ignition tuning campaign, an additional γ-ray line at 19.8 MeV, produced by hydrogen+tritium fusion with a branching ratio of unity, will increase the available γ-ray signal and may allow measurement of reacting fuel composition or ion temperature. Ablator areal density measurements with the GRH are also made possible by detection of 4.43 MeV γ-rays produced by inelastic scatter of DT fusion neutrons on (12)C nuclei in the ablating plastic capsule material.

Pulse-dilation enhanced gated optical imager with 5 ps resolution (invited)

A 5 ps gated framing camera was demonstrated using the pulse-dilation of a drifting electron signal. The pulse-dilation is achieved by accelerating a photoelectron derived information pulse with a time varying potential [R. D. Prosser, J. Phys. E 9, 57 (1976)]. The temporal dependence of the accelerating potential causes a birth time dependent axial velocity dispersion that spreads the pulse as it transits a drift region. The expanded pulse is then imaged with a conventional gated microchannel plate based framing camera and the effective gating time of the combined instrument is reduced over that of the framing camera alone. In the drift region, electron image defocusing in the transverse or image plane is prevented with a large axial magnetic field. Details of the unique issues associated with rf excited photocathodes were investigated numerically and a prototype instrument based on this principle was recently constructed. Temporal resolution of the instrument was measured with a frequency tripled femtosecond laser operating at 266 nm. The system demonstrated 20× temporal magnification and the results are presented here. X-ray image formation strategies and photometric calculations for inertial confinement fusion implosion experiments are also examined.

Modelling the effect of 3He in direct drive capsule implosions

D3He fuels are often used in ICF implosion experiments, either as a surrogate for DT to restrict the output neutron yield, or to produce protons for use in diagnosis of core conditions. Recent experiments have suggested that capsules filled with D3He do not behave as expected, but that both proton and neutron yields are anomalously degraded relative to the pure D2 case. We have performed direct drive implosion experiments using the Omega laser to examine the effect of 3He on DT-filled glass capsules. The use of DT fuel allows reaction history measurements to be obtained using the Gas Cherenkov diagnostic (GCD). It was hoped that the detailed information provided by GCD measurements would complement existing measurements to constrain modelling. We present recent modelling and analysis of the experiments using radiation-hydrocode simulations, and explore some of the hypotheses proposed to explain the results.

Heavy ion hollow beam formation at the energy of 1 AGeV for implosion experiments using an original RF system for fast rotation

For experiments at Facility for Antiprotons and Ion Research (FAIR) on high energy density states in matter produced by heavy ion beams, a hollow cylindrical target combined with cylindrical geometry of the ion beam energy deposition region is required. This combination facilitates extremely high densities and pressures on the axis of the imploding cylinder if a hollow or multi-layered target is imploded using a hollow beam with an annular focal spot. We report investigations on a new method for radio frequency rotation of the ion beam, which is proposed for the Laboratory Planetary Sciences (LAPLAS) experiment in order to produce reliably such a hollow beam of ∼2 mm in diameter. For this purpose the beam delivered from the synchrotron (SIS-100) will be transformed by means of fast rotation around a cylindrical axis. The specified RF system consisting of two deflecting cavities is currently under development. An operating rotation frequency of 325 MHz has been chosen. According to simulations and analytical models, this frequency is sufficient for uniform target illumination at the power deposition level of about 10 TW/g. The RF rotation system will be placed in the beam transport line at a distance from the target, which is equal to one quarter wavelength of the transverse beam oscillations. The design of the rotation system and layout of the target–rotation system and focusing elements are presented.

Dynamic Stark broadening as the Dicke narrowing effect

A very fast method to account for charged particle dynamics effects in calculations of spectral line shape emitted by plasmas is presented. This method is based on a formulation of the frequency fluctuation model (FFM), which provides an expression of the dynamic line shape as a functional of the static distribution of frequencies. Thus, the main numerical work rests on the calculation of the quasistatic Stark profile. This method for taking into account ion dynamics allows a very fast and accurate calculation of Stark broadening of atomic hydrogen high- n series emission lines. It is not limited to hydrogen spectra. Results on helium- beta and Lyman- alpha lines emitted by argon in microballoon implosion experiment conditions compared with experimental data and simulation results are also presented. The present approach reduces the computer time by more than 2 orders of magnitude as compared with the original FFM with an improvement of the calculation precision, and it opens broad possibilities for its application in spectral line-shape codes.

Angular distribution measurement and simulation of M band X-ray from the half- hohlraum

Measurement of radiation field near a implosion pellet M band X-ray is a important area in ICF research. On the SG Ⅱ laser facility, M band X-ray from Au plasma was measured in half-hohlraum radiation from the end. The experimental set-up and typical result were given. And weve also used the view-factor program to simulate this experiment. The results indicate that angular distribution of M band X-ray from the end of half-hohlraum doesnt assume linear relationship with the cosine of eradiating angle. The results are useful for the research of implosion experiment and target design.

High-mode Rayleigh-Taylor growth in NIF ignition capsules

An assessment of short wavelength hydrodynamic stability is an essential component in the optimization of NIF ignition target designs. Using highly-resolved massively-parallel 2D Hydra simulations [Marinak, M.M. et al., Physics of Plasmas (1998). 5(4): 1125], we routinely evaluate target designs up to mode numbers of 2000 ( λ∼2 μm) [Hammel, B.A. et al., Journal of Physics: Conference Series, 2008. 112(2): p. 02200]. On the outer ablator surface, mode numbers up to ∼300 ( λ∼20 μm) can have significant growth in CH capsule designs. At the internal fuel:ablator interface mode numbers up to ∼2000 are important for both CH and Be designs. In addition, “isolated features” on the capsule, such as the “fill-tube” (∼5 μm scale-length) and defects, can seed short wavelength growth at the ablation front and the fuel:ablator interface, leading to the injection of ∼10's ng of ablator material into the central hot-spot. We are developing methods to measure high-mode mix on NIF implosion experiments. X-ray spectroscopic methods are appealing since mix into the hot-spot will result in x-ray emission from the high-Z dopant (Cu or Ge) in the ablator material (Be or CH).

The effects of target mounts in direct-drive implosions on OMEGA

The effects of two types of target mounts, stalks and spider silks, on the implosion of both room-temperature D2-gas-filled shells and cryogenic D2-ice-filled shells have been studied both experimentally and by means of two-dimensional simulations. The simulations indicate that the hydrodynamic effect of the expanding plasma created by the ablation of material from the target mounts and refraction of laser light by this plasma induce perturbations in the imploding shell that are damaging to the implosion. The spider silks are the more-damaging type of mount since the silks (typically four) are arrayed over the target surface, whereas the stalk (typically one) meets the target at a single point. Stalks are therefore preferred over silks as a target mount. The scale and magnitude of the perturbations induced by the spider silks have been verified by planar-target experiments performed on the OMEGA laser [T. R. Boehly, D. L. Brown, R. S. Craxton et al., Opt. Commun. 133, 495 (1995)]. The perturbations predicted by simulations to arise from stalks qualitatively agree with the results of implosion experiments using Ti-doped plastic shells.

Two-plasmon-decay instability in direct-drive inertial confinement fusion experiments

The two-plasmon-decay (TPD) instability in direct-drive irradiation OMEGA [J. M. Soures, R. L. McCrory, C. P. Verdon, et al., Phys. Plasmas 3, 2108 (1996)] experiments is seen in the half-integer harmonic emission. Experimental time-resolved ω/2 and 3ω/2 spectra indicate that the linear theory for the absolute TPD instability reasonably predicts TPD thresholds. The plasma wave spectra do not, however, agree at all with the predictions of the linear theory. This is most likely a consequence of the nonlinear evolution of this instability once it is above threshold. This is demonstrated with spectral data obtained from spherical implosion experiments as well as planar target experiments. In the latter, Thomson scattering shows the importance of the Landau cutoff. For the TPD instability, the Landau cutoff is found to be respected in all spherical and planar target experiments. In addition, the maximum plasma wave amplitudes appear to occur near the Landau cutoff.

Modifying Wire-Array Z-Pinch Ablation Structure and Implosion Dynamics Using Coiled Wires

Coiled arrays, which are cylindrical arrays in which each wire is formed into a helix, suppress the modulation of ablation at the fundamental wavelength. Outside the vicinity of the wire cores, ablation flow from coiled arrays is modulated at the coil wavelength and has a two-stream structure in the r, thetas plane. Within the vicinity of the helical wires, ablation is concentrated at positions with the greatest azimuthal displacement, and plasma is axially transported from these positions such that the streams become aligned with the sections of the coil furthest from the array axis. The GORGON MHD code accurately reproduces this observed ablation structure, which can be understood in terms of J times B forces that result from the interaction of the global magnetic field with a helical current path as well as additional current paths suggested by the simulations. With this ability to control where the ablation streamers occur, large wavelength coils were constructed such that the breaks that form in the wires had sufficient axial separation to prevent perturbations in the implosion sheath from merging. This produced a new mode of implosion in which the global instability can be controlled, and perturbations correlated between all wires in an array. For large-wavelength eight-wire coiled arrays, this produced a dramatic increase in X-ray power, equaling that of a 32-wire straight array. These experiments were carried out on the mega ampere generator for plasma implosion experiments (1 MA, 240 ns) at Imperial College London, London, U.K.