Ablation Pressure Research Articles

Mitigation of cross-beam energy transfer: Implication of two-state focal zooming on OMEGA

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.

Developments in Direct Drive Laser Fusion

Recent designs for laser driven, direct drive inertial confinement fusion (ICF) indicate that substantial gains (G>100) might be achieved with lower total laser energy (E~500 kJ) than previously considered possible. A leading contender is the shock ignition approach which compresses low aspect ratio pellets with high intensity laser pulses (1015 W/cm2) before achieving ignition with a final higher intensity spike (1016 W/cm2). Excimer laser systems based on a krypton-fluoride (KrF) medium are particularly well suited to these new ideas as they operate in the ultraviolet (248 nm), provide highly uniform illumination, possess large bandwidth (1-3 THz), and can easily exploit beam zooming to improve laser-target coupling for the final spike pulse. This paper will examine target physics advantages of KrF lasers in relation to the new implosion designs and the balancing of hydrodynamic instability and laser-plasma instabilities. Supporting experimental and theoretical studies of are being conducted by the Nike laser group at the U. S. Naval Research Laboratory. Recent experimental work has also shown that the high ablation pressures and smooth profiles obtained with the Nike laser can be used to accelerate planar targets to velocities consistent with the requirements of impact ignition.

Dense plasma heating and Gbar shock formation by a high intensity flux of energetic electrons

Process of shock ignition in inertial confinement fusion implies creation of a high pressure shock with a laser spike having intensity of the order of a few PW/cm2. However, the collisional (Bremsstrahlung) absorption at these intensities is inefficient and a significant part of laser energy is converted in a stream of energetic electrons. The process of shock formation in a dense plasma by an intense electron beam is studied in this paper in a planar geometry. The energy deposition takes place in a fixed mass target layer with the areal density determined by the electron range. A self-similar isothermal rarefaction wave of a fixed mass describes the expanding plasma. Formation of a shock wave in the target under the pressure of expanding plasma is described. The efficiency of electron beam energy conversion into the shock wave energy depends on the fast electron energy and the pulse duration. The model is applied to the laser produced fast electrons. The fast electron energy transport could be the dominant mechanism of ablation pressure creation under the conditions of shock ignition. The shock wave pressure exceeding 1 Gbar during 200–300 ps can be generated with the electron pulse intensity in the range of 5–10 PW/cm2. The conclusions of theoretical model are confirmed in numerical simulations with a radiation hydrodynamic code coupled with a fast electron transport module.

Model of Laser-Induced Nano-Cavitation in the Surface of Gelatin Confronted with the Fast Rise Time of the Optical Attenuation Measurements

In the laser-polymer interaction a particular mechanism is evidenced when the polymer absorption coefficient is small. In this case the laser ablation is most of the time, but not necessarily, accompanied by intense bubbling, which results from the interplay between ablation gas, polymer melt and pressure wave. In this work, the inception of this phenomenon called cavitation, is monitored for the gelatin case (as a model polymer) by the fast extinction of a probe cwHeNe laser for several fluences of the KrF laser pulses (1.13, 0.65, 0.470 J/cm²) used to trigger ablation whose threshold is 0.5 J/cm². A new model combining the time dependence of temperature, pressure and viscosity is developed for the simulation of this extinction due to the cavitation phenomenon and used to fit the experimental data. The model concludes that the cavity precursors with initial radius R0 = 3 nm and concentration n0 = 5×1013 cm-3 are suddenly expanded by the tension wave launched immediately after the absorption of the laser pulse. The probe laser radiation is attenuated by Rayleigh or Mie scattering with a rise time of the order of 110 ns, depending on the ablation fluence, and corresponding to the transit of the tension wave. The dynamics of the radius increase is given by the Rayleigh-Plesset equation and the thermodynamic kinetics of the growth is given by the classical theory of the homogeneous nucleation. The model shows that complete darkening of the sample surface is achieved only for a fluence larger than 1.1 J/cm² and that the increase of the radius is limited to the range 3–100 nm, in this case. The approximations are discussed in the paper. In the experiments, the increase of the opacity of the sample surface occurs also at lower fluence (0.65 and 0.470 J/cm²) although with a slower dynamics. This reveals that another mechanism may be invoked to account for the slower opacity rise observed at lower fluences in the experimental data. The model excludes melting in these cases and indicates rather a solid phase transition due to the gas evolution.

Experimental research on X-ray radiation and ablation of an Ag foil targets irradiated by high intensity 2ω0 laser light beam

The characteristics of radiation and ablation are investigated for an Ag foil irradiated with 2 ns, ～5×1014 W/cm2, 526.5 nm laser at SGII laser facility. The flight trajectory and velocity of the Ag foil are measured by X-ray streak camera. The experimental results show that they are in good agreement with simulations of one-dimensional radiation hydrodynamics code multi-1d using a flux-limited f=0.01. A rocket model is used to fit the experimental data, and the mass ablation rate and ablation pressure are obtained. The L-shell X-ray conversion and spectra of the laser-produced Ag plasma are measured with a Bragg crystal spectrometer and an array of X-ray diodes. The design for X-ray backlighting radiograph experiments will benefit from these experimental results. The result and method presented in this article are significant for the ablation research on the capsule shell and cavity wall material in laser fusion ignition experiment.

Ablation Pressure Driven by an Energetic Electron Beam in a Dense Plasma

An intense beam of high energy electrons may create extremely high pressures in solid density materials. An analytical model of ablation pressure formation and shock wave propagation driven by an energetic electron beam is developed and confirmed with numerical simulations. In application to the shock-ignition approach in inertial confinement fusion, the energy transfer by fast electrons may be a dominant mechanism of creation of the igniting shock wave. An electron beam with an energy of 30 keV and energy flux 2-5 PW/cm(2) can create a pressure amplitude more than 300 Mbar for a duration of 200-300 ps in a precompressed solid material.

Ablation driven by hot electrons generated during the ignitor laser pulse in shock ignition

An analytical model for the ablation driven by hot electrons is presented. The hot electrons are assumed to be generated during the high intensity laser spike used to produce the ignitor shock wave in the shock ignition driven inertial fusion concept, and to carry on the absorbed laser energy in its totality. Efficient energy coupling requires to keep the critical surface sufficiently close to the ablation front and this goal can be achieved for high laser intensities provided that the laser wavelength is short enough. Scaling laws for the ablation pressure and the other relevant magnitudes of the ablation cloud are found in terms of the laser and target parameters. The effect of the preformed plasma assembled by the compression pulse, previous to the ignitor, is also discussed. It is found that a minimum ratio between the compression and the ignitor pulses would be necessary for the adequate matching of the corresponding scale lengths.

Driving high-gain shock-ignited inertial confinement fusion targets by green laser light

Standard direct-drive inertial confinement fusion requires UV light irradiation in order to achieve ignition at total laser energy of the order of 1 MJ. The shock-ignition approach opens up the possibility of igniting fusion targets using green light by reducing the implosion velocity and laser-driven ablation pressure. An analytical model is derived, allowing to rescale UV-driven targets to green light. Gain in the range 100–200 is obtained for total laser energy in the range 1.5–3 MJ. With respect to the original UV design, the rescaled targets are less sensitive to irradiation asymmetries and hydrodynamic instabilities, while operating in the same laser-plasma interaction regime.

Cryogenic thermonuclear fuel implosions on the National Ignition Facility

The first inertial confinement fusion implosion experiments with equimolar deuterium-tritium thermonuclear fuel have been performed on the National Ignition Facility. These experiments use 0.17 mg of fuel with the potential for ignition and significant fusion yield conditions. The thermonuclear fuel has been fielded as a cryogenic layer on the inside of a spherical plastic capsule that is mounted in the center of a cylindrical gold hohlraum. Heating the hohlraum with 192 laser beams for a total laser energy of 1.6 MJ produces a soft x-ray field with 300 eV temperature. The ablation pressure produced by the radiation field compresses the initially 2.2-mm diameter capsule by a factor of 30 to a spherical dense fuel shell that surrounds a central hot-spot plasma of 50 μm diameter. While an extensive set of x-ray and neutron diagnostics has been applied to characterize hot spot formation from the x-ray emission and 14.1 MeV deuterium-tritium primary fusion neutrons, thermonuclear fuel assembly is studied by measuring the down-scattered neutrons with energies in the range of 10 to 12 MeV. X-ray and neutron imaging of the compressed core and fuel indicate a fuel thickness of (14 ± 3) μm, which combined with magnetic recoil spectrometer measurements of the fuel areal density of (1 ± 0.09) g cm–2 result in fuel densities approaching 600 g cm–3. The fuel surrounds a hot-spot plasma with average ion temperatures of (3.5 ± 0.1) keV that is measured with neutron time of flight spectra. The hot-spot plasma produces a total fusion neutron yield of 1015 that is measured with the magnetic recoil spectrometer and nuclear activation diagnostics that indicate a 14.1 MeV yield of (7.5±0.1)×1014 which is 70% to 75% of the total fusion yield due to the high areal density. Gamma ray measurements provide the duration of nuclear activity of (170 ± 30) ps. These indirect-drive implosions result in the highest areal densities and neutron yields achieved on laser facilities to date. This achievement is the result of the first hohlraum and capsule tuning experiments where the stagnation pressures have been systematically increased by more than a factor of 10 by fielding low-entropy implosions through the control of radiation symmetry, small hot electron production, and proper shock timing. The stagnation pressure is above 100 Gbars resulting in high Lawson-type confinement parameters of Pτ≃10 atm s. Comparisons with radiation-hydrodynamic simulations indicate that the pressure is within a factor of three required for reaching ignition and high yield. This will be the focus of future higher-velocity implosions that will employ additional optimizations of hohlraum, capsule and laser pulse shape conditions.

First implosion experiments with cryogenic thermonuclear fuel on the National Ignition Facility**Part of the EPS 2011 special issue. Based on the plenary talk by S H Glenzer at the 38th EPS Plasma Physics meeting in Strassbourg, 2011.

Non-burning thermonuclear fuel implosion experiments have been fielded on the National Ignition Facility to assess progress toward ignition by indirect drive inertial confinement fusion. These experiments use cryogenic fuel ice layers, consisting of mixtures of tritium and deuterium with large amounts of hydrogen to control the neutron yield and to allow fielding of an extensive suite of optical, x-ray and nuclear diagnostics. The thermonuclear fuel layer is contained in a spherical plastic capsule that is fielded in the center of a cylindrical gold hohlraum. Heating the hohlraum with 1.3 MJ of energy delivered by 192 laser beams produces a soft x-ray drive spectrum with a radiation temperature of 300 eV. The radiation field produces an ablation pressure of 100 Mbar which compresses the capsule to a spherical dense fuel shell that contains a hot plasma core 80 µm in diameter. The implosion core is observed with x-ray imaging diagnostics that provide size, shape, the absolute x-ray emission along with bangtime and hot plasma lifetime. Nuclear measurements provide the 14.1 MeV neutron yield from fusion of deuterium and tritium nuclei along with down-scattered neutrons at energies of 10–12 MeV due to energy loss by scattering in the dense fuel that surrounds the central hot-spot plasma. Neutron time-of-flight spectra allow the inference of the ion temperature while gamma-ray measurements provide the duration of nuclear activity. The fusion yield from deuterium–tritium reactions scales with ion temperature, which is in agreement with modeling over more than one order of magnitude to a neutron yield in excess of 1014 neutrons, indicating large confinement parameters on these first experiments.

Pressure increase in foam-solid target from X-ray driven shock waves

A strong pressure increase is produced by X-ray heating a foam-solid target in comparison with heating directly the solid target, due to impedance mismatch at the foam-solid interface. We study the pressure increase of CH foam-aluminum target, and find that only by shock present in foam can the ablative pressure in aluminum increase. In this paper an analytical model based on strong wave assumption is developed to study the shock impedance mismatching crossing the interface and the interaction of rarefaction with shock. We evaluate the shock pressure amplified in the aluminum layer as a function of density and adiabatic exponent, and show that the amplified pressure is a short high pressure pulse which is destroyed by rarefactions. This is in agreement with simulation results by MULTI code.

Femtosecond Laser‐Induced Formation of Wurtzite Phase ZnSe Nanoparticles in Air

We demonstrate an effective method to prepare wurtzite phase ZnSe nanoparticles from zincblende ZnSe single crystal using femtosecond pulse laser ablation. The fabricated ZnSe nanoparticles are in spherical shape and uncontaminated while synthesized under ambient environment. By controlling the laser fluences, the average size of ZnSe nanoparticles can be varied from ~16 nm to ~22 nm in diameter. In Raman spectra, the surface phonon mode becomes dominant in the smaller average particle size with uniform size distribution. The interesting phase transition from the zinc blende structure of ZnSe single crystal to wurtzite structure of ZnSe nanoparticles may have been induced by the ultrahigh ablation pressure at the local area due to the sudden injection of high energy leading to solid‐solid transition.

The direct measurement of ablation pressure driven by 351-nm laser radiation

The instantaneous scaling of ablation pressure to laser intensity is directly inferred for ramp compression of diamond targets irradiated by 351-nm light. Continuously increasing pressure profiles from 100 to 970 GPa are produced by direct-drive laser ablation at intensities up to 7 × 1013 W/cm2. The free-surface velocity on the rear of the target is used to directly infer the instantaneous ablation-pressure profile at the front of the target. The laser intensity on target is determined by laser power measurements and fully characterized laser spots. The ablation pressure is found to depend on the laser intensity as P(GPa)=42(±3)[I(TW/cm2)]0.71(±0.01).

Outpatient Thermachoice Endometrial Balloon Ablation: Long-term, Prognostic and Quality-of-life Measures

Among the several endometrial ablation devices available for treatment of menorrhagia, there is wide variation in the preferred device. It is unclear whether patients should be treated in the outpatient or day-case general anesthesia setting. One device, thermal balloon endometrial ablation (TBEA), has become popular in the last decade. Several prognostic factors are associated with favorable outcome after TBEA: older age, higher intrauterine pressure, anteverted compared with retroverted uterus, shorter uterine length, and lower (<10 mL) intrauterine balloon volumes. This prospective observational study evaluated long-term treatment outcomes of local anesthetic-TBEA among women with menorrhagia and also investigated prognostic factors. The study was conducted at a teaching hospital in the United Kingdom between 2001 and 2005. A total of 102 premenopausal menorrhagic women underwent local anesthetic-TBEA using either Gynecare Thermachoice I (n = 51) or Thermachoice III (n = 51) TBEA in an outpatient setting under local anesthesia with no conscious sedation. The long-term effectiveness of therapy was assessed with a postal questionnaire sent to all participants in the summer of 2006. The primary study outcome measures included completion of treatment, pain and analgesia, duration of stay and follow-up, primary treatment success, use of secondary treatment, improvements in amenorrhea and other menstrual symptoms, level of patient satisfaction, and quality of life. The questionnaire response rate was 86% (88/102). The TBEA procedure was completed in 97% of patients. Mean duration of hospital stay was 8.0 hours, with a 95% confidence interval of 6.6 to 9.3. Mean treatment follow-up was 30 months (95% confidence interval: 26–32). Secondary treatment occurred in 19% (19/102) of patients and included levonorgestrel intrauterine system, repeat TBEA, or hysterectomy. Surgical re-intervention occurred in 50% of patients by 19 months. The rate of amenorrhea was 29%, and the level of treatment satisfaction was 76%. Higher mean intrauterine ablation pressure was associated with better long-term patient satisfaction. These findings show that TBEA is an effective day-case procedure that can be successfully performed in the outpatient setting. The data suggest that higher intrauterine ablation pressure is associated with improved long-term treatment outcome.

Dynamics of laser-induced surface phase explosion in silicon

Time-resolved ultrasonic studies revealed a second, delayed ablative pressure pulse after the first primary plasma pressure pulse in a silicon wafer irradiated by a UV nanosecond laser. The intensity-dependent delay time for the second pulse indicates the existence of a corresponding intensity-dependent homogeneous vapor bubble nucleation time in the superheated molten silicon prior to its phase explosion and ablative removal, since the integral pressure correlates with the ablation rate. A transient hot ablative plasma with calculated peak temperature ∼30–90 eV and pressure ∼20–110 GPa is suggested to superheat the bulk silicon via short-wavelength recombination and Bremsstrahlung emission.

Cavity formation in a liquid Sn droplet driven by laser ablation pressure for an extreme ultraviolet light source target

An in situ approach to the formation of cavities in liquid Sn droplets for the purpose of increasing ion density from Sn plasma produced by a CO2 laser is investigated. Two-dimensional hydrodynamic simulations, treating the laser as a pulsed pressure source, are compared both spatially and temporally to experimental shadowgraphs for verification of cavity formation. It is shown that a 15 ns pulse from a 1.064 μm laser with intensity of 2 × 1010 W/cm2 creates a cavity approximately 300 μm wide and 100 μm deep in approximately 1.4 μs. The presence of the cavity enhances the conversion of laser energy to 13.5 nm radiation from the plasma.

An Alternate Treatment of the Vapor-Plasma Transition

In this paper, we address the problem of predicting p(I), the variation of surface ablation pressure vs. incident pulsed laser intensity I, from the onset of ablation through the transition to its mediation by laser-induced plasma in vacuum. Despite its simplicity, the recently published approach of Sinko and Phipps [1] to this problem describes momentum coupling for many laser-target interactions quite well, for one material at a single wavelength where the ablation fluence threshold is clearly defined. Alternatively, if vapor pressure vs. temperature data p(T) is available for a material, e.g., using the SESAME tables, a different model can be used. In addition to the p(T) data, this model only requires knowledge of basic parameters for the material, such as specific heat, thermal conductivity, optical absorptivity, atomic weight, and its ionization state energies and their partition functions. Since all these parameters, except for optical absorptivity, are independent of laser wavelength, it is possib...

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 KrF excimer laser beam modification resulting from ablation under closed thick film flowing filtered water

The application of a closed thick film flowing filtered water to immerse the ablation etching mechanism of an excimer laser poses interesting possibilities concerning debris control, modification of machined feature topography and modification of the ablation rate. Furthermore, these parameters have been shown to be dependent on flow velocity; hence, offering further user control of machining characteristics. However, the impact of this technique requires investigation. This contribution offers comparison of the calculated ablation pressure and the effect on feature surface characteristics given for laser ablation of bisphenol A polycarbonate using KrF excimer laser radiation in ambient air against laser ablation of the same substrate under closed thick film flowing filtered water immersion. Also, an impact of such immersion equipment on the optical performance of the micromachining centre used is quantified and reviewed. The pressure is calculated to have risen by a magnitude of 48, when using the liquid immersed ablation technique. This increase in pressure is proposed to have an increased surface roughness, promoting the number of asperities with a surface area lower than 16 μm 2; resulting in a diffuse reflection of light and an apparent darkening of features. The focal length of the optical system was accurately predicted to increase by 2.958 mm, when using the closed flowing liquid immersion equipment. This equipment is predicted to have increased the optical depth of focus via reduction in the angle of convergence of the two defining image rays; yet the perceived focus, measured discretely by mean feature wall angle, was found to be 25% smaller when using the closed thick film flowing filtered water immersion technique instead of similar laser ablation in ambient air. A compressed plume interaction is proposed as a contributing factor in this change.

From ablation to radiation pressure in intense laser–matter interaction

Abstract The transition from ablation to radiation pressure with increasing laser intensity in linear polarization is studied in a steady state model. At moderate intensities and laser wavelength λ up to I λ 2 = 10 15 – 10 16 W / cm 2 μ m 2 the pressure is determined basically by the hot plasma and its flow dynamics. The contribution of the ponderomotive (radiative) potential to the total pressure is determined by plasma outflow inhibition which causes flow stagnation in the critical region and ablation pressure lowering already at ratios radiation/plasma pressure as low as a few percent. When approaching relativistic intensities the dominance of the ponderomotive pressure leads to a continuous transition from a I 2/3 to I 1 dependence. The location of this transition is governed by heat conduction into the dense target interior. We attribute the strong deviations of the measured exponents from 2 3 in the low intensity domain to energy losses also by electron heat flow. Basic features of the model have been tested by particle-in-cell simulations.