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Fast Ignition Experimental and Theoretical Studies

Description: We are becoming dependent on energy more today than we were a century ago, and with increasing world population and booming economies, sooner or later our energy sources will be exhausted. Moreover, our economy and welfare strongly depends on foreign oil and in the shadow of political uncertainties, there is an urgent need for a reliable, safe, and cheap energy source. Thermonuclear fusion, if achieved, is that source of energy which not only will satisfy our demand for today but also for centuries to come. Today, there are two major approaches to achieve fusion: magnetic confinement fusion (MFE) and inertial confinement fusion (ICF). This dissertation explores the inertial confinement fusion using the fast ignition concept. Unlike the conventional approach where the same laser is used for compression and ignition, in fast ignition separate laser beams are used. This dissertation addresses three very important topics to fast ignition inertial confinement fusion. These are laser-to-electron coupling efficiency, laser-generated electron beam transport, and the associated isochoric heating. First, an integrated fast ignition experiment is carried out with 0.9 kJ of energy in the compression beam and 70 J in the ignition beam. Measurements of absolute K{sub {alpha}} yield from the imploded core revealed that about 17% of the laser energy is coupled to the suprathermal electrons. Modeling of the transport of these electrons and the associated isochoric heating, with the previously determined laser-to-electron conversion efficiency, showed a maximum target temperature of 166 eV at the front where the electron flux is higher and the density is lower. The contribution of the potential, induced by charge separation, in opposing the motion of the electrons was moderate. Second, temperature sensitivity of Cu K{sub {alpha}} imaging efficiency using a spherical Bragg reflecting crystal is investigated. It was found that due to the shifting and broadening of ...
Date: October 20, 2006
Creator: Akli, K
Partner: UNT Libraries Government Documents Department

A Bremsstrahlung Spectrometer using k-edge and Differential Filters with Image plate dosimeters

Description: A Bremsstrahlung spectrometer using k-edge and differential filtering has been used with Image Plate dosimeters to measure the x-ray fluence from short-pulse laser/target interactions. An electron spectrometer in front of the Bremsstrahlung spectrometer deflects electrons from the x-ray line of sight and simultaneously measures the electron spectrum. The response functions were modeled with the Monte Carlo code Integrated Tiger Series 3.0 and the dosimeters calibrated with radioactive sources. Electron distributions with slope temperatures in the MeV range are inferred from the Bremsstrahlung spectra.
Date: May 2, 2008
Creator: Chen, C; Mackinnon, A; Beg, F; Chen, H; Key, M; King, J A et al.
Partner: UNT Libraries Government Documents Department

Electron-Heated Target Temperature Measurements in Petawatt Laser Experiments Based on Extreme Ultraviolet Imaging and Spectroscopy

Description: Three independent methods (XUV spectroscopy, imaging at 68 eV and 256 eV) have been used to measure planar target rear surface plasma temperature due to heating by hot electrons. The hot electrons are produced by ultra-intense laser plasma interactions using the 150 J, 0.5 ps Titan laser. Soft x-ray spectroscopy in the 50-400 eV region and imaging at the 68 eV and 256 eV photon energies were used to determine the rear surface temperature of planar CD targets. Temperatures were found to be in the 60-150 eV range, with good agreement between the three diagnostics.
Date: May 2, 2008
Creator: Ma, T; Beg, F; Macphee, A; Chung, H; Key, M; Mackinnon, A et al.
Partner: UNT Libraries Government Documents Department

Fast Ignition relevant study of the flux of high intensity laser generated electrons via a hollow cone into a laser-imploded plasma

Description: An integrated experiment relevant to fast ignition is described. A Cu doped CD spherical shell target is imploded around an inserted hollow Au cone by a six beam 600J, 1ns laser to a peak density of 4gcm{sup -3} and a diameter of 100 {micro}m. A 10 ps, 20TW laser pulse is focused into the cone at the time of peak compression. The flux of high-energy electrons through the imploded material is determined from the yield of Cu K{alpha} fluorescence by comparison with a Monte Carlo model and is estimated to carry 15% of the laser energy. Collisional and Ohmic heating are modeled. An electron spectrometer shows significantly greater reduction of the transmitted electron flux than is due to binary collisions and Ohmic potential. Enhanced scattering by instability-induced magnetic fields is suggested.
Date: October 11, 2005
Creator: Key, M; Adam, J; Akli, K; Borgheshi, M; Chen, M; Evans, R et al.
Partner: UNT Libraries Government Documents Department

Extreme Ultraviolet Imaging of Electron Heated Targets in Petawatt Laser Experiments

Description: The study of the transport of electrons, and the flow of energy into a solid target or dense plasma, is instrumental in the development of fast ignition inertial confinement fusion. An extreme ultraviolet (XUV) imaging diagnostic at 256 eV and 68 eV provides information about heating and energy deposition within petawatt laser-irradiated targets. XUV images of several irradiated solid targets are presented.
Date: November 29, 2007
Creator: Ma, T; MacPhee, A; Key, M; Akli, K; Mackinnon, A; Chen, C et al.
Partner: UNT Libraries Government Documents Department

Limitation on Pre-pulse Level for Cone-Guided Fast-Ignition ICF

Description: The viability of fast-ignition (FI) inertial confinement fusion hinges on the efficient transfer of laser energy to the compressed fuel via multi-MeV electrons. Pre-formed plasma due to laser pre-pulse strongly influences ultra-intense laser plasma interactions and hot electron generation in the hollow cone of an FI target. We induced a prepulse and consequent preplasma in copper cone targets and measured the energy deposition zone of the main pulse by imaging the emitted K{sub {alpha}} radiation. An integrated simulation of radiation hydrodynamics for the pre-plasma and particle in cell for the main pulse interactions agree well with the measured deposition zones and provide an insight into the enrgy deposition mechanism and electron distribution. It was demonstrated that a under these conditions a 100mJ pre-pulse completely eliminates the forward going component of {approx}2-4MeV electrons. Consequences for cone-guided fast-ignition are discussed.
Date: September 1, 2009
Creator: MacPhee, A G; Akli, K U; Beg, F N; Chen, C D; Chen, H; Divol, L et al.
Partner: UNT Libraries Government Documents Department

Diagnostics for Fast Ignition Science

Description: The concept for Electron Fast Ignition Inertial Confinement Fusion demands sufficient laser energy be transferred from the ignitor pulse to the assembled fuel core via {approx}MeV electrons. We have assembled a suite of diagnostics to characterize such transfer. Recent experiments have simultaneously fielded absolutely calibrated extreme ultraviolet multilayer imagers at 68 and 256eV; spherically bent crystal imagers at 4 and 8keV; multi-keV crystal spectrometers; MeV x-ray bremmstrahlung and electron and proton spectrometers (along the same line of sight); nuclear activation samples and a picosecond optical probe based interferometer. These diagnostics allow careful measurement of energy transport and deposition during and following laser-plasma interactions at extremely high intensities in both planar and conical targets. Augmented with accurate on-shot laser focal spot and pre-pulse characterization, these measurements are yielding new insight into energy coupling and are providing critical data for validating numerical PIC and hybrid PIC simulation codes in an area that is crucial for many applications, particularly fast ignition. Novel aspects of these diagnostics and how they are combined to extract quantitative data on ultra high intensity laser plasma interactions are discussed, together with implications for full-scale fast ignition experiments.
Date: May 6, 2008
Creator: MacPhee, A; Akli, K; Beg, F; Chen, C; Chen, H; Clarke, R et al.
Partner: UNT Libraries Government Documents Department

Fast Electron Generation in Cones with Ultra-Intense Laser Pulses

Description: Experimental results from copper cones irradiated with ultra-intense laser light are presented. Spatial images and total yields of Cu K{sub {alpha}} fluorescence were measured as a function of the laser focusing properties. The fluorescence emission extends into the cone approximately 300 {micro}m from the cone tip and cannot be explained by ray tracing including cone wall absorption. In addition the total fluorescence yield from cones is an order of magnitude higher than for equivalent mass foil targets. Indications are that the physics of the laser cone interaction is dominated by preplasma created from the long duration, low energy pre-pulse from the laser.
Date: December 7, 2007
Creator: Mackinnon, A; VanWoerkom, L; Akli, K; Bartal, T; Beg, F; Chawla, S et al.
Partner: UNT Libraries Government Documents Department

Laser heating of solid matter by light pressure-driven shocks

Description: Heating by irradiation of a solid surface in vacuum with 5 x 10{sup 20} W cm{sup -2}, 0.8 ps, 1.05 {micro}m wavelength laser light is studied by x-ray spectroscopy of the K-shell emission from thin layers of Ni, Mo and V. A surface layer is heated to {approx} 5 keV with an axial temperature gradient of 0.6 {micro}m scale length. Images of Ni Ly{sub {alpha}} show the hot region has a {approx} 25 {micro}m diameter, much smaller than {approx} 70 {micro}m region of K{sub {alpha}} emission. 2D particle-in-cell (PIC) simulations suggest that the surface heating is due to a light pressure driven shock.
Date: May 4, 2007
Creator: Akli, K; Hansen, S B; Kemp, A J; Freeman, R R; Beg, F N; Clark, D et al.
Partner: UNT Libraries Government Documents Department

Characterization of Ti K-alpha radiation resulting from interaction of a highly intense laser pulse with a thin titanium foil

Description: A first demonstration has recently been made of radiography of implosions using a nonthermal K{alpha} radiation source generated with a high intensity picosecond laser pulse [1]. Absolute source brightness is important in assessing the potential of this diagnostic and we present here measurements and Monte-Carlo simulations of the brightness of the Ti K{alpha} back-lighter source. The experiment was conducted at the Vulcan laser within the Rutherford Appleton Laboratory (RAL) in the UK. A set of radiographs were taken in which a back-lighting source was produced using a 1 ps CPA beam. The beam delivered an average of 49J, within an 800 {micro}m by 400 {micro}m elliptical spot, onto a 25 {micro}m thick Ti foil (Figure 1). The first of two instruments used to characterize the K{alpha} source was a spherical Bragg crystal imager (Quartz 20{bar 2}3, 2d of 0.2749 nm, radius of curvature 38 cm, aperture 1.6 cm) used to spatially resolve the emission of the K{alpha} back-lighter [2]. The crystal focused the 4.5 keV K{alpha} photons with 10 mm spatial resolution and 7.9 x magnification onto a cooled, 16-bit, 1'' x 1'', 1024 x 1024 pixel CCD chip. The instrument observation angle was normal to the rear axis of the foil. The 2nd instrument was a single hit CCD spectrometer which was used to measure the absolute K{alpha} yield from the Ti target. The spectrometer consisted of a back-thinned CCD with 2048 x 2048 13.5 {micro}m square pixels and a filter (100 or 150 {micro}m Ti) that placed the chip in the single hit photon regime. The angle of observation was 41{sup o} from the rear surface normal of the Ti foil.
Date: June 13, 2005
Creator: King, J A; Key, M H; Chen, C D; Freeman, R R; Phillips, T; Akli, K U et al.
Partner: UNT Libraries Government Documents Department

Temperature sensitivity of Cu K(alpha) imaging efficiency using a spherical Bragg reflecting crystal

Description: The Vulcan laser facility at the Rutherford Appleton Laboratory was used to study the interaction of a 75 J 10 ps, high intensity laser beam with low-mass solid, Cu targets. Two instruments were fielded as diagnostics of the Cu K-shell emission from the targets: A single photon counting CCD spectrometer provided the absolute K{sub {alpha}} yield and a spherically bent Bragg crystal recorded 2D monochromatic images with a spatial resolution of 10 {micro}m. Due to the shifting and broadening of the K{sub {alpha}} spectral lines with increasing temperature, there is a temperature dependence of the crystal collection efficiency. This provides a temperature diagnostic when cross calibrated against a single hit CCD spectrometer, and it affects measurements of the spatial pattern of electron transport. The experimental data showing changing collection efficiency are presented. The results are discussed in light of modeling of the temperature-dependent spectrum of Cu K-shell emission.
Date: August 7, 2006
Creator: Akli, K U; Key, M H; Chung, H K; Hansen, S B; Freeman, R R; Chen, M H et al.
Partner: UNT Libraries Government Documents Department

Experiment vs. theory on electric inhibition of fast electron penetration of targets

Description: A dominant force of inhibition of fast electrons in normal density matter is due to an axially directed electrostatic field. Fast electrons leave the critical density layer and enter the solid in an assumed relativistic Maxwellian energy distribution. Within a cycle of the solid density plasma frequency, the charge separation is neutralized by a background return current density j{sub b} = en{sub b}v{sub b} equal and opposite to the fast electron current density j{sub f} = en{sub f}v{sub f} [1] where it is assumed that the fast electron number density is much less than the background number density, n{sub f} << n{sub b} [2]. This charge and current neutralization allows the forward moving fast electron current to temporarily exceed the Alfven limit by many orders of magnitude [3]. During this period the cold return current, in passing through the material resistivity, ohmically generates an electric field in opposition to the fast current. As a result, the fast electron current loses its energy to the material, via the return current, in the form of heat [4]. So, although the highly energetic electrons suffer relatively little direct collisional loss of energy (owing to the inverse relation of the Coulomb cross section to velocity), their motion is substantially damped by ohmic heating of the slower return current. The equation for the ohmically generated electric field, E, is given by Ohm's law, E = j{sub c}{eta} where {eta} is the material resistivity.
Date: June 13, 2005
Creator: Freeman, R R; Akli, K U; Batani, D; Baton, S; Hatchett, S P; Hey, D et al.
Partner: UNT Libraries Government Documents Department

Proton Beam Focusing and Heating in Petawatt Laser-Solid Interactions

Description: It has recently been demonstrated that femtosecond-laser generated proton beams may be focused. These protons, following expansion of the Debye sheath, emit off the inner concave surface of hemispherical shell targets irradiated at their outer convex pole. The sheath normal expansion produces a rapidly converging proton beam. Such focused proton beams provide a new and powerful means to achieve isochoric heating to high temperatures. They are potentially important for measuring the equation of state of materials at high energy density and may provide an alternative route to fast ignition. We present the first results of proton focusing and heating experiments performed at the Petawatt power level at the Gekko XII Laser Facility at ILE Osaka Japan. Solid density Aluminum slabs are placed in the proton focal region at various lengths. The degree of proton focusing is measured via XUV imaging of Planckian emission of the heated zone. Simultaneous with the XUV measurement a streaked optical imaging technique, HISAK, gave temporal optical emission images of the focal region. Results indicate excellent coupling between the laser-proton conversion and subsequent heating.
Date: August 13, 2003
Creator: Snavely, R A; Gu, P; King, J; Hey, D; Akli, K; Zhang, B B et al.
Partner: UNT Libraries Government Documents Department

A Dual Channel X-ray Spectrometer for Fast Ignition Research

Description: A new Dual Channel Highly Ordered Pyrolytic Graphite (DC-HOPG) x-ray spectrometer was developed to study laser-generated electron beam transport. The instrument uses a pair of graphite crystals and has the advantage of simultaneously detecting self emission from low-Z materials in first diffraction order and high-Z materials in second order. The emissions from the target are detected using a pair of parallel imaging plates positioned in a such way that the noise from background is minimized and the mosaic focusing is achieved. Initial tests of the diagnostic on Titan laser (I {approx} 10{sup 20} W/cm{sup 2}, {tau} = 0.7 ps) show excellent signal-to-noise ratio (SNR) > 1000 for the low energy channel and SNR > 400 for the high energy channel.
Date: April 19, 2010
Creator: Akli, K U; Patel, P K; Van Maren, R; Stephens, R B; Key, M H; Higginson, D P et al.
Partner: UNT Libraries Government Documents Department

Hot surface ionic line emission and cold K-inner shell emission from petawatt-laser irradiated Cu foil targets

Description: A hot, T{sub e} {approx} 2- to 3-keV surface plasma was observed in the interaction of a 0.7-ps petawatt laser beam with solid copper-foil targets at intensities >10{sup 20} W/cm{sup 2}. Copper K-shell spectra were measured in the range of 8 to 9 keV using a single-photon-counting x-ray CCD camera. In addition to K{sub {alpha}} and K{sub {beta}} inner-shell lines, the emission contained the Cu He{sub {alpha}} and Ly{sub {alpha}} lines, allowing the temperature to be inferred. These lines have not been observed previously with ultrafast laser pulses. For intensities less than 3 x 10{sup 18} W/cm{sup 2}, only the K{sub {alpha}} and K{sub {beta}} inner-shell emissions are detected. Measurements of the absolute K{sub {alpha}} yield as a function of the laser intensity are in agreement with a model that includes refluxing and confinement of the suprathermal electrons in the target volume.
Date: December 13, 2005
Creator: Theobald, W; Akli, K; Clarke, R; Delettrez, J A; Freeman, R R; Glenzer, S et al.
Partner: UNT Libraries Government Documents Department

On Point Designs for High Gain Fast Ignition

Description: Fast ignition research has reached the stage where point designs are becoming crucial to the identification of key issues and the development of projects to demonstrate high gain fast ignition. The status of point designs for cone coupled electron fast ignition and some of the issues they highlight are discussed.
Date: September 27, 2007
Creator: Key, M; Akli, K; Beg, F; Betti, R; Clark, D S; Chen, S N et al.
Partner: UNT Libraries Government Documents Department

Studies of electron and proton isochoric heating for fast ignition

Description: Isochoric heating of inertially confined fusion plasmas by laser driven MeV electrons or protons is an area of great topical interest in the inertial confinement fusion community, particularly with respect to the fast ignition (FI) proposal to use this technique to initiate burn in a fusion capsule. Experiments designed to investigate electron isochoric heating have measured heating in two limiting cases of interest to fast ignition, small planar foils and hollow cones. Data from Cu K{alpha} fluorescence, crystal x-ray spectroscopy of Cu K shell emission, and XUV imaging at 68eV and 256 eV are used to test PIC and Hybrid PIC modeling of the interaction. Isochoric heating by focused proton beams generated at the concave inside surface of a hemi-shell and from a sub hemi-shell inside a cone have been studied with the same diagnostic methods plus imaging of proton induced K{alpha}. Conversion efficiency to protons has also been measured and modeled. Conclusions from the proton and electron heating experiments will be presented. Recent advances in modeling electron transport and innovative target designs for reducing igniter energy and increasing gain curves will also be discussed.
Date: October 2, 2006
Creator: Mackinnon, A.; Key, M.; Akli, K.; Beg, F.; Clarke, R.; Clarke, D. et al.
Partner: UNT Libraries Government Documents Department

Hot Electron Generation and Transport Using K(alpha) Emission

Description: We have conducted experiments on both the Vulcan and Titan laser facilities to study hot electron generation and transport in the context of fast ignition. Cu wires attached to Al cones were used to investigate the effect on coupling efficiency of plasma surround and the pre-formed plasma inside the cone. We found that with thin cones 15% of laser energy is coupled to the 40{micro}m diameter wire emulating a 40{micro}m fast ignition spot. Thick cone walls, simulating plasma in fast ignition, reduce coupling by x4. An increase of prepulse level inside the cone by a factor of 50 reduces coupling by a factor of 3.
Date: October 15, 2009
Creator: Akli, K U; Stephens, R B; Key, M H; Bartal, T; Beg, F N; Chawla, S et al.
Partner: UNT Libraries Government Documents Department

Overview of recent progress in US fast ignition research

Description: The Fast Ignition Program in the United States has enjoyed increased funding in various forms from the Office of Fusion Energy Sciences of the Department of Energy. The program encompasses experiments on large laser facilities at various world-wide locations, and benefits enormously from collaborations with many international scientists. The program includes exploratory work in cone-target design and implosion dynamics, high electron current transport measurements in normal density materials, development of diagnostics for heating measurements, generation of protons from shaped targets, theoretical work on high gain target designs, and extensive modeling development using PIC and hybrid codes.
Date: September 28, 2005
Creator: Freeman, R. R.; Akli, K.; Beg, F.; Betti, R.; Chen, S.; Clark, D. J. et al.
Partner: UNT Libraries Government Documents Department