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

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

Extreme Ultraviolet Imaging of Electron Heated Targets in Petawatt Laser Experiments

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

Fast Electron Generation in Cones with Ultra-Intense Laser Pulses

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

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

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

Diagnostics for Fast Ignition Science

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

On Point Designs for High Gain Fast Ignition

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

Studies of electron and proton isochoric heating for fast ignition

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

Studies of electron and proton isochoric heating for fast ignition Studies of electron and proton isochoric heating for fast ignition

Abstract 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 α 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 α . 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. This work was performed under the auspices of the U.S. Department of Energy by University of California Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, with the additional support of the Ohio State University, the Hertz Foundation and General Atomics. Introduction The Fast Ignition (FI) approach to Inertial Confinement Fusion (ICF) holds particular promise for fusion energy because the independently generated ignition pulse allows ignition with less compression, resulting in (potentially) higher gain. Designing targets able to exploit the FI scheme efficiently requires an understanding of the transport of electrons in prototypical geometries and at relevant densities and temperatures. We present an overview of recent research designed to investigate proton and electron isochoric heating in regimes of interest to fast ignition. This work, which is part of the US Fusion Energy Program, has been conducted through international collaborative experiments carried out on the Callisto and Titan laser facilities at LLNL in the USA, at the Vulcan laser facility in the UK and at the ILE Osaka Gekko PW facility in Japan. In the near term the goals of this program are to test theoretical models of isochoric heating with well diagnosed experiments. In the longer term we aim to carry out tests of the integrated problem where short pulse lasers isochorically heat shock-compressed materials. Finally we plan to carry out fast ignition experiments on ignition scale plasmas on the National Ignition Facility. In cone coupled electron fast ignition the fuel capsule is imploded onto the tip of a hollow cone. The short pulse laser is focused through the cone and relativistic electrons (of a few MeV average energy) are transported from the cone tip over a distance of the order of 100 µm into the assembled DT core at about 300 gcm -3 and confined within a diameter <40µm. The total energy deposited in the hot spot is about 20kJ in 10 ps. Overall coupling efficiency between laser energy and thermal energy in the ignition hot spot should exceed 10% and preferably reach 20% (as seen in the first small scale integrated experiments) to make the scheme practically attractive. Modeling of the laser accelerated electron sources, the transport of energy by electrons and the consequent isochoric heating have advanced considerably (studies into the sensitivity of the electron transport to the incident electron distribution are currently underway, while a number of new target concepts that may reduce the ignition energy and increase the gain curve are also under active investigation) but there is still no well established modeling capability to enable extrapolation from small scale experiments to full scale FI. The processes are complex and challenging from both experimental study and modeling aspects. We have recently used two limiting cases, which address specific issues in electron transport and offer good opportunities for comparison with modeling. (1) Thin foils of small area constrain electrons to reflux between the surfaces in a time short compared to the laser pulse duration so that there is always approximate cancellation of the net injected current by reflux current thus eliminating the effect of Ohmic heating by the return current of cold electrons, which is a dominant effect in initially cold solid targets in the absence of refluxing. (2) In the opposite limit a hollow cone couples the laser to a long thin wire and provides a situation where there is 100% compensation of the fast electron injection into the wire by the cold electron return current thus maximizing Ohmic effects. The geometry is simple in that the area of the current flow is constant and equal to the cross sectional area of the wire