The capability of the HELEN laser at the Atomic Weapons Establishment Aldermaston has been enhanced by the addition of a short-pulse laser beam to augment the twin opposing nanosecond time scale beams. The short-pulse beam utilizes the chirped pulse amplification (CPA) technique and is capable of delivering up to 60 J on target in a 500 fs pulse, around 100 TW, at the fundamental laser wavelength of 1.054 mu m. During the commissioning phase a number of diagnostic systems have been fielded, these include: x-ray pinhole imaging of the laser heated spot, charged particle time of flight, thermoluminescent dosimeter array, calibrated radiochromic film, and CR39 nuclear track detector. These diagnostic systems have been used to verify the performance of the CPA beam to achieve a focused intensity of around 10(19) W cm(-2) and to underwrite the facility radiological safety system.

The characteristics of 22–40keV Kα x-ray sources are measured. These high-energy sources are produced by 100TW and petawatt high-intensity lasers and will be used to develop and implement workable radiography solutions to probe high-Z and dense materials for the high-energy density experiments. The measurements show that the Kα source size from a simple foil target is larger than 60μm, too large for most radiography applications. The total Kα yield is independent of target thicknesses, verifying that refluxing plays a major role in photon generation. Smaller radiating volumes emit brighter Kα radiation. One-dimensional radiography experiments using small-edge-on foils resolved 10μm features with high contrast. Experiments were performed to test a variety of small volume two-dimensional point sources such as cones, wires, and embedded wires, measured photon yields, and compared the measurements with predictions from hybrid-particle-in-cell simulations. In addition to high-energy, high-resolution backlighters, future experiments will also need imaging detectors and diagnostic tools that are workable in the high-energy range. An initial look at some of these detector issues is also presented.

Time-resolved x-ray spectra from solid targets irradiated by the VULCAN Petawatt laser focused to 1020Wcm−2 show that material at solid density is heated to temperatures above 500 eV to a depth of about 15 μm and for a duration of more than 30 ps. Modeling with the implicit hybrid plasma code LSP shows that the heating is sensitive to the laser prepulse through resistive inhibition of the laser accelerated electrons in the blow off layer.

Introduction Atomic clusters are well known for their ability to absorb a very high fraction of the laser energy incident upon them (>90%). Consequently, a radically different laser interaction occurs in comparison to that found when gas targets or solid targets are used. Gases, which typically absorb <1% of the incident laser energy, produce relatively low temperature plasma, while solid density targets are heated far more efficiently through a variety of heating mechanisms.

X-ray radiography is an important tool for diagnosing and imaging planar and convergent hydrodynamics phenomena for laser experiments. Until now, hydrodynamics experiments at Omega and NIF utilize E{sub x-ray} 10{sup 17} W/cm{sup 2}. We have performed several experiments on the JanUSP, and the Vulcan 100TW, and Vulcan Petawatt lasers to understand K-{alpha} sources and to test radiography concepts. 1-D radiography using an edge-on foil and 2-D radiography using buried wires and cone-fiber targets were tested. We find that 1-D thin edge-on foils can have imaging resolution better than 10 {micro}m. Micro volume targets produce bright sources with measured conversion efficiency from laser energy to x-ray photons of {approx} 1 x 10{sup -5}. This level of conversion may not be enough for 2-D point projection radiography. A comparison of our experimental measurements of small volume sources with the LSP/PIC simulation show similarmore » K-{alpha} creation profiles but discrepancy in absolute yields.« less

We review a recent experimental campaign to study the interaction physics of petawatt laser pulses incident at relativistic intensities on solid targets. The campaign was performed on the 500 J sub-picosecond petawatt laser at the Rutherford Appleton Laboratory. An extensive suite of optical, x-ray, and particle diagnostics was employed to characterise the processes of laser absorption, electron generation and transport, thermal and K-alpha x-ray generation, and proton acceleration.

Summary form only given. Petawatt lasers have several important applications including fast ignition of fusion targets and the development of compact X-ray and charged particle sources. Understanding the transport and energy deposition of the relativistic electron beam produced by the absorption from ultra-intense lasers is a crucial part of these applications. We have used the thermal X-ray emission of aluminium tracer layers in a CH substrate to diagnose the temperature as a function of depth. With laser intensities around 10/sup 20/ W cm/sup -2/ and a pulse length of /spl sim/800 fsec we find temperatures of 650 eV-450 eV at depths of 8 micron to 17 micron. Deeper layers show strongly non-thermal emission characteristic of 'hollow atoms' with multiple inner shell vacancies. We have used the hybrid plasma simulation code LSP to model the transport and energy deposition from the relativistic electron beam and find broad agreement with our measurements which is improved if some laser pre-pulse is added to the model which has the effect of inhibiting the penetration of the relativistic electrons. The LSP simulations also show that the aluminium tracer layer should be significantly hotter than the CH substrate due to enhanced resistive heating, but with very thin tracer layers the electrons responsible for exciting the X-ray emission are not in equilibrium at the higher temperature due to finite mean free path effects (non-local transport).

A free-standing gold transmission grating with a period of 5000 Angstrom has been coupled to a soft x-ray sensitive streak camera with a limiting temporal resolution of 10 ps. The streak camera is equipped with a caesium iodide transmission photocathode and observations have been made in the 10-100 Angstrom regime. For a small source (200 mum diameter) the spectral resolution is predicted to be around 2.5 Angstrom. This has been confirmed by examination of the Lyman-alpha line in hydrogen-like laser heated boron. A recorded linewidth of 2.44 Angstrom is demonstrated. The instrument has been used to diagnose the soft x-ray emission from a plastic (CH) foil target heated by an ultra-intense (2x10(20) W cm(-2)) laser pulse. (C) 2004 American Institute of Physics.

We are studying the feasibility of utilizing Kalpha x-ray sources in the range of 20 to 100 keV as a backlighters for imaging various stages of implosions and high aerial density planar samples driven by the NIF laser facility. The hard x-ray Kalpha sources are created by relativistic electron plasma interactions in the target material after a radiation by short pulse high intensity lasers. In order to understand Kalpha source characteristics such as production efficiency and brightness as a function of laser parameters, we have performed experiments using the 10 J, 100 fs JanUSP laser. We utilized single-photon counting spectroscopy and x-ray imaging diagnostics to characterize the Kalpha source. We find that the Kalpha conversion efficiency from the laser energy at 22 keV is similar to3 x 10(-4).

Dynamic range measurements have been made on EEV P8307 (Photochron II type) streak image tubes with both internal microchannel plate (MCP), and externally coupled wafer intensifier tube, image amplification. When arranged for equal radiant gain at 560 nm, both configurations showed comparable dynamic range of similar to1100 and similar to2900 for 30 ps and 100 ps duration (fwhm) input pulses respectively. The variation in dynamic range with photocthode current density was also measured and the results compared to a computational model based on Coulomb repulsion in the image tube. Experimental data are compared to the model and shown to be in moderate agreement.

Electron–optic streak chronoscopy is an important diagnostic technique for the diagnosis of laser driven hydrodynamic and radiative phenomenon. To minimize the undesirable effects of excessive space charge in streak image tubes an image amplification stage is often used. Microchannel plate (MCP) electron amplifiers are frequently employed for this purpose. These devices may be utilized in two ways: either to amplify light from the streak image tube phosphor screen by externally coupling a proximity-focus MCP wafer intensifier tube, or by insertion inside the streak tube body to amplify the streaked electrons directly. To investigate how the operating regime of the MCP influences the dynamic range of the streak camera system, dynamic range measurements have been made on two identical streak image tubes (English Electric Valve Co. type P8307) one incorporating an internal MCP, the other, externally coupled image amplification. Dynamic range measurements have been made for 30 ps full width half maximum (FWHM) and 100 ps (FWHM) laser pulses and comparison made to a previous study of this type of image tube. For internal MCP and externally coupled intensifier camera systems of equal radiant gain the dynamic range for the pulse widths studied was found to be comparable.

Silicon p–n diodes have proved to be excellent soft x-ray detectors due to their high sensitivity, nominally flat response, and long term stability. Advances in fabrication techniques have overcome many of the limitations of older silicon diodes by minimizing the thickness of the surface dead layer which would otherwise absorb low energy x-ray photons. Silicon photodiodes with extremely thin (80 Å) surface dead layers are now available.1 One of these diodes has been characterized for spectral sensitivity at x-ray photon energies of 163 eV and 1.4 keV using a laser produced plasma soft x-ray source. Measurements have also been made to characterize the impulse response using fourth harmonic laser light from a short pulse (80 ps full width half maximum) NdYAG laser.

The AWE 1 TW Nd-glass laser HELEN has been used for a number of studies of material properties. Here we discuss measurement of preheat in our high pressure Hugoniot studies and present results for the principal Hugoniot of triacrylate foam at pressures up to 1 Mb. Michelson interferometer results demonstrate spallation in shocked aluminum enabling us to quantify the spall thickness and strength. High resolution spectral data has been obtained from niobium samples in which the M-shell approaches full occupancy.

In this report we discuss two experimental programmes recently carried out using the AWE 1 TW Nd-glass laser, HELEN, to determine the properties of materials under shock loading conditions relevant to hypervelocity impacts. In the first experiment a laser heated hohlraum is employed to generate a uniform steady planar shock which is used to derive high accuracy equation of state data using the impedance match method at pressures in excess of those generally available by conventional means and hitherto only achieved in the vicinity of nuclear explosions. The results are shown to be self consistent and reproducible and thus represent a significant new tool in the material modeler's armoury. In the second experiment direct laser illumination at 10(13) W.cm(-2) is used to generate an attenuating shock in an aluminium target. Direct evidence of a spall layer is obtained by means of x-ray backlighting for the first time in such laser driven experiments. A cumulative damage model is required to predict the spallation and a preliminary analysis shows that the Johnson model, with parameters derived from conventional experiments at much lower strain rates, can be used to predict the spall thickness at the higher rates associated with the laser experiment. (C) British Crown Copyright 1999/MOD. Published by Elsevier Science Ltd with the permission of the Controller of Her Britannic Majesty's Stationery Office.

We describe x-ray streak camera measurements of wall motion and plasma filling in hohlraum targets heated by the AWE HELEN laser. An x-ray streak camera using a transmission mode photocathode on a thin plastic substrate (1000 Å Parylene-N) was coupled to a 15° incidence gold mirror to define a spectral channel response of width 45 eV full width at half-maximum centered around 120 eV. A 20 μm diam pinhole was used to image the hohlraum interior onto the photocathode slit of the streak camera, via the gold reflector. Plasma expansion from the laser hot spots, and the indirectly heated wall, was recorded. The experimental data are compared with simulations using the AWE Lagrangian hydrocode NYM.

We review a recent experimental campaign to study the interaction physics of petawatt laser pulses incident at relativistic intensities on solid targets. The campaign was performed on the 500 J sub-picosecond petawatt laser at the Rutherford Appleton Laboratory. An extensive suite of optical, x-ray, and particle diagnostics was employed to characterise the processes of laser absorption, electron generation and transport, thermal and K-alpha x-ray generation, and proton acceleration.