Henry Tiedje

Laser-accelerated proton conversion efficiency thickness scaling

The conversion efficiency from laser energy into proton kinetic energy is measured with the 0.6ps, 9×1019W∕cm2 Titan laser at the Jupiter Laser Facility as a function of target thickness in Au foils. For targets thicker than 20μm, the conversion efficiency scales approximately as 1∕L, where L is the target thickness. This is explained by the domination of hot electron collisional losses over adiabatic cooling. In thinner targets, the two effects become comparable, causing the conversion efficiency to scale weaker than 1∕L; the measured conversion efficiency is constant within the scatter in the data for targets between 5 and 15μm, with a peak conversion efficiency of 4% into protons with energy greater than 3MeV. Depletion of the hydrocarbon contaminant layer is eliminated as an explanation for this plateau by using targets coated with 200nm of ErH3 on the rear surface. The proton acceleration is modeled with the hybrid-particle in cell code LSP, which reproduced the conversion efficiency scaling observed...

Kirkpatrick-Baez microscope for hard X-ray imaging of fast ignition experiments

A Kirkpatrick-Baez X-ray microscope has been developed for use on the Titan laser facility at the Lawrence Livermore National Laboratory in Fast Ignition experiments. It was developed as a broadband alternative to narrow band Bragg crystal imagers for imaging Kα emission from tracer layers. A re-entrant design is employed which allows for alignment from outside the chamber. The mirrors are coated with Pt and operate at a grazing incident angle of 0.5° providing higher resolution than an equal brightness pinhole and sufficient bandwidth to image thermally shifted characteristic Kα emission from heated Cu tracer layers in Fast Ignition experiments. The superpolished substrates (<1 Å rms roughness) had a final visible wavelength roughness of 1.7 Å after coating, and exhibited a reflectivity corresponding to an X-ray wavelength roughness of 7 ± 1 Å. A unique feature of this design is that during experiments, the unfiltered direct signal along with the one-dimensional reflections are retained on the detector in order to enable a live indication of alignment and incident angle. The broad spectral window from 4 to 9 keV enables simultaneous observation of emission from several spectral regions of interest, which has been demonstrated to be particularly useful for cone-wire targets. An experimentally measured resolution of 15 μm has been obtained at the center of the field of view.

On specular reflectivity measurements in high and low-contrast relativistic laser-plasma interactions

Using both experiment and 2D3V particle-in-cell (PIC) simulations, we describe the use of specular reflectivity measurements to study relativistic (Iλ2 > 1018 W/cm2⋅μm2) laser-plasma interactions for both high and low-contrast 527 nm laser pulses on initially solid density aluminum targets. In the context of hot-electron generation, studies typically rely on diagnostics which, more-often-than-not, represent indirect processes driven by fast electrons transiting through solid density materials. Specular reflectivity measurements, however, can provide a direct measure of the interaction that is highly sensitive to how the EM fields and plasma profiles, critical input parameters for modeling of hot-electron generation, evolve near the interaction region. While the fields of interest occur near the relativistic critical electron density, experimental reflectivity measurements are obtained centimeters away from the interaction region, well after diffraction has fully manifested itself. Using a combination of P...

Spectral calibration of EBT3 and HD-V2 radiochromic film response at high dose using 20 MeV proton beams

Radiochromic film is used extensively in many medical, industrial, and scientific applications. In particular, the film is used in analysis of proton generation and in high intensity laser-plasma experiments where very high dose levels can be obtained. The present study reports calibration of the dose response of Gafchromic EBT3 and HD-V2 radiochromic films up to high exposure densities. A 2D scanning confocal densitometer system is employed to carry out accurate optical density measurements up to optical density 5 on the exposed films at the peak spectral absorption wavelengths. Various wavelengths from 400 to 740 nm are also scanned to extend the practical dose range of such films by measuring the response at wavelengths removed from the peak response wavelengths. Calibration curves for the optical density versus exposure dose are determined and can be used for quantitative evaluation of measured doses based on the measured optical densities. It was found that blue and UV wavelengths allowed the largest dynamic range though at some trade-off with overall accuracy.

Characterisation and Modelling of a Passively Q-Switched Yb:CaF 2 Laser

A diode-pumped, passively Q-switched Yb:CaF2 laser with Cr:YAG saturable absorber has been developed and characterized. A maximum average output power of 737 mW at a repetition rate of 1060 Hz was obtained with slope efficiency of 20%. The output pulse energy was 0.7 mJ with a pulse width of 78 ns giving a peak pulse power of 8.9 kW with a very high beam quality. Laser threshold pump power was 8.6 W. Rate equation modeling was also performed for the Q-switched Yb:CaF2 laser and the output characteristics shown generally matched experimental values and trends observed.

Application of laser induced breakdown spectroscopy (LIBS) for detection of lead contaminants in water using wood sample substrates

Toxic heavy metals can be a significant risk to human health. Lead poisoning has been a recognized health hazard for more than 2000 years. Exposure can occur through drinking water, food, air, soil, and dust from old paint containing lead. Laser-induced breakdown spectroscopy (LIBS) is a powerful analytical technique to monitor heavy metals in all forms including aqueous solutions. Our goal is to develop a LIBS sampling technique for the measurement of heavy metal contaminants in water that has sufficient sensitivity to measure contamination at the allowable limit for drinking water (e.g. 15 ppb for lead). To perform fast and sensitive trace metal detection in aqueous solution with LIBS, a thin wood sample has been used as a liquid absorber to transform liquid sample analysis to solid sample analysis [1]. Initial studies have been carried out for lead in water. The plasma was generated by focusing Nd: YAG laser pulses with ~7 mJ energy at 1064nm on test samples in air. We also tried to further improve the sensitivity by carrying out experiments in argon gas in a sample chamber. The dependence of the elemental spectra on time delay and laser beam energy was also investigated. Initial results for lead in water indicate that a 3-sigma limit of detection (LOD) of the order of 10 ppb is achievable with a 1000 lasers shots, which is readily achievable with moderate repetition rate laser systems today. The experimental results will be presented and discussed.

Effect of preplasma on double pulse irradiation of targets for proton acceleration

Summary form only given. We report on the experimental and simulated characterization of proton acceleration from double-pulse irradiation of um-scale foil targets with varying preplasma conditions. Temporally separated pulses of less than a picosecond in duration have been shown to increase the conversion efficiency of laser energy to MeV protons1. The experiment utilized two 700 fs, 1054 nm pulses, separated by 1 to 5 ps; total beam energy was 100 J, with 5-20% of the total energy contained within the first pulse. In contrast to the ultraclean beams used in previous experiments1, prepulse energies on the order of 10 mJ were present. The resulting significant preplasma appears to have a moderating effect on the double pulse enhancement. Proton beam measurements were made with radiochromic film stacks and magnetic spectrometers.LSP 2D PIC simulations2 have been performed to better understand the double pulse enhancement mechanism, as well as the role of preplasma in modifying this effect. Simulation results will be shown for various target conditions, and compared to experimental data.

Inverse faraday effect magnetic field generation in laser induced plasma

Summary form only given. Laser plasma interactions with high intensity laser pulses can produce high magnetic fields in the 10s of MG range. One technique for generating high magnetic fields in under dense plasmas is via the Inverse Faraday Effect (IFE) which has been shown to induce axial magnetic fields in the MG range using circularly polarized light [1]. It is also proposed that similar fields could be induced using linearly polarized light where higher order angular momentum modes [2] are employed to couple the required angular momentum to the electrons. We wish to study this magnetic field generation in under dense plasma and are carrying out a simulation study of the expected fields.IFE is a phenomenon in which the circularly polarized light propagates through a non-linear medium, transmits angular momentum to the electrons and induces an axial magnetic field [3]. This picture gets more complicated with hot electron generation by the propagation of the intense laser pulse through the plasma, which in addition generates an axial current and solenoidal magnetic field. We have carried out initial Large Scale PIC (LSP) simulations to predict the scaling law for this hot electron generation and hence the expected magnetic field levels. Data will be shown for various parameters of laser plasma interaction with different densities and intensities in the threshold relativistic intensity range of 1017 to 1019 Wcm-2.

Interferometrio characterization of preplasma density for high intensity laser plasma interaction studies

Summary form only given. High intensity laser plasma interactions are often affected by the preplasma created by the inherent prepulse of the system due to leakage laser light (typically millijoules over nanoseconds) prior to the arrival of the main high energy laser pulse (typically 100s of joules). Even small scalelength preplasma can alter the absorption of laser light and the subsequent creation of high energy electrons and ions in the interaction process. This can have significant effects on the application of such pulses for areas such as Fast Ignition Laser Fusion Energy. In order to correctly model the interaction process it is important to know the plasma density profile accurately prior to the main interaction process. The current study is focussed on characterizing the expected preplasma conditions for experiments carried out at the Titan laser facility at the Lawrence Livermore National Laboratory (LLNL) and similar facilities. Interferometric measurements for accurate characterization of the plasma density profile of an aluminum plasma created by nanosecond duration 532nm Nd:YAG laser pulses are carried out using a femtosecond-laser probe pulse (400nm/130fs). The main laser pulse is focussed onto an aluminum rod using f/20 optics producing a 20μm diameter focal spot and the interferometry is carried out using a Mach Zehnder Interferometer. The interferograms are used to determine the evolution of the density profile of the plasma with time over a time scale of 1 to 5 nanoseconds. The results are then compared to 2D hydrodynamic modeling of the plasma expansion and used to determine the equation of state models which best fit the experiment. From these results, we expect that more accurate preplasma density models can be developed and incorporated into the analysis of high intensity laser plasma interaction experiments. The experimental and simulation results will be presented and discussed.

Characterization of MeV electron generation using 527nm laser pulses for fast ignition

Summary form only given. Fast Ignition [1] holds the promise of improved efficiency and reduced laser energy requirements for Laser Fusion Energy systems. The main approach proposed to date is by coupling a beam of 1 to 2 MeV electrons from the laser interaction spot to a 40 micron spot in the compressed fuel core using a metal cone insert to get close to the compressed core [2]. However, multi-millijoule level laser prepulse can create extended preplasmas within the cone, effectively moving the electron generation source region far back from the cone tip and core [3]. By employing second harmonic pulses much reduced levels of prepulse can be achieved and at the same time colder electron distribution can be obtained, closer to those required ultimately for Fast Ignition.