D. F. Price

University of British Columbia - Vancouver, Vancouver, British Columbia, Canada

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Publications (43)66.59 Total impact

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    ABSTRACT: Warm Dense Matter (WDM) physics has been a growing field of high energy density physics, driven by the fundamental urge to understand the convergence between plasma and condensed matter physics, and the practical need to understand dynamic behavior of materials under extreme conditions. A platform for creating and probing WDM by isochoric heating of free-standing nano-foils has been developed recently to study the non-equilibrium processes. Results of optical measurements reveal the existence of a quasi-steady state in the time history, during which the interband component of the dielectric function shows both enhancement and a red shift. First-principles calculations of the dielectric function suggest that the enhanced red shift of the interband transition peak might be explained by a positive charge state of the gold foil due to ejection of electrons by the high intensity laser pulse. The impact on optical properties by the formation of an electronic sheath was examined by the Thomas–Fermi theory with local equilibrium approximation.
    High Energy Density Physics. 01/2010;
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    ABSTRACT: This paper describes a new positron source using ultraintense short pulse lasers. Although it has been theoretically studied since the 1970s, the use of lasers as a valuable new positron source was not demonstrated experimentally until recent years, when the petawatt-class short pulse lasers were developed. In 2008 and 2009, in a series of experiments performed at the Lawrence Livermore National Laboratory, a large number of positrons were observed after shooting a millimeter thick solid gold target. Up to 2x10{sup 10} positrons/s ejected at the back of approximately millimeter thick gold targets were detected. The targets were illuminated with short (approx1 ps) ultraintense (approx1x10{sup 20} W/cm{sup 2}) laser pulses. These positrons are produced predominantly by the Bethe-Heitler process and have an effective temperature of 2-4 MeV, with the distribution peaking at 4-7 MeV. The angular distribution of the positrons is anisotropic. For a wide range of applications, this new laser-based positron source with its unique characteristics may complement the existing sources based on radioactive isotopes and accelerators.
    Physics of Plasmas 12/2009; 16(12). · 2.38 Impact Factor
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    ABSTRACT: The characteristics of 22-40 keV K alpha x-ray sources are measured. These high-energy sources are produced by 100 TW 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 K alpha source size from a simple foil target is larger than 60 mu m, too large for most radiography applications. The total K alpha yield is independent of target thicknesses, verifying that refluxing plays a major role in photon generation. Smaller radiating volumes emit brighter K alpha radiation. One-dimensional radiography experiments using small-edge-on foils resolved 10 mu m 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. (c) 2006 American Institute of Physics.
    Physics of Plasmas 04/2006; 13:056309. · 2.38 Impact Factor
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    ABSTRACT: An open question about the dynamical behavior of materials is how phase transition occurs in highly nonequilibrium systems. One important class of study is the excitation of a solid by an ultrafast, intense laser. The preferential heating of electrons by the laser field gives rise to initial states dominated by hot electrons in a cold lattice. Using a femtosecond laser pump-probe approach, we have followed the temporal evolution of the optical properties of such a system. The results show interesting correlation to nonthermal melting and lattice disordering processes. They also reveal a liquid-plasma transition when the lattice energy density reaches a critical value.
    Physical Review Letters 03/2006; 96(5):055001. · 7.94 Impact Factor
  • Conference Paper: The Titan laser at LLNL
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    ABSTRACT: The Titan laser, designed to produce up to 600 J with pulses of 0.4-200 ps, has begun initial target shots. Coupled with a kJ long-pulse beam, Titan will support high-energy-density physics research at LLNL.
    Lasers and Electro-Optics, 2006 and 2006 Quantum Electronics and Laser Science Conference. CLEO/QELS 2006. Conference on; 01/2006
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    ABSTRACT: We have demonstrated a 10 Hz Ni-like Mo x-ray laser operating at 18.9 nm with 150 mJ total pump energy by employing a novel pumping scheme. The grazing-incidence scheme is described, where a picosecond pulse is incident at a grazing angle to a Mo plasma column produced by a slab target irradiated by a 200 ps laser pulse. This scheme uses refraction of the short pulse at a predetermined electron density to increase absorption to pump a specific gain region. The higher coupling efficiency inherent to this scheme allows a reduction in the pump energy where 70 mJ long pulse energy and 80 mJ short pulse energy are sufficient to produce lasing at a 10 Hz repetition rate. Under these conditions and by optimizing the delay between the pulses, we achieve strong amplification and close to saturation for 4 mm long targets.
    Physical Review Letters 04/2005; 94(10):103901. · 7.94 Impact Factor
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    ABSTRACT: The first demonstration of the grazing incidence pumping (GRIP) scheme for laser-driven x-ray lasers (XRLs) is described utilizing 2-pulse pumping. A long pulse is incident normal to the target to produce a plasma with a particular density profile. Then a short pulse is incident at a grazing angle, chosen to optimally couple the short pulse laser energy into the specific density region where the inversion process will occur. The short pulse is simultaneously absorbed and refracted at a maximum electron density specified by the chosen pump angle then turns back into the gain region. The increased path length gives improved absorption allowing a reduction in the drive energy required for lasing. A Ni-like Mo XRL at 18.9 nm has been demonstrated with only 150 mJ total pump energy and a repetition rate of 10 Hz. We report high gains of 60 cm⁻¹ and the achievement of gain saturation for targets of 4 mm length.
    01/2005;
  • Y. Ping, T. Ao, H. Tam, K. Widmann, D.F. Price, A. Ng
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    ABSTRACT: form only given. The study of non-equilibrium phase transitions is a rapidly developing field. Non-thermal melting has been observed in femtosecond laser heated semiconductors such as silicon. This is thought to result from the excitation of valence electrons to the conduction band, giving rise to anti-bonding states. In metals, the process of melting under ultrafast laser excitation is not clearly understood. In our experiment, we measure the broadband (400-800 nm) optical reflectivity and transmissivity of freestanding, 30 nm-thick gold foils heated with 150 fs, 400 nm laser light. Prior to laser excitation the sample shows strong reflectivity for wavelengths above 500 nm. This is due to interband (d to s/p) transitions, thus giving gold its characteristic color. The reflectivity and transmissivity spectra of the heated sample (hence the color of gold) change substantially with laser excitation energy densities. Such spectral signatures offer a new means of probing electronic and structure behaviors associated with non-equilibrium phase transitions
    Plasma Science, 2005. ICOPS '05. IEEE Conference Record - Abstracts. IEEE International Conference on; 01/2005
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    ABSTRACT: The warm dense matter (WDM) regime poses a tremendous challenge on theoretical predictions and simulations. In general, WDM is too hot for characterizing its properties by means of condensed matter models and too cold and dense for applying plasma equation-of-state and transport models. Successful extension of the current models into the WDM regime depends on the availability of experimental data. For a direct, model independent comparison these measurements of the properties of WDM should be obtained on a uniformly heated target [PRE 58, 1248 (1998)]. Using 30--nm thin freestanding foils and a high--contrast 150--fs pulse laser, we have been able to generate such a uniformly heated WDM sample. In particular, we have measured the AC conductivity of a single state of warm dense gold at energy densities up to 2 × 10^7 J/kg. Our time--resolved measurements also reveal a quasi steady phase of several ps duration in which the heated foil does not show any measurable expansion.
    05/2004;
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    ABSTRACT: We report on a single-state measurement of electrical conductivity of warm dense gold in the solid to plasma transition regime. This is achieved using the idealized slab plasma approach of isochoric heating of ultrathin samples by a femtosecond laser, coupled with femtosecond probe measurements of reflectivity and transmission. The experiment also reveals the time scale associated with the disassembly of laser heated solid.
    Physical Review Letters 04/2004; 92(12):125002. · 7.94 Impact Factor
  • Y Ping, T. Ao, K Widmann, D F Price, A Ng
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    ABSTRACT: Summary form only given. The advance in ultrafast laser technology has enabled the observation of transient processes in many areas. One interesting phenomenon is laser-induced phase transition from solid to plasma. A detailed study of this could bridge the gap between condensed matter physics and plasma physics. A recent experiment on femtosecond laser heating of ultrathin freestanding gold foils revealed a quasi-steady-state behavior before the onset of hydrodynamic expansion. Further investigations with frequency domain interferometry (FDI) yielded the time scale of such a state, which agrees well with the electron-phonon coupling time calculated from a simple Two-Temperature Model (TTM). To provide further insight into the processes of electron-electron and electron-phonon relaxation, a fs wavelength-tunable probe generated by an optical parametric amplifier is used to follow the temporal evolution of interband transition in gold (5d-6s/p) under intense laser excitation through transient reflectivity and transmissivity measurements. The comparison of the measurements and the model calculation will be discussed.
    Plasma Science, 2004. ICOPS 2004. IEEE Conference Record - Abstracts. The 31st IEEE International Conference on; 01/2004
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    ABSTRACT: We report progress in developing efficient pumping of laser-driven x-ray lasers that opens new possibilities for both high average power x-ray lasers as well as producing progressively shorter wavelength lasers. The new scheme of grazing incidence pumping (GRIP) is described. In essence, a chosen electron density region of a pre-formed plasma column, produced by a longer pulse at normal incidence onto a slab target, is selectively pumped by focusing the short pulse âps laser at a determined grazing incidence angle to the target. The controlled use of refraction of the pumping laser in the plasma results in several benefits: The pump laser path length is longer and there is an increase in the laser absorption in the gain region for creating a collisional Ni-like ion x-ray laser. There is also an inherent traveling wave, close to c, that increases the overall pumping efficiency. The scheme requires careful tailoring of the pump and plasma conditions to the specific x-ray laser under investigation but the main advantage is a 3 - 30 times reduction in the laser pump energy for mid-Z materials. We report several examples of this new x-ray laser on two different laser systems. The first demonstrates a 10 Hz x-ray laser operating at 18.9 nm pumped with a total of 150 mJ of 800 nm wavelength from a Ti:Sapphire laser. The second case is shown where the COMET laser is used both at 527 nm and 1054 nm wavelength to pump higher Z materials with the goal of extending the wavelength regime of tabletop x-ray lasers below 10 nm.
    01/2004;
  • T. Ao, A. Ng, Y. Ping, K. Widmann, D.F. Price, E. Lee
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    ABSTRACT: form only given. Ultrafast laser heating of a solid offers a unique approach to examine the behavior of non-equilibrium high energy density states. Initially, the electrons are optically excited while the ions in the lattice remain cold. This is followed by electron-electron and electron-phonon relaxation. Numerous studies have been made at low laser fluences to investigate such processes together with solid-liquid transition. Recently, experiments were performed in which ultrathin freestanding, gold foils were heated by a femtosecond laser to a strongly overdriven regime with energy densities reaching 20 MJ/kg. Interestingly, femtosecond laser reflectivity and transmission measurements on the heated sample revealed a quasi-steady-state behavior before the onset of hydrodynamic expansion. This led to the conjecture of the existence of a metastable, disordered state prior to the disassembly of the solid. To further examine the dynamics of ultrafast laser heated solids, frequency domain interferometry (FDI) was used to provide an independent observation. The highly sensitive change in the phase shift of the FDI probe clearly showed evidence of the quasi-steady-state behavior. The new experiment also yielded a detailed measurement of the time scale of such a quasi-steady-state phase that may help elucidate the process of electron-phonon coupling and disassembly in a strongly overdriven regime.
    Plasma Science, 2004. ICOPS 2004. IEEE Conference Record - Abstracts. The 31st IEEE International Conference on; 01/2004
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    ABSTRACT: A new technique is described for the isochoric heating (i.e., heating at constant volume) of matter to high energy-density plasma states (>10(5) J/g) on a picosecond time scale (10(-12)sec). An intense, collimated, ultrashort-pulse beam of protons--generated by a high-intensity laser pulse--is used to isochorically heat a solid density material to a temperature of several eV. The duration of heating is shorter than the time scale for significant hydrodynamic expansion to occur; hence the material is heated to a solid density warm dense plasma state. Using spherically shaped laser targets, a focused proton beam is produced and used to heat a smaller volume to over 20 eV. The technique described of ultrafast proton heating provides a unique method for creating isochorically heated high-energy density plasma states.
    Physical Review Letters 09/2003; 91(12):125004. · 7.94 Impact Factor
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    ABSTRACT: Ultraintense laser-matter interactions provide a unique source of temporally short, broad spectrum electrons, which may be utilized in many varied applications. One such, which we are pursuing, is as part of a diagnostic to trace magnetic field lines in a magnetically confined fusion device. An essential aspect of this scheme is to have a detailed characterization of the electron angular and energy distribution. To this effect we designed and constructed a compact electron spectrometer that uses permanent magnets for electron energy dispersion and over 100 scintillating fibers coupled to a 1024×1024 pixel charge coupled device as the detection system. This spectrometer has electron energy coverage from 10 keV to 60 MeV. We tested the spectrometer on a high intensity (1017–1021 W/cm2) short pulse (<100 fs) laser, JanUSP, at Lawrence Livermore National Laboratory using various solid targets. The details of the spectrometer and the experimental results will be reported.
    Review of Scientific Instruments 03/2003; 74(3). · 1.60 Impact Factor
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    ABSTRACT: Experiments utilizing femtosecond laser pump-probe techniques are capable of producing nearly single warm dense matter (WDM) states. Such a single state can be described as an idealized slab plasma (ISP). Obtaining an ISP opens the window on the experimental determination of global properties - such as the AC conductivity - in a model independent manner. Hence, these experiments can provide guidance for the development and improvement of theoretical predictions of the optical properties and transport properties of WDM, i.e., in the strongly coupled, partially ionized, high-energy-density plasma regime. In this paper, we present the first measurement of the AC conductivity for well defined, single WDM states. Using measured values of the reflectivity and the transmission for an ISP produced by nearly isochoric heating of ultrathin free-standing gold foils with a femtosecond laser, the AC conductivity of warm dense gold was determined.
    IEEE International Conference on Plasma Science 01/2003;
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    ABSTRACT: Summary form only given. Heating ultrathin foils with femtosecond laser pulses provides an experimental test ground for the study of high-energy density matter, in particular, for warm dense matter (WDM). Investigating the optical properties with high temporal and spatial resolution yields information about the AC conductivity of these nearly isochorically heated states. Theoretical predictions for the optical properties in this WDM regime, i.e., the strongly coupled, partially ionized plasma regime, differ significantly depending on the conductivity and equation of state models used. Thus, gaining experimental access to the optical properties is an important step for guiding empirical models and for testing ab-initio calculations. Measurements - performed at the Ultrashort Pulse laser facility at the Lawrence Livermore National Laboratory - of the reflectivity and the transmission for S- and P-polarized 800-mn light of a 20 nm thin aluminum foil which is being heated by a 100-fs, 400-nm laser pulse will be presented. The measurements also provide the experimental determination of the energy density deposition which is an important constraint with respect to the comparison between the experiment and simulations.
    01/2002;
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    ABSTRACT: Summary form only given, as follows. The AC conductivity of aluminum in the warm dense matter regime of several electron-volts and solid density is of particular interest because of its proximity to the predicted region of conductivity minimum. In this paper, we will describe the first determination of AC conductivity of a well defined, single plasma state using measured values of reflectivity and transmission for a slab plasma produced by the isochoric heating of an ultrathin aluminum foil with a femtosecond laser.
    01/2002;
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    ABSTRACT: Simultaneous temporally and spatially resolved measurements of the phase change and reflectivity of S- and P-polarized femtosecond laser probes are obtained from hot expanded states produced by femtosecond laser heating of a solid aluminum target. The combined set of data provides an integral test of equation-of-state models in a regime up to 10 Mbar and densities of 0.01–1 times solid. The results suggest that target stoichiometry at the few Å level should be considered in the analysis of phase and reflectivity measurements in such experiments. © 2001 American Institute of Physics.
    Physics of Plasmas 08/2001; 8(9):3869-3872. · 2.38 Impact Factor
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    ABSTRACT: We report on the characterization of plasmas produced by ultrashort laser irradiation of 500Å thick layers of NaF buried at varying depths in plastic to reduce plasma gradients. The targets are irradiated with 130fs,≈170mJ laser pulses (λ=400nm) at an intensity of ≈2×1018Wcm−2. The data are spectrally and temporally resolved, with resolutions of λ/Δλ≈1000 and ≳500fs, respectively. We use X-ray emission spectroscopy to assess the electron density, Ne, and temperature, Te, in the plasma. The density is measured from Stark broadened line profiles of the He-like 1 1S–31P,Heβ, and H-like Lyβ lines of Na, while the temperature is determined from the intensity ratio of Na Heβ to Lyβ, and also from the dielectronic satellites to these lines. We find peak densities and temperatures of ≳1023cm−3 and ≈400eV, respectively, at 2–4ps after the laser pulse. The plasma conditions plateau near these values for 5ps after that. Atomic data for the kinetics simulations are generated with the HULLAC suite of codes. The Stark broadened line profiles, with full accounting of satellite transitions, are computed with the TOTAL code. Self-absorption effects are included along the observation line of sight using the radiation transport code CRETIN. Steady-state, non-LTE equilibrium plasma conditions are demonstrated suggesting that buried-layer experiments can be used as a test bed to study equation of state and opacity properties in hot, near-solid density matter.
    Journal of Quantitative Spectroscopy and Radiative Transfer 01/2001; 71(2):339-354. · 2.38 Impact Factor