[Show abstract][Hide abstract] ABSTRACT: We demonstrate strong stimulated inelastic x-ray scattering by resonantly exciting a dense gas target of neon with femtosecond, high-intensity x-ray pulses from an x-ray free-electron laser (XFEL). A small number of lower energy XFEL seed photons drive an avalanche of stimulated resonant inelastic x-ray scattering processes that amplify the Raman scattering signal by several orders of magnitude until it reaches saturation. Despite the large overall spectral width, the internal spiky structure of the XFEL spectrum determines the energy resolution of the scattering process in a statistical sense. This is demonstrated by observing a stochastic line shift of the inelastically scattered x-ray radiation. In conjunction with statistical methods, XFELs can be used for stimulated resonant inelastic x-ray scattering, with spectral resolution smaller than the natural width of the core-excited, intermediate state.
[Show abstract][Hide abstract] ABSTRACT: We present results on the realization of an atomic inner-shell x-ray laser in neon at 1.46 nm wavelength, by photo-ionization pumping and, alternatively, stimulated resonant Raman scattering with an x-ray free-electron laser source.
[Show abstract][Hide abstract] ABSTRACT: We present a method for treating quantum processes in a classical molecular dynamics (MD) simulation. The computational approach, called 'Small Ball' (SB), was originally introduced to model emission and absorption of free–free radiation. Here, we extend this approach to handle ionization/recombination reactions as well as nuclear fusion events. This method exploits the short-range nature of screened-particle interactions in a dense plasma to restrict consideration of quantum processes to a small region about a given ion, and carefully accounts for the effects of the plasma environment on two-particle interaction rates within that region. The use of a reduced set of atomic rates, corresponding to the bottleneck approximation, simplifies their implementation within an MD code. We validate the extended MD code against a collisional–radiative code for model systems under two scenarios: (i) solid-density carbon at conditions encountered in recent experiments, and (ii) high-density Xe-doped hydrogen relevant for laser fusion. We find good agreement for the time-dependent ionization evolution for both systems. We also simulate fast protons stopping in warm, dense carbon plasmas. Here, reasonable agreement with recent experimental data requires contributions from both bound electrons, as modeled by SB in the extended MD code, and free electrons; for the latter, use of the classical random phase approximation (RPA) formula instead of the MD prediction yields better agreement with the experiment, a result that can be attributed to the use of modified Coulomb potentials in MD simulations of electron–ion plasmas. Finally, we confirm that the fusion reaction rate obtained from an MD simulation agrees with analytical expressions for the reaction rate in a weakly screened plasma.
New Journal of Physics 01/2013; 15(1):015011. · 4.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A new class of high-speed detectors, called RadOptic detectors, measures ionizing radiation incident on a transparent semiconductor by sensing changes in the refractive index with an optical probe beam. We describe the role of radiation-initiated electron cascades in setting the sensitivity and the spatial and temporal resolution of RadOptic detectors. We model electron cascades with both analytical and Monte Carlo computational methods. We find that the timescale for the development of an electron cascade is less than of order 100 fs and is not expected to affect the time response of a detector. The characteristic size of the electron cloud is typically less than 2 μm, enabling high spatial resolution in imaging systems. The electron-hole pair density created by single x-rays is much smaller than the saturation density and, therefore, single events should not saturate the detector.
Journal of Applied Physics 01/2013; 114(15):154510-154510-8. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hot electrons created by short, intense laser pulses can heat solid density material to temperatures of order 500 eV. Inertial confinement can maintain such hot-dense plasmas for times of order 10 ps. This provides a platform for measurement of basic properties of hot dense matter, such as opacity and equation-of-state. In this paper we describe the role of computational modeling in the design and analysis of such opacity experiments. We describe a method to model the hot electron transport and deposition and the resulting target radiation-hydrodynamics. We present several design concepts to achieve uniform, long-lasting plasmas.
High Energy Density Physics 01/2013; 9(4):725–730. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present experimental results on the first realization of an atomic inner-shell x-ray laser and x-ray Raman laser in the KeV photon-energy regime in Neon. Extension of the scheme to diatomic molecules is discussed.
[Show abstract][Hide abstract] ABSTRACT: We report recent progress in the development of RadOptic detectors, radiation to optical converters, that rely upon x-ray absorption induced modulation of the optical refractive index of a semiconductor sensor medium to amplitude modulate an optical probe beam. The sensor temporal response is determined by the dynamics of the electron-hole pair creation and subsequent relaxation in the sensor medium. Response times of a few ps have been demonstrated in a series of experiments conducted at the LLNL Jupiter Laser Facility (JLF). This technology will enable x-ray bang-time and fusion burn-history measurements with ∼ ps resolution.
The Review of scientific instruments 10/2012; 83(10):10D307. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Hohlraums are employed at the national ignition facility to convert laser energy into a thermal x-radiation drive, which implodes a fusion capsule, thus compressing the fuel. The x-radiation drive is measured with a low spectral resolution, time-resolved x-ray spectrometer, which views the region around the hohlraum's laser entrance hole. This measurement has no spatial resolution. To convert this to the drive inside the hohlraum, the size of the hohlraum's opening ("clear aperture") and fraction of the measured x-radiation, which comes from this opening, must be known. The size of the clear aperture is measured with the time integrated static x-ray imager (SXI). A soft x-ray imaging channel has been added to the SXI to measure the fraction of x-radiation emitted from inside the clear aperture. A multilayer mirror plus filter selects an x-ray band centered at 870 eV, near the peak of the x-ray spectrum of a 300 eV blackbody. Results from this channel and corrections to the x-radiation drive are discussed.
The Review of scientific instruments 10/2012; 83(10):10E525. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The compact Wedge Range Filter (WRF) proton spectrometer was developed for OMEGA and transferred to the National Ignition Facility (NIF) as a National Ignition Campaign diagnostic. The WRF measures the spectrum of protons from D-(3)He reactions in tuning-campaign implosions containing D and (3)He gas; in this work we report on the first proton spectroscopy measurement on the NIF using WRFs. The energy downshift of the 14.7-MeV proton is directly related to the total ρR through the plasma stopping power. Additionally, the shock proton yield is measured, which is a metric of the final merged shock strength.
The Review of scientific instruments 10/2012; 83(10):10D901. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The understanding of the exchange of forward going power and energy
between two crossing beams in a plasma  is now sufficiently developed
that it can be used to enable access to new experimental configurations.
The existing models of the process allow the design of beam combiners
that will produce higher energy in individual beams for new applications
in ignition and HED physics. For example the Energy Partitioning and
Energy Coupling (EPEC)  program is simulating nuclear events in
various environments by delivering energy to the center of a chamber
through a narrow tube that allows minimal perturbation of the
surrounding region. We will describe the design of gas filled targets
that will allow a 2x to 5x increase in the energy in a single NIF quad
to enable higher yield events to be simulated in EPEC. These designs as
well as advanced ignition target designs will require models with
improved precision to predict their performance accurately. We will also
compare the predictions of existing and emerging models of wave
saturation  with the existing experimental data to determine the
uncertainty in the models.[4pt]  P. Michel Physics of Plasmas
2010.[0pt]  K. Fournier, these proceedings[0pt]  P. Michel, E.
Williams, these proceedings.
[Show abstract][Hide abstract] ABSTRACT: We present the results of an experiment where amorphous carbon undergoes a phase transition induced by femtosecond 830 eV x-ray free-electron laser pulses. The phase transition threshold fluence is found to be 282 ± 11 mJ/cm2. Atomic force microscopy, photoelectron microscopy, and micro-Raman spectroscopy give experimental evidence for the phase transition in terms of a volume expansion, graphitization, and change of local order of the irradiated sample area. The interaction is modeled by an accurate time-dependent treatment of the ionization dynamics coupled to a two-temperature model. At the phase transition fluence threshold the free-electron density Ne is found to be at maximum 9 × 1020 cm−3 while the ion (atom) temperature is found to be 1050 K, e.g., above the crystallization activation temperature reported in the literature. This low ionization rate and high atom temperature suggest a thermally activated phase transition.
Physical Review B 07/2012; 86(2). · 3.66 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We used photon pulses from an x-ray free-electron laser to study ultrafast x-ray-induced transitions of graphite from solid to liquid and plasma states. This was accomplished by isochoric heating of graphite samples and simultaneous probing via Bragg and diffuse scattering at high time resolution. We observe that disintegration of the crystal lattice and ion heating of up to 5 eV occur within tens of femtoseconds. The threshold fluence for Bragg-peak degradation is smaller and the ion-heating rate is faster than current x-ray-matter interaction models predict.
[Show abstract][Hide abstract] ABSTRACT: Achieving inertial confinement fusion ignition requires a symmetric, high velocity implosion. Experiments show that we can reach 95 plusmn 5% of the required velocity by using a 420 TW, 1.6 MJ laser pulse. In addition, experiments with a depleted uranium hohlraum show an increase in capsule performance which suggests an additional 18 plusmn 5 mum/ns of velocity with uranium hohlraums over gold hohlraums. Combining these two would give 99 plusmn 5% of the ignition velocity. Experiments show that we have the ability to tune symmetry using crossbeam transfer. We can control the second Legendre mode (P2) by changing the wavelength separation between the inner and outer cones of laser beams. We can control the azimuthal m = 4 asymmetry by changing the wavelength separation between the 23.5 and 30 degree beams on NIF. This paper describes our ldquofirst passrdquo tuning the implosion velocity and shape on the National Ignition Facility laser [Moses et al., Phys. Plasmas, 16, 041006 (2009)].
[Show abstract][Hide abstract] ABSTRACT: Since the invention of the laser more than 50 years ago, scientists have striven to achieve amplification on atomic transitions of increasingly shorter wavelength. The introduction of X-ray free-electron lasers makes it possible to pump new atomic X-ray lasers with ultrashort pulse duration, extreme spectral brightness and full temporal coherence. Here we describe the implementation of an X-ray laser in the kiloelectronvolt energy regime, based on atomic population inversion and driven by rapid K-shell photo-ionization using pulses from an X-ray free-electron laser. We established a population inversion of the Kα transition in singly ionized neon at 1.46 nanometres (corresponding to a photon energy of 849 electronvolts) in an elongated plasma column created by irradiation of a gas medium. We observed strong amplified spontaneous emission from the end of the excited plasma. This resulted in femtosecond-duration, high-intensity X-ray pulses of much shorter wavelength and greater brilliance than achieved with previous atomic X-ray lasers. Moreover, this scheme provides greatly increased wavelength stability, monochromaticity and improved temporal coherence by comparison with present-day X-ray free-electron lasers. The atomic X-ray lasers realized here may be useful for high-resolution spectroscopy and nonlinear X-ray studies.
[Show abstract][Hide abstract] ABSTRACT: We describe the status of a new time-dependent simulation capability for dense plasmas. The backbone of this multi-institutional effort—the Cimarron Project—is the massively parallel molecular dynamics (MD) code "ddcMD," devel-oped at Lawrence Livermore National Laboratory. The project's focus is material conditions such as exist in inertial confinement fusion experiments, and in many stellar interiors: high temperatures, high densities, significant elec-tromagnetic fields, mixtures of high-and low-Z elements, and non-Maxwellian particle distributions. Of particular importance is our ability to incorporate into this classical MD code key atomic, radiative, and nuclear processes, so that their interacting effects under non-ideal plasma conditions can be investigated. This paper summarizes progress in computational methodology, discusses strengths and weaknesses of quantum statistical potentials as effective in-teractions for MD, explains the model used for quantum events possibly occurring in a collision, describes two new experimental efforts that play a central role in our validation work, highlights some significant results obtained to date, outlines concepts now being explored to deal more efficiently with the very disparate dynamical timescales that arise in fusion plasmas, and provides a careful comparison of quantum effects on electron trajectories predicted by more elaborate dynamical methods.
High Energy Density Physics 01/2012; 6557. · 1.60 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of an experiment where amorphous carbon was irradiated
by femtosecond x-ray free electron laser pulses. The 830 eV laser pulses induce
a phase transition in the material which is characterized ex-situ. The phase
transition energy threshold is determined by measuring the surface of each
irradiated area using an optical Nomarski microscope. The threshold fluence is
found to be 282 +/- 11 mJ/cm^2, corresponding to an absorbed dose at the
surface of 131 +/-5 meV/atom. Atomic force microscopy measurements show volume
expansion of the irradiated sample area, suggesting a solid to solid phase
transition. Deeper insight into the phase transition is gained by using
scanning photoelectron microscopy and micro-Raman spectroscopy. Photoelectron
microscopy shows graphitization, i.e. modification from sp3 to sp2
hybridization, of the irradiated material. The micro-Raman spectra show the
appearance of local order, i.e. formation of graphite nanocrystals. Finally,
the nature of the phase transition is discussed, taking into account previous
theory and experimental results.
[Show abstract][Hide abstract] ABSTRACT: V. GLEBOV, P. RADHA, T. SANGSTER, LLE, R. OLSON, R. LEEPER, SNL, J.
KLINE, G. KYRALA, D. WILSON, LANL, J. KILKENNY, A. NIKROO, GA, C.
SANGSTER, LLE The Wedge Range Filter (WRF) proton spectrometer was
developed for OMEGA and transferred to the NIF as a National Ignition
Campaign (NIC) diagnostic. In tuning campaign implosions containing D
and ^3He gas, the WRFs measure the spectrum of protons from D-^3He
reactions. The energy downshift of the 14.7-MeV proton is directly
related to total ρR through the plasma stopping power. WRFs fielded
simultaneously on the pole and equator measure low-mode polar ρR
asymmetries due to drive inhomogeneity. We find no correlation between
shock ρR symmetry and x-ray self-emission symmetry near peak
compression for low polar modes. Adjacent WRFs are sensitive to
high-mode asymmetries due to hydro instabilities; these have not been
observed. This work was supported in part by the U.S. DOE, LLNL and LLE.
[Show abstract][Hide abstract] ABSTRACT: Measurements of optical backscattered SRS and SBS light from NIF
hohlraum targets are used with hydrodynamic modeling to develop a
consistent experimental understanding of the hohlraum plasma.
Measurements are made of the temporally resolved spectra, power, and
near field light distribution for a range of plasma conditions using a
FABS (full aperture backscatter system) and an NBI (near backscatter
imager). The measurements are combined with simulations to develop an
overall model for backscatter origin locations, SRS and SBS interaction,
hohlraum energetics, and the evolution of the hohlraum plasma in time.
We will describe the backscatter measurements and the modeling used to
infer plasma characteristics in the hohlraum targets.
[Show abstract][Hide abstract] ABSTRACT: The interaction of ultrashort high-intensity pulses from XFELs with neon gas at atmospheric pressures results in strong amplification of spontaneous emission of the neon K- line and self-stimulated resonant Raman scattering in the x-ray regime.
[Show abstract][Hide abstract] ABSTRACT: The National Ignition Facility at Lawrence Livermore National Laboratory was formally dedicated in May 2009. The hohlraum energetics campaign with all 192 beams began shortly thereafter and ran until early December 2009. These experiments explored hohlraum-operating regimes in preparation for experiments with layered cryogenic targets. The hohlraum energetic series culminated with an experiment that irradiated an ignition scale hohlraum with 1 MJ. The results demonstrated the ability to produce a 285 eV radiation environment in an ignition scale hohlraum while meeting ignition requirements for symmetry, backscatter and hot electron production. Complementary scaling experiments indicate that with ~1.3 MJ, the capsule drive temperature will reach 300 eV, the point design temperature for the first ignition campaign. Preparation for cryo-layered implosions included installation of a variety of nuclear diagnostics, cryogenic layering target positioner, advanced optics and facility modifications needed for tritium operations and for routine operation at laser energy greater than 1.3 MJ. The first cyro-layered experiment was carried out on 29 September 2010. The main purpose of this shot was to demonstrate the ability to integrate all of the laser, target and diagnostic capability needed for a successful cryo-layered experiment. This paper discusses the ignition point design as well as findings and conclusions from the hohlraum energetics campaign carried out in 2009. It also provides a brief summary of the initial cryo-layered implosion.