[Show abstract][Hide abstract] ABSTRACT: High-energy short-pulse lasers are pushing the limits of plasma-based particle acceleration, x-ray generation, and high-harmonic generation by creating strong electromagnetic fields at the laser focus where electrons are being accelerated to relativistic velocities. Understanding the relativistic electron
dynamics is key for an accurate interpretation of measurements. We present a unified and self-consistent modeling approach in quantitative agreement with measurements and differing trends across multiple target types acquired from two separate laser systems, which differ only in their nanosecond to picosecond-scale rising edge. Insights from high-fidelity modeling of laser-plasma interaction demonstrate that the ps-scale, orders of magnitude weaker rising edge of the main pulse measurably alters target evolution and relativistic electron generation compared to idealized pulse shapes. This can lead for instance to the experimentally observed difference between 45 MeV and 75 MeV maximum energy protons for two nominally identical laser shots, due to ps-scale prepulse variations. Our results show that the realistic inclusion of temporal laser pulse profiles in modeling efforts is required if predictive capability and extrapolation are sought for future target and laser designs or for other relativistic laser ion acceleration schemes.
Full-text · Article · Apr 2015 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: A model for nonlinear optical propagation is cast into a split-step numerical framework via a variable stencil-size Crank–Nicolson finite-difference method for the linear step and a choice of two different nonlinear integration schemes for the nonlinear step. The model includes Kerr, Raman scattering, and ionization effects (as well as linear and nonlinear shock, diffraction, and dispersion). We demonstrate the practical importance of numerical effects when interpreting computational studies of high-intensity optical pulse propagation in physical materials. Examples demonstrate the significant error that can arise in discrete, limited precision implementations as one attempts to improve practical operator accuracy through increased operator support size and sampling frequency. We also demonstrate the effect of the method used to obtain the finite-difference operator coefficients defining the equations ultimately used in the discrete model. Smooth, plausible, but incorrect solutions may result from these numerical effects. This implies the necessity of a complete, precise description of all numerical methods when reporting results of computational physics investigations in order to ensure proper interpretation and reproducibility.
[Show abstract][Hide abstract] ABSTRACT: We have designed and produced an optical coating suitable for broad bandwidth high reflection (BBHR) at 45° angle of incidence (AOI), P polarization (Ppol) of petawatt (PW) class fs laser pulses of ∼ 900 nm center wavelength. We have produced such BBHR coatings consisting of TiO2/SiO2 layer pairs deposited by ion assisted e-beam evaporation using the large optics coater at Sandia National Laboratories. This paper focuses on laser-induced damage threshold (LIDT) tests of these coatings. LIDT is difficult to measure for such coatings due to the broad range of wavelengths over which they can operate. An ideal test would be in the vacuum environment of the fs-pulse PW use laser using fs pulses identical to of the PW laser. Short of this ideal testing would be tests over portions of the HR band of the BBHR coating using ns or sub-ps pulses produced by tunable lasers. Such tests could be over ∼ 10 nm wide wavelength intervals whose center wavelengths could be tuned over the BBHR coating's operational band. Alternatively, the HR band of the BBHR coating could be adjusted by means of wavelength shifts due to changing the AOI of the LIDT tests or due to absorbed moisture by the coating under ambient conditions. We conduct LIDT tests on the BBHR coatings at selected AOIs to gain insight into the coatings' laser damage properties, and analyze how the results of the different LIDT tests compare.
No preview · Article · Jan 2015 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: Advanced z-pinch accelerators require precise timing of multiple mega-ampere drivers to deliver terawatt power. The triggering of these drivers is now largely initiated by laser ionization of gas switches. In this paper, we discuss detailed fully kinetic simulation of the Z laser-triggered gas switch involving detailed finite-difference time-domain particle-in-cell Monte Carlo modeling of the trigger section of the switch. Other components of the accelerator from the Marx bank through the pulse-forming line are described as circuit elements. The simulations presented here build on a recently developed model of electro-negative gas breakdown and streamer propagation that included photons produced from de-excited neutrals. New effects include multi-photon ionization of the gas in a prescribed laser field. The simulations show the sensitivity of triggering to laser parameters including focal plane within the anode-cathode gap of the trigger section of the switch, intensity at focus, and laser pulse length. Detailed electromagnetic simulations of the trigger section with circuit modeling of the upstream and downstream components are largely in agreement with Z data and demonstrate a new capability.
No preview · Article · Jun 2013 · Physics of Plasmas
[Show abstract][Hide abstract] ABSTRACT: We present the results of 2D and 3D fully-kinetic electromagnetic particle-in-cell Monte Carlo (PICMC) simulations of triggered three-electrode gas switches using dry air as a gas (at pressures greater than 1 atm). In such switches the AK gap voltage is set slightly below the breakdown threshold. A voltage pulse applied to a trigger needle placed in the AK gap allows breakdown to occur between, initially, the trigger and anode, followed by the trigger and cathode. We demonstrate that a fully-kinetic PICMC approach can be used to follow the entire evolution of the switch, from the initial avalanche and streamer formation up to the fully conducting phase. We utilize an 18-species air chemistry model which is shown to agree with swarm parameters (breakdown threshold, drift velocity) obtained by experiment. Photon transport and photo-ionization are also included to permit the modeling of cathode directed streamers. This computational model will be used to help design closing switches for pulsed-power systems.
[Show abstract][Hide abstract] ABSTRACT: form only given. We present the results of 2D and 3D Fully kinetic electromagnetic particle-in-cell Monte-Carlo (PICMC) simulations of triggered three-electrode gas switches using dry air as a gas (at pressures greater than 1 AT M). In such switches the AK gap voltage is set slightly below the breakdown threshold. A voltage pulse applied to a trigger needle placed in the AK gap allows breakdown to occur between, first, the trigger and anode, followed by the trigger and cathode. We demonstrate that a fully kinetic PICMC approach can be used to follow the entire evolution of the switch, from the initial avalanche and streamer formation up to the fully conducting phase. We utilize an 18-species air chemistry model which is shown to agree with swarm parameters (breakdown threshold, drift velocity) obtained by experiment. Photon transport and photoionization are also included to permit the modeling of cathode directed streamers. This computational model will be used to help design closing switches for pulsed-power systems.
[Show abstract][Hide abstract] ABSTRACT: High current pulsed-power generators efficiently store and deliver magnetic energy to z-pinch targets. We review applications of magnetically driven implosions (MDIs) to inertial confinement fusion. Previous research on MDIs of wire-array z-pinches for radiation-driven indirect-drive target designs is summarized. Indirect-drive designs are compared with new targets that are imploded by direct application of magnetic pressure produced by the pulsed-power current pulse. We describe target design elements such as larger absorbed energy, magnetized and pre-heated fuel, and cryogenic fuel layers that may relax fusion requirements. These elements are embodied in the magnetized liner inertial fusion (MagLIF) concept [Slutz “Pulsed-power-driven cylindrical liner implosions of laser pre-heated fuel magnetized with an axial field,” Phys. Plasmas, 17, 056303 (2010), and Stephen A. Slutz and Roger A. Vesey, “High-Gain Magnetized Inertial Fusion,” Phys. Rev. Lett., 108, 025003 (2012)]. MagLIF is in the class of magneto-inertial fusion targets. In MagLIF, the large drive currents produce an azimuthal magnetic field that compresses cylindrical liners containing pre-heated and axially pre-magnetized fusion fuel. Scientific breakeven may be achievable on the Z facility with this concept. Simulations of MagLIF with deuterium-tritium fuel indicate that the fusion energy yield can exceed the energy invested in heating the fuel at a peak drive current of about 27 MA. Scientific breakeven does not require alpha particle self-heating and is therefore not equivalent to ignition. Capabilities to perform these experiments will be developed on Z starting in 2013. These simulations and predictions must be validated against a series of experiments over the next five years. Near-term experiments are planned at drive currents of 16 MA with D2 fuel. MagLIF increases the efficiency of coupling energy (=target absorbed energy/driver stored energy) to targe- s by 10-150X relative to indirect-drive targets. MagLIF also increases the absolute energy absorbed by the target by 10-50X relative to indirect-drive targets. These increases could lead to higher fusion gains and yields. Single-shot high yields are of great utility to national security missions. Higher efficiency and higher gains may also translate into more compelling (lower cost and complexity) fusion reactor designs. We will discuss the broad goals of the emerging research on the MagLIF concept and identify some of the challenges. We will also summarize advances in pulsed-power technology and pulsed-power driver architectures that double the efficiency of the driver.
Full-text · Article · Dec 2012 · IEEE Transactions on Plasma Science
[Show abstract][Hide abstract] ABSTRACT: We report on experiments with the Z-Petawatt laser at Sandia National
Labs using mm-sized foils and mass-limited targets of various
thicknesses. Rear side accelerated proton beam measurements, in
combination with simulation results, were used to infer hot electron
transport in presence of preplasma. Full-scale, 3D
radiation-hydrodynamics simulations of the ns to ps prepulse were
performed. Preplasma properties (density profiles, temperatures, charge
states) where then imported into a fully explicit and kinetic 2D
particle-in-cell code to simulate, 10 ps of the main laser pulse
interaction with the preplasma and target at full scale. A comparison of
experimental data and numerical data shows outstanding agreement in all
measured proton beam parameters, which gives confidence in the
simulation results of hot electron transport. Sandia National Labs is a
multi-program laboratory managed and operated by Sandia Corporation, a
wholly owned subsidiary of Lockheed Martin Corp., for the U.S.
Department of Energy's National Nuclear Security Administration under
[Show abstract][Hide abstract] ABSTRACT: Since October 2007 Sandia National Laboratories has operated the
refurbished Z machine at an improved load current of 26 MA yielding 400
TW of x-ray power. The current pulse shape to the load is controlled by
36 independently timed laser triggered gas switches. As part of the
refurbishment effort, a fiber coupled laser spark detector system has
been installed which is able to detect the laser generated plasma in
situ inside the trigger section of the high voltage switch. In this
paper we describe how this detection system can be used to characterize
the discharge dynamics of these 5.9 MV, 820 kA switches.
Preview · Article · Jul 2012 · Review of Modern Physics
[Show abstract][Hide abstract] ABSTRACT: A wavefront sensor has been used to measure the Kerr nonlinear focal shift of a high intensity ultrashort pulse beam in a focusing beam geometry while accounting for the effects of plasma-defocusing. It is shown that plasma-defocusing plays a major role in the nonlinear focusing dynamics and that measurements of Kerr nonlinearity and ionization are coupled. Furthermore, this coupled effect leads to a novel way that measures the laser ionization rates in air under atmospheric conditions as well as Kerr nonlinearity. The measured nonlinear index n₂ compares well with values found in the literature and the measured ionization rates could be successfully benchmarked to the model developed by Perelomov, Popov, and Terentev (PPT model) [Sov. Phys. JETP 50, 1393 (1966)].
[Show abstract][Hide abstract] ABSTRACT: Experiments investigating fracture and resistance to plastic deformation at fast strain rates (>106 s−1) were performed via laser ablation on thin sheets of aluminum and aluminum alloys. Single crystal high purity aluminum (Al-HP) and a single crystal 1100 series aluminum alloy (AA1100) were prepared to investigate the role of impurity particles. Specimens of aluminum alloy +3 wt. % Mg (Al+3Mg) at three different grain sizes were also studied to determine the effect of grain size. In the present experiments, high purity aluminum (Al-HP) exhibited the highest spall strength over 1100 series aluminum alloy (AA1100) and Al+3Mg. Fracture characterization and particle analysis revealed that fracture was initiated in the presence of particles associated with impurity content in the AA1100 and at both grain boundaries and particles in Al+3Mg. The Al+3Mg specimens exhibited the greatest resistance to plastic deformation likely resulting from the presence of magnesium atoms. The Al-HP and AA1100, both lacking a strengthening element such as Mg, were found to have the same Hugoniot elastic limit (HEL) stress. Within the single crystal specimens, orientation effects on spall strength and HEL stress appear to be negligible. Although the fracture character shows a trend with grain size, no clear dependence of spall strength and HEL stress on grain size was measured for the Al+3Mg. Hydrodynamic simulations show how various strength and fracture models are insufficient to predict material behavior at fast strain rates, and a revised set of Tuler-Butcher coefficients for spall are proposed.
No preview · Article · Nov 2011 · Journal of Applied Physics
[Show abstract][Hide abstract] ABSTRACT: Many high energy/high power lasers such as Z-Petawatt at Sandia National
Laboratories utilize extremely heavy and sensitive optical assemblies
for final focusing. Redirecting the beams is very difficult if not
impossible, and setups also often require long focal lengths, which may
compromise the pointing stability. We suggest that the application of
plasma mirrors can be very useful for HEDP experiments with such
systems, regardless of the contrast enhancing feature (e.g. using a high
reflector as plasma mirror substrate). Applications of plasma mirrors
can be deflection of the beam close to the target for more convenient
experimental geometries or debris mitigation for the last large sized
optics. In case of the more advanced concept of ellipsoidal geometries,
plasma mirrors can even be used for f# translation, leading to focus
intensity enhancement and improved pointing stability (if f# is
reduced). This presentation will explain applications along with laser
requirements and performance challenges. -- *Sandia National Labs is a
multi-program laboratory managed and operated by Sandia Corporation, a
wholly owned subsidiary of Lockheed Martin Corp., for the U.S.
Department of Energy's National Nuclear Security Administration under
[Show abstract][Hide abstract] ABSTRACT: Streamer and leader formation in high pressure devices is dynamic process involving a broad range of physical phenomena. These include elastic and inelastic particle collisions in the gas, radiation generation, transport and absorption, and electrode interactions. Accurate modeling of these physical processes is essential for a number of applications, including high-current, laser-triggered gas switches. Towards this end, we present a new 3D implicit particle-in-cell simulation model of gas breakdown leading to streamer formation in electronegative gases. The model uses a Monte Carlo treatment for all particle interactions and includes discrete photon generation, transport, and absorption for ultra-violet and soft x-ray radiation. Central to the realization of this fully kinetic particle treatment is an algorithm that manages the total particle count by species while preserving the local momentum distribution functions and conserving charge [D. R. Welch, T. C. Genoni, R. E. Clark, and D. V. Rose, J. Comput. Phys. 227, 143 (2007)]. The simulation model is fully electromagnetic, making it capable of following, for example, the evolution of a gas switch from the point of laser-induced localized breakdown of the gas between electrodes through the successive stages of streamer propagation, initial electrode current connection, and high-current conduction channel evolution, where self-magnetic field effects are likely to be important. We describe the model details and underlying assumptions used and present sample results from 3D simulations of streamer formation and propagation in SF6.
[Show abstract][Hide abstract] ABSTRACT: We discuss upgrades currently underway at Sandia National Labs Z-Backlighter facility. Among them: A new OPCPA front end, 94 cm × 42 cm MLD gratings, laser beam combination studies, advanced debris mitigation techniques, and a major target area expansion.
[Show abstract][Hide abstract] ABSTRACT: We discuss upgrades and development currently underway at the Z-Backlighter facility. Among them are a new optical parametric chirped pulse amplier (OPCPA) front end, 94 cm 42 cm multi layer dielectric (MLD) gratings, dichroic laser beam transport studies, 25 keV x-ray source development, and a major target area expansion. These upgrades will pave the way for short/long pulse, multi-frame, multi-color x-ray backlighting at the Z-Accelerator.
No preview · Article · May 2011 · Proceedings of SPIE - The International Society for Optical Engineering
[Show abstract][Hide abstract] ABSTRACT: Experiments dedicated to the characterization of plasma mirrors with a high energy, single shot short-pulse laser were performed at the 100 TW target area of the Z-Backlighter Facility at Sandia National Laboratories. A suite of beam diagnostics was used to characterize a high energy laser pulse with a large aperture through focus imaging setup. By varying the fluence on the plasma mirror around the plasma ignition threshold, critical performance parameters were determined and a more detailed understanding of the way in which a plasma mirror works could be deduced. It was found, that very subtle variations in the laser near field profile will have strong effects on the reflected pulse if the maximum fluence on the plasma mirror approaches the plasma ignition threshold.
No preview · Article · May 2011 · The Review of scientific instruments
[Show abstract][Hide abstract] ABSTRACT: We compare designs and laser-induced damage thresholds (LIDTs) of hafnia/silica antireflection (AR) coatings for 1054 nm or dual 527 nm/1054 nm wavelengths and 0° to 45° angles of incidence (AOIs). For a 527 nm/1054 nm, 0° AOI AR coating, LIDTs from three runs arbitrarily selected over three years are ∼20 J/cm2 or higher at 1054 nm and <10 J/cm2 at 527 nm. Calculated optical electric field intensities within the coating show two intensity peaks for 527 nm but not for 1054 nm, correlating with the lower (higher) LIDTs at 527 nm (1054 nm). For 1054 nm AR coatings at 45° and 32° AOIs and S and P polarizations (Spol and Ppol), LIDTs are high for Spol (>35 J/cm2) but not as high for Ppol (>30 J/cm2 at 32° AOI; ∼15 J/cm2 at 45° AOI). Field intensities show that Ppol discontinuities at media interfaces correlate with the lower Ppol LIDTs at these AOIs. For Side 1 and Side 2 dual 527 nm/1054 nm AR coatings of a diagnostic beam splitter at 22.5° AOI, Spol and Ppol LIDTs (>10 J/cm2 at 527 nm; >35 J/cm2 at 1054 nm) are consistent with Spol and Ppol intensity behaviors.
[Show abstract][Hide abstract] ABSTRACT: Laser-accelerated proton beams can be used in a variety of applications, e.g. ultrafast radiography of dense objects or strong electromagnetic fields. Therefore high energies of tens of MeV are required. We report on proton-acceleration experiments with a 150 TW laser system using mm-sized thin foils and mass-reduced targets of various thicknesses. Thin- foil targets yielded maximum energies of 50 MeV. A further reduction of the target dimensions from mm-size to 250 x250x25 microns increased the maximum proton energy to >65 MeV, which is comparable to proton energies measured only at higher-energy, Petawatt-class laser systems. The dependence of the maximum energy on target dimensions was investigated, and differences between mm-sized thin foils and mass-reduced targets will be reported.
[Show abstract][Hide abstract] ABSTRACT: To extend the backlighting capabilities for Sandia's Z- Accelerator, Z-Petawatt, a laser which can provide laser pulses of 500 fs length and up to 120 J (100TW target area) or up to 450 J (Z / Petawatt target area) has been built over the last years. The main mission of this facility focuses on the generation of high energy X-rays, such as tin Kalpha at 25 keV in ultra-short bursts. Achieving 25 keV radiographs with decent resolution and contrast required addressing multiple problems such as blocking of hot electrons, minimization of the source, development of suitable filters, and optimization of laser intensity. Due to the violent environment inside of Z, an additional very challenging task is finding massive debris and radiation protection measures without losing the functionality of the backlighting system. We will present the first experiments on 25 keV backlighting including an analysis of image quality and X-ray efficiency.