Stilbene Array for Portable Neutron Imaging
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46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
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Results for “pulse shaping”
Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.
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High-Z Nanocomposites for Fast and Radiation Hard Scintillation
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73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
Temporal shaping of narrow-band picosecond pulses via noncolinear sum-frequency mixing of dispersion-controlled pulses
Not Available
43 PARTICLE ACCELERATORS↗
Spatio-temporally shaped deep UV laser source for ultrabright photocathodes using novel upconversion techniques (Final Report)
The goal of this project was to take a VUV laser system designed by Kapteyn-Murnane Labs (KMLabs)—the Hyperion VUV, and increase its functionality, efficiency, and applicability by further probing the fundamental physics taking place in the system and optimizing the system performance. More specifically, DOE accelerator applications—particularly for XFELs that require low-emittance electron beams, require precisely controlled ultrashort pulses in the deep-UV at 250-260 nm to drive the photocathode electron source. This is conventionally done using an ultrafast infrared laser, with subsequent stages of nonlinear upconversion in solid-state nonlinear-optical crystal media (principally BBO, which is commonly used for UV upconversion). However, maintaining high spatial mode beam quality, as well as precisely controlled temporal profile, has proven a persistent challenge. As a possible alternative to this approach, KMLabs has demonstrated a system that generates vacuum ultraviolet light (VUV) by upconverting pulses from an infrared ultrafast laser in a gas-filled hollow waveguide, driving a process of highly cascaded harmonic generation (HCHG). Because HCHG upconversion is done in a gas, rather than a solid, at high intensity in a guided-wave geometry, it retains an excellent beam spatial profile as well as a large spectral bandwidth that allows for control over the temperal structure of the pulse. A challenge, however, is in energy scaling of this HCHG technique to the ~1-20 μJ pulses required for photocathode applications. To-date, the pulse energies used in published HCHG work are sub-10-nJ. This project proposed to explore ways to generate higher-energy pulses using HCHG, as well as to investigate spectral and temporal shaping of the deep UV pulses by pulse shaping the infrared laser used to seed the process. Although we showed that our approach works for generating light at 259 nm, the total pulse energy achieved seems limited to approximately the same value as past work. However, the techniques we investigated may well have benefits for extending this source to much shorter wavelength output.
43 PARTICLE ACCELERATORS↗
Novel Hot-Spot Ignition Designs for Inertial Confinement Fusion with Liquid-Deuterium-Tritium Spheres
A new class of ignition designs is proposed for inertial confinement fusion (ICF) experiments. These designs are based on the hot-spot-ignition approach, but instead of conventional targets that comprise of spherical shells with thin frozen deuterium{tritium (DT) layers, homogeneous-density (solid or liquid) DT spheres are used where the lower-density central region and higher-density shell are created dynamically by appropriately shaping the laser pulse. These offer several advantages, including simplicity in target production and lower sensitivity to both laser imprint and physics uncertainty in shock interaction with the ice-vapor interface. The design evolution starts by launching an ~1-Mbar shock into a homogeneous DT sphere. After bouncing from the center, the reflected shock reaches the outer surface of the sphere and the shocked material starts to expand outward until its pressure drops below the ablation pressure. At this point, an adjustment shock is launched inward by supporting ablation pressure. This shock compresses the ablator and fuel, forming a shell. The shell is then accelerated and compressed by appropriately shaping the drive laser pulse, similar to the conventional thin-shell, hot-spot designs. Here, this paper demonstrates the feasibility of the new concept using hydrodynamic simulations and discusses the advantages and disadvantages of the concept compared with more-traditional ICF designs.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Drilling of sub-100 μm hourglass-shaped holes in diamond with femtosecond laser pulses
We present images of microholes drilled in diamond using a homemade femtosecond Yb : KGW laser. We use a femtosecond laser source emitting pulses with a duration of 230 fs at a wavelength of 1030 nm, the focusing spot size amounting to 8.9 μm. The effect of the pulse energy and the number of pulses on the microhole geometry (hole diameter, circularity, taper angle, and drilling quality) is evaluated. The obtained results demonstrate the feasibility of drilling of hourglass-shaped holes in a diamond sample, which have similar diameters at the hole entrance (92 μm) and exit (95 μm), but a much smaller diameter (28 μm) at a certain waist section inside the hole. (paper)
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Neural network-based control of an ultrafast laser
With the recent advances in machine learning (ML) and data science (DS), the control, modeling, and analysis of these complex systems continues to improve. In this work, we report on the optimization of the intensity of a femtosecond laser using feedforward neural networks (FFNN) that model the input–output relationships of the data. The input parameters of the system were optimized to achieve the required performance of the femtosecond laser. We propose a neural network-based control system to model the relationship between the spectral amplitude and phase of the input laser pulse at the amplifier input and the shape of the output pulse. Low-jitter laser parameter inputs and the resulting laser pulse duration were modeled, and the resulting correlation between the input and output data was used to optimize the laser pulse. Here, we demonstrate improved processing and laser control performance.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Energy density and spectrum of single-cycle and sub-cycle electromagnetic pulses
Based on the exact solution of the Maxwell equations in the form of a collapsing spherical vector wave specified by an arbitrary function of time, we have calculated the maximum energy density that can be achieved when focusing extremely short pulses of various shapes. It is shown that our earlier formula expressing the maximum energy density in terms of the spectrum parameters for Gaussian quasi-monochromatic pulses is approximately (with an accuracy of ±40 %) valid for the main types of extremely short pulses. (extremely short laser pulses)
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Observation of attosecond electron dynamics in the photoelectron momentum distribution of atoms using few-cycle laser pulses
The method of time-resolved measurement with ultrashort laser pulses is vital to the development of attosecond science. Pump-probe measurements using a train of attosecond pulses in combination with a near-infrared (NIR) multicycle driving laser have been successful in capturing the intercycle electron dynamics which repeats every optical cycle and leads to above-threshold ionization (ATI) spectra in the frequency domain. Here in this work, we study the effect of a carrier-envelope phase (CEP) in a few-cycle (< 6 fs) NIR laser pulse on the photoelectron momentum distribution (PMD) of a hydrogen atom and show that interference patterns in a PMD change dramatically with CEPs in the few-cycle regime. When the few-cycle driving laser pulse has a sine shape, the double-slit interference with characteristic modulation of ATI peaks dominates the PMD. On the other hand, when the driving pulse has a cosine shape, the holographical interference featured by a spider-like pattern is isolated. Our results suggest that the CEP-stable few-cycle laser pulses can be used to identify different types of intracycle interference structures in a PMD which reveal the underlying subcycle electron dynamics on an attosecond timescale.
74 ATOMIC AND MOLECULAR PHYSICS↗
Time-walk and jitter correction in SNSPDs at high count rates
Superconducting nanowire single-photon detectors (SNSPDs) are a leading detector type for time correlated single photon counting, especially in the near-infrared. When operated at high count rates, SNSPDs exhibit increased timing jitter caused by internal device properties and features of the RF amplification chain. Variations in RF pulse height and shape lead to variations in the latency of timing measurements. To compensate for this, we demonstrate a calibration method that correlates delays in detection events with the time elapsed between pulses. The increase in jitter at high rates can be largely canceled in software by applying corrections derived from the calibration process. We demonstrate our method with a single-pixel tungsten silicide SNSPD and show it decreases high count rate jitter. The technique is especially effective at removing a long tail that appears in the instrument response function at high count rates. At a count rate of 11.4 MCounts/s, we reduce the full width at 1% maximum level (FW1%M) by 45%. The method, therefore, enables certain quantum communication protocols that are rate-limited by the FW1%M metric to operate almost twice as fast.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Light field–controlled PHz currents in intrinsic metals
Oriented electric currents in metals are routinely driven by applying an external electric potential. Although the response of electrons to the external electric fields occurs within attoseconds, conventional electronics do not use this speed potential. Ultrashort laser pulses with controlled shapes of electric fields that switch direction at petahertz frequencies open perspectives for driving currents in metals. Light field–driven currents were demonstrated in various media including dielectrics, semiconductors, and topological insulators. Now, our research question is whether we can drive and control orders of magnitude more charge carriers in metals enabling ultrafast switching with practically low-energy, picojoule-level pulses. Here, we demonstrate the interaction of light with nanometer-thick metallic layers, which leads to a generation of light field–controlled electric currents. We show that the implantation of metallic layers into a dielectric matrix leads to up to 40 times increase of the sensitivity in contrast to a bare dielectric, decreasing the intensity threshold for lightwave electronics.
Fehér, Beatrix [HUN-REN Wigner Research Centre for↗
Development and validation of a gamma-ray detection response model for neutron-diagnosed subcritical experiments
In a neutron-diagnosed subcritical experiment, a highly sensitive and fast detector is required to accurately determine the true fission γ-ray emission rate from an object composed of special nuclear material under interrogation, in addition to a proper detector response model for use in forward modeling. We present the development of a detector response model that is based in GEANT4 and uses post-processing techniques to include photomultiplier tube time and pulse height response effects. We also define a set of detector performance evaluation metrics that emphasized our experimental objectives. A suite of time and pulse height response measurements for several diverse detector assemblies are described. These assemblies were simulated with this response model and benchmarked against the laboratory measurements. This response model reproduces the pulse height curve shapes and performance hierarchy of the measured detectors well. Additionally, the simulated time response curves exhibit reasonable agreement with counterpart measurements. These measurements and simulation results together suggest that a large volume EJ-299-49 plastic scintillator that is mated to an ultraviolet transmitting plastic light guide and relies on total internal reflection for light transport to the photocathode will provide significantly faster timing, increase pulse height by a factor of 3, and require 25% less γ rays to meet experimental needs, compared to the candidate detector design developed prior to the availability of this response model. Application of this response model and associated performance metrics in the context of large-scale design sensitivity studies is also discussed.
46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
Optical shock-enhanced self-photon acceleration
Photon accelerators can spectrally broaden laser pulses with high efficiency in moving electron density gradients driven in a rapidly ionizing plasma. When driven by a conventional laser pulse, the group velocity walk-off experienced by the accelerated photons and deterioration of the gradient from diffraction and plasma-refraction limit the extent of spectral broadening. In this work, we show that a laser pulse with a shaped space-time and transverse intensity profile overcomes these limitations by creating a guiding density pro le at a tunable velocity. Self-photon acceleration in this profile leads to dramatic spectral broadening and intensity steepening, forming an optical shock that further enhances the rate of spectral broadening. In this new regime, multi-octave spectra extending from 400nm to 60nm wavelengths, which support near-transform limited < 400 as pulses, are generated over < 100 μm of interaction length.
74 ATOMIC AND MOLECULAR PHYSICS↗
Collaborative Research Proposal: Time Resolved Optical Emissions Spectroscopy and Laser Induced Fluorescence Spectroscopy of Nanosecond Pulsed Discharges in a Gas-Liquid Water Film Reactor
Electrical discharge plasma formed in contact with liquid water is of interest for a wide range of applications in chemical, biomedical, agricultural, electrical, and materials science and engineering. Such plasma reactors are of very timely importance since they also have significant disinfection capability by inactivating bacteria, viruses and other pathogens. Many types of plasma sources including those driven by AC, DC, RF, microwave, and pulsed electrical power supplies coupled to a wide range of different electrode configurations and reactor designs have been developed and explored which contact the plasma with liquid water. Recent roadmaps have recommended that further work is needed to develop our understanding of the fundamental chemical and physical process which occur at the interface of non-thermal plasma with liquid water solutions in order to advance this large diversity of applications which ultimately depend upon efficient production of key reactive chemical species. There is a wide range of interacting factors that affect the chemical reactions that occur in the plasma, in the liquid phase, and at the interface. These factors ultimately govern the key reactive chemical species formed and used in the various applications and they include a) the reactor design and input parameters, b) the discharge and transport processes, c) the plasma properties, and d) the resulting chemical reactions. For pulsed discharges, the power supply design and output parameters control the applied voltage, frequency, rise time, and width (duration) of the applied pulses. The reactor design involves specification of the gas-liquid contacting methods, electrode gap distance, reactor volume and shape, and gas and liquid flow rates achievable. The gas and liquid compositions as well as the liquid properties such as pH and conductivity are also of key importance in determination of the resulting chemical reactions. In addition, the important plasma properties include plasma gas temperature, electron density, electron energy (distribution), and size of the plasma channels which are all affected by the discharge and transport properties. Many studies have focused on specific aspects of these various processes. Of particular important and relevance to the proposed work is the utilization of nanosecond pulses to generate plasma in gas-liquid systems, liquid bubbles, underwater, and in gases. Recent advances in nanosecond pulses provide significant advantages in utilization with liquid water. For example, fast rise time and short pulses are less sensitive to water conductivity, such short pulses may provide advantages in fast temporal quenching of the plasma, and fundamental analysis of pulse properties, including pulse shape and width, may be facilitated by investigation of single filamentary fast pulses. Many studies have also dealt with the role of the gas composition on formation of reactive oxygen species (ROS) (i.e., hydroxyl radicals – ·OH, hydrogen peroxide – H 2 O 2 , various atomic oxygen species, ozone-O 3 , hydroperoxyl radicals HO 2 ·) and reactive nitrogen species (RNS) (i.e., nitrogen oxides – NO, NO 2 , N 2 O - collectively termed NO X , nitrite-NO 2 - , nitrate-NO 3 - , peroxynitrite-ONOO - ). The hydroxyl radical is the critical species in many chemical oxidation reactions for chemical degradation of toxic compounds in water and gases and for synthesis of some compounds. The mixture of various nitrogen oxide species is important for many biochemical and biological processes involved in biomedical and agricultural applications including disinfection and fertilizer production. The present proposal focuses on determination of the effects of time resolved electron density and hydroxyl radicals on plasma chemical reactions through collaboration with the Princeton Collaborative Low Temperature Plasma Research Facility (PCRF) at the Princeton Plasma Physics Laboratory (PPPL). In order to further investigate the role of the plasma generated electrons and hydroxyl radicals on the overall formation of the key species including hydrogen peroxide, hydroxyl radicals, and nitrogen oxides, the proposed work, thus seeks to determine high resolution time resolved electron density by optical emissions spectroscopy and time resolved hydroxyl radicals using laser induced fluorescence in the nanosecond discharge reactor. The combination of data on electron density and hydroxyl radical concentration will be utilized in the present work to more fully characterize the chemical reaction processes in this system and to advance the design, development, and operation of such chemical reactors for a wide range of applications.
70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
Ultrafast control of the LnF + /LnO + ratio from Ln(hfac) 3
Shaped fs laser pulses control the fragmentation products of a lanthanide complex.
Chemistry↗
Laser-plasma acceleration beyond wave breaking
Laser wakefield accelerators rely on the extremely high electric fields of nonlinear plasma waves to trap and accelerate electrons to relativistic energies over short distances. When driven strongly enough, plasma waves break, trapping a large population of the background electrons that support their motion. Aside from limiting the maximum electric field, this trapping can lead to accelerated electron bunches with large energy spreads. Here we introduce a novel regime of plasma wave excitation and wakefield acceleration that allows for arbitrarily high electric fields while avoiding the deleterious effects of unwanted trapping. Here, the regime, enabled by spatiotemporal shaping of laser pulses, exploits the property that nonlinear plasma waves with superluminal phase velocities cannot trap charged particles and are therefore immune to wave breaking.
36 MATERIALS SCIENCE↗
Real-time observation of multi-electron processes in atoms and diatomic molecules
We report on the implementation of a reliable experimental approach to observe, in real-time, electron dynamics in atoms and molecules at the attosecond time scale. The experimental approach combines a source of shape-controlled attosecond pulses, a stabilized XUV/IR interferometer for time-resolved measurements, and a multi-particle imaging system (Coltrims). In addition, we report new capabilities to control electron dynamics in quantum systems.
74 ATOMIC AND MOLECULAR PHYSICS↗
FORTE Measurements of Global Optical Lightning Waveforms and Implications for Optical Lightning Detection
Lightning processes generate a diverse collection of optical pulses from current traversing the lightning channels. These signals are then broadened spatially and temporally via scattering in the clouds. The resulting waveforms measured from space with instruments like the photodiode detector (PDD) on the Fast On-orbit Recording of Transient Events (FORTE) satellite have a variety of shapes. In this study, we use coincident optical and Radio-Frequency measurements to document the properties of optical PDD waveforms associated with different types of lightning, estimate delays from scattering in the clouds, and comment on how pulse shape impacts optical lightning detection. We find that the attributes of optical pulses recorded by the PDD are consistent with prior studies, but vary globally and with event amplitude. The most powerful lightning tends to be single-peaked with faster rise times (median: ~100 µs) and shorter effective widths (median: ~400 µs) than normal lightning. Particularly dim events, meanwhile, include cases of broad optical waveforms with sustained optical emission throughout the PDD record, which the pixelated FORTE Lightning Location System (LLS) instrument has difficulty detecting. We propose that this is due to the optical signal being divided between individual LLS pixels that are each, individually, not bright enough to trigger. We use PDD waveforms and Monte Carlo radiative transfer modeling to demonstrate that increasing the temporal and spatial resolution of a pixelated lightning imager will make it more difficult to detect these broad/dim pulses as their energy becomes divided between additional pixels/integration frames.
54 ENVIRONMENTAL SCIENCES↗