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At least 883 records · Page 49
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Tunnelling approximation for estimating the amplitude of high harmonic generation in intense laser fields: analysis of ionisation and recombination times

We consider in the low-frequency approximation the quasi-classical equations for the ionisation and recombination times that arise in the analysis of high harmonic generation in the quasi-classical approximation. Based on the assumption that the ratio of the time of the subbarrier motion of an atomic electron, Δ t ′, to the characteristic period of the laser pulse, T , is small, we have constructed a formal perturbation theory with respect to the parameter Δ t ′/ T to calculate the ionisation and recombination times. The accuracy of the constructed perturbation theory is analysed. Using the approach developed, we suggest extending the classical theory of rescattering for linear polarisation to the case of fields with two spatial components. (paper)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2025

Parametric raytracing modeling for NSTX-U scenario development with high harmonic fast waves and neutral beam injection

High harmonic fast waves (HHFW) are a versatile heating and current drive tool for scenario development. Extensive modeling scans were performed to find optimal parameters for different uses of HHFW in National Spherical Tokamak Experiment (NSTX-U). Scans of plasma density, temperature, magnetic field, and antenna phasing were performed both with and without neutral beam injection. For speed of calculation, the ray-tracing code GENRAY coupled to the quasilinear Fokker–Planck code CQL3D was used. CQL3D allows for a more accurate description of the fast ion population, as well as for quasilinear effects such as HHFW-induced modifications of the distribution function. Best current drive results are obtained at elevated electron temperatures and with the lowest k φ phasing. Adding neutral beams however typically strongly reduces the HHFW current drive efficiency at the low density cases due to HHFW absorption on beam ions. Results of this parametric study will feed into scenario development and predict-first whole-shot modeling of NSTX-U discharges.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Direct reconstruction of isolated XUV or soft x-ray attosecond pulses from high-harmonic generation streaking spectra

Characterization of an isolated attosecond pulse (IAP) in the extreme ultraviolet (XUV) or soft x-ray (SXR) region is essential for its applications. Here we propose to retrieve an IAP in the time domain directly through the modulation of high-harmonic generation (HHG) spectra in the presence of a time-delayed intense few-cycle infrared or mid-infrared laser. The retrieval algorithm is derived based on the strong-field approximation and an extended quantitative rescattering model. We show that both isolated XUV pulses with a narrow spectral bandwidth and isolated SXR pulses with a broad bandwidth can be well characterized through the HHG streaking spectra. Such an all-optical method for characterizing the IAP differs from the commonly used approach based on the streaked photoelectron spectra that would require electron spectrometers. We check the robustness of the retrieval method by changing the dressing laser or by adjusting the steps of time delay. We also show that the XUV pulse can be accurately retrieved by treating the HHG streaking spectra calculated from solving the time-dependent Schrödinger equation for single atoms as the ‘experimental’ data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

ELM-free and inter-ELM divertor heat flux broadening induced by edge harmonics oscillation in NSTX

A new n =1 dominated edge harmonic oscillation (EHO) has been found in NSTX. The new EHO, rotating toroidally in the counter-current direction and the opposite direction of the neutral beam, was observed during certain inter-ELM and ELM-free periods of H-mode operation. This EHO is associated with a significant broadening of the integral heat flux width (${{\lambda}_{\operatorname{int}}}$ ) by up to 150%, and a decrease in the divertor peak heat flux by >60%. An EHO induced filament was also observed by the gas puff imaging diagnostic. The toroidal rotating filaments could change the edge magnetic topology resulting in toroidal rotating strike point splitting and heat flux broadening. Finally, experimental result of the counter current rotation of strike points splitting is consistent with the counter-current EHO.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Radial index of Laguerre-Gaussian modes in high-order-harmonic generation

High-order-harmonic generation (HHG) is a tabletop and tunable source of extreme ultraviolet (XUV) light. Its flexibility was lately evidenced by the production of Laguerre-Gaussian (LG) modes in the XUV with a known azimuthal index. Here we investigate the role of the radial index of LG modes in HHG. We show both numerically and experimentally that the mode content of the emitted XUV radiation can be tuned by controlling the weight of the different quantum trajectories involved in the process. Furthermore, the appearance of high-order radial modes is finally linked to the atomic dipole phase of HHG. These results extend the capabilities of shaping the spatial mode of ultrashort XUV pulses of light.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Dispersion of the Nonlinear Susceptibility of MoS2 and WS2 from Second-Harmonic Scattering Spectroscopy

Dispersion of the absolute second-order susceptibility of both MoS2 and WS2 is assessed on a wide spectral excitation range (710–1300 nm) by using second-harmonic scattering spectroscopy (SHS). SHS is an accurate ensemble measurement here applied on well-dispersed suspensions of monodisperse liquid-exfoliated nanosheets showing a high monolayer content. The as-derived, high susceptibility values shed light on the discrepancies between available literature values while evidencing resonances associated with the main excitonic transitions.

CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SU↗
DOE OSTI2023

Hidden anomalies in topological t – PtBi 2 – x probed by second harmonic generation

The noncentrosymmetric Weyl semimetal PtBi 2–x (t-PtBi 2–x ) exhibits various interesting technologically important physical properties. We report the experimental investigation of PtBi 1.6 via second harmonic generation (SHG), single-crystal x-ray diffraction, magnetic susceptibility, and electrical resistivity measurements. While bulk structural, magnetic, and electrical properties show no phase transitions below room temperature, the temperature dependence of the SHG intensity reveals two anomalies: one at T* ~ 60 K and another at T x ~ 200 K. Quantitative analysis indicates that the SHG signal results from both the buckled Bi1 surface termination with the 3m symmetry and flat Bi2 surface termination with the m symmetry. However, the anomalies are mainly driven by Bi1 on the surface: (1) T* marks the onset of surface states which is also manifested in the c-axis resistivity drop and (2) T x corresponds to the lowest thermal contraction of the structure and enhanced magnetic susceptibility. Furthermore, this study demonstrates that SHG is a powerful technique for probing surface properties even for noncentrosymmetric materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy

In this paper, we report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of ${\mathrm{N}}_{2}$ in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength $1.3\text{ }\text{ }{\mu}\mathrm{m}$ driving laser to isolate the HOMO in the near threshold region, 19–67 eV. Finally, the precision of our group delay measurements reveals previously unseen angle-resolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2020

Electron-Nanobunch-Width-Dominated Spectral Power Law for Relativistic Harmonic Generation from Ultrathin Foils

Relativistic high-order harmonic generation from solid-density plasma offers a compact source of coherent ultraviolet and x-ray light. For solid targets much thinner than the laser wavelength, the plasma thickness can be tuned to increase conversion efficiency; a reduction in total charge allows for balancing the laser and plasma driving forces, producing the most effective interaction. Unlike for semi-infinite plasma surfaces, in this study we find that for ultrathin foil targets the dominant factor in the emission spectral shape is the finite width of the electron nanobunches, leading to a power-law exponent of approximately 10/3. Ultrathin foils produce higher-efficiency frequency conversion than solid targets for moderately relativistic (1 0 <40) interactions and also provide unique insight into how the trajectories of individual electrons combine and interfere to generate reflected attosecond pulses.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2024

Insulating moiré homobilayers lack a threefold symmetric second-harmonic generation

Atoms within moiré bilayers relax in plane to minimize elastic energy; such relaxation brings their space group symmetries down to P1. Here, the ab initio second harmonic generation (SHG) of twisted and atomistically optimized hBN bilayers was determined at four twist angles (θ = 38.21°, 60.00°, 73.17°, and 98.21°) and for three displacements τ measured away from the ground state AA' configuration. All moiré bilayers have a P1 space symmetry after structural optimization. This situation is quite different to monolayers with hexagonal lattices, which retain a threefold symmetry. We point out that the actual symmetries of the SHG reported for hBN bilayers on two experimental works do not coincide with the sixfold symmetric theoretical profiles they provide [either sin 2 ⁡(3⁢Φ) or cos 2 ⁡(3⁢Φ)], and show that the intrinsic low structural symmetry of (atomically optimized) hBN bilayer moirés can in fact be read out from experimental SHG intensity profiles—which are tunable by θ and by the frequency of light ω: The SHG is most definitely not sixfold symmetric because moirés do not retain a threefold symmetry. Furthermore, an extrinsic twofold symmetry of the SHG emission is realized by tilting the pump by an angle α away from the 2D material's normal, regardless of θ and ω. Furthermore, the design of in-plane and ultrathin sources of SHG with low symmetry could be useful for the eventual creation of entanglement sources from 2D materials.

2-dimensional systems↗
DOE OSTI2019

A Classical Field Theory Formulation for the Numerical Solution of Time Harmonic Electromagnetic Fields

Finite element representations of Maxwell's equations pose unusual challenges inherent to the variational representation of the “curl-curl” equation for the fields. We present a variational formulation which solves for the four-potential instead, based on classical field theory. Borrowing from quantum electrodynamics, we modify the Lagrangian by adding an implicit gauge-fixing term. This reformulation explicitly accounts for Gauss’ law through the coupling between $\phi$ and $\rho$ , and enables the use of nodal basis functions instead of edge elements for time-harmonic problems. Here, we demonstrate how this formulation, adhering to the deeper underlying symmetries of the four-dimensional covariant field description, provides a highly general, robust numerical framework.

43 PARTICLE ACCELERATORS↗
DOE OSTI2018

Tunable, Ultrafast Fiber-Laser between 1.15 μm and 1.35 μm for Harmonic Generation Microscopy in Human Skin

We demonstrate a fiber-optic ultrafast tunable source for harmonic generation microscopy (HGM) in human skin. The source is based on a 31-MHz Er-fiber laser followed by self-phase modulation enabled spectral selection (SESS). The resulting pulses are tunable between 1.15 and 1.35 μm with up to >10-nJ pulse energy and ~100-fs pulse duration. Here we employ this source to drive a scanning microscope for HGM imaging of ex vivo human skin. A systematic investigation on imaging depth versus excitation wavelength reveals that excitation wavelengths in the 1.15-1.25 μm range exhibit low optical attenuation within the tissue and allows larger imaging depth for HGM in human skin. HGM driven by fiber-based SESS sources constitutes an enabling tool for noninvasive virtual skin biopsy in clinical applications.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2021

Test Results of a High-Gradient 2.856-GHz Negative Harmonic Accelerating Waveguide

We report the high-gradient tests results of a novel traveling wave accelerating structure for β=0.3 based on a novel approach of operating at the first negative spatial harmonic. Accelerating gradients of 50 MV/m and peak electric fields of 160 MV/m were achieved in a single structure consisting of 15 coupled cells during tests at the advanced photon source. This work was performed by RadiaBeam, in collaboration with the Argonne National Laboratory, as a part of a Research and Development Program for the development of an ultrahigh-gradient linear accelerator, the Advanced Compact Carbon Ion Linac, for hadron therapy.

42 ENGINEERING↗
DOE OSTI2019

A 220-$\mu$ W –83-dBm 5.8-GHz Third-Harmonic Passive Mixer-First LP-WUR for IEEE 802.11ba

A 40-nm CMOS IEEE 802.11ba low-power wake-up receiver (LP-WUR) is presented. It receives 802.11ba messages generated by an 802.11 orthogonal frequency-division multiplexing (OFDM) transmitter operating at 5.8 GHz. The power consumption of the RF front-end is minimized by removing active RF gain stages and using a third harmonic passive mixer with a ring-based local oscillator (LO) operating at one-third of the RF frequency. The noise figure is 14 dB, taking advantage of a 1:3 transformer that provides passive voltage gain in front of the high switch loss mixer-first RF front-end, and matching across 5.5-5.8 GHz with <; -12 dB S11. Here, the receiver achieves a sensitivity of -83-dBm and -20-dB adjacent channel signal-tointerference ratio (SIR) while consuming 220 μW at a bit error rate (BER) of 10-3 and data rate of 62.5 kb/s, which shows the best sensitivity-power tradeoff among >3-GHz receivers.

42 ENGINEERING↗
DOE OSTI2020

Filtered Backprojection in Compton Imaging using a Spherical Harmonic Wiener Filter with Pixelated CdZnTe

Filtered backprojection is an image reconstruction technique for Compton imaging that provides reasonably high resolution at much lower computational costs when compared to iterative methods. Here, this work applies a Wiener filter that has been derived for spherical harmonics on Compton imaging using the OrionUM pixelated CdZnTe imaging-spectrometer. To regularize the filter, an investigation is made into the power spectral density of the signal and noise to develop an appropriate spectral signal-to-noise ratio model for the restoration process. Experimental measurements were conducted with two 228Th sources placed 30° apart. The resulting filtered image of the two sources have an average full-width-at-half-maximum (FWHM) of 9.8° or 7.5° when using a mean squared error and structural similarity optimization approach respectively; an improvement from the 29.0° FWHM image when using simple backprojection.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2014

Two-dimensional equations of the surface harmonics method for solving problems of spatial neutron kinetics in square-lattice reactors

Two-dimensional time-dependent finite-difference equations of the surface harmonics method (SHM) for the description of the neutron transport are derived for square-lattice reactors. These equations are implemented in the SUHAM-TD code. Verification of the derived equations and the developed code are performed by the example of known test problems, and the potential and efficiency of the SHM as applied to the solution of the time-dependent neutron transport equation in the diffusion approximation in two-dimensional geometry are demonstrated. These results show the substantial advantage of SHM over direct finite-difference modeling in computational costs.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2014

Calculation of spin resonance harmonics in an accelerator with Snakes

In this work, the ASPIRRIN code has been extended to provide the calculation of resonance harmonics for the case of machine with Siberian Snakes and spin rotators. Examples are shown for RHIC accelerator, including the case of the configuration with six Snakes per RHIC ring.

43 PARTICLE ACCELERATORS↗