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At least 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Efficient and tunable blue light generation using lithium niobate nonlinear photonics

Thin-film lithium niobate (LN) has recently emerged as a playground for chip-scale nonlinear optics and leads to efficient frequency conversions from near-infrared to near-visible bands. For many nonlinear and quantum photonics applications, it is desirable to operate deep into the visible band within LN's transparency window. However, the strong material dispersion at short wavelengths makes phase-matching difficult, necessitating sub-micrometer scale control of domain structures for efficient phase-matching. Here, we report the operation of thin film LN in the blue wavelength and high fidelity poling of the thin-film LN waveguide to this regime. As a result, quasi-phase matching is realized between IR (871 nm) and blue (435.5 nm) wavelengths in a straight waveguide and prompts strong blue light generation with a conversion efficiency (1040% ± 140%/W). This blue second harmonic generator exhibits stable temperature tunability, which is important for applications that require precise frequency alignment, such as atomic clocks.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2022

Restored Passivation after Complete Removal of Front Poly-Si Between the Grid in Poly-Si/SiO2 Front/Back Cells

This work explores the use of wet chemical etching to completely remove the front poly-Si layer between grid lines of a front/back poly-Si/SiO2 passivated contact device. The purpose of the work is to provide excellent passivation under the metal grid, but to also maximize Jsc by eliminating optical absorption in the front poly-Si. We show that after a high temperature anneal to distribute carriers in the poly-Si/SiO2/c-Si interface all of the poly-Si can be removed selectively with TMAH, using the SiO2 as an etch-stop layer. The etched surface can be repassivated back to its original level with the addition of a SiNx/Al2O3 stack grown on the preserved tunnelling oxide layer followed by a short forming gas anneal. Using symmetric 100 - 200 nm intrinsically doped poly-Si/SiO2 on textured n-type wafer templates we show absolute device efficiency enhancement from 15% to 21% by increasing the Jsc by 12 mA/cm2 after removing the front poly-Si and repassivating while effectively preserving the Voc, and FF of the device. IQE values are similar to PERC devices in the short wavelength range.

etching↗
DOE OSTI2022

Theory of the magnetothermal instability in coronal plasma flows

The theory of the magnetothermal instability (MTI) is revisited through the lens of the stability of uniform systems. The linear stability analysis includes flow advection and Nernst transport. The instability criteria derived distinguish between the convective and the absolute nature of the perturbation growth. It is proven that, in the region where the Nernst and plasma blowoff velocities cancel, the MTI can be absolute and wave-packet perturbations grow in situ. This instability is mediated by the internal feedback between the Biermann battery and Righi–Leduc terms. Here, the analysis is extended to derive the dispersion relation for short-wavelength perturbations developing in nonuniform profiles with the application to coronal plasmas. It is found that the condition for MTI requires the net B-field convection velocity to be small at the isothermal sonic section, and the plasma conditions in this section govern the dynamics of the instability. Analysis of hydro-equivalent implosions suggests that unstable perturbations undergo more e-foldings of growth in larger-size targets.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

3D simulation of lower-hybrid drift waves in strong guide field asymmetric reconnection in laboratory experiments

The magnetic reconnection experiment has recently seen short wavelength ([Formula: see text]) lower-hybrid waves near the electron diffusion region in strong guide field reconnection. Based on plasma parameters from the experiment, we perform a three-dimensional fully kinetic simulation in order to investigate the generation of the lower-hybrid waves and their effects on the reconnection process. We find that the low-beta regions around the reconnection site are unstable to the lower-hybrid drift instability propagating in the outflow direction, driven by the difference between the electron and ion outflows. The waves modify the electron distributions, leading to periodic opening and closing of gaps in electron velocity space, and provide a small contribution to the anomalous resistivity. Finally, the simulation results are discussed in the context of space observations and laboratory experiments.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2023

Supersonically expanded sodium metal-dilute halogen gas interactions. The importance of reaction populated and energy storing reservoir states and population inversion created amplification in Na2

The reactions of Cl, Br, and I with Nan=2,3 produced in a supersonic expansion form Na2* and Na* excited states extending across the visible and ultraviolet regions. Emission in the region extending from 410 to 600 nm indicates selectively formed excited state Na2 emission features. Experimental evidence suggests that this emission is associated with Na3 + X reactions. Broadband (0.5 cm−1) laser measurements demonstrate gain (population inversion) for select features at∼524–528(1%), ∼492(0.3%), and ∼458.7–461(0.8%) nm. Single mode (0.007 cm−1) measurements extending from 528.03 to 527.63 nm demonstrate amplification involving five to six individual rovibronic levels with a maximum gain close to 3% recorded at 527.9 nm. The observed gain is associated with select transitions from levels of the Na2 11Πu state populated, via identified curve crossings, through collision induced transfer from long-lived Na2 21Σg+ and 11Πg reservoir states. Collision induced population buildup in the lowest vibrational levels of these reservoir states and collision induced transfer to the Na2 11Πu state create a population inversion in transitions to the X 1Σg+ state of Na2. The observed amplification is aided by rapid vibrational and rotational relaxation in both the Na2 ground and excited reservoir states producing amplifiers in the visible region like the HF amplifier in the infrared. This study suggests the importance of reaction populated and energy storing long-lived reservoir states in small sodium molecule combustion processes and indicates the potential for providing new short wavelength visible and ultraviolet amplifiers for future laser-based chemical propulsion concepts.

Chemistry↗
DOE OSTI2024

Localized phase contrast imaging at the Wendelstein 7-X stellarator

In its basic form, phase contrast imaging (PCI) provides line-integrated measurements of electron density fluctuations in plasmas. As turbulent fluctuations in magnetically confined plasmas have wave vectors almost perpendicular to the background magnetic field, the signals scattered by fluctuations from different parts of the PCI line-of-sight (LoS) are spatially separated in focal planes of the plasma. This allows localized PCI measurements by placing a mask in such a plane, to only permit signals from specific parts of the LoS to reach the PCI detectors. The present paper describes modeling and design of localization masks for the PCI system at the Wendelstein 7-X (W7-X) stellarator as well as the first results obtained using the masks in the recent long-pulse W7-X experimental campaign. During this project, we have extended the theory describing the mask response within the Fraunhofer diffraction model. As a novel development, we show from first principles that the mask response is determined by the fraction of power of the scattered beam spots that passes the mask. These insights have been used to select the W7-X mask design, consisting of a circular cutout, allowing the unscattered beam spot to pass the mask, with wedges covering a fixed angular range outside the central cutout. In the recent W7-X experimental campaign, the masks have verified the location of the main turbulence features observed by the PCI system and provided new information about the location of short-wavelength magnetohydrodynamic modes.

Physics↗
DOE OSTI2025

Finite ion temperature effects on electrostatic instabilities in partially magnetized plasmas

In this paper, the effects of warm ions on electrostatic, kinetic instabilities observed in partially magnetized plasmas, such as the electron cyclotron drift instability (ECDI) and modified two-stream instability (MTSI), are investigated. The kinetic dispersion relation that accounts for warm, non-magnetized ions and warm, magnetized electrons is solved using a complex root-finding technique [A. C. Denig and K. Hara, Phys. Plasmas 30, 032108 (2023)]. It is observed that finite ion temperature introduces ion Landau damping effects, decreasing the growth rate of most unstable modes. The high-frequency, short-wavelength ECDI modes are more strongly affected by ion Landau damping than the low-frequency, long-wavelength MTSI modes. The solutions of the kinetic dispersion relation for MTSI are compared with the fluid dispersion relation of MTSI, which was originally proposed by McBride et al. [Phys. Fluids 15 2367–2383 (1972)]. The differences in the dispersion characteristics between the kinetic and fluid MTSI limits are assessed, illustrating the effects of electron and ion Landau damping on kinetic instabilities in partially magnetized plasmas.

Denig, A. C. (ORCID:0000000253339572)↗
DOE OSTI2026

Wetted-foam target stability for laser-direct-drive inertial confinement fusion

Using highly resolved three-dimensional radiation-hydrodynamic simulations, the effects of wetted-foam (WF) ablators in direct-drive inertial confinement fusion cryogenic implosions on the OMEGA laser were investigated. The performance of these implosions was compared with that of implosions using targets with solid-plastic ablators. Simulations considering WF ablators as a homogeneous material show no mitigation of the effects of laser imprint, where the absence of this mitigation can be attributed to the use of picket pulses. Heterogeneous random-structure WF ablators introduce large short-wavelength perturbations, which are seeded by WF inhomogeneities. These perturbations exceed the perturbations seeded by the imprint and cause an additional reduction in implosion performance when the average pore size in the WF exceeds ~0.2 μm. Foam-seeded perturbations are expected to be less damaging in more stable high-adiabat (⁠ ≳ 5⁠) implosions. Here, the perturbations introduced by the WF ablators can be substantially reduced by using a foam matrix with a layered structure instead of a random one. Further study of the effects of WFs in ignition designs of the MJ laser energy scale is required in the perspective of future inertial fusion energy mass production.

Igumenshchev, I. V. [University of Rochester, NY (↗
DOE OSTI2022

The X-ray Focusing System at the Time-Resolved AMO Instrument

The emergence of X-ray Free Electron Lasers (XFELs) in recent years has opened up new fields of research, with the merging of short wavelengths and ultrafast timescales. As XFEL facilities including the Linac Coherent Light Source (LCLS) have matured, there is renewed interest in pushing the performance boundaries of experimental endstations, as well as improving ease of operation for experiments. One of the major areas of improvement is that of X-ray micro- and nano-focusing. The focus quality is directly linked to the peak intensity at the interaction point with a sample, and is especially important for intensity-dependent (e.g. x-ray pump) studies and imaging efforts such as single particle imaging.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2020

Millimeter wave traveling wave tubes for the 21st Century

Traveling wave tubes are rapidly evolving to provide unprecedented power level in comparison to solid state devices in the millimeter waves region of the spectrum (80–300 GHz) thus enabling a wide range of applications. Wireless communications, imaging, plasma diagnostics, healthcare and many others will gain substantial features if high power at millimeter waves would be available from compact sources. The development of fabrication technologies is proving crucial for introducing new topologies and structures for millimeter wave vacuum electronic devices, compatible with the dimensions dictated by the short wavelength that poses substantial manufacturing challenges due to tight tolerances. This review paper will provide an overview of the principles, evolution and state of the art of one of the most widely utilized vacuum electronic device, the traveling wave tube (TWT). The wide band, high gain features of TWTs make those devices the most promising solutions for high power at millimeter waves and THz frequencies.

42 ENGINEERING↗
DOE OSTI2024

Protocols for x-ray transient grating pump/optical probe experiments at x-ray free electron lasers

Abstract Transient grating spectroscopy is a specialized application of the four-wave-mixing methodology and constitutes a versatile technique for investigating the dynamics of vibrational, magnetic and electronic degrees of freedom of matter in a background-free fashion. Recent developments in free-electron laser sources have enabled the extension of this technique into the extreme ultraviolet range. Ongoing efforts to expand transient grating spectroscopy into the x-ray regime promise numerous advantages: (1) substantial penetration depths that allow for probing bulk material properties, (2) element specificity via specific core-excited states, and (3) short wavelengths that allow for excitation gratings with higher momentum transfer and improved spatial resolution. In this study, we comprehensively outline the procedures for conducting x-ray transient grating pump/optical probe experiment. The process encompasses the design and alignment of the experimental setup, as well as the subsequent steps involved in data acquisition and analysis. This paper is intended as a comprehensive guide for researchers interested in implementing x-ray transient grating spectroscopy, providing valuable insights into the intricacies of the experimental workflow required for this novel technique. Furthermore, we discuss the potential for extending this methodology to an x-ray pump/x-ray probe scheme, envisioning a future direction that holds promise for enhancing the capabilities and scope of x-ray transient grating spectroscopy, opening new opportunities for studying ultrafast processes with unprecedented temporal and spatial resolutions.

Optics↗
DOE OSTI2024

Role of gas composition in weakened nonlinear standing wave excitation and improved plasma radial uniformity in very-high-frequency asymmetric capacitive Ar/CF 4 discharges

Abstract The higher harmonics generated by nonlinear sheath motion would enhance the standing wave effect, and thus lead to center-peaked plasma density profile in very-high-frequency (VHF) capacitive discharges. In this work, a nonlinear transmission line (NTL) model introduced in Zhouet al(2021Plasma Sources Sci. Technol.30125017) has been extended, with radial transport of various particles and nonlinear sheath motion into account, to investigate the effects of CF 4 fraction α on the nonlinear standing wave excitation and plasma radial uniformity in VHF (60 MHz) capacitively coupled Ar/CF 4 plasmas at 3 Pa. The results indicate that for pure Ar discharges (i.e. α = 0%), the nonlinearly excited harmonics with short wavelength significantly enhance the electron power absorption at the radial center, resulting in a pronounced central-high plasma density profile. As α increases, the high-order harmonics are gradually damped due to the increase of resistance, as well as the longer wavelength caused by thicker sheath thickness. Thus, the radial profile of the electron absorbed power density shifts from center-peak to edge-high. Besides, at the radial center, the electron density and Ar + ion density decrease with α , CF 3 + ion density shows an increasing trend, while F − ion density initially rises and then decreases. Moreover, the density profiles of all the species become more uniform at higher CF 4 fraction, due to the suppressed nonlinear standing wave excitation and the longer wavelength of the nonlinear harmonics.

Physics↗
DOE OSTI2024

Effects of low-frequency voltage on nonlinear standing wave excitation, plasma uniformity, and ion dynamics in dual-frequency asymmetric capacitive discharges

Abstract It is known that in very-high-frequency (VHF) capacitively coupled plasmas, the higher harmonics generated by nonlinear sheath motion can enhance the standing wave effect (SWE), which can lead to center-peaked plasma density profiles. In this work, an improved nonlinear electromagnetic model incorporating a transmission line model, an electron momentum balance model, a bulk plasma model, a collisionless nonlinear numerical sheath model, and an ion Monte-Carlo collision (MCC) model is developed to study the effects of low-frequency (LF) voltage V L on the nonlinear standing wave excitation, plasma uniformity, and ion energy and angular distribution functions (IEDFs and IADFs) in dual-frequency (DF) asymmetric capacitive argon discharges at relatively low pressure of 3 Pa. The plasma diffusion in the radial direction and ion dynamics within the LF oscillating sheath are self-consistently considered. The LF voltage V L at 2 MHz varies from 0 to 700 V while the HF voltage V H at 60 MHz is fixed at 100 V. Simulation results indicate that without the addition of an LF source (i.e. V L = 0 V), there are a considerable number of high-order harmonics with short wavelengths, leading to significant SWE and central peak in the radial plasma density profile. Nevertheless, the high-order harmonic excitations tend to be weakened and merely occur around the phase of the full LF sheath collapse due to a shorter characteristic damping time of the surface waves as V L increases. This, combined with increased surface wavelengths of both the driving frequency and the higher harmonics at a higher V L , leads to suppressed standing waves and improved plasma uniformity. Meanwhile, the simulations show that both the low and the high energy peaks of IEDF move towards higher energies, and the energy peak separation width ΔEbecomes wider with the increase of V L . The IEDF at the radial center of the powered electrode exhibits a broader ΔEthan that at the edge. For the IADF, an increased V L results in more ions incident on the electrode with a smaller deflection angle. Because of a thinner sheath and a higher sheath voltage at the electrode center, the peak value of IADF at the electrode center is greater than that at the edge.

Physics↗
DOE OSTI2021

Anomalous two-photon Compton scattering

Abstract X-ray free-electron lasers can generate radiation pulses with extreme peak intensities at short wavelengths. This enables the investigation of laser–matter interactions in a regime of high fields, yet at a non-relativistic ponderomotive potential, where ordinary rules of light–matter interaction may no longer apply and nonlinear processes are starting to become observable. Despite small cross-sections, first nonlinear effects in the hard x-ray regime have recently been observed in solid targets, including x-ray-optical sum-frequency generation (XSFG), x-ray second harmonic generation (XSHG) and two-photon Compton scattering (2PCS). Nonlinear interactions of bound electrons in the x-ray range are fundamentally different from those dominating at optical frequencies. Whereas in the optical regime nonlinearities are predominantly caused by anharmonicities of the atomic potential in the chemical bonds, x-ray nonlinearities far above atomic resonances are expected to be due to nonlinear oscillations of quasi-free electrons, including inner-shell atomic electrons. While the quasi-free-electron model agrees reasonably well with the experimental data for XSFG and XSHG, 2PCS measurements have led to unexpected results: the energy of the nonlinearly scattered photons from non-relativistic electrons shows a substantial unexpected red shift in addition to the Compton shift that is well beyond that predicted by a nonlinear quantum electrodynamics model for free electrons. A potential explanation for the spectral broadening is based on a previously unexplored scattering process that involves the whole atom rather than just quasi-free electrons. A first simulation that includes the atomic binding potential was successful in describing a broadening of the spectrum of the nonlinearly scattered photons to longer wavelengths for soft x-rays. However, the same model does not show any broadening at hard x-ray wavelengths, which is in agreement with other simulation approaches. To this point no calculation has been able to reproduce the experimentally observed broadening. Here we present further experimental data of 2PCS for an extended parameter range using additional diagnostics. In particular, we present measurements of the electron momentum distribution during the interaction that strongly suggest that the spectral broadening is not caused by an increased plasma temperature. We extend our measurement of the magnitude of the red shift in beryllium to > 1.9 k e V in addition to the Compton shift expected for free electrons and expand the measurement of the angular distribution to include forward scattering angles. We also present first measurements of 2PCS from diamond.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Experimental studies on photoabsorption by endohedral fullerene ions with a focus on Xe@C 60 + confinement resonances

A brief overview on the status of experimental investigations into photoionization and fragmentation of fullerenes and endohedral fullerenes by absorption of a single short-wavelength photon is presented. The focus of this paper is on the endohedral Xe@C 60 + molecular ion in which a xenon atom is centrally encapsulated inside a C$^{+}_{60}$ fullerene cage. Confinement resonances that result from the interference of Xe photoelectron matter waves emerging from the C$^{+}_{60}$ cavity were studied by exposing Xe@C 60 + ions to synchrotron radiation of 60 to 150 eV energy which is the region of the well-known giant atomic Xe 4d excitation resonance. Photoions Xe@C n q+ (with final charge states q = 2,3,4 of the product ions and numbers of carbon atoms left in the cage) were recorded as a function of photon energy and cross sections for the individual reaction channels were determined. In addition to previous work, new final channels (q = 2; n = 60, 58 and q = 4; n = 58, 56) were observed, and thus, about 70% of the oscillator strength expected for the encapsulated Xe atom in the investigated energy range could be recovered. The present results establish the first and, so far, only conclusive experimental observation of confinement resonances in an endohedral fullerene. The data are in remarkable agreement with relativistic R-matrix calculations that accompanied the previous experimental work.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

The large scale polarization explorer (LSPE) for CMB measurements: performance forecast

The measurement of the polarization of the Cosmic Microwave Background (CMB) radiation is one of the current frontiers in cosmology. In particular, the detection of the divergence-free component of the polarization field, the B-mode component, reveals the presence of gravitational waves in the early Universe. The detection of such component is at the moment the most promising technique to probe the inflationary theory describing the very early evolution of the Universe. The measurement of the polarization of the Cosmic Microwave Background (CMB) radiation is one of the current frontiers in cosmology. In particular, the detection of the primordial divergence-free component of the polarization field, the B-mode, could reveal the presence of gravitational waves in the early Universe. The detection of such a component is at the moment the most promising technique to probe the inflationary theory describing the very early evolution of the Universe. We present the updated performance forecast of the Large Scale Polarization Explorer (LSPE), a program dedicated to the measurement of the CMB polarization. LSPE is composed of two instruments: LSPE-Strip, a radiometer-based telescope on the ground in Tenerife-Teide observatory, and LSPE-SWIPE (Short-Wavelength Instrument for the Polarization Explorer) a bolometer-based instrument designed to fly on a winter arctic stratospheric long-duration balloon. The program is among the few dedicated to observation of the Northern Hemisphere, while most of the international effort is focused into ground-based observation in the Southern Hemisphere. Measurements are currently scheduled in Winter 2022/23 for LSPE-SWIPE, with a flight duration up to 15 days, and in Summer 2022 with two years observations for LSPE-Strip. In this work, we describe the main features of the two instruments, identifying the most critical aspects of the design, in terms of impact on the performance forecast. We estimate the expected sensitivity of each instrument and propagate their combined observing power to the sensitivity to cosmological parameters, including the effect of scanning strategy, component separation, residual foregrounds and partial sky coverage. We also set requirements on the control of the most critical systematic effects and describe techniques to mitigate their impact. LSPE will reach a sensitivity in tensor-to-scalar ratio of σr < 0.01, set an upper limit r < 0.015 at 95% confidence level, and improve constraints on other cosmological parameters.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Magnetized directly-driven ICF capsules: increased instability growth from non-uniform laser drive

Simulations anticipate increased perturbation growth from non-uniform laser heating for magnetized direct-drive implosions. At the capsule pole, where the magnetic field is normal to the ablator surface, the field remains in the conduction zone and suppresses non-radial thermal conduction; in unmagnetized implosions this non-radial heat-flow is crucial in mitigating laser heating imbalances. Single-mode simulations show the magnetic field particularly amplifying short wavelength perturbations, whose behavior is dominated by thermal conduction. The most unstable wavelength can also become shorter. 3D multi-mode simulations of the capsule pole reinforce these findings, with increased perturbation growth anticipated across a wide range of scales. Finally, the results indicate that high-gain spherical direct-drive implosions require greater constraints on the laser heating uniformity when magnetized.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2026

Advancing extreme pulsed lasers through measurements: summary of the pulsed laser metrology workshop

In this report we present consensus findings produced during the pulsed laser metrology (PLM) workshop (Boulder, CO, USA, August 2025). The workshop was a meeting of 35 scientists from private industry, academia, and U.S. government to compile a list of the metrology needs to support extreme pulsed laser development and implementation. We define ‘extreme pulsed lasers’ loosely as those with peak powers above 1 TW or multi-Joule pulse energies. Such lasers are being increasingly used and investigated for use in high energy particle acceleration, short wavelength radiation generation, laser-based fusion, etc. The rapid growth of these fields and the proliferation of extreme lasers for these applications, coupled with the extreme electric fields they generate and relative scarcity of such lasers, presents unique challenges to accuracy in their measurement. The PLM workshop was a two-day event designed to hear from users, developers, and researchers on the current state of optical metrology associated with these extreme pulsed lasers. The goal was to identify the most important measurement challenges limiting development and implementation of these lasers in practical applications. We report here the conclusions and recommendations of this workshop. The hope is that this document will become the first of many iterations which track the status of measurement needs supporting extreme pulsed lasers.

Lasers↗