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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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At least 919 records · Page 51
All sourcesNASA NTRSDOE OSTI
DOE OSTI2016

Upgrading a high-throughput spectrometer for high-frequency (<400 kHz) measurements

The upgraded spectrometer used for charge exchange recombination spectroscopy on the Madison Symmetric Torus resolves emission fluctuations up to 400 kHz. The transimpedance amplifier’s cutoff frequency was increased based upon simulations comparing the change in the measured photon counts for time-dynamic signals. We modeled each signal-processing stage of the diagnostic and scanned the filtering frequency to quantify the uncertainty in the photon counting rate. This modeling showed that uncertainties can be calculated based on assuming each amplification stage is a Poisson process and by calibrating the photon counting rate with a DC light source to address additional variation.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2022

Faster network disruption from layered oscillatory dynamics

Nonlinear complex network-coupled systems typically have multiple stable equilibrium states. Following perturbations or due to ambient noise, the system is pushed away from its initial equilibrium, and, depending on the direction and the amplitude of the excursion, it might undergo a transition to another equilibrium. It was recently demonstrated [M. Tyloo, J. Phys. Complex. 3 03LT01 (2022)] that layered complex networks may exhibit amplified fluctuations. Here, I investigate how noise with system-specific correlations impacts the first escape time of nonlinearly coupled oscillators. Interestingly, I show that, not only the strong amplification of the fluctuations is a threat to the good functioning of the network but also the spatial and temporal correlations of the noise along the lowest-lying eigenmodes of the Laplacian matrix. Finally, I analyze first escape times on synthetic networks and compare noise originating from layered dynamics to uncorrelated noise.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2024

Spatiotemporal dispersion compensation for a 200-THz noncollinear optical parametric amplifier

A noncollinear optical parametric amplifier (NOPA) can produce few-cycle femtosecond laser pulses that are ideally suited for time-resolved optical spectroscopy measurements. However, the nonlinear-optical process giving rise to ultrabroadband pulses is susceptible to spatiotemporal dispersion problems. Here, we detail refinements, including chirped-pulse amplification (CPA) and pulse-front matching (PFM), that minimize spatiotemporal dispersion and thereby improve the properties of ultrabroadband pulses produced by a NOPA. Here, the description includes a rationale behind the choices of optical and optomechanical components, as well as assessment protocols. We demonstrate these techniques using a 1 kHz, second-harmonic Ti:sapphire pump configuration, which produces ~5-fs duration pulses that span from about 500 to 800 nm with a bandwidth of about 200 THz. To demonstrate the utility of the CPA-PFM-NOPA, we measure vibrational quantum beats in the transient–absorption spectrum of methylene blue, a dye molecule that serves as a reference standard.

47 OTHER INSTRUMENTATION↗
DOE OSTI2024

Radioisotope production using lasers: From basic science to applications

The discovery of chirped pulse amplification has led to great improvements in laser technology, enabling energetic laser beams to be compressed to pulse durations of tens of femtoseconds and focused to a few micrometers. Protons with energies of tens of MeV can be accelerated using, for instance, target normal sheath acceleration and focused on secondary targets. Under such conditions, nuclear reactions can occur, with the production of radioisotopes suitable for medical application. The use of high-repetition lasers to produce such isotopes is competitive with conventional methods mostly based on accelerators. In this paper, we study the production of 67Cu, 63Zn, 18F, and 11C, which are currently used in positron emission tomography and other applications. At the same time, we study the reactions 10B(p,α)7Be and 70Zn(p,4n)67Ga to put further constraints on the proton distributions at different angles, as well as the reaction 11B(p,α)8Be relevant for energy production. The experiment was performed at the 1 PW laser facility at Vega III in Salamanca, Spain. Angular distributions of radioisotopes in the forward (with respect to the laser direction) and backward directions were measured using a high purity germanium detector. Our results are in reasonable agreement with numerical estimates obtained following the approach of Kimura and Bonasera [Nucl. Instrum. Methods Phys. Res., Sect. A 637, 164–170 (2011)].

11C↗
DOE OSTI2011

Laser and thermal characteristics of Yb : YAG crystals in the 80 - 300 K temperature range

The spectral and thermooptical characteristics of an Yb : YAG crystal were measured in the 80 - 300 K temperature range. On cooling the crystal, the amplification cross section increases fivefold, the peak of the cross section spectrum shifts from 1030.1 to 1029.3 nm, and its width lowers from {approx}10 to 1.3 nm. These effects must be taken into account when operating with cryogenic amplifiers. Also measured were thermooptical constants P and Q as well as the photoelastic anisotropy parameter. These material constants completely determine the thermal and phase polarisation distortions in an active element when the heat release and the thermal conductivity coefficient are known. The thermoinduced depolarisation was shown to lower by a factor of 1000 and the optical strength by a factor of 50 on cooling the Yb : YAG crystal to a temperature of 78 K. (active media)

36 MATERIALS SCIENCE↗
DOE OSTI2011

Carbon dioxide laser with an e-beam-initiated discharge produced in the working gas mixture at a pressure up to 5 atm

A high-pressure CO{sub 2} laser with a discharge initiated by an electron beam of sub-nanosecond duration in the laser gas mixture at a pressure up to 5 atm is fabricated. For the 20-ns pulses the energy from the active volume {approx} 4 cm{sup 3} amounted to 40 mJ. The laser operation at a pulse repetition rate up to 5 Hz is demonstrated. In the gas mixture CO{sub 2}:N{sub 2}:He = 1:1:6 at a pressure 5 atm, the specific energy deposition of {approx} 0.07 J cm{sup -3} atm{sup -1} is obtained in the process of a non-self-sustained discharge with ionisation amplification.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

High-power thulium-doped fibre laser with intracavity dispersion management

This paper reports a scheme for the generation and amplification of pico- and femtosecond pulses in the range 1.93-1.97 {mu}m using thulium-doped silica fibres. Group velocity dispersion (GVD) management in the cavity of the thulium-doped fibre laser oscillator is ensured by a single-mode germanosilicate fibre (75 mol % GeO{sub 2} in the core) with a positive GVD. Pulses are obtained down to 200 fs in duration and up to 56 nJ in energy.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Multiterawatt femtosecond hybrid system based on a photodissociation XeF(C-A) amplifier in the visible range

This paper describes a multiterawatt hybrid laser system based on a Ti : sapphire front end and 25-cm-aperture photodissociation XeF(C - A) amplifier. The front end generates 50-fs pulses with second-harmonic ({lambda} = 475 nm) energies of up to 5 mJ. The gainmedium of the amplifier is produced in a XeF{sub 2} - N{sub 2} mixture by VUV radiation excited in xenon by a high-power electron beam. Through amplification of a negatively chirped 1-ps pulse, followed by pulse compression in bulk fused silica, a peak power of 14 TW, record-high for the visible range, has been achieved.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2012

Phasing of multichannel laser radiation upon stimulated Brillouin scattering

We investigated the phasing of pulsed two- and fourchannel laser beams due to phase conjugation upon transient stimulated Brillouin scattering (SBS) in a double-pass amplification scheme. A high quality of beam phasing was experimentally demonstrated with the use of a microlens raster and an angular selector in the SBS-mirror scheme. The data obtained in the numerical simulation of transient SBS are in good agreement with experimental ones.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

All-fibre high-energy chirped-pulse laser in the 1 {mu}m range

We report an all-fibre 1030-nm ultrashort (100 ps with the possibility of compression to 430 fs) chirped pulse laser configuration based on a seed oscillator that includes a semiconductor saturable absorber mirror, and demonstrate amplification of its pulses to an energy of 0.6 {mu}J, with the possibility of pulse compression to 650 fs. (extreme light fields and their applications)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Fibre amplifier based on an ytterbium-doped active tapered fibre for the generation of megawatt peak power ultrashort optical pulses

We report a new ytterbium-doped active tapered fibre used in the output amplifier stage of a fibre laser system for the generation of megawatt peak power ultrashort pulses in the microjoule energy range. The tapered fibre is single-mode at its input end (core and cladding diameters of 10 and 80 μm) and multimode at its output end (diameters of 45 and 430 μm), but ultrashort pulses are amplified in a quasi-single-mode regime. Using a hybrid Er/Yb fibre system comprising an erbium master oscillator and amplifier at a wavelength near 1.5 μm, a nonlinear wavelength converter to the 1 μm range and a three-stage ytterbium-doped fibre amplifier, we obtained pulses of 1 μJ energy and 7 ps duration, which were then compressed by a grating-pair dispersion compressor with 60% efficiency to a 130 fs duration, approaching the transform-limited pulse duration. The present experimental data agree well with numerical simulation results for pulse amplification in the threestage amplifier. (extreme light fields and their applications)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Influence of the input radiation pulse characteristics on the parameters of a XeF(C – A) amplifier in a THL-100 laser system

We report the results of experimental and theoretical investigations on the influence of spatial and energy parameters of input radiation with a pulse duration of 50 ps on output characteristics of a XeF(C – A) amplifier in a visible-range, multi-terawatt THL-100 laser system. Dynamics of the energy density radial distribution for laser radiation passing through the amplifier is studied. Results of numerical simulation are presented for amplification of laser beams with Gaussian and super-Gaussian radial energy density distributions. It is shown that the laser energy of 3.2 J obtained experimentally is not the limiting value. According to calculations, the output energy of the amplifier with such mirror configuration may reach 4.1 J, which in the case of a pulse compressed down to 50 fs corresponds to the radiation power of 82 TW. (paper)

42 ENGINEERING↗
DOE OSTI2017

Brillouin optical reflectometer with a Brillouin active filter

A new scheme of a fibre-optic Brillouin reflectometer is experimentally studied, in which the spectral line of spontaneous Brillouin scattering is selected by an active Brillouin filter represented by the tested fibre itself. To improve the reflectometer characteristics, a cyclic code and Raman amplification of the scattering signal are applied. With an averaging time of 5 min, scanning of 25 km of fibre with a spatial resolution of 4 m and a sampling resolution of 1 m are provided. The root-mean-square deviation in determining the Brillouin frequency is less than 1.1 MHz. The reflectometer sensitivity is evaluated with respect to the temperature changes and mechanical deformation. (paper)

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Propagation of a light pulse with a duration of less than one period in a resonant amplifying medium

The propagation of a unipolar light pulse with a duration of less than one period in a two-level resonant amplifying medium is studied theoretically. In the process of amplification, the unipolar pulse becomes bipolar. The effect of the relaxation time on the shape and duration of the amplified pulse is demonstrated. It is found that the electric pulse area (integral of electric field strength with respect to time) is conserved, in contrast to the area under the long pulse envelope (integral of slowly varying field amplitude with respect to time), which, in the case of long pulses, satisfies the McCall – Hahn area theorem. (extremely short pulses)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Contactless method for studying temperature within the active element of a multidisk cryogenic amplifier

A new original method has been developed and experimentally implemented, allowing temperature fields to be contactlessly measured in the pump region of active elements in high-power-diode-pumped laser amplifiers, including those operating at cryogenic temperatures. The presence of a temperature gradient of ∼57 K mm{sup −1} along the pump beam axis at the centre of the active element of the laser amplification unit operating at cryogenic temperatures with a pulse repetition rate up to 1 kHz is simulated and experimentally confirmed. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Stimulated emission of AlGaN layers grown on sapphire substrates using ammonia molecular beam epitaxy

By optimising the growth temperature of the AlGaN layers and using high-temperature AlN buffer layers, high-quality Al{sub x}Ga{sub 1 – x}N layers (x = 0.15, 0.21, 0.26, and 0.3) were obtained, in which stimulated emission in the UV spectral range 330 – 297 nm was implemented with the threshold intensity of excitation I{sub th} ≈ 0.7 – 1.4 MW cm{sup −2}, respectively. It is found that the threshold value of the stimulated emission of AlGaN layers grown by molecular beam epitaxy is largely determined by the intensity of the process of thermal decomposition of GaN, which affects the surface morphology and, consequently, the amount of optical scattering loss. It is shown that no pronounced localisation of nonequilibrium charge carriers occurs in the AlGaN layers, which is manifested in the absence of a large Stokes shift and in the realisation of optical amplification at transitions in an electron-hole plasma, and also indicates a relatively homogeneous material composition. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Laser and thermooptical characteristics of a laser head based on a thin Yb : YAG slab

Amplification and thermally induced phase distortions in a cw pumped laser head with an active element in the form of a thin Yb : YAG slab are experimentally studied. Lasing characteristics of an emitter based on this laser head are examined. Laser radiation is obtained with an average power of 32 W and a slope efficiency with respect to the absorbed pump power η = 68 %. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2020

Study of saturated-absorption resonances on the {sup 3}P{sub 0, 1, 2} – {sup 3}D{sub 1, 2, 3} transitions of magnesium atoms in a hollow-cathode discharge cell

The saturated-absorption resonances on the {sup 3}P{sub 0, 1, 2} – {sup 3}D{sub 1, 2, 3} transitions of magnesium atoms in a hollow-cathode discharge cell are studied. The line width (FWHM) for the observed saturated-absorption resonance on the 3{sup 3}P{sub 0} → 3{sup 3}D{sub 1} transition turns out to be ∼220 MHz. Experiments are performed using a system based on a 766-nm diode laser with amplification and frequency doubling in a nonlinear BiBO crystal. The results obtained are of interest for sub-Doppler cooling on the 3{sup 3}P{sub 2} → 3{sup 3}D{sub 3} transition. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗