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The fast luminescent decay of the MV-50 GaAs doped Si light-emitting diode has been studied. This diode is found to provide a fast, inexpensive, bright, and convenient light source for the calibration of fast optical timing systems. A simple passive electronic module is described that allows driving the light source directly by a laboratory pulse generator.
A model is proposed for the Vela pulsar in which the radio emission originates near the surface of the neutron star while the pulsed gamma-ray emission is produced by synchrotron radiation near the speed-of-light cylinder. This model can explain the energy flux, double pulse structure, and phase shift (with respect to the radio) of the gamma-ray emission, and offers approximate quantitative predictions for other X- and gamma-ray fluxes.
Current and future lidar applications to meteorological studies are presented. In water vapor, temperature, and pressure measurement applications, differential absorption lidar (DIAL) techniques are used, employing a minimum of two wavelengths. The DIAL technique has proven particularly accurate in pressure measurements. For wind measurements, lidar investigations generally use the Doppler shifting of laser light backscattered from aerosols, and a pulsed low-power CO2 Doppler lidar is being developed for airborne platform applications. At visible to near-infrared wavelengths, spatial distribution of aerosol and clouds can be obtained from lidar, and this information can help determine such atmospheric parameters as mixed layer height and cloud height distributions. A Shuttle lidar facility, being developed for the end of the 1980's, will enable laser remote sensing techniques to be applied to studies of the lower atmosphere.
Conceptual electro-optical shutter subsystem regulates intensity of light entering video camera from variable-plasma-pulse-arc weld in process. Camera part of video system that enables welding technician to obtain better view of weld bead and puddle in light of arc. Electro-optical shutter opened during low-illumination viewing intervals near zero crossings of welding current, preventing overloading of video camera.
Recent progress toward the development of light weight, long life coolers is presented. In particular, programs underway in the United States to develop Stirling cycle coolers and pulse tube coolers are described. The potential availability of reliable, long lifetime coolers for space applications is discussed.
A batch of four samples were received and chemical analysis was performed of the surface and near surface regions of the samples by the surface analysis by laser ionization (SALI) method. The samples included four one-inch diameter optics labeled windows no. PR14 and PR17 and MgF2 mirrors 9-93 PPPC exp. and control DMES 26-92. The analyses emphasized surface contamination or modification. In these studies, pulsed desorption by 355 nm laser light and single-photon ionization (SPI) above the sample by coherent 118 nm radiation (at approximately 5 x 10(exp 5) W/cm(sup 2)) were used, emphasizing organic analysis. For the two windows with an apparent yellowish contaminant film, higher desorption laser power was needed to provide substantial signals, indicating a less volatile contamination than for the two mirrors. Window PR14 and the 9-93 mirror showed more hydrocarbon components than the other two samples. The mass spectra, which show considerable complexity, are discussed in terms of various potential chemical assignments.
In order to meet the need for light weight, long lifetime, efficient, short pulse lasers, a diode-pumped, Nd:YLF oscillator and regenerative amplifier is being developed. The anticipated output is 20 mJ per 10 picosecond pulse, running at a repetition rate of 40 Hz. The fundamental wavelength is at 1047 nm. The oscillator is pumped by a single laser diode bar and mode locked using an electro-optic, intra-cavity phase modulator. The output from the oscillator is injected as a seed into the regenerative amplifier. The regenerative amplifier laser crystal is optically pumped by two 60W quasi-cw laser diode bars. Each diode is collimated using a custom designed micro-lens bar. The injected 10 ps pulse from the oscillator is kept circulating within the regenerative amplifier until this nanojoule level seed pulse is amplified to 2-3 millijoules. At this point the pulse is ejected and sent on to a more standard single pass amplifier where the energy is boosted to 20 mJ. The footprint of the entire laser (oscillator-regenerative amplifier-amplifier) will fit on a 3 by 4 ft. optical pallet.
Various beam imaging and/or sheet forming optics delivered light at 1064 nm from a pulsed Nd:YAG laser for use either as a beam of 3 mm radius or as a laser sheet. Imaging measurements were performed with a grated intensified array camera equipped with an ultraviolet f4.5 lens and a 40 mm extension tube. Point measurements were performed using an ultraviolet 250 mm focal length lens to collect and focus the laser induced incandescence (LII) signal into a 1 meter long quartz optical fiber which directed the LII signal to a 1/4 meter monochromator. An aperture preceding the lens restricted the signal collection region to 1 cm along the laser beam at the center of the gravimetric chimney. Signals from the PMT were processed by a boxcar integrator whereas the images were captured digitally using a frame-grabber with 16 MByte of on-board memory. Both 'point' and planar measurements were made with detector gates of 250 ns to minimize possible morphology bias in collection of the LII signal. Additionally, the imaging measurements were performed with broadband spectral collection of the LII signal to maximize the signal and again minimize any potential effects of morphology dependent heating and/or cooling rates. Digital delay generators controlled the firing of he laser, detector gates and data acquisition. Neutral density filters were used for both sets of measurements to maintain signal levels within linear dynamic ranges of the detectors, the range being determined prior to experiments.
The thermoluminescence (TL) and optically stimulated luminescence (OSL) response of Al2O3 dosimeters to high-energy heavy charged particles (HCP) has been studied using the heavy ion medical accelerator at Chiba, Japan. The samples were Al2O3 single-crystal chips, of the type usually known as TLD-500, and Luxel(TM) dosimeters (Al2O3:C powder in plastic) from Landauer Inc. The samples were exposed to 4He (150 MeV/u), 12C (400 MeV/u), 28Si (490 MeV/us) and 56Fe (500 MeV/u) ions, with linear energy transfer values covering the range from 2.26 to 189 keV/micrometers in water and doses from 1 to 100 mGy (to water). A 90Sr/90Y beta source, calibrated against a 60Co secondary standard, was used for calibration purposes. For OSL, we used both continuous-wave OSL measurements (CW-OSL, using green light stimulation at 525 nm) and pulsed OSL measurements (POSL, using 532 nm stimulation from a Nd:YAG Q-switched laser). The efficiencies (eta HCP, gamma) of the different HCPs at producing OSL or TL were observed to depend not only upon the linear energy transfer (LET) of the HCP, but also upon the sample type (single crystal chip or Luxel(TM)) and the luminescence method used to define the signal--i.e. TL, CW-OSL initial intensity, CW-OSL total area, or POSL. Observed changes in shape of the decay curve lead to potential methods for extracting LET information of unknown radiation fields. A discussion of the results is given, including the potential use of OSL from Al2O3 in the areas of space radiation dosimetry and radiation oncology. c2004 Elsevier Ltd. All rights reserved.
We report the discovery of 105-ms X-ray pulsations from the compact central object (CCO) in the supernova remnant \snr\ using data acquired with the {\it Newton X-Ray Multi-Mirror Mission). Using two observations of the pulsar taken 6-days apart we derive an upper limit on its spin-down rate of $\dot P < 9 \times 10"{-14}$-s-~$~{-l)$,a nd find no evidence for binary orbital motion. The implied energy loss rate is $\dot E < 3 \times 10A{36)$-ergs-s$A{-1)$, polar magnetic field strength is $B-{\rm p) < 3 \times 10A{12)$-G, and spin-down age is $\tau > 18.5$-kyr. The latter exceeds the remnant's estimated age, suggesting that the pulsar was born spinning near its current period. The X-ray spectrum of \psr\ is best characterized as a blackbody of temperature $kT {BB) =, 0.43\pm0.02$ keV, radius $R-{BB) \approx 1.3$-km, and $~I{\rm bol) = 5.2 \times 10A{33)$ ergs-sSA{-1)$ at $d = 7.1$-kpc. The sinusoidal light curve is modulated with a pulsed fraction of $>45\%$, suggestive of a small hot spot on the surface of the rotating neutron star. The lack of a discernible pulsar wind nebula is consistent with an interpretation of \psr\ as a rotation-powered pulsar whose spin-down luminosity falls below the empirical threshold for generating bright wind nebulae, $\dot E-{\rm c) = 4 \times 10A{36)$-ergs-sSA{-I)$. The age discrepancy suggests that its $\dot E$ has always been below $\dot E c$, perhaps a distinguishing property of the CCOs. Alternatively, the X-ray spectrum of \psr\ suggests a low-luminosity AXP, but the weak inferred $B-{\rm p)$ field is incompatible with a magnetar theory of its X-ray luminosity. The ordinary spin parameters discovered from \psr\ highlight the inability of existing theories to explain the high luminosities and temperatures of CCO thermal X-ray spectra.
Direct upward remote sensing of the moon’s exosphere from a surface vantage can address production of water-related lunar volatiles as well as their exospheric loss, ballistic transport, and ultimate adsorption in permanently shadowed regions (PSRs). Far UV (FUV) dayside measurements of atomic oxygen, liberated from regolith by energetic solar protons and micrometeorite impact, can provide critical insight into the endogenic lunar water cycle by constraining total column density [O] at site of production. A notional Lunar-surface UV Photometric Investigation of Exospheres (LUPINE) instrument is designed to exploit solar-pumped atomic oxygen fluorescence at 130.4-nm, in a manner similar to the Apollo 17 UV Spectrometer (UVS) experiment [Fastie 1973; Feldman and Morrison, 1991] and the LRO Lyman-alpha Mapping Project (LAMP) spectrograph [Cook et al., 2013], by implementing a zenith-directed FUV photometer from the lunar surface at low (± 10°) selenographic latitude during the lunar day. Atomic oxygen production from solar energetic particle impact sources is thought to maximize in near solar noon [Sarantos et al., 2012], and the lander-embedded zenith-directed LUPINE photometer, in contrast to UVS and LAMP twilight measurements from orbit, can potentially capture the full column abundance of lunar regolith liberated oxygen. Herein we describe adaptation of FUV reflective optics, pulse-counting electronics, and scattered-light-suppression technologies developed for LEO FUV photometry for the challenging thermal environment of the daylit lunar surface. Preliminary thermal modeling and TVAC measurements of heritage FUV photometer components suggests that, if allowed to shed ~8W of waste heat into the bus of a reference Commercial Lunar Payload Services (CLPS) lander, the LUPINE photometer will be kept sufficiently cool to limit dark current to less than 20 counts/s. This level of dark signal enables an OI 130.4-nm 3-sigma detection threshold of ~1 mR for assumed 2-hour integrations.
No cryogenic Reaction Control System (RCS) has ever flown in space. Cryogenic propellants are baselined by HLS and CLPS partners and are the key use case for future ISRU manufactured propellants. A reliable LOX/LCH 4 RCS is an enabling technology for human Lunar and Mars exploration. In previous thermal vacuum (<275 F, <10 torr) testing at NASA GRC Plum Brook, the team uncovered anomalous LOX/LCH4 engine ignition phenomena where flame kernels quenched at these ultra cold hardware temperatures, leading to many pulses where the engines would not light. In 2022, the project was able to recreate the no light condition with an upgraded Frost-Mint test stand at JSC. This project implemented test stand improvements and procured propellants and follow on engine components for a hot fire campaign beginning in November 2024 which will attempt to ignite the RCS engine at ultra cold vacuum conditions by modifying the engine mixture ratio. New modeling techniques were implemented to account for past no lights and are an enabling method to make lunar surface go no go predictions based on hardware conditions. This was enabled by significant improvements to the Frost Mint system, which reduced moisture build up and leaks, increased propellant availability, leading to more attempts at thermal vacuum hot-fires.
Free-electron lasers (FEL), with their ultrashort pulses, ultrahigh intensities, and high repetition rates at short wavelength, have provided new approaches to Atomic and Molecular Optical Science. One such approach is following the birth of a photo electron to observe ion dynamics on an ultrafast timescale. Such an approach presents the opportunity to decipher the photon-initiated structural dynamics of an isolated atomic and molecular species. It is a fundamental step towards understanding single- and non-linear multi-photon processes and coherent electron dynamics in atoms and molecules, ultimately leading to coherent control following FEL research breakthroughs in pulse shaping and polarization control. A key aspect for exploring photoinduced quantum phenomena is visualizing the collective motion of electrons and nuclei in a single reaction process, as dynamics in atoms/ions proceed at femtosecond (10 –15 s) timescales while electronic dynamics take place in the attosecond timescale (10 –18 s). Here, we report on the design of a Dynamic Reaction Microscope (DREAM) endstation located at the second interaction point of the Time-Resolved Molecular and Optical (TMO) instrument at the Linac Coherent Light Source (LCLS) capable of following the photon–matter interactions by detecting ions and electrons in coincidence. The DREAM endstation takes advantage of the pulse properties and high repetition rate of LCLS-II to perform gas-phase soft X-ray experiments in a wide spectrum of scientific domains. With its design ability to detect multi-ions and electrons in coincidence while operating in step with the high repetition rate of LCLS-II, the DREAM endstation takes advantage of the inherent momentum conservation of reaction product ions with participating electrons to reconstruct the original X-ray photon–matter interactions. In this report, we outline in detail the design of the DREAM endstation and its functionality, with scientific opportunities enabled by this state-of-the-art instrument.
Free-electron-laser-based beamlines utilize fully coherent laser pulses with extremely narrow bandwidth allowing direct use of X-rays without monochromators. This could be very beneficial for all users of current and future fourth-generation diffraction-limited synchrotron light sources (DL-SLSs) who need narrowband full-coherence high-brightness X-ray pulses. Based on our previous finding, i.e. that separating the two stages of echo-enabled harmonic generation (EEHG) with a few extra bending-magnet sections provides an effective way to increase the momentum compaction of chicane 1, one can simultaneously achieve adequate prebunching at extremely high harmonics as well as keep the energy modulation to the ideal minimum. This could open the door for cascaded EEHG, toward fully coherent tender and hard X-ray wavelengths. Built on our compact design of a twin-pulse seeding electron beam with an adjustable delay and timing jitter at the level of a few femtoseconds, a cascaded EEHG can be implemented, which includes two EEHG beamlines, where the radiation pulse generated by the first beamline with harmonic $h$ 1 could be used as the input seed laser pulse to the second beamline with harmonic $h$ 2 . Hence, the second radiator could potentially reach very high harmonics [$h$ = $h$ 1 (20)$h$ 2 (25–100)] from 500 to 2000, corresponding to tender and hard X-ray wavelengths. It is demonstrated that the cascaded EEHG scheme is compatible with almost any current or planned fourth-generation DL-SLS, with significant benefits for space-limited storage rings in particular. The main advantage is that this scheme requires almost no change of the storage-ring lattice and is fully compatible with other beamlines. Current proposals for rings with much longer straight sections would add self-amplified spontaneous emission as another viable option for storage-ring-based free-electron lasers.
High-order harmonic generation (HHG) based transient extreme ultraviolet (XUV) absorption spectroscopy is an emerging technique to trace photoinduced charge carrier dynamics in condensed phase materials with femtosecond and even attosecond temporal resolution and elemental specificity. However, its application to nanoparticulate samples that are relevant, for example, for novel photocatalytic light harvesting concepts, has been limited. This is in part due to the challenge to produce residual-free samples on ultrathin, XUV-transparent substrates as well as a widespread understanding that sparsely distributed nanoparticles do not provide sufficient contrast for XUV absorption measurements. Here, we present static XUV absorption spectra of partially oxidized Co nanowire-structures with diameters of approximately 4.5 nm and lengths between 10 and 40 nm, recorded with an ultrashort pulse HHG light source. Nanoparticles are synthesized by the agglomeration of Co atoms inside superfluid helium droplets, followed by surface deposition and oxidation in ambient air. The method is uniquely suited for residual-free synthesis of transition metal nanowires and their deposition on ultrathin substrates. Analysis by high-resolution transmission electron microscopy reveals the formation of CoO nanowires with regions of unoxidized Co in their interior. The nanoparticle samples are investigated in an HHG-driven ultrafast XUV absorption setup. Despite the low surface coverage of only 23%, the recorded spectrum exhibits a distinct absorption feature at the Co M 2,3 (2p) edge near 60 eV with a peak height of about 40 mOD. Finally, the results support the feasibility of table-top ultrafast transient XUV absorption studies of photoinduced dynamics in transition metal oxide nanoparticles with sub-monolayer surface coverage.
Recent theoretical studies have suggested that avalanches of electron-positron pairs are produced by spark discharges near the magnetic poles of pulsars. This pair production mechanism may result in a double-peaked particle distribution function. We investigate collective effects caused by a high-energy electron-positron beam traversing a low-energy electron-positron plasma for an astrophysically interesting range of physical parameters. The principal result of this paper is to identify transverse unstable waves whose phase propagation may occur at any arbitrary angle (other than zero) with respect to the magnetic field but whose group velocity is along the magnetic field and approximately the speed of light. It is suggested that the observed radio pulses from pulsars may be caused by the focusing of these transverse waves along the open field lines extending from a pulsar's polar cap to the light cylinder.
The Geoscience Laser Altimeter System (GLAS) on board ICESat spacecraft measures the surface height (altimetry) via the time of flight of its 1064 nm laser pulse. The GLAS laser transmitter produces 6 ns wide pulses with 70 mJ energy at 1064 nm at a 40 Hz rate. The altimeter receiver consists of a telescope, aft optics, a silicon avalanche photodiode, and electronic amplifiers. The transmitted and echo pulse waveforms are digitized at 1 GHz rate. The laser pulse time of flight is determined on the ground from the two digitized pulse waveforms and their positions in the full waveform record (about 5.4 ms ong) by computing the pulse centroids or by curve fitting. The GLAS receiver algorithms in on board software selects the two waveform segments containing the transmitted and the echo pulses and sends them to ground. The probability of echo pulse detection and the accuracy of time of flight measurement depend on the received signal level, the background light within the receiver field of view, the inherent detector and amplifier noise, the quantization of the digitizer, and some times by cloud obscurations. A receiver model has been developed to calculate the probability of detection and accuracy of the altimeter measurements with these noise sources. From prelaunch testing, the minimum detectable echo pulse energy for 90% detection probability was about 0.1 fj/pulse onto the detector. Such a receiver sensitivity allows GLAS to measure the surface height through clouds with optical density less than 2. The echo pulse energy required to achieve 10 cm ranging accuracy was found to be about 3 times higher than the minimum detectable signal level. The smallest single shot range measurement error, which was determined by ranging to a fixed target with strong echo pulses and no background light, was 2 to 3cm. The maximum linear response echo pulse energy was 10 fJ/pulse for the strongest echo signals, assuming a Lambertian scattering snow surface, clear sky atmosphere transmission, and no pulse width spreading due to surface slopes. Results from the initial 38 days of GLAS operation on-orbit with Laser 1, showed the echo pulse energies varied, as expected, over a wide dynamic range due to the variability of atmosphere transmission and Earth surface characteristics. The receiver was able to reliably detect the ground surface echoes and measure time of flight under such dynamic conditions, except when there were dense clouds. When over Antarctica, the echo pulse energies were several times stronger than predicted. This suggests that the 1064 nm backscattering from the snow pack and ice-sheet &ce are not completely Lambertian but is somewhat peaked at opposit.ion. With Laser 1, the peak amplitude of the echo pulses from flat ice sheets under clear sky conditions exceeded the linear response range of the receiver, causing some pulse waveform distortion due to saturation. We have characterized the effects of receiver saturation on the time of flight, pulse width, and pulse energy measurements for flat surface by testing a night spare detector in the lab with simulated echo pulses. A data processing algorithm that minimizes the errors due to saturation for these measurements are described.
Fully characterizing high energy density (HED) phenomena using pulsed power facilities (Z machine) and coherent light sources is possible only with complementary numerical modeling for design, diagnostic development, and data interpretation. The exercise of creating numerical tests, that match experimental conditions, builds critical insight that is crucial for the development of a strong fundamental understanding of the physics behind HED phenomena and for the design of next generation pulsed power facilities. The persistence of electron correlation in HED materials arising from Coulomb interactions and the Pauli exclusion principle is one of the greatest challenges for accurate numerical modeling and has hitherto impeded our ability to model HED phenomena across multiple length and time scales at sufficient accuracy. An exemplar is a ferromagnetic material like iron, while familiar and widely used, we lack a simulation capability to characterize the interplay of structure and magnetic effects that govern material strength, kinetics of phase transitions and other transport properties. Herein we construct and demonstrate the Molecular-Spin Dynamics (MSD) simulation capability for iron from ambient to earth core conditions, all software advances are open source and presently available for broad usage. These methods are multi-scale in nature, direct comparisons between high fidelity density functional theory (DFT) and linear-scaling MSD simulations is done throughout this work, with advancements made to MSD allowing for electronic structure changes being reflected in classical dynamics. Main takeaways for the project include insight into the role of magnetic spins on mechanical properties and thermal conductivity, development of accurate interatomic potentials paired with spin Hamiltonians, and characterization of the high pressure melt boundary that is of critical importance to planetary modeling efforts.