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Birefringence in thermally anisotropic relativistic plasmas and its impact on laser–plasma interactions
One of the paradigm-shifting phenomena triggered in laser–plasma interactions at relativistic intensities is the so-called relativistic transparency. As the electrons become heated by the laser to relativistic energies, the plasma becomes transparent to the laser light even though the plasma density is sufficiently high to reflect the laser pulse in the non-relativistic case. This paper highlights the impact that relativistic transparency can have on laser-matter interactions by focusing on a collective phenomenon that is associated with the onset of relativistic transparency: plasma birefringence in thermally anisotropic relativistic plasmas. Here, the optical properties of such a system become dependent on the polarization of light, and this can serve as the basis for plasma-based optical devices or novel diagnostic capabilities.
Infrared spectroscopic method for uranium isotopic analysis
A high performance infrared (HPIR) system was developed and demonstrated for the infrared absorption analysis of {sup 235}U and {sup 238}U isotopes in uranium hexafluoride gas samples. Sweeping the quantum cascade laser light source over the spectral range and sampling via a high-rate analog-to-digital converter provided 0.0005 cm-1 spectral resolution, which allowed for high-precision measurements of the isotopic peak shift. A data analysis method was developed using principal component analysis to predict the isotope weight % content of {sup 235}U. The HPIR precision, accuracy, and error were evaluated for a wide range of isotopic ratio samples (0.287 – 93.7 weight % {sup 235}U), and the results were compared to the International Target Values (ITVS) set forth by the International Atomic Energy Agency (IAEA) for non-destructive and destructive analytical techniques. The method meets or surpasses the IAEA ITVs for non-destructive analysis of samples with isotopic content of depleted to highly enriched. The results also demonstrated the capability of the HPIR system to correctly predict the {sup 235}U weight % content of a mislabeled sample whose isotopic distribution was validated by mass spectroscopic measurements. The HPIR measurement is nondestructive and, thus, allows for confirmatory analyses of the exact sample at a designated IAEA lab if higher-resolution or a certified analysis is needed. (author)
Light element ($$Z=1,2$$) production from spontaneous ternary fission of $$^{252}$$Cf
Abstract The yields of light elements ( $$Z=1,2$$ Z = 1 , 2 ) obtained from spontaneous ternary fission of $$^{252}$$ 252 Cf are treated within a nonequilibrium approach, and the contribution of unstable nuclei and excited bound states is taken into account. These light cluster yields may be used to probe dense matter, and to infer in-medium corrections such as Pauli blocking which is determined by the nucleon density. Continuum correlations are calculated from scattering phase shifts using the Beth-Uhlenbeck formula, and the effect of medium modification is estimated. The relevant distribution is reconstructed from the measured yields of isotopes. This describes the state of the nucleon system at scission and cluster formation, using only three Lagrange parameters which are the nonequilibrium counterparts of the temperature and chemical potentials, as defined in thermodynamic equilibrium. We concluded that a simple nuclear statistical equilibrium model neglecting continuum correlations and medium effects is not able to describe the measured distribution of H and He isotopes. Moreover, the freeze-out concept may serve as an important ingredient to the nonequilibrium approach using the relevant statistical operator concept.
Circadian phase relationships in monkeys
Two adult male pigtail monkeys were placed in an isolated, soundproofed chamber (entered for cleaning only) for a period of six months, during which time their deep body temperatures T sub DB, telemetered from transmitters implanted in the abdominal cavity), fluid intake, urinary output (UV), urinary sodium and potassium were continuously monitored. During the first 3 1/2 months, lights (L) were turned on at 0000 hours, off at 1200 hours. Photoperiod phase was then delayed (light span prolonged) 6 hours to a new schedule: L on at 0600 hours, off at 1800 hours. Six weeks later, photoperiod phase was advanced 6 hours to return to the original schedule. Prior to shift, T sub DB typically began a steep rise 0-5 hours prior to L on, a steep fall 3-4 hours prior to L off, relative plateaus in between. Urinary Na typically peaks 2 hours prior to L off, has a minimum 2-4 hours prior to L on; K tends both to peak and show a minimum 2-8 hours earlier than Na; in contrast, UV peaks at L on, has a minimum 2-6 hours after L off. Upon delaying photoperiod phase, T sub DB shift was completed in 8 days. UV shifted more rapidly but tended to overshoot the new phase. Within 5 days, UV and K completed their shifts, although Na did not fully resynchronize within the 6 week period monitored.
Self-precession and frequency shift for electromagnetic waves in homogeneous plasmas
The nonlinear propagation of an arbitrarily polarized electromagnetic wave in a uniform plasma is studied. It is shown that nonlinear effects cause precession of the polarization ellipse as the wave propagates. The ellipticity remains constant, but the orientation of the principal axes is rotated relative to its initial value. A relativistic Vlasov model is used to study nonlinear frequency shifts as well as self-precession, in a plasma of arbitrary temperature. Even when the electron temperature is much greater than the product of the electron mass times the square of the velocity of light, the qualitative nature of these two processes remains unchanged, although their dependence on the plasma density is altered in significant ways. Implications of these effects for plasma instabilities driven by strong electromagnetic waves are briefly discussed.
Full-Duplex Digital Communication on a Single Laser Beam
A proposed free-space optical communication system would operate in a full-duplex mode, using a single constant-power laser beam for transmission and reception of binary signals at both ends of the free-space optical path. The system was conceived for two-way data communication between a ground station and a spacecraft in a low orbit around the Earth. It has been estimated that in this application, a data rate of 10 kb/s could be achieved at a ground-station-to-spacecraft distance of 320 km, using a laser power of only 100 mW. The basic system concept is also applicable to terrestrial free-space optical communications. The system (see figure) would include a diode laser at one end of the link (originally, the ground station) and a liquid-crystal- based retroreflecting modulator at the other end of the link (originally, the spacecraft). At the laser end, the beam to be transmitted would be made to pass through a quarter-wave plate, which would convert its linear polarization to right circular polarization. For transmission of data from the laser end to the retroreflector end, the laser beam would be modulated with subcarrier phase-shift keying (SC-PSK). The transmitted beam would then pass through an aperture- sharing element (ASE) - basically, a mirror with a hole in it, used to separate the paths of the transmitted and received light beams. The transmitted beam would continue outward through a telescope (which, in the original application, would be equipped with a spacecraft-tracking system) that would launch the transmitted beam along the free-space optical path to the retroreflector end.
Luminescent solar concentrators and related methods of manufacturing
Luminescent solar concentrators in accordance with various embodiments of the invention can be designed to minimize photon thermalization losses and incomplete light trapping using various components and techniques. Cadmium selenide core, cadmium sulfide shell (CdSe/CdS) quantum dot (“QD”) technology can be implemented in such devices to allow for near-unity QDs and sufficiently large Stokes shifts. Many embodiments of the invention include a luminescent solar concentrator that incorporates CdSe/CdS quantum dot luminophores. In further embodiments, anisotropic luminophore emission can be implemented through metasurface/plasmonic antenna coupling. In several embodiments, red-shifted luminophores are implemented. Additionally, top and bottom spectrally-selective filters, such as but not limited to selectively-reflective metasurface mirrors and polymeric stack filters, can be implemented to enhance the photon collection efficiency. In some embodiments, luminescent solar concentrator component is optically connected in tandem with a planar Si subcell, forming a micro-optical tandem luminescent solar concentrator.
Effects of Gravity on Insect Circadian Rhythmicity
Circadian rhythms - endogenous daily rhythmic fluctuations in virtually all characteristics of life - are generated and coordinated by the circadian timing system (CTS). The CTS is synchronized to the external 24-hour day by time cues such as the light/dark cycle. In an environment without time cues, the length of an animal's day is determined by the period of its internal pacemaker (tau) and the animal is said to be free-running. All life on earth evolved under the solar day; the CTS exists as an adaptation that allows organisms to anticipate and to prepare for rhythmic environmental fluctuations. All life on earth also evolved under the force of earth's gravitational environment. While it is therefore not surprising that changes in the lighting environment affect the CTS, it is surprising that changes in the gravitational environment would do so. However, recent data from one of our laboratories using the brn-3.1 knockout mouse revealed that this model, which lacks the sensory receptor hair cells within the neurovestibular system, does not respond to exposure to a hyperdynamic environment in the same fashion as normal mice. The brn-3.1 mice did not show the expected suppression of circadian rhythmicity shown by control mice exposed to 2G. Exposure to altered ambient force environments affects the amplitude, mean and timing of circadian rhythms in species from unicellular organisms to man. In addition, there is a circadian influence on the homeostatic response to acute 2G acceleration and pulses of 2G can act as a time cue, synchronizing the CTS. This is of significance because maintenance of internal and external temporal coordination is critical for normal physiological and psychological function. Typically, during adaptation to an increased gravitational environment (+G), an initial acute reaction is followed by adaptation and, eventually, a new steady state (14-16), which can take weeks to months to establish. Until the development of space stations, exposure to microgravity was, of necessity, relatively short in duration. In early spaceflight experiments an organism's internal rhythms often expressed periods that were different from each other, even in the presence of a 24.0 hour light-dark cycle, suggesting that the organism was experiencing internal desynchronization (17, 18). In (micro)G, the body temperature rhythm was delayed with respect to other body rhythms and to the light-dark cycle in rhesus macaques (19) and man (20, 21). In the absence of a light-dark cycle, the circadian rhythm of spore formation persisted in Neurospora crassa, however, both the variability and average period of the rhythm increased (22). The beetle Trigonoscelis gigas, exhibited changes in period during and following 11-13 days in (micro)G (23, 24). Resynchronization of the urinary calcium rhythm following a 1800 phase shift of the LID cycle was retarded in rats exposed to (micro)G compared to 1G controls (25). With the development of the Russian Mir Space Station, long-term controlled microgravity exposure became possible. We recorded activity rhythms from black-bodied Tenebrionid beetles, Trigonoscelis gigas, in (micro)G (spaceflight). Each insect was housed individually within an activity monitor (26) and data (activity counts) were collected and stored in five-minute bins. Thirty-two individual activity monitors were housed within each of 2 experimental kits. The beetles within each kit were divided into two groups and the lighting was controlled separately for each group.
Structural comparison of allophycocyanin variants reveals the molecular basis for their spectral differences
Abstract Allophycocyanins are phycobiliproteins that absorb red light and transfer the energy to the reaction centers of oxygenic photosynthesis in cyanobacteria and red algae. Recently, it was shown that some allophycocyanins absorb far-red light and that one subset of these allophycocyanins, comprising subunits from the ApcD4 and ApcB3 subfamilies (FRL-AP), form helical nanotubes. The lowest energy absorbance maximum of the oligomeric ApcD4-ApcB3 complexes occurs at 709 nm, which is unlike allophycocyanin (AP; ApcA-ApcB) and allophycocyanin B (AP-B; ApcD-ApcB) trimers that absorb maximally at ~ 650 nm and ~ 670 nm, respectively. The molecular bases of the different spectra of AP variants are presently unclear. To address this, we structurally compared FRL-AP with AP and AP-B, performed spectroscopic analyses on FRL-AP, and leveraged computational approaches. We show that among AP variants, the α-subunit constrains pyrrole ring A of its phycocyanobilin chromophore to different extents, and the coplanarity of ring A with rings B and C sets a baseline for the absorbance maximum of the chromophore. Upon oligomerization, the α-chromophores of all AP variants exhibit a red shift of the absorbance maximum of ~ 25 to 30 nm and band narrowing. We exclude excitonic coupling in FRL-AP as the basis for this red shift and extend the results to discuss AP and AP-B. Instead, we attribute these spectral changes to a conformational alteration of pyrrole ring D, which becomes more coplanar with rings B and C upon oligomerization. This study expands the molecular understanding of light-harvesting attributes of phycobiliproteins and will aid in designing phycobiliproteins for biotechnological applications.
Simultaneous mapping of the unsteady flow fields by Particle Displacement Velocimetry (PDV)
Current experimental and computational techniques must be improved in order to advance the prediction capability of the longitudinal vortical flows shed by underwater vehicles. The generation, development, and breakdown mechanisms of the shed vortices at high Reynolds numbers are not fully understood. The ability to measure hull separated vortices associated with vehicle maneuvering does not exist at present. The existing point-by-point measurement techniques can only capture approximately the large 'mean' eddies but fail to meet the dynamics of small vortices during the initial stage of generation. A new technique, which offers a previously unavailable capability to measure the unsteady cross-flow distribution in the plane of the laser light sheet, is called Particle Displacement Velocimetry (PDV). PDV consists of illuminating a thin section of the flowfield with a pulsed laser. The water is seeded with microscopic, neutrally buoyant particles containing imbedded fluorescing dye which responds with intense spontaneous fluorescence with the illuminated section. The seeded particles in the vortical flow structure shed by the underwater vehicle are illuminated by the pulse laser and the corresponding particle traces are recorded in a single photographic frame. Two distinct approaches were utilized for determining the velocity distribution from the particle traces. The first method is based on matching the traces of the same particle and measuring the distance between them. The direction of the flow can be identified by keeping one of the pulses longer than the other. The second method is based on selecting a small window within the image and finding the mean shift of all the particles within that region. The computation of the auto-correlation of the intensity distribution within the selected sample window is used to determine the mean displacement of particles. The direction of the flow is identified by varying the intensity of the laser light between pulses. Considerable computational resources are required to compute the auto-correction of the intensity distribution. Parallel processing will be employed to speed up the data reduction. A few examples of measured unsteady vortical flow structures shed by the underwater vehicles will be presented.
Controlling the size of gold nanoparticles grown on indium tin oxide substrates prepared by seed mediated growth method
One of the unique optical properties of gold nanoparticles is the enhanced absorption and scattering light around metal nanoparticles commonly called the Localized Surface Plasmon Resonance (LSPR) effect of gold nanoparticles. This property is determined by the shape and size of gold nanoparticles. In this work, we observed the role of three materials used in synthesis process on the morphology and the LSPR effect of gold nanoparticles. The gold nanoparticles were directly grown on indium tin oxide (ITO) coated glass substrates using the seed mediated growth method with three different concentrations of trisodium citrate (Na{sub 3}C{sub 6}H{sub 5}O{sub 7}), C{sub 16}TAB and ascorbic acid (C{sub 6}H{sub 8}O{sub 6}). Based on the FESEM image and optical absorption spectrum of gold nanoparticles, it was found that the higher concentration of those materials has decreased the size of gold nananoparticles and shifted the LSPR peaks to lower wavelength.
Water Ordering Controls the Dynamic Equilibrium of Micelle-Fiber Formation in Self-Assembly of Peptide Amphiphiles
Understanding the role of water in governing the kinetics of the self-assembly processes of amphiphilic peptides has still remained elusive. Here, using a multi-stage atomistic-coarse-grained approach, complemented by circular dichroism/infra-red spectroscopy and dynamic light scattering experiments, we highlight the dual nature of water in dictating the mechanism and dynamics of self-assembly of peptide amphiphiles (PAs). Our computational study shows that (i) Water cage formation and breakage near the hydrophobic groups controls the fusion dynamics and aggregation of PAs in the micellar stage, and (ii) Enhanced structural ordering of vicinal water near the hydrophilic amino acids shifts the equilibrium towards the fiber phase and stimulates structure and order in the PAs when they assemble into a hexagonal nanofiber architecture. Finally, spectroscopy and microscopy studies authenticate our computational observation that water ordering near the PAs increases with increase in time. The measured infra-red O-H bond stretch frequency reminiscent of ice-like suggests that the solvated water becomes increasingly solid-like with increased structural order in the assembled peptide network – thus shedding light on the role of water in a self-assembly process.
Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer
We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of Y x Sc 3-x N@C 80 (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S 1 –T 1 gap decrease in this row. For Sc 3 N@C 80 , we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with 14 N, 45 Sc, and 89 Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y 3 N to Sc 3 N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.
Reactor cell neutron dose for the molten salt breeder reactor conceptual design
The private sector’s interest in the active development of molten salt reactors has led to the need to develop and test advanced modeling and simulation tools to analyze various advanced reactor types under numerous conditions. This paper discusses the effort undertaken to model the Oak Ridge National Laboratory (ORNL) Molten Salt Breeder Reactor (MSBR) design using ORNL’s Shift Monte Carlo code. The MSBR model integrates a Monte Carlo N-Particle (MCNP) MSBR core model with an MCNP model that was generated from a CAD model of the external components and the reactor building, which was subsequently run in Shift. This paper focuses on development of the fully integrated model and its use in performing neutron transport calculations in the reactor cell area. This model is intended to aid in understanding radiological dose conditions during operation, as well as the iron dpa rates in the reactor vessel. The neutron biological dose rates and flux calculated in the reactor cell are much higher in the MSBR than in typical light-water reactors. The implications of these results and future work are also discussed in this paper.
Correlation at two-dimensional charge-transfer FeSe interface
The charge transfer and spin coupling effects are explored at the interface of two-dimensional (2D) superconducting FeSe nanosheets and molecular photochromic potassium-7,7,8,8-tetracyanoquinodimethane (KTCNQ). Light-induced conductivity in 2D FeSe nanosheets is enhanced by the electron doping from KTCNQ by the destabilized spin-Peierls phase through their interface. Furthermore, the spin coupling at the interface of FeSe and KTCNQ shifts the dimerization transition temperature of KTCNQ. Our results suggest 2D exfoliated FeSe nanosheets as a versatile strongly correlated platform for the study of interfacial electron doping and spin coupling.
Electrical and optical properties of nitrile rubber modified by ion implantation
Implantation of N{sup +} ion beams are performed on to a non-conjugated elastomer, acrylonirtle butadiene rubber (NBR) with energy 60 keV in the fluence range of 10{sup 14} to 10{sup 16} ions/cm{sup 2}. A decrease in the resistivity of the sample by about eight orders of magnitude is observed in the implanted samples along with color changes. The ion exposed specimens were characterized by means of UV/Vis spectroscopy which shows a shift in the absorption edge value for the as deposited polymer towards higher wavelengths. The band gap is evaluated from the absorption spectra and is found to decrease with increasing fluence. This study can possibly throw light on ion induced changes in the polymer surface.
Initial beam emission spectroscopy diagnostic system on HL-2A tokamak
A beam emission spectroscopy system is being developed and deployed on the HL-2A tokamak to measure local low wavenumber (k ⊥ ρ i < 1) density fluctuations by measuring the Doppler-shifted emission from a 50 kV deuterium heating neutral beam. High spatial resolution (Δr ≤ 1 cm, Δz ≤ 1.5 cm) measurements are achieved with customized in-vacuum optics. High frequency, high-gain preamplifiers sample the light intensity at a Nyquist frequency of 1 MHz and achieve a high S/N ratio via high optical throughput, low-noise preamplifiers, and high quantum efficiency photodiodes. A first set of 16 detector channels [configured in an 8 (radial) × 2 (poloidal) array] has been installed and tested at HL-2A, covering the radial range r/a = 0.8–1.1. The frequency and wavenumber spectra have been measured under different plasma conditions. Initial measurements have demonstrated the capability of measuring edge plasma density fluctuation spectra and the poloidal flow velocity fields with a high S/N ratio.