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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 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Universality of Poisson-driven plasma fluctuations in the Alcator C-Mod scrape-off layer

Large-amplitude, intermittent fluctuations are ubiquitous in the boundary region of magnetically confined plasmas and lead to detrimental plasma-wall interactions in next-generation, high duty cycle fusion power experiments. Using gas puff imaging data time series from the scrape-off layer in the Alcator C-Mod device, it is demonstrated in this work that the large-amplitude fluctuations can be described as a super-position of pulses with a fixed shape and a constant duration. By applying a new deconvolution algorithm on the data time series with a two-sided exponential pulse function, the arrival times and amplitudes of the pulses can be estimated, and the measurement time series can be reconstructed with high accuracy. The pulse amplitudes are shown to follow an exponential distribution. The waiting times between pulses are uncorrelated, their distribution has an exponential tail, and the number of arrivals is a linear function of time. This demonstrates that pulse arrivals follow a homogeneous Poisson process. Identical statistical properties apply to both ohmic and high confinement mode plasmas, clearly demonstrating the universality of the fluctuation statistics in the boundary region of Alcator C-Mod.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

THL-100 Multi-Terawatt Laser System of Visible Range

Results of investigation of the parameters of the THL-100 hybrid laser system of visible range developed at the IHCE SB RAS in collaboration with the LPI RAS are presented. The laser system consists of the Ti:Sa Start-480M starting complex and the photodissociation XeF(C-A) amplifier with aperture of 24 cm. The starting complex provides 50-fs spectrally limited radiation pulses with energy of the second harmonic (475 nm) up to 20 mJ and 50-ps positively chirped radiation pulses with energy up to 5 mJ. The active medium of the XeF(CA) amplifier is created in the XeF{sub 2}/N{sub 2} mixture upon exposure to VUV radiation of xenon excited by a highpower electron beam. It is demonstrated that record power of 14 TW is attained by amplification of negatively chirped radiation pulse with duration of 1.6 ps in the XeF(C-A) amplifier, and maximum energy of ~3.2 J is realized by amplification of positively chirped radiation pulse.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Use of heavily doped germanosilicate fibres with a small core diameter in stretchers of ultrashort laser pulses at a wavelength of 1.03 μm

The use of a heavily doped germanosilicate fibre with a W-profile refractive index and small core diameter in stretchers of ultrashort laser pulses with their subsequent amplification and compression in all-fibre laser systems is considered. The application of fibres of this type makes it possible to stretch, amplify, and then compress a laser pulse with minimum distortions of its initial shape and width. Due to the dispersion properties of these fibres, which allow the pulse duration to be increased significantly at a small fibre length and the third-order positive dispersion of the diffraction-grating-based output compressor to be compensated for, amplified pulses with an energy of 2 μJ and width of 250 fs, free of a picosecond pedestal, are obtained. Several types of fibres intended for the use in stretchers of ultrashort laser pulses are comparatively analysed from the point of view of their dispersion compatibility with a diffraction-grating-based output compressor. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1973

Isolated output for class-D dc amplifiers

Transformer-coupled output stage is used with pulse-width modulated class-D dc amplifiers. Circuit is comprised of two channels corresponding to negative and positive input signals. Amplitude of secondary-current triangular pulse is function of duration of driving pulse. Therefore, circuit converts pulse-width modulated driving signal to pulse-amplitude modulated signal.

Honnel, M. A.↗
DOE OSTI2015

Anomalous memory effect in the breakdown of low-pressure argon in a long discharge tube

The characteristics of breakdown of argon in a long tube (with a gap length of 75 cm and diameter of 2.8 cm) at pressures of 1 and 5 Torr and stationary discharge currents of 5–40 mA were studied experimentally. The breakdown was initiated by paired positive voltage pulses with a rise rate of ∼10{sup 8}–10{sup 9} V/s and duration of ∼1–10 ms. The time interval between pairs was varied in the range of Τ ∼ 0.1–1 s, and that between pulses in a pair was varied from τ = 0.4 ms to ≈Τ/2. The aim of this work was to detect and study the so-called “anomalous memory effect” earlier observed in breakdown in nitrogen. The effect consists in the dynamic breakdown voltage in the second pulse in a pair being higher than in the first pulse (in contrast to the “normal” memory effect, in which the relation between the breakdown voltages is opposite). It is found that this effect is observed when the time interval between pairs of pulses is such that the first pulse in a pair is in the range of the normal memory effect of the preceding pair (under the given conditions, Τ ≈ 0.1–0.4 s). In this case, at τ ∼ 10 ms, the breakdown voltage of the second pulse is higher than the reduced breakdown voltage of the first pulse. Optical observations of the ionization wave preceding breakdown in a long tube show that, in the range of the anomalous memory effect and at smaller values of τ, no ionization wave is detected before breakdown in the second pulse. A qualitative interpretation of the experimental results is given.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Numerical simulation of amplification of frequency-modulated radiation in a gas amplifier THL-100 laser system

Numerical simulation methods have been used to study the effect of frequency modulation of input laser radiation with a pulse duration of 250 ps on the gain characteristics in a gas XeF (C – A) amplifier of the hybrid THL-100 laser system at a pump energy of 270 – 400 J. Using the Wigner distribution function, local radiation spectra are calculated at various points of the input laser beam with positive and negative chirps. The effect of the pump energy of the amplifier on the energy, intensity, and duration of the output laser radiation pulses is investigated. It is shown that with an insignificant change in energy, the pulse duration of the output radiation turns out to be longer with a negative chirp of the input radiation. (lasers)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2006

Sequence Detection for PPM Optical Communication With ISI

A method of sequence detection has been proposed to mitigate the effects of inter-slot interference and inter-symbol interference (both denoted ISI) in the reception of M-ary pulse-position modulation (PPM) optical signals. The method would make it possible to reduce the error rate for a given slot duration, to use a shorter slot duration (and, hence, to communicate at a higher rate) without exceeding a given error rate, or to use a lower-bandwidth (and, hence, less-expensive) receiver to receive a signal of a given slot width without exceeding a given error rate. In M-ary PPM, a symbol period is divided into M time slots, each of duration T(sub s), and a symbol consists of a binary sequence ones and zeros represented by pulses or the absence of pulses, respectively, in the time slots. At the transmitter, the bit stream is used to modulate a laser, the output of which is constant (either full power or zero power, representing 1 or 0, respectively) during each time slot. However, the signal becomes attenuated (signal photons are lost) in propagation from the transmitter to the receiver and noise enters at the receiver, complicating the problem of determining the timing of the symbol periods and slots and identifying the symbols.

Moision, Bruce↗
DOE OSTI2019

Measurements of temporally- and spatially-resolved neutron production in a sheared-flow stabilized Z-pinch

A novel approach using multiple scintillator detectors is applied to measure temporally- and spatially-resolved neutron production in the Fusion Z-pinch Experiment (FuZE) device, a Sheared-Flow Stabilized (SFS) Z-pinch. Diagnosing neutron production from FuZE is important for determining if fusion is thermonuclear and whether the FuZE device can be scaled toward reactor conditions. Absolute yields of up to 2 × 10 5 neutrons per discharge are measured with calibrated plastic scintillator detectors operating in pulse-counting mode. Neutron emission durations of up to ~8 μs are inferred by recording the time difference between the first and last pulses for each discharge. Multiple scintillator detectors located at different positions with respect to the fusing plasma are used to demonstrate that the axial extent of the neutron producing region is comparable to the device volume. Scintillator detectors are well-suited as neutron diagnostics for FuZE and other plasma devices with similar yields and emission durations. Increasing the neutron yield, duration, and volume of the neutron emitting region within the plasma column are significant experimental objectives for FuZE.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
NASA NTRS2012

Search for a Correlation Between Very-High-Energy Gamma Rays and Giant Radio Pulses in the Crab Pulsar

We present the results of a joint observational campaign between the Green Bank radio telescope and the VERITAS gamma-ray telescope, which searched for a correlation between the emission of very-high-energy (VHE) gamma rays ( E(sub Gamma) > 150 GeV) and giant radio pulses (GRPs) from the Crab pulsar at 8.9 GHz. A total of 15,366 GRPs were recorded during 11.6 hr of simultaneous observations, which were made across four nights in 2008 December and in 2009 November and December. We searched for an enhancement of the pulsed gamma-ray emission within time windows placed around the arrival time of the GRP events. In total, eight different time windows with durations ranging from 0.033 ms to 72 s were positioned at three different locations relative to the GRP to search for enhanced gamma-ray emission which lagged, led, or was concurrent with, the GRP event. Furthermore, we performed separate searches on main pulse GRPs and interpulse GRPs and on the most energetic GRPs in our data sample. No significant enhancement of pulsed VHE emission was found in any of the preformed searches. We set upper limits of 5-10 times the average VHE flux of the Crab pulsar on the flux simultaneous with interpulse GRPs on single-rotation-period timescales. On approx. 8 s timescales around interpulse GRPs, we set an upper limit of 2-3 times the average VHE flux. Within the framework of recent models for pulsed VHE emission from the Crab pulsar, the expected VHE-GRP emission correlations are below the derived limits.

RADIO↗
DOE OSTI2014

Aluminium plasma production at high laser intensity

Thick and thin films of Al targets were irradiated in vacuum with iodine laser at 1315 nm wavelength, 300 ps pulse duration at a maximum intensity of about 10{sup 16} W/cm{sup 2} by varying the pulse energy and focal position. The laser-generated plasma was monitored in forward and backward directions by using ion collectors, SiC detectors, Thomson parabola spectrometer, and X-ray streak camera. Ion emission shows maximum proton energy of about 4 MeV in self-focusing conditions and a maximum Al ion energy of about 50 MeV. An evaluation of the electric field driving ions in conditions of target normal sheath acceleration is given.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Emission dynamics of an expanding ultrafast-laser produced Zn plasma under different ambient pressures

We report time and space resolved spectral measurements of neutral Zn emission from an ultrafast laser produced plasma, generated by the irradiation of a Zn target with laser pulses of 100 femtoseconds duration, carried out in a broad ambient pressure range of 0.05 to 100 Torr. The measurement is done for three different axial positions in the expanding plume. The spectra are rich in neutral Zn (Zn I) emissions at 334.5 nm, 468 nm, 472 nm, 481 nm, and 636 nm, respectively, depicting the characteristic triplet structure of Zn. Fast as well as slow peaks are observed in the time of flight data of 481 nm emission, which arise from recombination and atomic contributions, respectively, occurring at different time scales. Average speeds of the fast atomic species do not change appreciably with ambient pressure. The plasma parameters (electron temperature and number density) are evaluated from the measured optical emission spectra. The rates of ionization and recombination can be enhanced by a double-pulse excitation configuration in which optical energy is coupled to the ultrafast plasma through a delayed laser pulse.

36 MATERIALS SCIENCE↗
DOE OSTI2014

One-dimensional electromagnetic band gap structures formed by discharge plasmas in a waveguide

We demonstrate the ability to develop one-dimensional electromagnetic band gap structure in X-band waveguide solely by using the positive columns of glow discharges in neon at the middle pressure. Plasma inhomogeneities are distributed uniformly along a typical X-band waveguide with cross section of 23×10 mm². It is shown that electron densities larger than 10¹⁴ cm ⁻³ are needed in order to create an effective one-dimensional electromagnetic band gap structure. Some applications for using the one-dimensional electromagnetic band gap structure in waveguide as a control of microwave (broadband filter and device for variation of pulse duration) are demonstrated.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2015

High average power, highly brilliant laser-produced plasma source for soft X-ray spectroscopy

In this work, a novel laser-produced plasma source is presented which delivers pulsed broadband soft X-radiation in the range between 100 and 1200 eV. The source was designed in view of long operating hours, high stability, and cost effectiveness. It relies on a rotating and translating metal target and achieves high stability through an on-line monitoring device using a four quadrant extreme ultraviolet diode in a pinhole camera arrangement. The source can be operated with three different laser pulse durations and various target materials and is equipped with two beamlines for simultaneous experiments. Characterization measurements are presented with special emphasis on the source position and emission stability of the source. As a first application, a near edge X-ray absorption fine structure measurement on a thin polyimide foil shows the potential of the source for soft X-ray spectroscopy.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
NASA NTRS1983

Triac failure detector

A failure detector is provided for detecting unidirectional failures in triacs, particularly as used in power factor controllers for induction motors. In a first embodiment, the triac voltage waveform is sensed and upon detection of an unbalanced signal, corresponding to failure of the triac in either the positive or negative direction, the triac is turned full on in both directions. In a second embodiment, a pair of pulsed signals are derived, the pulse durations of which are proportional to the phase difference between the load current and voltage for each half cycle, and the triac is turned full on responsive to a difference in pulse duration between the half cycle signals. An unidirectional open circuit detector is adapted to use a signal from either of the first and second embodiment to turn the triac off in response to an open circuit failure in either direction.

Nola, F. J.↗
DOE OSTI2016

Ultrahigh charging of dust grains by the beam−plasma method for creating a compact neutron source

Generation of high-voltage high-current electron beams in a low-pressure (P = 0.1–1 Torr) gas discharge is studied experimentally as a function of the discharge voltage and the sort and pressure of the plasma-forming gas. The density of the plasma formed by a high-current electron beam is measured. Experiments on ultrahigh charging of targets exposed to a pulsed electron beam with an energy of up to 25 keV, an electron current density of higher than 1 A/cm{sup 2}, a pulse duration of up to 1 μs, and a repetition rate of up to 1 kHz are described. A numerical model of ultrahigh charging of dust grains exposed to a high-energy electron beam is developed. The formation of high-energy positive ions in the field of negatively charged plane and spherical targets is calculated. The calculations performed for a pulse-periodic mode demonstrate the possibility of achieving neutron yields of higher than 10{sup 6} s{sup –1} cm{sup –2} in the case of a plane target and about 10{sup 9} s{sup –1} in the case of 10{sup 3} spherical targets, each with a radius of 250 μm.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2014

Profiling of the injected charge drift current transients by cross-sectional scanning technique

The electric field distribution and charge drift currents in Si particle detectors are analyzed. Profiling of the injected charge drift current transients has been implemented by varying charge injection position within a cross-sectional boundary of the particle detector. The obtained profiles of the induction current density and duration of the injected charge drift pulses fit well the simulated current variations. Induction current transients have been interpreted by different stages of the bipolar and monopolar drift of the injected carriers. Profiles of the injected charge current transients registered in the non-irradiated and neutron irradiated Si diodes are compared. It has been shown that the mixed regime of the competing processes of drift, recombination, and diffusion appears in the measured current profiles on the irradiated samples. The impact of the avalanche effects can be ignored based on the investigations presented. It has been shown that even a simplified dynamic model enabled us to reproduce the main features of the profiled transients of induced charge drift current.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2015

Theory of hysteresis during electron heating of electromagnetic wave scattering by self-organized dust structures in complex plasmas

Dust structuring is a natural and universal process in complex plasmas. The scattering of electromagnetic waves by dust structures is governed by the factor of coherency, i.e., the total number of coherent electrons in a single structure. In the present paper, we consider how the factor of coherency changes due to additional pulse electron heating and show that it obeys a hysteresis. After the end of the pulse heating, the scattering intensity differs substantially from that before heating. There are three necessary conditions for scattering hysteresis: first, the radiation wavelength should be larger than the pattern (structure) size; second, the total number of coherent electrons confined by the structure should be large; and third, the heating pulse duration should be shorter than the characteristic time of dust structure formation. We present the results of numerical calculations using existing models of self-consistent dust structures with either positively or negatively charged dust grains. It is shown that, depending on the grain charge and the ionization rate, two types of hysteresis are possible: one with a final increase of the scattering and the other with a final decrease of the scattering. It is suggested that the hysteresis of coherent scattering can be used as a tool in laboratory experiments and that it can be a basic mechanism explaining the observed hysteresis in radar scattering by noctilucent clouds during active experiments on electron heating in mesosphere.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Inception of electronic damage of matter by photon-driven post-ionization mechanisms

“Probe-before-destroy” methodology permitted diffraction and imaging measurements of intact specimens using ultrabright but highly destructive X-ray free-electron laser (XFEL) pulses. The methodology takes advantage of XFEL pulses ultrashort duration to outrun the destructive nature of the X-rays. Atomic movement, generally on the order of >50 fs, regulates the maximum pulse duration for intact specimen measurements. In this contribution, we report the electronic structure damage of a molecule with ultrashort X-ray pulses under preservation of the atoms' positions. A detailed investigation of the X-ray induced processes revealed that X-ray absorption events in the solvent produce a significant number of solvated electrons within attosecond and femtosecond timescales that are capable of coulombic interactions with the probed molecules. The presented findings show a strong influence on the experimental spectra coming from ionization of the probed atoms' surroundings leading to electronic structure modification much faster than direct absorption of photons. This work calls for consideration of this phenomenon in cases focused on samples embedded in, e.g., solutions or in matrices, which in fact concerns most of the experimental studies.

43 PARTICLE ACCELERATORS↗