Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “frequency”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Combined effects of gas pressure and exciting frequency on electron energy distribution functions in hydrogen capacitively coupled plasmas

The combined effects of the variation of hydrogen pressure (40-400 mTorr) and exciting frequency (13.56-50 MHz) on the electron energy probability function (EEPF) and other plasma parameters in capacitively coupled hydrogen H{sub 2} discharge at fixed discharge voltage were investigated using rf-compensated Langmuir probe. At a fixed exciting frequency of 13.56 MHz, the EEPF evolved from Maxwellian-like distribution to a bi-Maxwellian distribution when the H{sub 2} pressure increased, possibly due to efficient vibrational excitation. The electron density largely increased to a peak value and then decreased with the increase of H{sub 2} pressure. Meanwhile, the electron temperature and plasma potential significantly decrease and reaching a minimum at 120 mTorr beyond, which saturated or slightly increases. On the other hand, the dissipated power and electron density markedly increased with increasing the exciting frequency at fixed H{sub 2} pressure and voltage. The electron temperatures negligibly dependent on the driving frequency. The EEPFs at low pressure 60 mTorr resemble Maxwellian-like distribution and evolve into a bi-Maxwellian type as frequency increased, due to a collisonless (stochastic) sheath-heating in the very high frequency regime, while the EEPF at hydrogen pressure {>=}120 mTorr retained a bi-Maxwellian-type distribution irrespective of the driving frequency. Such evolution of the EEPFs shape with the driving frequency and hydrogen pressure has been discussed on the basis of electron diffusion processes and low threshold-energy inelastic collision processes taking place in the discharge. The ratio of stochastic power to bulk power heating ratio is dependent on the hydrogen pressure while it is independent on the driving frequency.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Nonequilibrium atmospheric pressure plasma jet using a combination of 50 kHz/2 MHz dual-frequency power sources

An atmospheric pressure plasma jet is generated by dual sinusoidal wave (50 kHz and 2 MHz). The dual-frequency plasma jet exhibits the advantages of both low frequency and radio frequency plasmas, namely, the long plasma plume and the high electron density. The radio frequency ignition voltage can be reduced significantly by using dual-frequency excitation compared to the conventional radio frequency without the aid of the low frequency excitation source. A larger operating range of α mode discharge can be obtained using dual-frequency excitation which is important to obtain homogeneous and low-temperature plasma. A larger controllable range of the gas temperature of atmospheric pressure plasma could also be obtained using dual-frequency excitation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

Distinctive plume formation in atmospheric Ar and He plasmas in microwave frequency band and suitability for biomedical applications

Distinctive discharge formation in atmospheric Ar and He plasmas was observed in the microwave frequency band using coaxial transmission line resonators. Ar plasmas formed a plasma plume whereas He formed only confined plasmas. As the frequency increased from 0.9 GHz to 2.45 GHz, the Ar plasma exhibited contraction and filamentation, and the He plasmas were constricted. Various powers and gas flow rates were applied to identify the effect of the electric field and gas flow rate on plasma plume formation. The He plasmas were more strongly affected by the electric field than the Ar plasmas. The breakdown and sustain powers yielded opposite results from those for low-frequency plasmas (∼kHz). The phenomena could be explained by a change in the dominant ionization process with increasing frequency. Penning ionization and the contribution of secondary electrons in sheath region reduced as the frequency increased, leading to less efficient ionization of He because its ionization and excitation energies are higher than those of Ar. The emission spectra showed an increase in the NO and N{sub 2} second positive band in both the Ar and He plasmas with increasing frequency whereas the hydroxyl radical and atomic O peaks did not increase with increasing frequency but were highest at particular frequencies. Further, the frequency effect of properties such as the plasma impedance, electron density, and device efficiency were presented. The study is expected to be helpful for determining the optimal conditions of plasma systems for biomedical applications.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

A comparative study on the frequency effects of the electrical characteristics of the pulsed dielectric barrier discharge in He/O{sub 2} and in Ar/O{sub 2} at atmospheric pressure

In this work, a comparative study on the frequency effects of the electrical characteristics of the pulsed dielectric barrier discharges in He/O{sub 2} and in Ar/O{sub 2} at atmospheric pressure has been performed by means of the numerical simulation based on a 1-D fluid model at frequencies below 100 kHz. The frequency dependences of the characteristic quantities of the discharges in the two gases have been systematically calculated and analyzed under the oxygen concentrations below 2%. The characteristic quantities include the discharge current density, the averaged electron density, the electric field, and the averaged electron temperature. Especially, the frequency effects on the averaged particle densities of the reactive species have also been calculated. This work gives the following significant results. For the two gases, there are two bipolar discharges in one period of applied voltage pulse under the considered frequency range and oxygen concentrations, as occurred in the pure noble gases. The frequency affects the two discharges in He/O{sub 2}, but in Ar/O{sub 2}, it induces a strong effect only on the first discharge. For the first discharge in each gas, there is a characteristic frequency at which the characteristic quantities reach their respective minimum, and this frequency appears earlier for Ar/O{sub 2}. For the second discharge in Ar/O{sub 2}, the averaged electron density presents a slight variation with the frequency. In addition, the discharge in Ar/O{sub 2} is strong and the averaged electron temperature is low, compared to those in He/O{sub 2.} The total averaged particle density of the reactive species in Ar/O{sub 2} is larger than those in He/O{sub 2} by about one order of magnitude.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2019

Real frequency tearing layers with parallel dynamics and the effect on error field locking and resistive wall modes

The response to tearing perturbations of a resonant layer within a toroidal plasma often includes real frequencies. These real frequencies are of importance not only for the stability of the tearing mode but also for determining the response to an error field, including locking torques, and the stability properties when interacting with a resistive wall. Unfortunately, including the physics which drives these frequencies can make the model highly complicated and mask the root physics mechanisms driving the response, making it difficult to draw connections between the physics mechanisms occurring in different regimes. Here, we present a simple and intuitive method to derive the tearing mode layer properties in various regimes of resistive MHD with the key physics effects of pressure gradient, toroidal field line curvature and parallel dynamics, focusing on two important regimes: The resistive-inertial (RI) and the viscoresistive (VR) regimes. The usual Glasser effect, a toroidal effect which involves real frequencies, has been discussed only in the context of the RI regime. We find that it occurs in the VR regime as well. Based on these results and the presence of real frequencies in many other tearing regimes, we reach the conclusion that real frequencies in tearing modes are the rule rather than the exception. We also find that the existence of tearing modes with real frequencies in the VR and RI regimes is related to nearby electrostatic resistive interchange modes, themselves with real frequencies. Another important effect we find is that the threshold for the destabilization of resistive wall tearing modes is significantly lowered by plasma rotation in the presence of real frequency layers.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2012

Absolute frequency measurement for the emission transitions of molecular iodine in the 982 - 985 nm range

We report high-precision frequency measurements of the separate hyperfine structure (HFS) components of the emission B - X system transitions of {sup 127}I{sub 2} molecules in the 982 - 985 nm range. To resolve the HFS of the emission lines, advantage was taken of the method of three-level laser spectroscopy. The function of exciting radiation was fulfilled by the second harmonic of a cw Nd : YAG laser, and the probe radiation in the 968 - 998 nm range was generated by an external-cavity diode laser. The output Nd : YAG laser frequency was locked to an HFS component of the absorption transition and the probing laser radiation to the emission transition component. When both frequencies were locked to HFS components with a common upper level, the output diode laser frequency was precisely equal to the emission transition frequency. The output frequency of the thus stabilised diode laser was measured with the help of a femtosecond optical frequency synthesiser based on a Ti : sapphire laser. We present the results of the absolute frequency measurements of 20 HFS components belonging to six vibrational - rotational transitions of the B - X system of iodine [R56(32 - 48)a1, P58(32 - 48)a1, P85(33 - 48)a1, R87(33 - 48a1, R88(33 - 48)a10] and all 15 components of the R86(33 - 48) line. The relative measurement uncertainty is equal to 7 Multiplication-Sign 10{sup -10} and is determined by the frequency instability of the diode laser radiation.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Impact of Frequency-Watt Control on the Dynamics of a High DER Penetration Power System

Inverter-coupled generation is slowly displacing synchronous generators in the power system, resulting in reduced system inertia and primary frequency response, which makes the system susceptible to larger frequency deviations. Such deviations may result in load shedding or tripping of inverters, thus reducing the reliability. Frequency-watt control of inverters is an autonomous approach to support grid frequency at a fast timescale by adjusting the inverter output with changes in frequency. This study investigates the dynamics of a power system with a high penetration of distributed PV generation with frequency-watt droop control enabled. A simple aggregated reduced-order two-source model of the O'ahu Island (Hawaii) power system is developed and validated against a full dynamic model, and the small-signal stability of the reduced model is then examined the under various PV penetrations. The impact of frequency-watt parameters on the system frequency dynamics is investigated by observing trajectories of small- signal eigenvalues. The results of the eigenvalue analysis are validated through time-domain simulation. Considerations for frequency-watt parameter selection are discussed.

14 SOLAR ENERGY↗
NASA NTRSDate not supplied

In-Orbit Selection of Cryocooler Drive Frequencies for XRISM/Resolve

The XRISM/Resolve instrument cooling system uses adiabatic demagnetization refrigerators (ADRs) to cool the detectors to 50 mK and Joule-Thomson and Stirling cryocoolers to reduce heat load on the He tank supporting the ADRs. Resolve was designed with tunable cryocooler drive frequencies so that interference could be avoided. The cryocooler generated micro-vibration changes with drive frequency and that micro-vibration causes interference and degrades instrument performance. Poor drive frequency choices have been shown to dramatically impact ADR cooling power, temperature stability, detector noise, and degrading spectroscopic performance. However, some choices are free of these features, allowing the instrument to satisfy its performance requirements with significant margin. Thus, the choice of drive frequencies is critical to achieving peak instrument performance. The drive frequencies of the Joule-Thomson cooler, near 52 Hz, and the Stirling coolers, near 15 Hz, were adjusted in three scan sequences while measuring interference to narrow candidate frequency options. Choices were based on stability of the 50 mK control thermometer, changes in the ADR demagnetization rate, and changes in detector noise. These scans were performed routinely during ground testing. Surprisingly though, the results were not repeatable after the Dewar was warmed and re-cooled but were repeatable when remeasured during the same test campaign. Since the good/bad drive frequencies change during Dewar warmup/cooldown, and possibly after vibration events, the drive frequencies could not be finalized before launch. This paper describes the cryocooler frequency scan measurements and compares the results during instrument commissioning to those performed during ground tests.

XRISM↗
DOE OSTI2015

Investigating Power System Primary and Secondary Reserve Interaction under High Wind Penetration Using Frequency Response Model

Power system frequency needs to be maintained close to its nominal value at all times to avoid machine damage, under-frequency load-shedding and even blackouts. Adequate primary frequency response and secondary frequency response are the primary forces to correct an energy imbalance at the second to minute level. As wind energy becomes a larger portion of the world's energy portfolio, there are greater oppotunities for wind to provide frequency response services. This paper addresses one area of frequency control that has been missing in previous work - the reliabilty impacts and interactions between primary and secondary frequency control. The lack of a commercially available tools to simulate the interaction of these two responses has limited the energy industry's understanding of when the depletion of primary control reserve will impact the performance of secondary conrol response or vice versa. To investigate this issue, in this paper we develop a multi-area frequency response integration model with combined primary and secondary frequency control capabilities.

17 WIND ENERGY↗
DOE OSTI2020

Impedance-Based Prediction of SSR-Generated Harmonics in Doubly-Fed Induction Generators

Doubly-fed induction generators (DFIGs) are prone to subsynchronous resonance (SSR) with series-compensated transmission lines and experiencing SSR-generated harmonics of wide-ranging magnitudes. This paper presents a large-signal impedance-based approach for the prediction of the magnitude of SSR-generated harmonics in DFIGs. Note that the large-signal impedance of a network shows variation in its impedance response with the magnitude of perturbation at its terminals. It is discovered in this paper that the impedance of a series-compen- sated transmission line starts changing with the magnitude of SSR because of the metal-oxide varistor-based protection of series capacitors in the line. The changing line impedance halts the growth of the SSR beyond a certain point, where the DFIG forms a limit-cycle mode and emits harmonics at the SSR frequency. The discovered role of the protection of series capacitors in SSR not only enables the prediction of SSR-generated harmonics but also can be used for the timely detection of an SSR event to avoid system disruption and sudden loss of bulk generation. The large- signal impedance-based prediction of SSR-generated harmonics in DFIG-based wind and hydro generators is demonstrated using numerical simulations of a 2.5-MVA, 690-V DFIG supplying to a series-compensated transmission line.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2019

Impedance-Based Characterization of Power System Frequency Response: Preprint

This paper presents an impedance-based noninvasive approach for the characterization of power system frequency response in real time in the absence of a transient event. It measures transfer function response from the injected active power to the frequency at the point of interconnection for the estimation of inertia and primary frequency response of the system. The so-called frequency response function also shows the effects of the speed of primary frequency control. The paper also derives the relationship between the frequency response function and the network impedance seen from the point of interconnection. This relationship highlights how impedance captures the frequency response behavior of a network. The proposed methodology is demonstrated on a modified IEEE 9-bus system with 25% penetration of wind and PV generation; a 5-MW battery energy storage system is used for the injection of active power perturbations for the measurement of the frequency response function.

24 POWER TRANSMISSION AND DISTRIBUTION↗
DOE OSTI2018

The Effects of Low Frequency on the Microstructure and Mechanical Properties of High-Strength Al-Mg Aluminum Alloys by Wire and Double-Pulsed Arc Additive Manufacturing

An innovative and high deposition efficiency process, the double-pulsed arc additive manufacturing process, is used to produce high-strength Al-Mg wall components. This dissertation investigates the electrical parameter acquisition, bead appearance, porosity morphology, microstructure and mechanical properties of additively manufactured high-strength Al-Mg wall components and how these are affected by the low frequency of the double-pulsed arc additive manufacturing process. Then, using this process at different low frequencies of 0, 1, 3, 5, 7 and 9 Hz, multilayer deposition experiments were performed. It can be observed that the deposit porosity is obviously eliminated compared with cold metal transfer (CMT) process. The bead appearance of the deposited aluminum wall varies clearly with low frequencies, and at a low frequency of 3 Hz, its appearance is the best of all. The optical observation of microstructure proves that the average grain size varies significantly with low frequency, and at a low frequency of 1 Hz, it is the smallest. In addition, the deposited wall showed maximum mean microhardness and maximum ultimate tensile strength at a low frequency of 3 Hz. Considering the deposited porosity and mechanical priorities, double-pulsed arc additive manufacturing with a frequency of 3 Hz is more suitable for making high-strength Al-Mg wall components than at other low frequencies.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Tensor-to-scalar ratio forecasts for extended LiteBIRD frequency configurations

LiteBIRD is a planned JAXA-led cosmic microwave background (CMB) B-mode satellite experiment aiming for launch in the late 2020s, with a primary goal of detecting the imprint of primordial inflationary gravitational waves. Its current baseline focal-plane configuration includes 15 frequency bands between 40 and 402 GHz, fulfilling the mission requirements to detect the amplitude of gravitational waves with the total uncertainty on the tensor-to-scalar ratio, δr, down to δr < 0.001. A key aspect of this performance is accurate astrophysical component separation, and the ability to remove polarized thermal dust emission is particularly important. In this paper we note that the CMB frequency spectrum falls off nearly exponentially above 300 GHz relative to the thermal dust spectral energy distribution, and a relatively minor high frequency extension can therefore result in even lower uncertainties and better model reconstructions. Specifically, we compared the baseline design with five extended configurations, while varying the underlying dust modeling, in each of which the High-Frequency Telescope (HFT) frequency range was shifted logarithmically toward higher frequencies, with an upper cutoff ranging between 400 and 600 GHz. In each case, we measured the tensor-to-scalar ratio r uncertainty and bias using both parametric and minimum-variance component-separation algorithms. When the thermal dust sky model includes a spatially varying spectral index and temperature, we find that the statistical uncertainty on r after foreground cleaning may be reduced by as much as 30–50% by extending the upper limit of the frequency range from 400 to 600 GHz, with most of the improvement already gained at 500 GHz. We also note that a broader frequency range leads to higher residuals when fitting an incorrect dust model, but also it is easier to discriminate between models through higher χ 2 sensitivity. Even in the case in which the fitting procedure does not correspond to the underlying dust model in the sky, and when the highest frequency data cannot be modeled with sufficient fidelity and must be excluded from the analysis, the uncertainty on r increases by only about 5% for a 500 GHz configuration compared to the baseline.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2011

Time-Frequency Approach for Stochastic Signal Detection

The detection of events in a stochastic signal has been a subject of great interest. One of the oldest signal processing technique, Fourier Transform of a signal contains information regarding frequency content, but it cannot resolve the exact onset of changes in the frequency, all temporal information is contained in the phase of the transform. On the other hand, Spectrogram is better able to resolve temporal evolution of frequency content, but has a trade-off in time resolution versus frequency resolution in accordance with the uncertainty principle. Therefore, time-frequency representations are considered for energetic characterisation of the non-stationary signals. Wigner Ville Distribution (WVD) is the most prominent quadratic time-frequency signal representation and used for analysing frequency variations in signals.WVD allows for instantaneous frequency estimation at each data point, for a typical temporal resolution of fractions of a second. This paper through simulations describes the way time frequency models are applied for the detection of event in a stochastic signal.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

Simple-design ultra-low phase noise microwave frequency synthesizers for high-performing Cs and Rb vapor-cell atomic clocks

We report on the development and characterization of novel 4.596 GHz and 6.834 GHz microwave frequency synthesizers devoted to be used as local oscillators in high-performance Cs and Rb vapor-cell atomic clocks. The key element of the synthesizers is a custom module that integrates a high spectral purity 100 MHz oven controlled quartz crystal oscillator frequency-multiplied to 1.6 GHz with minor excess noise. Frequency multiplication, division, and mixing stages are then implemented to generate the exact output atomic resonance frequencies. Absolute phase noise performances of the output 4.596 GHz signal are measured to be −109 and −141 dB rad{sup 2}/Hz at 100 Hz and 10 kHz Fourier frequencies, respectively. The phase noise of the 6.834 GHz signal is −105 and −138 dB rad{sup 2}/Hz at 100 Hz and 10 kHz offset frequencies, respectively. The performances of the synthesis chains contribute to the atomic clock short term fractional frequency stability at a level of 3.1 × 10{sup −14} for the Cs cell clock and 2 × 10{sup −14} for the Rb clock at 1 s averaging time. This value is comparable with the clock shot noise limit. We describe the residual phase noise measurements of key components and stages to identify the main limitations of the synthesis chains. The residual frequency stability of synthesis chains is measured to be at the 10{sup −15} level for 1 s integration time. Relevant advantages of the synthesis design, using only commercially available components, are to combine excellent phase noise performances, simple-architecture, low-cost, and to be easily customized for signal output generation at 4.596 GHz or 6.834 GHz for applications to Cs or Rb vapor-cell frequency standards.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2011

THEORETICAL p-MODE OSCILLATION FREQUENCIES FOR THE RAPIDLY ROTATING {delta} SCUTI STAR {alpha} OPHIUCHI

A rotating, two-dimensional stellar model is evolved to match the approximate conditions of {alpha} Oph. Both axisymmetric and nonaxisymmetric oscillation frequencies are computed for two-dimensional rotating models which approximate the properties of {alpha} Oph. These computed frequencies are compared to the observed frequencies. Oscillation calculations are made assuming the eigenfunction can be fitted with six Legendre polynomials, but comparison calculations with eight Legendre polynomials show the frequencies agree to within about 0.26% on average. The surface horizontal shape of the eigenfunctions for the two sets of assumed number of Legendre polynomials agrees less well, but all calculations show significant departures from that of a single Legendre polynomial. It is still possible to determine the large separation, although the small separation is more complicated to estimate. With the addition of the nonaxisymmetric modes with |m| {<=} 4, the frequency space becomes sufficiently dense that it is difficult to comment on the adequacy of the fit of the computed to the observed frequencies. While the nonaxisymmetric frequency mode splitting is no longer uniform, the frequency difference between the frequencies for positive and negative values of the same m remains 2m times the rotation rate.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Low-frequency QPOS and possible changes in the accretion geometry during the outbursts of Aquila X−1

We have studied the evolution of low-frequency quasi-periodic oscillations (LFQPOs) during the rising phase of seven outbursts of the neutron star soft X-ray transient Aql X−1 observed with the RXTE. A frequency correlation between the low-frequency break and the LFQPO sampled on a timescale of ∼2 days was observed. Except for the peculiar 2001 outburst, the frequency of LFQPOs increased with time before the hard-to-soft state transition up to a maximum ν{sub max} at ∼31 Hz, a factor of ∼5 higher than those seen in BH transients such as GX 339−4, making the maximum quasi-periodic oscillation (QPO) frequency a likely indicator of the mass of the central compact object. The characteristic frequencies increased by around 10% per day in the early rising phase and accelerated to nearly 100% per day since ∼2 days before the hard-to-soft state transition. We examined the dependence of the frequency ν{sub LF} on the source flux f and found an anti-correlation between the maximum frequency of the LFQPOs and the corresponding X-ray luminosity of the hard-to-soft transition (or outburst peak luminosity) among the outbursts. We suggest that the X-ray evaporation process cannot be the only mechanism that drives the variation of the inner disk radius if either of the twin kHz QPOs corresponds to the Keplerian frequency at the truncation radius.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

FREQUENCY SHIFTS OF RESONANT MODES OF THE SUN DUE TO NEAR-SURFACE CONVECTIVE SCATTERING

Measurements of oscillation frequencies of the Sun and stars can provide important independent constraints on their internal structure and dynamics. Seismic models of these oscillations are used to connect structure and rotation of the star to its resonant frequencies, which are then compared with observations, the goal being that of minimizing the difference between the two. Even in the case of the Sun, for which structure models are highly tuned, observed frequencies show systematic deviations from modeled frequencies, a phenomenon referred to as the “surface term.” The dominant source of this systematic effect is thought to be vigorous near-surface convection, which is not well accounted for in both stellar modeling and mode-oscillation physics. Here we bring to bear the method of homogenization, applicable in the asymptotic limit of large wavelengths (in comparison to the correlation scale of convection), to characterize the effect of small-scale surface convection on resonant-mode frequencies in the Sun. We show that the full oscillation equations, in the presence of temporally stationary three-dimensional (3D) flows, can be reduced to an effective “quiet-Sun” wave equation with altered sound speed, Brünt–Väisäla frequency, and Lamb frequency. We derive the modified equation and relations for the appropriate averaging of 3D flows and thermal quantities to obtain the properties of this effective medium. Using flows obtained from 3D numerical simulations of near-surface convection, we quantify their effect on solar oscillation frequencies and find that they are shifted systematically and substantially. We argue therefore that consistent interpretations of resonant frequencies must include modifications to the wave equation that effectively capture the impact of vigorous hydrodynamic convection.

79 ASTRONOMY AND ASTROPHYSICS↗