Broad Power Spectrum Research Articles

Integration of fixed-frequency and FM-CW (frequency-modulated continuous-wave) reflectometers for coincident turbulence measurements on LTX-β (Lithium Tokamak eXperiment-β).

The fixed-frequency and frequency-modulated continuous-wave (FM-CW) reflectometers on LTX-β (Lithium Tokamak eXperiment-β) have been configured to use the same transmission lines and antenna arrays for coincident views of the core and edge plasma. The fixed-frequency channels (13.1-20.5 and 20-40GHz, tunable between discharges) provide time-resolved measurements of density fluctuations, while the FM-CW channels (13.1-20.2 and 19.5-33.5GHz) measure the density profile and fluctuations, with high spatial resolution and a sampling rate determined by the frequency sweep interval (5 μs). Data from both reflectometers are synchronously acquired to simultaneously leverage the wide bandwidth and high spatial resolution of the respective systems. Experiments showed that mutual crosstalk interference is momentary and does not diminish the capability of either system. Spectral analysis indicated broad power spectra (several hundreds of kHz) and suggests that the signals from the FM-CW system are consistent with under-sampled fixed-frequency signals. Radial correlations were explored using data from the two reflectometers, as well as from the FM-CW system alone. The core channels showed high levels of agreement between these two comparisons, suggesting that the data from the reflectometers are interchangeable for statistical estimates. For the edge channels, comparisons using data from the FM-CW reflectometer alone showed significant decorrelation due to time lag caused by the finite frequency up-sweep duration. Alternatively, this effect is eliminated when cross-correlating data from the different reflectometers. These results highlight the advantages of operating the fixed-frequency and FM-CW reflectometers in this manner, where the combined system can overcome the limitations of each separate system.

A genetic circuit on a single DNA molecule as an autonomous dissipative nanodevice

Realizing genetic circuits on single DNA molecules as self-encoded dissipative nanodevices is a major step toward miniaturization of autonomous biological systems. A circuit operating on a single DNA implies that genetically encoded proteins localize during coupled transcription-translation to DNA, but a single-molecule measurement demonstrating this has remained a challenge. Here, we use a genetically encoded fluorescent reporter system with improved temporal resolution and observe the synthesis of individual proteins tethered to a DNA molecule by transient complexes of RNA polymerase, messenger RNA, and ribosome. Against expectations in dilute cell-free conditions where equilibrium considerations favor dispersion, these nascent proteins linger long enough to regulate cascaded reactions on the same DNA. We rationally design a pulsatile genetic circuit by encoding an activator and repressor in feedback on the same DNA molecule. Driven by the local synthesis of only several proteins per hour and gene, the circuit dynamics exhibit enhanced variability between individual DNA molecules, and fluctuations with a broad power spectrum. Our results demonstrate that co-expressional localization, as a nonequilibrium process, facilitates single-DNA genetic circuits as dissipative nanodevices, with implications for nanobiotechnology applications and artificial cell design.

Multiband absorber design and optimization with Lorenz chaotic attractor

Metamaterials are artificial materials that don’t come from nature and have unusual electromagnetic properties. It has applications in many fields such as medical, image processing, signal absorption, invisibility cloak, sensor, and antenna. In this study, signal absorption of metamaterials was discussed and a simulation study was carried out. The most important difference of the proposed structure is that the pattern obtained from the Lorenz chaotic attractor was used in the resonator’s design, unlike the geometric shaped works. The chaos pattern consists of units that have both random shapes and sizes. Chaos can be defined as the order of disorder, which is extremely sensitive to initial conditions and has a broad power spectrum like noise. In addition, this study aims to contribute to the science of chaos. The resonator structure used in the proposed structure showed similar results with the studies in the literature. Rogers RT5870 material was used as substrate in the design. The proposed design is an eight-band structure with excellent signal absorption, partially insensitive to polarization and angle of incidence. The results show that the proposed design absorbs signals in the infrared region of the spectrum at 14.24, 15.44, 19.72, 21.04, 33.92, 35.16, 36.44 and 38.79 THz. Near-perfect signal absorption at the indicated frequencies is 97.98%, 98.73%, 99.85%, 92.03%, 95.62%, 92.56%, 96.21% and 97.09%, respectively. The proposed design includes heat sensor and thermal imaging etc can be used in application areas.

Primordial non-Gaussianity up to all orders: Theoretical aspects and implications for primordial black hole models

We develop an exact formalism for the computation of the abundance of primordial black holes (PBHs) in the presence of local non-gaussianity (NG) in the curvature perturbation field. For the first time, we include NG going beyond the widely used quadratic and cubic approximations, and consider a completely generic functional form. Adopting threshold statistics of the compaction function, we address the computation of the abundance both for narrow and broad power spectra. While our formulas are generic, we discuss explicit examples of phenomenological relevance considering the physics case of the curvaton field. We carefully assess under which conditions the conventional perturbative approach can be trusted. In the case of a narrow power spectrum, this happens only if the perturbative expansion is pushed beyond the quadratic order (with the optimal order of truncation that depends on the width of the spectrum). Most importantly, we demonstrate that the perturbative approach is intrinsically flawed when considering broad spectra, in which case only the non-perturbative computation captures the correct result. Finally, we describe the phenomenological relevance of our results for the connection between the abundance of PBHs and the stochastic gravitational wave (GW) background related to their formation. As NGs modify the amplitude of perturbations necessary to produce a given PBHs abundance and boost PBHs production at large scales for broad spectra, modelling these effects is crucial to connect the PBH scenario to its signatures at current and future GWs experiments.

Confronting the primordial black hole scenario with the gravitational-wave events detected by LIGO-Virgo

Adopting a binned method, we model-independently reconstruct the mass function of primordial black holes (PBHs) from GWTC-3 and find that such a PBH mass function can be explained by a broad red-tilted power spectrum of curvature perturbations if all the events (or the small effective spin events) have a primordial origin. Even though GW190521 with component masses in upper mass gap (m>65M⊙) can be naturally interpreted in the PBH scenario, the events (including GW190814, GW190425, GW200105, and GW200115) with component masses in the light mass range (m<3M⊙) are quite unlikely to be explained by binary PBHs although there are no electromagnetic counterparts because the corresponding PBH merger rates are much smaller than those given by LIGO-Virgo. Furthermore, we predict that both the gravitational-wave (GW) background generated by the binary PBHs and the scalar-induced GWs accompanying the formation of PBHs should be detected by the ground-based and space-borne GW detectors and pulsar timing arrays in the future.

Primordial black holes in the excursion set theory

ABSTRACT We study primordial black holes (PBHs) formation in the excursion set theory (EST) in a vast range of PBHs masses with and without confirmed constraints on their abundance. In this work, we introduce a new concept of the first touch in the context of EST for PBHs formation. This new framework takes into account the earlier horizon re-entry of smaller masses. Our study shows that in the EST, it is possible to produce PBHs in different mass range, with enhanced power spectrum, which could make up all dark matter. We also show that in a broad blue-tilted power spectrum, the production of PBHs is dominated by smaller masses. Our analysis puts an upper limit $\sim \,$0.1 on the amplitude of the curvature power spectrum at length scales relevant for PBHs formation.

Gravitational waves from type II axion-like curvaton model and its implication for NANOGrav result

The recent report of NANOGrav is gathering attention since its signal can be explained by a stochastic background of gravitational waves (GWs) with an abundance of ΩGW ∼ 10-9 at the reference frequency f ∼ 10-8.The PBH formation scenario is one of the candidates for the NANOGrav signal, which can simultaneously explain the observed 30M☉ black holes in the binary merger events in LIGO-Virgo collaboration.We focus on the type-II axion-like curvaton model of the PBH formation.In the type II model, the complex field whose phase part is the axion rolls down from the origin of the potential.It is found that type II model achieves the broad power spectrum of the density perturbations and can simultaneously explain the LIGO-Virgo events and the NANOGrav signal.We also improve the treatment of the non-Gaussianity of perturbations in our model to accurately estimate the amplitude of the induced GWs.

Implications of the NANOGrav result on primordial gravitational waves in nonstandard cosmologies

Recently, the NANOGrav collaboration has reported evidence for a common-spectrum stochastic process, which might be interpreted as the first ever detection of stochastic gravitational wave (GW) background. We discuss the possibility of the signal arising from the first and second-order GWs in nonstandard cosmological history. We show that the NANOGrav observation can be explained by the first order GWs in the nonstandard thermal history with an early matter-dominated era, whereas the parameter space required to explain the NANOGrav observation in the standard cosmology or in the nonstandard epoch of kination domination is ruled out by the BBN and CMB observations. For the second-order GWs arising from the large primordial scalar fluctuations, we study the standard radiation domination and two specific nonstandard cases with a few forms of the primordial power spectrum $P_{\zeta}(k)$ to achieve abundant primordial black hole (PBH) production. We find that the NANOGrav observation can be explained with standard radiation domination for all of these $P_{\zeta}(k)$. Furthermore, a dustlike epoch leads to abundant PBH formation for a lower amplitude of $P_{\zeta}(k)$ than the radiation dominated case and complies with the NANOGrav observation only for a few of the all $P_{\zeta}(k)$ forms considered here, where the peak wavenumber is larger than the wavenumber range probed by the NANOGrav. In this nonstandard epoch, for a broad power spectrum, PBHs are produced in a wide mass range in the planetary mass regime. A nonstandard epoch of kination domination cannot produce enough PBH for any of the $P_{\zeta}(k)$ if the NANOGrav result is to be satisfied.

Abundance of primordial black holes in peak theory for an arbitrary power spectrum

Abstract We modify the procedure for estimating the primordial black hole (PBH) abundance proposed in [C.-M. Yoo, T. Harada, J. Garriga, and K. Kohri, Prog. Theor. Exp. Phys. 2018, 123E01 (2018)] so that it can be applied to a broad power spectrum such as the scale-invariant flat power spectrum. In the new procedure we focus on peaks of the Laplacian of the curvature perturbation $\Delta \zeta$, and use the values of $\Delta \zeta$ and $\Delta \Delta \zeta $ at each peak to specify the profile of $\zeta$ as a function of the radial coordinate; the values of $\zeta$ and $\Delta \zeta$ are used in the previous paper. The new procedure decouples the larger-scale environmental effect from the estimate of PBH abundance. Because the redundant variance due to the environmental effect is eliminated, we obtain a narrower shape of the mass spectrum compared to the previous procedure. Furthermore, the new procedure allows us to estimate the PBH abundance for the scale-invariant flat power spectrum by introducing a window function. Although the final result depends on the choice of the window function, we show that the $k$-space tophat window minimizes the extra reduction of the mass spectrum due to the window function. That is, the $k$-space tophat window has the minimum required property in the theoretical PBH estimation. Our procedure makes it possible to calculate the PBH mass spectrum for an arbitrary power spectrum by using a plausible PBH formation criterion with the nonlinear relation taken into account.

Lower Hybrid Current Drive in High Aspect Ratio Tokamaks

A multi-machine study has been carried out to investigate the impact of a strongly bounded wave propagation domain on the Lower Hybrid current drive, a condition which occurs principally in high aspect ratio tokamaks. In this regime, the condition of kinetic resonance can be far above the upper boundary of the propagation domain, and may not be achieved by the usual toroidal upshift. Therefore no tail of fast electrons can be pulled out from the thermal bulk. Nevertheless, while tokamak plasmas are in principle almost transparent to the wave in this regime so-called “unbridgeable spectral gap”, full current drive is well achieved for the two tokamaks considered in this study, TRIAM-1M (Zushi et al. Nucl Fusion 43:1600, 2003) and WEST (Bourdelle et al. Nucl Fusion 55:063017, 2015), both characterized by a very large aspect ratio \(R/a>5.5\). The case of the high aspect ratio tokamak HL-2A (Liu et al. Nucl Fusion 45:S239, 2005) for which the wave propagation domain has also an upper boundary, but close to the resonance condition, is considered by comparison. First principles modeling of the rf-driven current and the fast electron bremsstrahlung using the ALOHA/C3PO/LUKE/R5-X2 chain of codes shows unambiguously that the spectral gap must be already filled at the separatrix in order to reproduce quantitatively observations and some important parametric dependencies. This result is an important milestone in the physics understanding of the Lower Hybrid current drive, highlighting the existence of a powerful and likely universal alternative mechanism to bridge the spectral gap, that is not related to toroidal magnetic refraction. With an initially broad power spectrum, lobes with low parallel refractive indexes that carry most of the plasma current can be absorbed in almost single pass, restoring the full validity of the ray-tracing approximation for describing the propagation of the Lower Hybrid wave in cold plasmas.

The ineludible non-Gaussianity of the primordial black hole abundance

We study the formation of primordial black holes when they are generated by the collapse of large overdensities in the early universe. Since the density contrast is related to the comoving curvature perturbation by a nonlinear relation, the overdensity statistics is unavoidably non-Gaussian. We show that the abundance of primordial black holes at formation may not be captured by a perturbative approach which retains the first few cumulants of the non-Gaussian probability distribution. We provide two techniques to calculate the non-Gaussian abundance of primordial black holes at formation, one based on peak theory and the other on threshold statistics. Our results show that the unavoidable non-Gaussian nature of the inhomogeneities in the energy density makes it harder to generate PBHs. We provide simple (semi-)analytical expressions to calculate the non-Gaussian abundances of the primordial black holes and show that for both narrow and broad power spectra the gaussian case from threshold statistics is reproduced by increasing the amplitude of the power spectrum by a factor \U0001d4aa(2÷ 3).

Hematocrit-insensitive Absolute Blood Flow Rate Measurement in 0.5-mm-diameter Flow Channel Using MEMS-based Laser Doppler Velocimeter with Signal Modification for Detecting Beat Frequency from Broad Power Spectrum

Hematocrit-insensitive Absolute Blood Flow Rate Measurement in 0.5-mm-diameter Flow Channel Using MEMS-based Laser Doppler Velocimeter with Signal Modification for Detecting Beat Frequency from Broad Power Spectrum

Characterization of translating tornado-induced pressures and responses of a low-rise building frame based on measurement data

This study presents an analysis and characterization of dynamic wind pressures and internal forces of a low-rise building frame using measurement data obtained from a tornado simulator. The deterministic time-varying mean pressure component is extracted from the time history record using discrete wavelet transformation (DWT) approach. A continuous wavelet transformation (CWT) based approach is employed for determining the evolutionary power spectral density (EPSD) functions and time-varying standard deviation (STD) as well as co-variance of dynamic pressures. The results showed that the characteristics of dynamic pressures are strongly affected by the location of tornado, and are very different from those under straight-line wind. The pressure drop caused by tornado vortex plays a key role in defining the pressure characteristics. The building frame responses are also calculated from the statistics of pressures. The equivalent static wind loads (ESWLs) causing peak responses are defined by using the gust response factor approach. The effect of translation speed of tornado vortex is also investigated. The translating tornado vortex leads to a delay of occurrence of maximum time-varying mean and STD, and reduction in maximum STD thus peak response. It also makes the energy distribution of pressure fluctuations shifted to higher frequencies with a broader power spectrum. Finally, it is shown that the tornado-induced responses are significantly higher than those from ASCE 7-10.

Generation of Langmuir wave supercontinuum by phase-preserving equilibration of plasmons with irreversible wave–particle interaction

We report the observation of supercontinuum of Langmuir plasma waves, that exhibits broad power spectrum having significant spatio-temporal coherence grown from a monochromatic seed-wave, in one-dimensional particle-in-cell simulations. The Langmuir wave supercontinuum (LWSC) is formed when the seed wave excites side-band fields efficiently by the modulational instabilities. Its identification is achieved by the use of the tricoherence analysis, which detects four wave mixings (FWMs) of plasmons (plasma wave quanta), and evaluation of the first order coherence, which is a measure of temporal coherence, of the wave electric fields. The irreversible evolution to the coherent LWSC from the seed wave is realized by the wave–particle interactions causing stochastic electron motions in the phase space and the coherence of LWSC is maintained by the phase-preserving FWMs of plasmons. The LWSC corresponds to a quasi Bernstein-Greene-Kruskal mode.

Stochastic generation of MAC waves and implications for convection in Earth’s core

Author(s): Buffett, B; Knezek, N | Abstract: Convection in Earth's core can sustain magnetic-Archemedes-Coriolis (MAC) waves through a variety of mechanisms. Buoyancy and Lorentz forces are viable sources for wave motion, together with the effects of magnetic induction. We develop a quantitative description for zonal MAC waves and assess the source mechanisms using a numerical dynamo model. The largest sources at conditions accessible to the dynamo model are due to buoyancy forces and magnetic induction. However, when these sources are extrapolated to conditions expected in Earth's core, the Lorentz force emerges as the dominant generation mechanism. This source is expected to produce wave velocities of roughly 2 km yr-1 when the internal magnetic field is characterized by a dimensionless Elsasser number of roughly Λ ≈ 10 and the root-meansquare convective velocity defines a magnetic Reynolds number of Rm ≈ 103. Our preferred model has a radially varying stratification and a constant (radial) background magnetic field. It predicts a broad power spectrum for the wave velocity with most power distributed across periods from 30 to 100 yr.

Inflationary theory and pulsar timing investigations of primordial black holes and gravitational waves

The gravitational waves measured at LIGO are presumed here to come from merging primordial black holes. We ask how these primordial black holes could arise through inflationary models while not conflicting with current experiments. Among the approaches that work, we investigate the opportunity for corroboration through experimental probes of gravitational waves at pulsar timing arrays. We provide examples of theories that are already ruled out, theories that will soon be probed, and theories that will not be tested in the foreseeable future. The models that are most strongly constrained are those with a relatively broad primordial power spectrum.

Features of parkinsonian and essential tremor of the human hand

Using spectral, wavelet, multifractal, and recurrence analyses we examined the features of involuntary shaking (tremor) arising during the performance of a given motor task. The task was to maintain the efforts of fingers under isometric conditions by a healthy subject, a patient with primary bilateral parkinsonism, and a patient with a syndrome of the essential tremor. The physiological tremor was characterized by the minimal amplitude, the broad power spectrum, the minimal energy of the wavelet spectrum, the maximal degree of multifractality, the minimal degree of determinism, and the maximal entropy of recurrence time density. In the case of the essential tremor we observed the significant enhancement of the wavelet spectrum energy and the decrease of the oscillation complexity. It was evident as the appearance of clear peaks in the power spectra, the fall in the multifractality degree, the emergence of a quasiperiodic structure in the recurrence diagrams, the grow of determinism and the decrease of the entropy of recurrence time density. All these trends were enhanced for the parkinsonian tremor. The suggested characteristics enable us to estimate the degree of deviation of the motor function from the healthy one.

Adjustable, (super)hydrophobicity by e-beam deposition of nanostructured PTFE on textured silicon surfaces

Polytetrafluoroethylene (PTFE)-like films, produced by electron beam (e-beam) deposition, have shown higher hydrophobicity than those deposited by RF sputtering under similar deposition rates. It was found that this results from both surface chemical composition and nano-roughness. X-ray photoelectron spectroscopy measurements revealed that larger moieties of CF2 and CF3 groups were present to reduce surface energy in the e-beam deposited films. RF sputtering led to a higher degree of PTFE target fragmentation producing a different perfluorinated film on the Si substrate. Scanning electron microscopy and atomic force microscopy measurements revealed a much larger rms roughness on the film surfaces produced by e-beam (25.13 nm, at 20 mA) than those by RF sputtering (2.42 nm, at 100 W), and allowed a broad power spectrum density analysis with determination of the κ B wetting parameter. In addition, the e-beam deposited films presented a linear increase of contact angle with applied electron current in the range under study (5–20 mA). This allows easy water repellency adjustment, up to 159 ± 2°. For a superhydrophobic state with self-cleaning, a micro-pyramid structure was wet etched on the Si wafer, followed by PTFE deposition, and a very low contact angle (163 ± 2°) and hysteresis was attained (<3°). These first results indicate that e-beam PTFE deposition with adjustable hydrophobicity may become a useful technique for integrated production with present Si microelectronics technology and for Si solar cells.

WAVE TRANSITION AND TRAPPING BY SUDDENLY CREATED PERIODIC PLASMA

Theory, numerical simulation, and experiment on the interaction of electromagnetic wave with suddenly created periodic plasma layers are presented. In the experiment, frequency-downshifted signals of considerably large spectral width and enhanced spectral intensity were detected. Numerical simulation of the experiment, that the plasma has a finite periodic structure and is created much faster than its decay, shows that the frequency downshifted waves have a broad power spectrum and are trapped in this plasma crystal until the plasma frequency drops to become less than the wave frequency. The spectral power increases exponentially with the frequency of the frequency downshifted wave, consistent with the experiment. The simulation reveals that wave trapping results in accumulating the frequency-downshifted waves generated in the finite transition period of plasma creation and decay. Though frequency-upshifted signals were missing in the experimental measurement, it might be attributed to the collision damping of the plasma.

Perceptual Sensitivity to High-Frequency Interaural Time Differences Created by Rustling Sounds

Interaural time differences (ITDs) can be used to localize sounds in the horizontal plane. ITDs can be extracted from either the fine structure of low-frequency sounds or from the envelopes of high-frequency sounds. Studies of the latter have included stimuli with periodic envelopes like amplitude-modulated tones or transposed stimuli, and high-pass filtered Gaussian noises. Here, four experiments are presented investigating the perceptual relevance of ITD cues in synthetic and recorded "rustling" sounds. Both share the broad long-term power spectrum with Gaussian noise but provide more pronounced envelope fluctuations than Gaussian noise, quantified by an increased waveform fourth moment, W. The current data show that the JNDs in ITD for band-pass rustling sounds tended to improve with increasing W and with increasing bandwidth when the sounds were band limited. In contrast, no influence of W on JND was observed for broadband sounds, apparently because of listeners' sensitivity to ITD in low-frequency fine structure, present in the broadband sounds. Second, it is shown that for high-frequency rustling sounds ITD JNDs can be as low as 30 μs. The third result was that the amount of dominance for ITD extraction of low frequencies decreases systematically with increasing amount of envelope fluctuations. Finally, it is shown that despite the exceptionally good envelope ITD sensitivity evident with high-frequency rustling sounds, minimum audible angles of both synthetic and recorded high-frequency rustling sounds in virtual acoustic space are still best when the angular information is mediated by interaural level differences.