Incoherent Fluctuations Research Articles

Variability of 22 GHz H2O masers in circumstellar shells

We have monitored different types of late-type stars in the 22 GHz maser line of water vapor with single-dish telescopes and the Very Large Array over several years. We find that the emission pattern in the circumstellar shells of Semi-Regular (SR) and Mira variables mapped by the VLA remain stable on the scale of several years while the maser profiles from the single-dish telescopes vary strongly. Thus the strong variability is mainly due to incoherent intensity fluctuations of the individual maser lines, which have lifetimes less than a year.In contrast to this, the variability in OH/IR stars and M-Supergiants is more regular and is mainly a response to the long-period variations of the central star. H2O maser in many OH/IR stars are “extinguished” most of the time, as the excitation temperature is only high enough during the phases close to the maximum of the variability.The strength of the profile variability is decreasing as a function of the radial distance of the maser shell from the star. This gives a natural explanation for the increasing regularity of the maser variations along the sequence SR-, Mira variables, OH/IR stars, M-Supergiants.

Low-frequency electromagnetic fluctuations in a plasma with strong external magnetic field

A general approach to the description of the fluctuations of the electric and magnetic fields in a plasma placed in a strong external magnetic field is developed. The wave dispersion is considered and the dependences of the damping rates of the Alfvén and magnetosonic waves on the propagation angle and parameters of the plasma are analysed. The spectral distributions of the fluctuations of the electric and magnetic fields in the frequency ranges and of the spectrum for the plasma with strong external magnetic field are studied in detail. The spectral distributions of the fluctuations of the electric and magnetic fields in non-equilibrium plasmas are treated. Effective temperatures have been introduced both for the collective Alfvén and magnetosonic fluctuations and for the incoherent fluctuations in low-frequency range; they can be used as convenient parameters for the description of the non-equilibrium state of plasmas in diagnostics in facilities devised for realization of the controlled thermonuclear fusion.

The dispersion and attenuation of zero sound in normal fermi-liquid due to the spin fluctuations

The scattering of zero-sound phonons by incoherent spin fluctuations (SF) and its influence on the velocity dispersion and attenuation of zero sound in exchange enhanced Fermi-liquid are considered.

Imaging detector of temporally coherent radiation

We present an optical architecture and image processor capable of detecting and locating temporally coherent radiation that may be dominated by incoherent background radiation. The optical architecture makes use of a coherent light modulator that modulates light of sufficient coherence length while it leaves light of short coherence length unmodulated. The design of the coherent light modulator offers a substantially wider field of view than did past designs, permitting its application within an imaging system. The image processor synchronously detects the modulation imposed on coherent light while it rejects incoherent light fluctuations. Results of a laboratory test are presented. The system tested in the laboratory had a 26 degrees field of view and was able to detect and locate coherent radiation >30 dB below the background incoherent light level.

The near wake region of a high solidity mesh strip

A pattern‐recognition analysis of the near region of a plane turbulent wake generated by a high solidity mesh strip reveals a markedly regular vortex street. In fact, the ratio of the ‘‘coherent’’ to ‘‘incoherent’’ (or ‘‘random’’) fluctuations is significantly larger than for many other plane turbulent wakes of similar Reynolds number. Since these incoherent fluctuations are so weak, the vortex street persists. This is in contrast to what is observed to happen in many other wakes, where secondary vortices, which are associated with the incoherent fluctuations, act to deform the main vortices of the vortex street into horseshoe‐like vortices very early in the development. Here, the secondary structures must be too weak in the near region to influence the vortex street to any significant effect.

Structure of Doppler peaks induced by active perturbations.

We investigate how the qualitative structure of Doppler peaks in the angular power spectrum of the cosmic microwave anisotropy is affected by basic assumptions going into theories of structure formation. We define the concepts of ``coherent'' and ``incoherent'' fluctuations, and also of ``active'' and ``passive'' fluctuations. In these terms inflationary fluctuations are passive and coherent while topological defects are active incoherent fluctuations. Causality and scale invariance are shown to have different implementations in theories differing in the above senses. We then extend the formalism of Hu and Sugiyama to treat models with cosmic defects. Using this formalism we show that the existence or absence of secondary Doppler peaks and the rough placing of the primary peak are very sensitive to the fundamental properties defined. We claim therefore that even a rough measurement of the angular power spectrum $C_l$ shape at $100<l<1500$ ought to tell us which are the basic ingredients to be used in the right structure formation theory. We also apply our formalism to cosmic string theories. These are shown to fall into the class of active incoherent theories for which one can robustly predict the absence of secondary Doppler peaks. The placing of the cosmic strings' primary peak is more uncertain, but should fall in $l\approx400-600$.

Spin dynamics of chromium. I. Formalism and commensurate alloys.

The spin dynamics of chromium alloys are produced by the fluctuations about a spin-density wave (SDW) consisting of bound electron-hole pairs. While commensurate (C) alloys have a single SDW with wave vector G/2=2\ensuremath{\pi}/a, incommensurate (I) alloys have two SDW's with wave vectors on either side of G/2. Spin fluctuations with frequency \ensuremath{\omega} and wave vector q correspond to possible quasiparticle transitions between the two (C) or three (I) bands of hybridized electron and hole energies with energy change \ensuremath{\omega} and momentum change q. This paper develops the random-phase approximation for the spin dynamics of both C and I alloys. The collective excitations of C alloys consist of a transverse spin-wave (SW) mode with linear dispersion and a longitudinal amplitude mode. Incoherent spin fluctuations lie above the amplitude mode frequency and below the SW frequency. While the SW mode involves the rigid rotation of the local magnetic moments, the amplitude mode involves the oscillation of the SDW amplitude, which decays in time according to a power law. \textcopyright{} 1996 The American Physical Society.

Experimental Observation of Magnetic Fluctuations in NBI Heated Plasmas in CHS

The magnetic fluctuations with the frequency f ≤100 kHz in NBI heated plasmas have been investigated in the Compact Helical System (CHS). The observed coherent fluctuations are identified as global modes with m ≤4, n ≤3. The dependences of these modes on the magnetic configuration and the plasma beta are well interpreted phenomenologically by the theoretical models of the resistive and the ideal interchange modes. Both these modes and the incoherent fluctuations increase with the beta value , however, saturate when reaches about 1%. The present accessible limit of =2.0% is determined by the confinement of which a key mechnism is not the instabilities investigated here. The nonlinear saturation mechanism must be clarified, because the change of the fluctuation amplitude can not be explained from the model that the saturation level is proportional to the linear growth rate of an instability.

Low-Frequency electrostatic fluctuations in magnetized plasma: microscopic theory

The classical approach together with the quasiparticle method are used to elaborate a microscopic theory of low-frequency (with respect to the ion cyclotron frequency) fluctuations in magnetized plasmas. The spectral distributions of electric field and density fluctuations are calculated in the electrostatic approximation, and the conditions for the transition from incoherent (spontaneous) fluctuations to collective fluctuations in the low-frequency oscillating magnetized plasma are studied. The significance of particle polarization drifts and finite-Larmor-radius effects, as well as of ion motion in such transition is explained.

Observation of density fluctuations from the infrared bremsstrahlung emission on TFTR

The infrared emission of beam heated TFTR discharges is studied using the high collection efficiency TFTR Thompson scattering system. It is found that emission in the vicinity of 9800 Å can be used to study fluctuations in the bremsstrahlung emission. Low-frequency (&amp;lt;40 kHz) fluctuations that are associated with MHD modes and high-frequency (≳200 kHz) bursts of fluctuations are observed. Both coherent and incoherent fluctuations exist. In some cases, the observed coherent fluctuations can be interpreted as being due to density fluctuations as high as 5%. The rms value of broadband density turbulence in the near central regions of the plasma is estimated to be about 1%.

Absorption at the ion–ion hybrid resonance in the presence of incoherent density fluctuations

A numerical study is made of the effects caused by incoherent fluctuations in the minority density concentration on the absorption of fast Alfven waves at the ion–ion hybrid resonance. This work extends to the incoherent regime of a previous study by Romero and Morales [Phys. Fluids B 2, 2007 (1990)], which surveyed in detail the effects of single‐mode (coherent) fluctuations. The system is represented by a Budden equation that properly describes the resonance and cutoff structure arising from spatial variations of the equilibrium magnetic field. For wave incidence such that the cutoff is encountered before the resonance layer, the absorption coefficient increases monotonically as the fluctuation amplitude is increased. A specific case is presented in which the absorption changes from ∼0% to ∼70% for minority density fluctuations δn/n0∼60% that correspond to electron density variations of less than 5%. For wave incidence such that the resonance is encountered before the cutoff, the reflection coefficient ...

Detection of pulse to pulse timing jitter in periodically gain-switched semiconductor lasers

Methods to assess pulse to pulse timing jitter in periodicallly gain-switched semiconductor lasers are investigated. The power spectrum of the optical pulse train for mutually incoherent fluctuations of the turn-on delay time and the pulse amplitude is calculated. The result shows that a measurement of the power spectrum alone is not sufficient for the characterization of the inherent timing jitter. Additional knowledge of the optical waveform and the transfer characteristics of the detection system is necessary.

Far-infrared scattering experiment on a bumpy torus

A far-infrared scattering experiment was installed on the ELMO bumpy torus experiment at Oak Ridge National Laboratory [Plasma Physics and Controlled Nuclear Fusion Research (IAEA, Vienna, 1975), Vol. II, p. 141]. The design allowed for coupled scanning of spatial and wave vector parameters while recording the frequency spectrum. Coherent waves and incoherent density fluctuations were observed for varying plasma conditions. Also, coherent waves were recorded in conjunction with ion cyclotron heating fast wave heating experiments, which demonstrated fast wave penetration into the plasma and ion-Bernstein wave conversion between harmonic zones.

Incoherent turbulence structure in the near wake of a normal plate

In this paper experimentally studied characteristics of various frequency components of incoherent velocity fluctuations in the near wake behind a thin normal plate immersed in a uniform flow are described. Measurements were made at a position 8 plate heights downstream of the plate where the wake had a marked periodicity, so that coherent vortices shed from the plate are expected to have a small dispersion in streamwise spacing, transverse location, strength and shape. Shearing stress associated with the incoherent fluctuations is mainly contributed by components with frequencies around half of the vortex-shedding frequency fs on one side of the wake. The ½fs frequency component appears to be caused by the spanwise locations of ribs, which are connected to the coherent vortices, being different from vortex to vortex. A probable spanwise arrangement of the ribs is suggested.

Spectral signatures of jumps and turbulence in interplanetary speed and magnetic field data

The low‐frequency power spectra of the magnitude of the magnetic and velocity fields in the solar wind frequently have a somewhat steeper than f−2 dependence, whereas the vector components of these quantities often have spectra characteristic of turbulence (f−5/3) with a break to a flatter spectrum at low frequencies. It is shown here that, consistent with a suggestion of Burlaga and Mish, the f−2 spectra in the magnitudes result from jumps due to various rapid changes in the time series for these quantities. If these jumps are removed from the data, the spectra of the resulting “difference” time series have the f−5/3 form. We conclude that f−2 spectra in the magnitudes of the magnetic field and velocity in the solar wind arise from phase coherent structures that can be distinguished clearly from incoherent turbulent fluctuations.

Self-consistency constraints on turbulent magnetic transport and relaxation in a collisionless plasma.

Novel constraints on collisionless relaxation and transport in drift-Alfv\'en turbulence are reported. These constraints arise as a result of the effects of mode coupling and incoherent fluctuations as manifested by the proper application of self-consistency conditions. The result that electrostatic fluctuations alone regulate transport in drift-Alfv\'en turbulence follows directly. Quasilinear transport predictions are discussed in light of these constraints.

Fluctuation-induced spinodal decomposition in coherent metal-hydrogen systems

The behavior of a coherent metal-hydrogen system quenched into the unstable two-phase region is examined. A stochastic theory including thermal fluctuations describes the coupling of the density modes to the environment. We conclude that the thermal fluctuations could be the dominant mechanism inducing the spinodal decomposition. In order to stimulate further studies, we give specific results for a niobium-hydrogen solution. The role of point defects in spoiling the effect of the incoherent thermal fluctuations is also analyzed.

Small-angle neutron scattering from Al-Zn and AI-Zn-Mg alloys during Guinier-Preston zone formation

Small-angle neutron scattering from A1-6.8 at % Zn and Al-6.8 at % Zn-0.1 at % Mg alloys during Guinier-Preston zone formation have been measured at KUR (Kyoto University Reactor). The samples were quenched from 300 °C into brine at -20 °C and kept at 77 K. They were aged at 40 °C for various periods ranging from 0 up to 1000 minutes each with subsequent neutron measurement at 77 K. The observed temporal evolution of the cross section can be interpreted qualitatively in terms of Saito's formula which consists of two distinct parts due to the incoherent anomalous fluctuation and to the coherent ordered structure of the concentration.

Simple spinless mixed-valence model. II. Solution of the isolatedf-level problem

We consider an isolated spinless localized $4f$ level interacting with a spinless Fermi sea through a hybridization and a Coulomb repulsion. The problem is mapped onto the anisotropic Kondo problem in an external magnetic field. Using the Ward identity for the total charge conservation we derive an exact relation between the specific heat and the charge susceptibility at low temperatures. Using a renormalization-group approach and the analogy to the x-ray threshold problem, we show that the impurity behaves like a resonant level with a renormalized resonance width. We give expressions for the charge susceptibility and the specific heat. The extension to a compound of noninteracting $f$-level impurities shows that discontinuous valence transitions are possible in models involving local and incoherent valence fluctuations.

Equivalent strain range due to random thermal fluctuations: Effect of spatial incoherence

Metal components in LMFBR's are subjected to random temperature fluctuations. These are caused by the turbulence in the liquid sodium coolant, and the existence of jets at differing inlet temperatures. In this paper the mean square stresses, and the mean square equivalent strain range are calculated for tubes and beams subjected to spatially incoherent temperature fluctuations. This may be used as a measure of crack initiation likelihood. The results depend on the thermal and mechanical boundary conditions, and on the integral scale of the spatial coherence. The mean square equivalent strain range is not necessarily reduced by the reduced spatial coherence.