Azimuthal Gradient Research Articles

Focusing limit of a cyclotron: Axial betatron instability against beam dynamics approach

In an isochronous relativistic cyclotron, axial defocusing of a beam caused by the radial growth of the isochronous magnetic field is compensated by the azimuthal field gradient introduced by sectors. The focusing capabilities of sectors set the maximum ion energy obtainable from the machine. Usually, the focusing limit of a machine is determined by using the criterion for axial beam instability evolving from the equations of betatron oscillations. The obtained value of the focusing limit is approximate because the equations of betatron oscillations it originates from are approximate as well. The accurate value of the focusing limit is obtained by simulating accelerated beam dynamics in the extraction region. It is shown that the focusing limit of a cyclotron resulting from the two methods could differ for more than 9%. The suggested third method for focusing limit computation relies on the beam dynamics simulation along the critical equilibrium orbit rather than the acceleration orbit and thus it is less time consuming although equally accurate.

Ion precipitation in the dawn sector during geomagnetic storms

Three southward turnings of the IMF during the magnetic storm of 29–31 October 2003 led to ring current intensifications characterized by Dst minima. Defense Meteorological Satellite Program (DMSP) satellites detected low‐energy ions precipitating equatorward of auroral electrons in the dawn/morning local time sector. These ion fluxes weakened and/or vanished subsequent to Dst recoveries. A survey of the DMSP database reveals that near‐dawn ion precipitation is a main‐phase characteristic of all large magnetic storms. DMSP and Combined Release and Radiation Effects Satellite (CRRES) satellites also detected similar ion precipitation during the main phase of the March 1991 magnetic storm. To reconcile these observations with elementary concepts of ion drifts, the data suggest two source populations. The lowest‐energy ions were initially energized earthward of the plasma sheet electron boundary in the evening sector then corotated eastward. Higher‐energy ions originated in the plasma sheet and drifted close to the Earth under the combined influences of time‐varying convective electric fields and azimuthal gradients in the Earth's magnetic field. We developed a modified Volland Stern model to study test‐particle trajectories in asymmetric magnetic fields and variable electric fields compatible with CRRES measurements. Ions with magnetic moments μ ≥ μc ≈ 14 eV/nT drift duskward. Ions with μ < μc are injected to low L shells during a spike in the convective electric field. As the electric field diminishes and shielding increases, these ions drift toward dawn much closer to Earth than any test electron trajectories. Precipitation in the dawn sector results from a combination of pitch‐angle scattering by intense ambient waves and the dictates of ion drift paths.

Heat transfer and crisis in swirl flow boiling

The lecture considers general features of swirl flows. For single-phase convection, both theoretical and experimental results obtained up to-day give rather clear understanding of the main mechanisms of energy and momentum transfer in tubes with twisted tape inserts. At one-side heating the high temperature azimuthal gradient exists at the inside surface, that requires the well-founded choice of reference temperature for physical properties used in calculations of HTC. Heat transfer in boiling of subcooled liquid in swirl flow at uniform heating can be successfully described by superposition of the known predicting equations for single-phase convection and nucleate boiling. Under one-side heating condition different heat transfer modes are observed along the circumference of the cooled channel. Practical recommendation on the prediction of HTC at nucleate boiling of subcooled liquid in swirl flow at one-side heating has been given in this paper. Boiling crisis in swirl flow at uniform heating can be prevented either by centrifugal body forces or by single-phase convection from the bubbly layer surface at the heated wall to the cold flow core. The greater from heat fluxes controlled by these two mechanisms determines CHF. At one-side heating in swirl flow CHF is essentially higher than under uniform heating condition. Based on the experimental measurements, the conclusion is stated that at high flow velocity and high liquid subcooling in swirl flow the thermodynamical limit of CHF has been achieved.

A Fine Point on Light’s Angular Momentum

Padgett and Allen reply: In our experiment, the particle was Teflon and had an absorption of a few percent, which meant it could be trapped within optical tweezers. In the review, we called it transparent to contrast it with the earlier work of H. He and coworkers. 1 1. H. He, M. E. J. Friese, N. R. Heckenberg, H. Rubinsztein-Dunlop, Phys. Rev. Lett. 75, 826 (1995). https://doi.org/10.1103/PhysRevLett.75.826 In hind-sight, as Anthony Siegman suggests, we should have described it as “slightly, or partially, absorbing.”Both the spin and orbital angular momentum of a beam can always be calculated from the transverse components of linear momentum. This transverse linear momentum arises both from the azimuthal phase gradient (orbital AM) and a combination of the beam’s intensity gradient and polarization (spin AM). For circularly polarized annular beams, as Siegman correctly describes, the azimuthal linear momentum from the spin contribution has equal and opposite senses on the inner and outer edges of the ring. As a consequence the particle spins, as shown in figure 4 of our article. A detailed consideration can be found in reference 2 2. L. Allen, M. J. Padgett, Opt. Commun. 184, 67 (2000). https://doi.org/10.1016/S0030-4018(00)00960-3 .REFERENCESSection:ChooseTop of pageREFERENCES <<1. H. He, M. E. J. Friese, N. R. Heckenberg, H. Rubinsztein-Dunlop, Phys. Rev. Lett. 75, 826 (1995). https://doi.org/10.1103/PhysRevLett.75.826 , Google ScholarCrossref2. L. Allen, M. J. Padgett, Opt. Commun. 184, 67 (2000). https://doi.org/10.1016/S0030-4018(00)00960-3 , Google ScholarCrossref© 2005 American Institute of Physics.

Effect of Nonuniform Beam Filling on the Propagation of Radar Signals at X-Band Frequencies. Part II: Examination of Differential Phase Shift

Abstract An analysis of the effects of nonuniform beam filling on the measurement of the propagation phase shift for X-band radar is presented. An analytical expression is derived for the measured differential phase shift. It is shown that the phase measurement is weighted by the reflectivities and the attenuation within the sampling volume and that this may lead to negative values in the retrieved specific differential phase shift KDP. These effects are compared with those due to the influence of the backscattering phase shift δ. Both effects are quantified using numerical simulations. A very simple rain model with one cell is used to analyze and understand the problem. Comparison of X and S bands shows that the effects of azimuthal gradients are stronger at X band. The problem is also illustrated with radar measurements carried out with an X-band coherent polarimetric radar. For practical purposes, the problem appears to be small for beamwidths of less than 1° and the cell sizes considered here, or if t...

苗場山ブナ樹冠における光合成、蒸散特性の垂直、水平方向および方位による変異

苗場山ブナ樹冠における光合成、蒸散特性の垂直、水平方向および方位による変異

Spatiotemporal intermittency in the torsional Couette flow between a rotating and a stationary disk

This work is devoted to the experimental study of the transition to turbulence of a flow confined in a narrow gap between a rotating and a stationary disk. When the fluid layer thickness is of the same order of magnitude as the boundary layer depths, the azimuthal velocity axial gradient is nearly constant and this rotating disk flow tends to be a torsional Couette flow. As in the plane Couette flow or the Taylor–Couette flow, transition to turbulence occurs via the appearance of turbulent domains inside a laminar background. In the rotating disk case, the nucleation of turbulent spirals, previously called “solitary waves” in the rotating disk flow literature, is connected to the birth of structural defects in a periodic underlying roll pattern. As the rotation rate is increased, the lifetime of these turbulent structures increases until a threshold is reached where they then form permanent turbulent spirals arranged nearly periodically all around a circumference. However, since the number of these turbulent spirals decreases with the rotational frequency, the transition to a fully turbulent regime is not achieved. Thus the turbulent fraction of the pattern saturates to a value lower than 0.5. After a geometrical description of the structures, we present a statistical analysis of sizes and lifetimes of the turbulent and laminar domains in order to compare this transition to already observed spatiotemporal intermittent behavior.

Search for non-helical disc dynamos in simulations

The origin of large scale magnetic elds in accretion discs is investigated. Using global three-dimensional simulations of accretion disc turbulence, a recent suggestion of Vishniac & Cho (2001, ApJ 550, 752) is re-examined, according to which large scale elds in accretion discs could be understood without explicitly invoking the usual helicity eect. Particular emphasis is placed on a certain correlation between vorticity and azimuthal velocity gradient which has been predicted to drive large scale dynamo action, independent of the presence or absence of kinetic helicity. In the global disc simulations two types of behaviours are found: those which do show this type of velocity correlation and those which do not. The former ones are typically also the cases where the resistivity is larger. The latter ones show signs typical of dynamo action based on the usual helicity eect. In the idealized simulations without rotation and just shear the above correlation is found to be particularly strong. In both cases there is, as expected, a systematic flux of magnetic helicity through the midplane. However, very little magnetic helicity leaves the domain through the top and bottom boundaries. The idealized simulations reveal that much of this systematic flux comes from the rotational component of the helicity flux and does not contribute to its divergence.

Detection of Bulk Motions in the Intracluster Medium of the Centaurus Cluster

Several recent numerical simulations of off-center cluster mergers predict that significant angular momentum with associated velocities of a few ?103 km?s-1 can be imparted to the resulting cluster. Such gas bulk velocities can be detected by the Doppler shift of X-ray spectral lines with ASCA spectrometers. Using two ASCA observations of the Centaurus cluster (Abell 3526), we produced a velocity map for the gas in the cluster's central regions. We also detected radial and azimuthal gradients in temperature and metal abundance distributions, which seem to be associated with the infalling subgroup centered at NGC 4709 (Cen 45). More importantly, we found a significant (>99.8% confidence level) velocity gradient along a line near-perpendicular to the direction of the incoming subgroup and with a maximum velocity difference of ~3.4 ? 1.1 ? 103 km s-1. It is unlikely (P < 0.002) that the observed velocity gradient is generated by gain fluctuations across the detectors. While the observed azimuthal temperature and abundance variations can be attributed to the interaction with Cen 45, we argue that the intracluster gas velocity gradient is more likely due to a previous off-center merging event in the main body of the Centaurus cluster.

Global dynamics of the plasmasphere and ring current during magnetic storms

Simultaneous global images of the plasmasphere and the ring current were obtained with the Imager for Magnetopause‐to‐Aurora Global Exploration (IMAGE) spacecraft during the geomagnetic storms of 24 May 2000 and 29 July 2000. The plasmasphere images confirm the theoretically predicted development of long plasma tails in the dusk hemisphere during magnetic storms. They also reveal several unexpected structures, including a sharp azimuthal gradient or shoulder, which forms on the night side and corotates with the Earth for many hours, and a narrow ion trough, which is located well inside the main plasmapause in the late evening sector. The ring current images show the peak of the ring current to be nearly centered on the plasmapause, which is consistent with ring current decay models that include coulomb scattering and wave‐particle interactions as well as charge exchange.

The Biconical Outflow in the Seyfert Galaxy NGC 2992

We report on a detailed kinematic study of the galactic-scale outflow in the Seyfert galaxy NGC 2992. The TAURUS-2 imaging Fabry-Perot interferometer was used on the 3.9 m Anglo-Australian Telescope to derive the two-dimensional velocity field of the Hα-emitting gas over the central arcminute of NGC 2992. The complete two-dimensional coverage of the data combined with simple kinematic models of rotating axisymmetric disks allow us to differentiate the outflowing material from the line-emitting material associated with the galactic disk. The kinematics of the disk component out to R = 3.0 kpc are well modeled by pure circular rotation in a plane inclined at i = 68° ± 3° from the plane of the sky and with kinematic major axis along P.A. = 32° ± 3°. The outflow component is distributed into two wide cones with opening angle ≈125°–135° and extending ~2.8 kpc (18'') on both sides of the nucleus at nearly right angles ( ≈ 116° ± 5°) to the disk kinematic major axis. The outflow on the southeastern side of the nucleus is made of two distinct kinematic components interpreted as the front and back walls of a cone. The azimuthal velocity gradient in the back-wall component reflects residual rotational motion, which indicates either that the outflowing material was lifted from the disk or that the underlying galactic disk is contributing slightly to this component. A single outflow component is detected in the northwestern cone. A biconical outflow model with velocities ranging from 50 to 200 km s-1 and oriented nearly perpendicular to the galactic disk can explain the data. The broad-line profiles and asymmetries in the velocity fields suggest that some of the entrained line-emitting material may lie inside the biconical structure rather than only on the surface of the bicone. The mass involved in this outflow is of order ~1 × 107 n M⊙, and the bulk and turbulent kinematic energies are ~6 × 1053 n ergs and ~3 × 1054 n ergs, respectively. The most likely energy source is a hot, bipolar, thermal wind powered on a subkiloparsec scale by the active galactic nucleus and diverted along the galaxy's minor axis by the pressure gradient of the ISM in the host galaxy. The data are not consistent with a starburst-driven wind or a collimated outflow powered by radio jets.

Optical Measurement of Resonant Oscillation and Marangoni Convection-Induced Oscillation in a Molten Silicon Surface

Surface oscillation due to oscillatory Marangoni flow in a liquid bridge of molten silicon was observed through interferometry. To obtain higher resolution, interferograms taken from a molten silicon surface and a reference sample were converted to phase-distribution profiles by using spatial phase-measurement. We analyzed the phase-distribution profiles by measuring the radial displacement, azimuthal phase value gradient, and axial phase value gradient. The total radial displacement was about 1.5 µm over a 2.0-s period. The radial displacement and axial phase value gradient showed in-phase correlation, which suggests a traveling wave in the azimuthal direction. The Fourier spectrum of the radial displacement showed several peaks from 0.5 to 5 Hz. These frequencies corresponded to the frequencies of Marangoni flow reported based on other methods. The spectrum also showed two single peaks at 21 and 24.5 Hz. These frequencies agreed well with theoretical resonant frequencies of the liquid bridge.

Interaction in Abell 2256: The BeppoSAX View.

We present results from a spatially resolved spectral analysis of the merging cluster Abell 2256. The long integration time (135 ks) and the good spatial resolution of the Medium-Energy Concentrator/Spectrometer on board BeppoSAX allow us to derive a new and substantially improved measurement of the temperature structure. We find that, within a central region of the cluster, where the effects of the merger are visible in the ROSAT surface brightness image, the azimuthally averaged projected temperature is remarkably flat; outside this region, the projected temperature rapidly declines. The BeppoSAX data also show clear evidence of an azimuthal temperature gradient in the 4&arcmin;-8&arcmin; (0.4-0.8 Mpc) radial bin, oriented in the same direction as the merger itself. Our metal abundance profile shows, for the first time, firm evidence of an abundance gradient in a rich merging cluster. Intriguingly, the abundance map shows, in the southeast sector, i.e., the one farthest away from the merger, a sharp factor of 2 drop in abundance at a radius comparable to the core radius of the cluster. A possible interpretation is that prior to the merger event, a cooling flow had already developed in the core of the infalling subgroup, as suggested by Fabian & Daines. The interaction between the substructures would have disrupted the cooling flow, thereby reheating and remixing the gas. Since the merger in A2256 is still in a relatively early stage, the gas located on the side opposite the merger event would still retain the low abundances associated with the intracluster medium prior to the cooling flow disruption.

Testing models for traveling convection vortices: Two case studies

We use geosynchronous magnetic field observations to discriminate between two models for traveling convection vortices (TCVs) generated by pressure variations applied to the magnetosphere. According to the model of Glassmeier [1992], an azimuthal gradient in the pressure generates a single field‐aligned current into the high‐latitude ionosphere and a single TCV. According to the model of Southwood and Kivelson [1990], an azimuthal gradient in the pressure generates a pair of oppositely directed currents and TCVs. We reexamine two well‐documented examples of TCVs and show that their signatures correspond better to the predictions of the Southwood/Kivelson model than to the predictions of the Glassmeier model.

Flow braking and the substorm current wedge

Recent models of magnetotail activity have associated the braking of earthward flow with dipolarization and the reduction and diversion of cross‐tail current, that is, the signatures of the substorm current wedge. Estimates of the magnitude of the diverted current by Haerendel [1992] and Shiokawa et al. [1997, 1998] tend to be lower than results from computer simulations of magnetotail reconnection and tail collapse [Birn and Hesse, 1996], despite similar underlying models. An analysis of the differences between these estimates on the basis of the simulations gives a more refined picture of the diversion of perpendicular into parallel currents. The inertial currents considered by Haerendel [1992] and Shiokawa et al. [1997] contribute to the initial current reduction and diversion, but the dominant and more permanent contribution stems from the pressure gradient terms, which change in connection with the field collapse and distortion. The major effect results from pressure gradients in the z direction, rather than from the azimuthal gradients [Shiokawa et al., 1998], combined with changes in By and Bz. The reduction of the current density near the equatorial plane is associated with a reduction of the curvature drift which overcompensates changes of the magnetization current and of the gradient B drift current. In contrast to the inertial current effects, the pressure gradient effects persist even after the burst of earthward flow ends.

Experiments on Pool Boiling of Water from Downward-Facing Hemispheres

Quenching experiments were conducted to investigate pool boiling of saturated water on downward-facing aluminum and 303e stainless steel hemispheres. Test sections had an outer diameter of 0.152 m and a wall thickness of 0.020 m. Destabilization of film boiling and wetting of the stainless steel surface occurred earlier than with aluminum (15 s versus 92 s), at 20 K higher wall superheat and ~10% higher minimum film boiling heat flux qmin. Wetting of the stainless steel surface occurred first near the edge of the test section and then gradually propagated azimuthally inward, followed by the maximum heat flux (MHF) front. Conversely, wetting of the aluminum surface occurred first at the lowermost position (θ = 0 deg) and then propagated azimuthally outward. The azimuthal propagation of the MHF front on the stainless steel surface (~5 deg/s for 60 deg < θ < 90 deg decreasing to ~1.8 deg/s for 10 deg < θ < 60 deg and then increasing slightly to ~2 deg/s for 0 deg < θ < 60 deg) was much slower than on aluminum (~22.5 deg/s on average). The MHF front traversed the entire stainless steel boiling surface in ~40 s versus only 4 s for aluminum. When MHF for the latter first occurred at θ = 0 deg, the radial and near-boiling surface azimuthal temperature gradients were 4 K/mm and 0.6 K/deg, respectively, compared to 12 to 15 K/mm and 1.5 K/deg for stainless steel. For both surfaces, the MHF and qmin values displayed parabolic dependencies on azimuthal angle. The local MHF at θ = 0 deg was 0.81 and 0.40 MW/m2 for aluminum and stainless steel, respectively, decreasing with increased azimuthal angle to minimums of 0.47 and 0.256 MW/m2 at θ = 45 deg. Beyond 45 deg, the local MHF increased with increased azimuthal angle to 0.76 and 0.47 MW/m2, respectively, near the edge of the surface (θ = 80 deg). However, the wall superheats corresponding to the MHF (30 K for aluminum and 80 K for stainless steel) and the qmin (125 K for aluminum and 145 K for stainless steel) were independent of azimuthal angle.

Transient states during spin-up of a Rayleigh–Bénard cell

The influence of impulsive spin-up on Rayleigh–Bénard convection in a cylindrical cell with radius-to-height ratio Γ=0.5 is investigated. Velocity and temperature fields in the thin layer adjacent to the top of the cell are measured by means of particle image velocimetry and thermochromic liquid crystal visualization. The cell, initially in steady nonrotating convection, is rapidly accelerated to steady rotation about its vertical axis with dimensionless rotation rates 4×103⩽Ω⩽8×104. Rayleigh numbers Ra vary from 5×107 to 5×108. In a large part of this domain of the parameter space, axisymmetric structures appear near the top of the cell. In most cases, a single ring forms with a radius close to 3/4 of the radius of the cell. The same experiment without heating produces typical Ekman spin-up devoid of ring structures. The ring is characterized by a local decrease in temperature and azimuthal velocity. As the flow evolves, an azimuthal velocity gradient forming across the ring causes shear instability and leads to the emergence of Kelvin–Helmholz vortices.

Thermal convection in a rotating fluid annulus blocked by a radial barrier

There have been extensive previous laboratory studies of thermal convection in a vertical cylindrical annulus of fluid that rotates about its axis with angular velocity Ω (say) with respect to an inertial frame and is subject to an axisymmetric horizontal temperature gradient, as well as associated theoretical and numerical work. The relative flow produced by concomitant buoyancy forces is strongly influenced by Coriolis forces, which give rise to azimuthal circulations and promote, through the process of “baroclinic instàbility”, regimes of non-axisymmetric sloping convection which can be spatially and temporally regular or irregular (“chaotic geostrophic turbulence”). It is also known from previous work that such flows are changed dramatically by the presence of a thin rigid impermeable radial barrier blocking the cross-section of the annulus, and capable of supporting a net azimuthal pressure gradient and associated net azimuthal temperature gradient within the fluid. The presence of the barrier can thus render convective heat transport across the fluid annulus (as measured by the Nusselt number, Nu) virtually independent of Ω (as measured by the so-called Ekman or Taylor number) and dependent only on the Grashof number, G. The present study reports further systematic determinations of heat transport and of velocity and temperature fields in the presence of a radial barrier, with emphasis on the Ω-dependence of the crucially-important net azimuthal temperature gradient supported by the barrier and the physical interpretation of that dependence.

Azimuthal pressure gradient as driving force of substorm currents

We have studied the azimuthal pressure gradient in the central plasma sheet during substorms using plasma and magnetic field data obtained by the AMPTE/IRM satellite at nightside in radial distances of 9–15 RE. The pressure gradient is statistically estimated for the interval when the magnetic field shows a dipolar configuration (elevation angle &gt;45°). It is found that by this criterion, most data are obtained during and after the passage of high‐speed ion flow in the vicinity of the neutral sheet during magnetically active times. We show that there is an azimuthal gradient of plasma pressure in the dipolar field region. The pressure gradient can drive a substantial amount of field‐aligned current (4.1×105 A per 2‐hour local time sector). We suggest that this current is a source of the substorm current system after the high‐speed ion flow stops.

Negative shuttle charging during TSS 1R

We studied 21 intervals during the TSS 1R deployment with a 15 Ω or 25 kΩ resistor connecting the tether to shuttle ground. Ion spectral peaks detected by the Shuttle Potential and Return Electron Experiment indicate that the shuttle consistently charged negatively with respect to the local plasma. With the 15 Ω shunt in the circuit, shuttle potential, Φs, decreased from −17 to −245 V as tether length, L, increased to 2.6 km. Current in the circuit depended strongly on ionospheric density. With the 25 kΩ resistor in place, Φs ≈ −300 V in the low density, nightside ionosphere with L = 5.1 km. Near local noon Φs ≈ −80 V with L = 17.2 km. The shuttle charged to ∼−600 V during two dawn terminator crossings, one with and one without thruster firings. As on TSS 1, firings of two aft vernier thrusters significantly increased |Φs|. In the case without thruster firings, simultaneous variations of Φs, tether current, and the inferred satellite potential are consistent with strong azimuthal and vertical ionospheric density gradients. These are the first known direct measurements of strong negative shuttle charging.