Confinement Zone Research Articles

Transient confinement zones: A type of lipid raft?

Many important signaling events are initiated at the cell membrane. To facilitate efficient signal transduction upon stimulation, membrane microdomains, also known as lipid rafts, are postulated to serve as platforms to recruit components involved in the signaling complex, but few methods exist to study rafts in vivo. Single particle tracking provides an approach to study the plasma membrane of living cells on the nano-scale. The trajectories of single gold particles bound to membrane proteins and lipids are characterized in terms of both random and confined diffusion; the latter occurs in "transient confinement zones". Here we review transient confinement zones and some of their implications for membrane structure and function.

Seasonal polar cap radiation zones in dayside magnetosphere

The phenomenon of quasi-stable trapping of charged particles in the keV to MeV energy range within the polar cusp region of the Earth’s magnetosphere is explored. The remote equatorial magnetic field lines on the dayside magnetosphere are compressed by the solar wind and exhibit two local minima in the geomagnetic field strength along the field line in high latitudes. These minima, on both sides of the equator, result in stable confinement structures. Numerical modeling of charged particle orbits that pass through the regions of these local field minima has been carried out using different seasonal Earth tilt and different magnetospheric disturbance level. These orbit tracings show when and where these off-equatorial trapped radiation zones would be situated. The existence and extent of these confinement zones depend on the tilt angle. Indeed, the northern cusp confinement zone appears only at the northern summer solstice, while the southern cusp particle capture zone appears around winter solstice. The particle orbits that pass through opposite off-equatorial field minimum during solstices reveal a bound of the geomagnetic equatorial plane on the day sector. During equinox, the particle confinement zones exist in both cusps at times of disturbed magnetosphere conditions. The trapped particles drift within the trapping zones with periods of the several minutes, conserving the 1st and 2nd adiabatic invariants.

Relationship of Lipid Rafts to Transient Confinement Zones Detected by Single Particle Tracking

Relationship of Lipid Rafts to Transient Confinement Zones Detected by Single Particle Tracking

Relationship of Lipid Rafts to Transient Confinement Zones Detected by Single Particle Tracking

We examined the physical and chemical characteristics of transient confinement zones (TCZs) that are detected in single particle trajectories of molecules moving within the membrane of C3H 10T1/2 murine fibroblasts and their relationship to “rafts.” We studied the lateral movement of different membrane molecules thought to partition to varying degrees into or out of the putative lipid domains known as rafts. We found that lipid analogs spend significantly less time in TCZs compared with Thy-1, a glycosylphosphatidylinositol-anchored protein, and GM1, a glycosphingolipid. For Thy-1, we found that zone abundance was markedly reduced by cholesterol extraction, suggesting that a major source of the observed temporary confinement is related to the presence of raft domains. More detailed analysis of particle trajectories reveals that zones can be revisited even tens of seconds after the original escape and that diffusion within the zones is reduced by a factor of ∼2, consistent with the zone being a cholesterol-rich liquid-ordered phase. Surprisingly, transient confinement was not strongly temperature dependent. Overall, our data demonstrate that there are raft-related domains present in certain regions of the plasma membrane of C3H cells, which can persist for tens of seconds.

Characterization of a miniature Paul-Straubel trap

Frequency standard applications and ultra-high resolution spectroscopy of a confined single ion require traps of drastically reduced dimensions (about or below 1 mm). These small dimensions increase the sensitivity of the trapping behavior to imperfections in the trap geometry and to patch potentials. For the aim of the metrological laser interrogation of a single Ca+ ion, a miniature cylindrical ring trap was built. In order to optimize the laser cooling process and to reach strong binding conditions, the boundaries of the stability diagram and the zones of low confinement as well as the ion motion properties were characterized.

Badly Arbitraged Markets and Negative Martingale Measures

In this paper, we seek to measure the impact of arbitrage opportunities on some key results of asset pricing theory. We do this by showing how a market that is viable, but potentially badly arbitraged, can exist if the agents' preferences is modified. In this case, the pricing measure is not necessarily positive and can no longer be treated as a risk-neutral probability. Unless such arbitrage opportunities are of the cash-and-carry kind, prices remain martingale under this potentially negative measure. This engenders generic phenomena such as information loss and price confinement zones. To better understand these phenomena, we construct an example of a martingale under a signed measure. In conclusion, we remind an empirical test for arbitrage detection, which we apply to CME options on the S\&P 500 index options during the turbulent aftermath of the bankruptcy of Russia.

Weak Dependence of Mobility of Membrane Protein Aggregates on Aggregate Size Supports a Viscous Model of Retardation of Diffusion

Proteins in plasma membranes diffuse more slowly than proteins inserted into artificial lipid bilayers. On a long-range scale (>250 nm), submembrane barriers, or skeleton fences that hinder long-range diffusion and create confinement zones, have been described. Even within such confinement zones, however, diffusion of proteins is much slower than predicted by the viscosity of the lipid. The cause of this slowing of diffusion on the micro scale has not been determined and is the focus of this paper. One way to approach this question is to determine the dependence of particle motion on particle size. Some current models predict that the diffusion coefficient of a membrane protein aggregate will depend strongly on its size, while others do not. We have measured the diffusion coefficients of membrane glycoprotein aggregates linked together by concanavalin A molecules bound to beads of various sizes, and also the diffusion coefficients of individual concanavalin A binding proteins. The measurements demonstrate at most a weak dependence of diffusion coefficient on aggregate size. This finding supports retardation by viscous effects, and is not consistent with models involving direct interaction of diffusing proteins with cytoskeletal elements.

Recent developments in the Baker‐Strehlow VCE analysis methodology

AbstractThe Baker‐Strehlow methodology was developed to provide an objective approach to prediction of blast pressures from vapor cloud explosions. The complete methodology was first published in 1994 [1]. Since then, it has evolved through ongoing research and use in VCE hazard analyses, facility siting studies and accident investigations. This article gives a brief overview of a paper on recent developments in the Baker‐Strehlow methodology presented at the 31st Loss Prevention Symposium in Houston on March 9‐13, 1997. Because the entire paper is too lengthy to be presented here, the following discussions may be lacking in some details. A copy of the complete paper can be obtained from the American Institute of Chemical Engineers (AIChE).Since the Baker‐Strehlow method was first published, it has been used extensively in VCE hazard assessments in refineries and chemical plants. As expected, many practical lessons have been learned during the course of the hazard assessments, and the Baker‐Strehlow method has evolved as a result. The changes have been evolutionary, not revolutionary. In keeping with the goals of the original study in which the methodology was developed, all changes have been incorporated with the intent of achieving an objective methodology to provide consistent prediction of VCE blast effects.The revisions to the Baker‐Strehlow method resulting from experience gained during plant walk‐downs and hazard assessments include: Systematic identification of “potential explosion sites” or “PESs,” Selection of the level of confinement for mixed zones of 2D and 3D confinement, Deciding on flame expansion when confinement is elevated above the vapor cloud, Selecting the reactivity for a fuel that is a mixture of fuels with differing reactivities, Predicting blast loads when there are multiple PES's within a vapor cloud considering different ignition source locations.

`Profile consistency' features in shaped sawtoothing ohmic TCV plasmas

A large variety of plasma conditions has been created in TCV (Tokamak à Configuration Variable, m, a < 0.25 m). They include limited and diverted discharges with elongations in the range 1-2.58, triangularities between -0.7 and 1 as well as `square' shapes with plasma currents in the range 0.1-1 MA. Over the entire range of quasi-stationary ohmic conditions investigated we observe a correlation between electron pressure profiles and conductivity profiles, suggesting that , where refers to an average over the volume or respectively, the cross sectional area of the plasma. The profiles become broader as the average current density is increased. These `profile consistency' features are in apparent agreement with theoretical considerations based on minimum energy states of the plasma or on stationary entropy. Further analysis of the experimental evidence, together with a model of the current profile consistent with neoclassical conductivity in the confinement zone and corresponding to a safety factor inside the sawtoothing core, however, shows that the observations can be accounted for by the effects of sawtooth activity.

Proposal for a new UV-light generating device based on cold electron emission

A new device intended for UV-light generation and based on cold electron emission is proposed and analyzed theoretically. The device operation is based on the excitation by the field emitted electrons of the surrounding nitrogen (N2) molecules, which in turn emit UV light. The device should operate at low pressures compatible with field emission. The proposed device involves electrons moving in crossed electric E and magnetic B fields. Equations for the electron time evolution (both radial and angular position) are obtained. When moving inside the device, the electron has a periodical cycloid-like trajectory which is always confined between two coaxial cylinders. The inner cylinder has the same radius and position with the circular array of emitters used for electron generation. The position of the outer cylinder (and so the width of the confinement zone) is controlled by E and B. The described analytical-numerical approach is used to analyze the device operation as function of model parameters.

On transport barriers and low–high mode transitions

Transport barriers and transitions between modes of low and high confinement in tokamak plasmas are often attributed to suppression of turbulence by a shear flow related to a plasma gradient, e.g., of density. However, such shear flow is also affected by the second derivative of density. When this is introduced there is no unique relation between flux and gradient—it depends on the source distribution within the plasma and on conditions at the plasma edge (e.g., imposed by the scrape-off layer). This edge gradient must lie within prescribed limits if a stationary plasma profile (which may include an improved confinement zone) is to exist.

On the properties of strange modes

Properties of the so-called strange modes occurring in linear stability calculations of stellar models are discussed. The behaviour of these modes is compared for two different sets of stellar models, for very massive zero-age main-sequence stars and for luminous hydrogen-deficient stars, both with high luminosity-to-mass ratios. We have found that the peculiar behaviour of the frequencies of the strange modes with the change of a control parameter is caused by the pulsation amplitude of a particular eigenmode being strongly confined to the outer part of the envelope, around the density inversion zone. The frequency of a strange mode changes because the depth of the confinement zone changes with the control parameter. Weakly non-adiabatic strange modes tend to be overstable because the amplitude confinement quenches the effect of radiative damping. On the other hand, extremely non-adiabatic strange modes become overstable because the perturbation of radiation force (gradient of radiation pressure) provides a restoring force that can be out of phase with the density perturbation. We discuss this mechanism by using a plane-parallel two-zone model.

On the properties of strange modes

ABSTRA C T Properties of the so-called strange modes occurring in linear stability calculations of stellar models are discussed. The behaviour of these modes is compared for two different sets of stellar models, for very massive zero-age main-sequence stars and for luminous hydrogendeficient stars, both with high luminosity-to-mass ratios. We have found that the peculiar behaviour of the frequencies of the strange modes with the change of a control parameter is caused by the pulsation amplitude of a particular eigenmode being strongly confined to the outer part of the envelope, around the density inversion zone. The frequency of a strange mode changes because the depth of the confinement zone changes with the control parameter. Weakly non-adiabatic strange modes tend to be overstable because the amplitude confinement quenches the effect of radiative damping. On the other hand, extremely non-adiabatic strange modes become overstable because the perturbation of radiation force (gradient of radiation pressure) provides a restoring force that can be out of phase with the density perturbation. We discuss this mechanism by using a plane-parallel two-zone model.

Magnetic shear effects on confinement and electron heat transport in Tore Supra discharges with electron heating

Various steady state non-inductive plasmas, with strong electron heating and significant modification of the current density profile, have been routinely obtained in Tore Supra discharges with lower hybrid current drive (LHCD) and/or fast wave electron heating (FWEH) experiments. The dependence of the electron heat diffusivity χe on the electron temperature gradient ∇Te, the magnetic shear s and the safety factor q is demonstrated. The increase of χe with ∇Te indicates the existence of a critical temperature gradient. Moreover, the current density profile affects the global confinement and the local transport. The electron heat flux qe is found to be roughly proportional to q2. The effect of magnetic shear on χe is studied in the improved confinement discharges obtained by modifying the current profile. When the magnetic shear increases in the confinement zone and/or vanishes in the plasma centre, χe decreases. The effect of the current profile is also observed in the saturated ohmic regime. The results for χe do not agree with the local transport models of Taroni et al. or Rebut-Lallia-Watkins (RLW) for all ranges of electronheating.

Similarity Invariance of Magnetic States and Nonlocal Transient Effects on Heat Transport in Tokamaks

States of stationary magnetic entropy are connected by a similarity transformation that describes a poloidal flux-conserving rearrangement of the equilibrium and a global invariance of the magnetic structure. It is conjectured that the only permissible states in the confinement zone of a tokamak are magnetically similar states of stationary entropy, and that the physical properties of transport in these states are the same, apart from a similarity transformation. This conjecture, which establishes a connection between transport and magnetic structure, allows one to write down a power balance equation in the confinement zone that describes the temperature evolution in transient processes driven by rapid changes of the magnetic equilibrium. In the present paper the following experimental facts are considered in the light of this equation: the sensitivity of the core temperature of the tokamak to edge perturbations of the current channel, the nonlocal, nondiffusive behaviour of heat pulses from the mixing zone observed in heat modulated experiments and the global temperature evolution observed immediately after the L-H transition.

Power thresholds of the L–H transition described by a global bifurcation of tokamak equilibrium

The magnetic configuration in the confinement zone of a tokamak admits a magnetic flux-bifurcation for critical values of the edge parameters and given injected power when the initial L-state is so defined as to be associated with a stationary magnetic entropy. Assuming the ITER89-P scaling of the energy confinement time in the L state, it is shown that the power is changing along the bifurcation threshold, for fixed values of the edge parameters, according to the law P=Cn0.77B0.97 (where n is the average density and B is the toroidal magnetic field), where the numerical value of the constant C follows from ITER89-P and is consistent with observations of the L–H transition. It is also shown that the observed independence of the threshold with respect to the current is a consequence of the fact that, at the threshold point, the ratio between the auxiliary power and the Ohmic power PΩ in the confinement region is constant when the current is changing and that the value of PΩ at the threshold is a very slow function of the current.

Enhanced shear damping and ion transit resonance damping of electron drift waves in low collisionality tokamaks

The overlap of potentially unstable electron drift modes, driven by trapped electrons scattering, is negligible in low collisionality tokamaks. The width, and hence the ion damping of the (then) uncoupled toroidal modes exceed those obtained in cylindrical geometry, owing to the ion excursion across the flux surfaces associated with the magnetic drift. Two regimes have to be considered: ωl > ci/⟨ν⟩χR where "shear damping" is merely enhanced, and ωl < ci/⟨ν⟩χR where even larger ion transit resonance damping takes over (ωl is the mode frequency, l the torodial mode numer, ci the ion thermal velocity, ⟨ν⟩χ the safety factor and R the major radius of the device). A contrario, the broader the eigenmode, the weaker the Landau damping on passing electrons. The ratio of the sum of the revisted stabilizing rates to the trapped electron destabilizing rate depends mainly on the ratio of the characteristic length-scales across and along the field lines, not on the absolute temperatures; it turns out to be approximately constant and of order unity throughout the confinement zone, confirming the relevance of the dissipative trapped electron mode to anomalous transport, even in low colisionality tokamak plasmas. In this confinement zone, the Landau damping rate is typically smaller than the shear damping rate, suggesting an even structure of the potential eigenfunction (and an odd structure for the associated radial magnetic field).

Transport simulations of ITER with broad density profiles and high radiative fraction

Special versions of the 1.5-D BALDUR predictive transport code are used to explore the confinement in the ignited ITER EDA by self-consistent calculations. The code computes 2-D equilibria and solves 1-D transport equations in the bulk and scrape-off layer with empirical transport coefficients for the ohmic, L and ELMy H mode regimes. The emphasis is on scenarios with flat density profiles and high, fixed radiative power in the main chamber due to the seeded impurities argon and neon. It is shown that self-sustained steady state thermonuclear burn is achieved for 370 s and is compatible with the flat density profiles and strong radiative cooling. The necessary local energy and particle transport are presented. In the argon and neon scenarios, the required radiation corrected energy confinement times are 4.1 and 3.5 s, respectively, which are achievable according to the ITER ELMy H mode scaling. The advantage of neon originates from its smaller radiative loss within the separatrix of 37% of the total radiation in the main chamber, compared with 60% in the case of argon. A significant radiative loss from the confinement zone, mainly by bremsstrahlung, cannot be avoided. It raises the required energy confinement time and is the price to be paid for reduction of the divertor heat load by radiative cooling in the main chamber. In steady state, a helium fraction of 5% is computed. The fractions of helium, argon and neon and the resulting fuel dilution are considerably lower than with peaked density profiles

Enhanced shear damping and ion transit resonance damping of electron drift waves in toroidal plasmas

Shear stabilization of low mode number, toroidally uncoupled electron drift waves is shown to be larger in axisymmetric tori than in cylinders, in sharp contrast to theoretical predictions obtained at higher mode numbers. When the wave frequency is larger than the thermal ion transit frequency, the enhancement factor, which is related to the toroidicity induced ∇B and curvature drifts of passing ions across the flux surfaces, is naturally reminiscent of the results of neoclassical diffusion theory in the Pfirsch–Schlüter regime. Even larger ion transit resonance damping arises in the opposite limit. The ratio of the sum of the revisited shear and Landau damping rates (resulting from interaction with passing ions and passing electrons, respectively) to the trapped electron excitation rate of these low frequency modes turns out to be approximately constant throughout the confinement zone of tokamaks with parabolic density profiles. An attempt is made to justify this remarkable result, as well as the transition from global gyro-Bohm to global Bohm scaling at very low collisionality.

Quantitative predictions of tokamak energy confinement from first-principles simulations with kinetic effects

A first-principles model of anomalous thermal transport based on numerical simulations is presented, with stringent comparisons to experimental data from the Tokamak Fusion Test Reactor (TFTR) [Fusion Technol. 21, 1324 (1992)]. This model is based on nonlinear gyrofluid simulations, which predict the fluctuation and thermal transport characteristics of toroidal ion-temperature-gradient-driven (ITG) turbulence, and on comprehensive linear gyrokinetic ballooning calculations, which provide very accurate growth rates, critical temperature gradients, and a quasilinear estimate of χe/χi. The model is derived solely from the simulation results. More than 70 TFTR low confinement (L-mode) discharges have been simulated with quantitative success. Typically, the ion and electron temperature profiles are predicted within the error bars, and the global energy confinement time within ±10%. The measured temperatures at r/a≂0.8 are used as a boundary condition to predict the temperature profiles in the main confinement zone. The dramatic transition to the improved confinement in the supershot regime is also qualitatively explained. Further work is needed to extend this model of core heat transport to include particle and momentum transport, the edge region, and other operating regimes besides the ITG-dominated L mode. Nevertheless, the present model is very successful in predicting thermal transport in the main plasma over a wide range of parameters.