Observation of coherent sheared turbulence flows in the DIII-D Tokamak

Time-resolved measurements of the turbulent density flow field in a tokamak plasma reveal low-frequency ( approximately 15 KHz), coherent oscillations in the poloidal flow, v(theta). These flow oscillations have a long poloidal wavelength (m<3) and narrow radial extent (k(r)rho(i) approximately 0.2). The estimated flow-shearing rate is of the same order of magnitude as the turbulence decorrelation rate and may thus regulate the turbulence amplitude. These features are consistent with theoretically predicted axisymmetric, self-regulating, sheared flows recognized as geodesic acoustic modes.     

Signature of turbulent zonal flows observed in the DIII-D tokamak

The spectrum of turbulent density fluctuations at long poloidal wavelengths in the edge plasma of the DIII-D tokamak peaks at nonzero radial wave number. The associated electric-potential fluctuations cause sheared E x B flows primarily in the poloidal direction. These zonal flows have been predicted by theory and are believed to regulate the overall level of turbulence and anomalous transport. This study provides the first indirect experimental identification of zonal flows.     

Role of poloidal E × B drift in divertor heat transport in DIII-D

Simulations for DIII-D high confinement mode plasmas with the multifluid code UEDGE show a strong role of poloidal E × B drifts on divertor heat transport, challenging the paradigm of conduction-limited scrape-off layer (SOL) transport. While simulations with reduced drift magnitude are well aligned with the assumption that electron heat conduction dominates the SOL heat transport, simulations with drifts predict that the poloidal convective E × B heat transport dominates over electron heat conduction in both attached and detached conditions. As poloidal E × B flow propagates across magnetic field lines, poloidal transport with shallow magnetic pitch angles can reach values that are of the same order as would be provided by sonic flows parallel to the field lines. These flows can lead to strong convection-dominated divertor heat transport, increasing the poloidal volume of radiative power front, consistent with previous measurements at DIII-D. Due to these convective flows, the Lengyel integral approach, assuming zero convective fraction, is expected to provide a pessimistic estimate for the radiative capability of impurities in the divertor. For the DIII-D simulations shown here, the Lengyel integral approach underestimates the radiated power by a factor of 6, indicating that, for reliable DIII-D divertor power exhaust predictions, full two-dimensional (2D) calculations, including drifts, would be necessary.     

Detection of zero-mean-frequency zonal flows in the core of a high-temperature tokamak plasma

A low-frequency, spectrally broad (Deltaf approximately 10 kHz) poloidal flow structure that peaks near zero frequency is observed in time-resolved measurements of the turbulence velocity field in the core region (r/a approximately 0.6-0.9) of DIII-D tokamak plasmas. These flows exhibit a long poloidal wavelength (low m) and a short radial coherence length comparable to the ambient turbulence decorrelation length. Characteristics of these observed poloidal flows are consistent with the theoretically predicted residual or zero-mean-frequency zonal flows.     

Solution-state dynamics of sugar-connected spin probes in sucrose solution as studied by multiband (L-, X-, and W-band) electron paramagnetic resonance

A multiband (L-band, 0.7GHz; X-band, 9.4GHz; and W-band, 94GHz) electron paramagnetic resonance (EPR) study was performed for two glycosidated spin probes, 4-(alpha,beta-D-glucopyranosyloxy)-TEMPO (Glc-TEMPO) and 4-(alpha,beta-D-lactopyranosyloxy)-TEMPO (Lac-TEMPO), and one non-glycosylated spin probe, 4-hydroxy-TEMPO (TEMPOL), where TEMPO=2,2,6,6-tetramethyl-1-piperidinyloxyl, to characterize fundamental hydrodynamic properties of sugar-connected spin probes. The linewidths of these spin probes were investigated in various concentrations of sucrose solutions (0-50wt%). The multiband approach has allowed full characterization of the linewidth parameters, providing insights into the molecular shapes of the spin probes in sucrose solution. The analysis based on the fast-motional linewidth theory has yielded anisotropy parameters of rho(x) approximately 2.6 and rho(y) approximately 0.9 for Glc-TEMPO, and rho(x) approximately 4.2 and rho(y) approximately 0.9 for Lac-TEMPO. These values indicate that the glycosidated spin probes have a prolate-type molecular shape elongated along the x-axis (NO(rad) axis) with Lac-TEMPO elongated more remarkably, consistent with their molecular structures. The interaction parameters k (the ratios of the effective hydrodynamic volumes to the real ones) corrected for the difference in molecular shape have been estimated and found to have the relation k(TEMPOL)相似文献   

Transport reduction by rotation shear in tokamak-edge turbulence

Effects of externally imposed and self-generated poloidal flows on turbulent transport in the edge region of a tokamak are investigated using 3D nonlinear global simulations of resistive pressure-gradient-driven turbulence. Transport reduction is found to be due to synergetic changes in the fluctuation amplitude and in the dephasing of the fluctuations. A scaling of the fluctuation level and turbulent diffusivity with E x B flow shear strength is deduced from these simulations. These scalings agree with recent experimental observations on edge biasing as well as with analytical models.     

Rates, polarizations, and asymmetries in charmless vector-vector B meson decays

With a sample of approximately 89 x 10(6) B(-)B pairs collected with the BABAR detector, we perform a search for B meson decays into pairs of charmless vector mesons (phi, rho, and K*). We measure the branching fractions, determine the degree of longitudinal polarization, and search for CP violation asymmetries in the processes B+-->phiK(*+), B0-->phiK(*0), B+-->rho(0)K(*+), and B+-->rho(0)rho(+). We also set an upper limit on the branching fraction for the decay B0-->rho(0)rho(0).     

Evidence of diffusion regions at a subsolar magnetopause crossing

On 1 April 2001, the Polar satellite crossed a subsolar magnetopause associated with antiparallel magnetic fields. Over a width approximately 6 magnetosheath ion skin depths (approximately 3 magnetospheric ion skin depths), perpendicular ion flows different from E x B/B(2) as well as Hall magnetic and electric field signatures were observed. At a smaller scale, the electron flow decoupled from the magnetic field near a deep minimum in the magnetic field strength. Separatrices were identified as boundaries of low frequency electric field turbulence associated with density minima and parallel electric fields. The reconnection rate was less than 2% of the asymptotic Alfvén speed.     

Distribution of fixed-point energies of a quasiperiodic Hamiltonian flow

Energy distributions rho(+/-)(E) for the elliptic and hyperbolic fixed points of the Hamiltonian H(x,y)= summation operator (k=0) (4) cos [x cos(2pik/5)+y sin(2pik/5)] are calculated as integrals over a one-dimensional manifold M(E) in five-dimensional space. Singular points of M(E) produce three logarithmic singularities of rho(+/-)(E), and vanishing of connected components of M(E) gives rise to three discontinuities. The strengths of the singularities and discontinuities of rho(+/-)(E) are determined analytically, and the distributions are evaluated numerically for representative points in the nonsingular intervals. The calculation provides an explicit realization of general theorems concerning the critical points of infinitely smooth functions defined on an n-dimensional torus and restricted to a k-dimensional linear subset. Formally the calculation resembles the determination of the density of states of a dynamical system with one degree of freedom on a 2-torus, but with important differences due to topology and symmetry.     

Arrested swelling of highly entangled polymer globules

Upon aging, a collapsed long chain evolves from a crumpled state to a self-entangled globule which can be thought of as a large knot. Swelling of an equilibrium globule in good solvent is a two-step process: (i) fast swelling into an arrested stretched structure with conserved entanglement topology followed by (ii) slow disentanglement. Using computer simulation, we found both mass-mass (m-m) and entanglement-entanglement (e-e) power law correlations inside the swollen globule. The m-m correlations are characterized by a set of two exponents in agreement with a Flory-type argument. The e-e correlations are also characterized by two exponents, both of them larger (by approximately 0.3) than the related m-m exponents. We interpret this difference as evidence of distance-dependent repulsion E=-0.3ln((rho)k(B)T between entanglements sliding along the polymer chain.     

Inverse statistics of smooth signals: the case of two dimensional turbulence

The problem of inverse statistics (statistics of distances for which the signal fluctuations are larger than a certain threshold) in differentiable signals with power law spectrum, E(k) approximately k(-alpha), 3< or =alpha<5, is discussed. We show that for these signals, with random phases, exit-distance moments follow a bifractal distribution. We also investigate two dimensional turbulent flows in the direct cascade regime, which display a more complex behavior. We give numerical evidences that the inverse statistics of 2D turbulent flows is described by a multifractal probability distribution; i.e., the statistics of laminar events is not simply captured by the exponent alpha characterizing the spectrum.     

Anomalous self-similarity in a turbulent rapidly rotating fluid

Our velocity measurements on quasi-two-dimensional turbulent flow in a rapidly rotating annulus yield self-similar (scale-independent) probability distribution functions for longitudinal velocity differences, deltav(l) = v(x+l)-v(x). These distribution functions are strongly non-Gaussian, suggesting that the coherent vortices play a significant role. The structure functions <[deltav(l)](p)> approximately l(zeta)p exhibit anomalous scaling: zeta(p) = p / 2 rather than the expected zeta(p) = p / 3. Correspondingly, the energy spectrum is described by E(k) approximately k(-2) rather than the expected E(k) approximately k(-5/3).     

Crossover from weak localization to Shubnikov-de Haas oscillations in a high-mobility 2D electron gas

We study the magnetoresistance deltarho(xx)(B)/rho(0) of a high-mobility 2D electron gas in the domain of magnetic fields B, intermediate between the weak localization and the Shubnikov-de Haas oscillations, where deltarho(xx)(B)/rho(0) is governed by the interaction effects. Assuming short-range impurity scattering, we demonstrate that in the second order in the interaction parameter lambda a linear B dependence, deltarho(xx)(B)/rho(0) approximately lambda(2)omega(c)/E(F) with a temperature-independent slope, emerges in this domain of B (here omega(c) and E(F) are the cyclotron frequency and the Fermi energy, respectively). Unlike previous mechanisms, the linear magnetoresistance is unrelated to the electron executing the full Larmour circle, but rather originates from the impurity scattering via the B dependence of the phase of the impurity-induced Friedel oscillations.     

Resonant nucleation

We investigate the role played by fast quenching on the decay of metastable (or false vacuum) states. Instead of the exponentially slow decay rate per unit volume, Gamma(HN) approximately exp([-E(b)/k(B)T] (E(b) is the free energy of the critical bubble), predicted by homogeneous nucleation theory, we show that under fast enough quenching the decay rate is a power law Gamma(RN) approximately [E(b)/k(B)T](-B), where B is weakly sensitive to the temperature. For a range of parameters, large-amplitude oscillations about the metastable state trigger the resonant emergence of coherent subcritical configurations. Decay mechanisms for different E(b) are proposed and illustrated in a (2+1)-dimensional scalar field model.     

Measurement of branching fractions and charge asymmetries in B+/--->rho+/-pi0 and B+/--->rho0pi+/- decays, and search for B0-->rho0pi0

We present measurements of branching fractions and charge asymmetries in B-meson decays to rho(+)pi(0), rho(0)pi(+), and rho(0)pi(0). The data sample comprises 89x10(6) Upsilon(4S)-->BBmacr; decays collected with the BABAR detector at the PEP-II asymmetric-energy B Factory at SLAC. We find the charge-averaged branching fractions B(B+-->rho(+)pi(0))=[10.9+/-1.9(stat)+/-1.9(syst)]x10(-6) and B(B+-->rho(0)pi(+))=(9.5+/-1.1+/-0.9)x10(-6), and we set a 90% confidence-level upper limit B(B0-->rho(0)pi(0))<2.9x10(-6). We measure the charge asymmetries ACP(pi(0))(rho(+))=0.24+/-0.16+/-0.06 and ACP(pi(+))(rho(0))=-0.19+/-0.11+/-0.02.     

Branching fraction measurements of B+-->rho+gamma, B0-->rho0gamma, and B0-->omegagamma

We present a study of the decays B+-->rho+gamma, B0-->rho0gamma, and B0-->omegagamma. The analysis is based on data containing 347 x 10(6) BB[over ] events recorded with the BABAR detector at the PEP-II asymmetric B factory. We measure the branching fractions B(B+-->rho+)gamma)=(1.10_(-0.33)(+0.37)+/-0.09)x10(-6) and B(B0-->rho0gamma)=(0.79(-0.20)(+0.22)+/-0.06)x10(-6), and set a 90% C.L. upper limit B(B0-->omegagamma)<0.78 x 10(-6). We also measure the isospin-averaged branching fraction B(B-->(rho/omega)gamma)=(1.25(-0.24)(+0.25)+/-0.09)x10(-6), from which we determine |Vtd/Vts|=0.200(-0.020)(+0.021)+/-0.015, where the first uncertainty is experimental and the second is theoretical.     

T1rho Dispersion profile of rat tissues in vitro at very low locking fields

The purpose of this study was to show the T(1rho) dispersion profile in various rat tissues (liver, brain, spleen, kidney, heart and skeletal muscle) at low (0.1 T) B(0) field at very low locking field B1, starting from 10 microT. The T(1rho) dispersion profile showed a quite similar pattern in all tissues. The highest R(1rho) relaxation rates were seen in the liver and muscle followed by the heart, whereas the values for spleen, kidney and brain were rather similar. The greatest difference between R2 relaxation rate and R(1rho) relaxation rate at B1=10 microT was seen in the liver and muscle. The steepest slope for a dispersion curve was seen in the muscle. The value of T(1rho) approximately approached the value of T2 when the locking field B1 approached 0. Except for the liver, the calculated apparent relaxation rate R2' was slightly larger than the calculated one. The potential value of T(1rho) imaging is to combine high R1 contrast of low-field imaging with the high signal-to-noise ratio (SNR) of high static field imaging. T(1rho) relaxation and dispersion data presented in the current study help to optimize the rotating-frame MR imaging.     

Search for the radiative decays B-->rhogamma and B0-->omegagamma

A search of the exclusive radiative decays B-->rho(770)gamma and B0-->omega(782)gamma is performed on a sample of about 84x10(6) BBmacr; events collected by the BABAR detector at the SLAC PEP-II asymmetric-energy e+e- storage ring. No significant signal is seen in any of the channels. We set upper limits on the branching fractions B of B(B0-->rho(0)gamma)<1.2 x 10(-6), B(B+-->rho+gamma)<2.1 x 10(-6), and B(B0-->omegagamma)<1.0 x 10(-6) at 90% confidence level (C.L.). Using the assumption that Gamma(B-->rhogamma)=Gamma(B+-->rho(+)gamma)=2 x Gamma(B0-->rho(0)gamma), we find the combined limit B(B-->rhogamma)<1.9 x 10(-6), corresponding to B(B-->rhogamma)/B(B-->K*gamma)<0.047 at 90% C.L.     

Analysis of the radial and longitudinal effect in a double TE(104) and a single TE(102) rectangular cavity

The response of the cavity to the rotation of a point-like sample in the horizontal (y-z) plane passing through the center of the Bruker double TE(104) and single TE(102) rectangular cavities in concentric circles of radii rho = 0, 1, 2, 3, 4, and 5 mm from the cavity center (radial effect) has been analyzed. The experimentally observed dependencies of the EPR signal intensity, I(pp), showed the following: (i) for rho = 0 mm (a sample position in the cavity center), I(pp) is independent of the angle of rotation; (ii) for rho = 1, 2, and 3 mm, the I(pp) dependence progressively changes from circular to oval; (iii) when the radius is further increased to rho = 4 and 5 mm, the I(pp) dependence changes dramatically, giving a figure eight shape. These experimental observations are in very good agreement with the theoretical calculations, in which the response is modeled using modified Cassinian curves, K(rho, phi). Similar trends were observed for any position of the horizontal (y-z) plane at which the sample is situated along the vertical x axis of the cavity; however, the amplitude of the signal decreases with increase in the absolute value of the x coordinate, ||x ||. The variation in the signal amplitude along the cavity x axis (longitudinal effect) can be calculated theoretically using a modified sine-squared curve, G(x). In general, the response of the cavity to a point-like sample situated at any position, P(rho, phi, x), can be represented as a product of the mentioned Cassinian curve, K(rho, phi), and sine-squared curve, G(x), giving for the signal intensity I(pp)(rho, phi, x) approximately K(rho, phi)G(x). The response to a large cylindrical sample which is concentrically situated on the cavity x axis can then be obtained by integrating the above product, K(rho, phi)G(x), over the sample volume. The nonlinear radial effect may give rise to a serious source of systematic error in quantitative EPR spectroscopy and shows that accurate and precise positioning of the sample in the microwave cavity is essential.     

Measurement of branching fractions, isospin, and CP-violating asymmetries for exclusive b-->dgamma modes

We report new measurements of the decays B+-->rho+gamma, B0-->rho0gamma, and B0-->omegagamma using a data sample of 657x10(6) B meson pairs accumulated with the Belle detector at the KEKB e+e- collider. We measure branching fractions B(B+-->rho+gamma)=(8.7_-2.7-1.1;+2.9+0.9)x10(-7), B(B0-->rho0gamma)=(7.8_-1.6-1.0;+1.7+0.9)x10(-7), and B(B0-->omegagamma)=(4.0_-1.7;+1.9+/-1.3)x10(-7). We also report the isospin asymmetry Delta(rhogamma)=-0.48_-0.19-0.09;+0.21+0.08 and the first measurement of the direct CP-violating asymmetry ACP(B+-->rho+gamma)=-0.11+/-0.32+/-0.09, where the first and second errors are statistical and systematic, respectively.