BRST cohomology of the Chapline-Manton model

We completely compute the local BRST cohomology $H(s|d)$ of the combined Yang-Mills-2-form system coupled through the Yang-Mills Chern-Simons term ("Chapline-Manton model"). We consider the case of a simple gauge group and explicitely include in the analysis the sources for the BRST variations of the fields ("antifields"). We show that there is an antifield independent representative in each cohomological class of $H(s|d)$ at ghost number 0 or 1. Accordingly, any counterterm may be assumed to preserve the gauge symmetries. Similarly, there is no new candidate anomaly beside those already considered in the literature, even when one takes the antifields into account. We then characterize explicitly all the non-trivial solutions of the Wess-Zumino consistency conditions. In particular, we provide a cohomological interpretation of the Green-Schwarz anomaly cancellation mechanism.Comment: Latex file, no figures, 15 page

On the instability of the magnetohydrodynamic pipe flow subject to a transverse magnetic field

The linear stability of a fully-developed liquid-metal MHD pipe flow subject to a transverse magnetic field is studied numerically. Because of the lack of axial symmetry in the mean velocity profile, we need to perform a BiGlobal stability analysis. For that purpose, we develop a two-dimensional complex eigenvalue solver relying on a Chebyshev-Fourier collocation method in physical space. By performing an extensive parametric study, we show that in contrast to Hagen-Poiseuille flow known to be linearly stable for all Reynolds numbers, the MHD pipe flow with transverse magnetic field is unstable to three-dimensional disturbances at sufficiently high values of the Hartmann number and wall conductance ratio. The instability observed in this regime is attributed to the presence of velocity overspeeds in the so-called Roberts layers and the corresponding inflection points in the mean velocity profile. The nature and characteristics of the most unstable modes are investigated, and we show that they vary significantly depending on the wall conductance ratio. A major result of this paper is that the global critical Reynolds number for the MHD pipe flow with transverse magnetic field is $Re=45230$, and it occurs for a perfectly conducting pipe wall and the Hartmann number $Ha=19.7$

A Theorem on First-Order Interaction Vertices for Free p-Form Gauge Fields

The complete proof of a theorem announced in [1] on the consistent interactions for (non-chiral) exterior form gauge fields is given. The theorem can be easily generalized to the analysis of anomalies. Its proof amounts to computing the local BRST cohomology H^0(s|d) in the space of local n-forms depending on the fields, the ghosts, the antifields and their derivatives.Comment: 18 pages, no figures, misquotes in references correcte

Couplings of gravity to antisymmetric gauge fields

We classify all the first-order vertices of gravity consistently coupled to a system of 2-form gauge fields by computing the local BRST cohomology H(s|d) in ghost number 0 and form degree n. The consistent deformations are at most linear in the undifferentiated two-form, confirming the previous results of [1] that geometrical theories constructed from a nonsymmetric gravity theory are physically inconsistent or trivial. No assumption is made here on the degree of homogeneity in the derivatives nor on the form of the gravity action.Comment: 11 pages, no figures, Latex2.0

Thermal Conduction in Magnetized Turbulent Gas

Using numerical methods, we systematically study in the framework of ideal MHD the effect of magnetic fields on heat transfer within a turbulent gas. We measure the rates of passive scalar diffusion within magnetized fluids and make the comparisons a) between MHD and hydro simulations, b) between different MHD runs with different values of the external magnetic field (up to the energy equipartition value), c) between thermal conductivities parallel and perpendicular to magnetic field. We do not find apparent suppression of diffusion rates by the presence of magnetic fields, which implies that magnetic fields do not suppress heat diffusion by turbulent motions.Comment: 4 pages; 2 figures; submitted to Ap

The Effect of Coherent Structures on Stochastic Acceleration in MHD Turbulence

We investigate the influence of coherent structures on particle acceleration in the strongly turbulent solar corona. By randomizing the Fourier phases of a pseudo-spectral simulation of isotropic MHD turbulence (Re $\sim 300$), and tracing collisionless test protons in both the exact-MHD and phase-randomized fields, it is found that the phase correlations enhance the acceleration efficiency during the first adiabatic stage of the acceleration process. The underlying physical mechanism is identified as the dynamical MHD alignment of the magnetic field with the electric current, which favours parallel (resistive) electric fields responsible for initial injection. Conversely, the alignment of the magnetic field with the bulk velocity weakens the acceleration by convective electric fields - \bfu \times \bfb at a non-adiabatic stage of the acceleration process. We point out that non-physical parallel electric fields in random-phase turbulence proxies lead to artificial acceleration, and that the dynamical MHD alignment can be taken into account on the level of the joint two-point function of the magnetic and electric fields, and is therefore amenable to Fokker-Planck descriptions of stochastic acceleration.Comment: accepted for publication in Ap

Du choix du précurseur pour la synthèse de poudres d’oxydes La1-xSrxMnO3

Différentes poudres précurseurs (carbonates, oxalates, citrates) de manganites de lanthane dopés au strontium ont été préparées par chimie douce. Après calcination, ces précurseurs conduisent aux oxydes mixtes à structure perovskite LaxSr1–xMnO3, dont la morphologie et la taille des particules dépendent à la fois de celles du précurseur et des conditions du traitement thermique. Des analyses thermiques de la décomposition des précurseurs montrent que seuls les citrates sont des précurseurs mixtes, ce qui permet d’obtenir les oxydes à structure perovskite dès 600 °C, au lieu de 850 °C pour les oxalates et 900 °C pour les carbonates. De plus, seule la méthode des citrates conduit à des poudres d’oxydes dont la morphologie, la surface spécifique, la non-stœchiométrie peuvent être modulée dans une large gamme

The Wess-Zumino Consistency Condition For p-Form Gauge Theories

The general solution of the antifield-independent Wess-Zumino consistency condition is worked out for models involving exterior form gauge fields of arbitrary degree. We consider both the free theory and theories with Chapline-Manton couplings. Our approach relies on solving the full set of descent equations by starting from the last element down ("bottom").Comment: latex 2.09, 46 pages, no figures, requires multibox style, available at ftp://ftp.tex.ac.uk/tex-archive/macros/latex209/contrib/misc/multibox.sty -- misquotes in references correcte

Characteristic cohomology of pp-form gauge theories

The characteristic cohomology $H^k_{char}(d)$ for an arbitrary set of free $p$-form gauge fields is explicitly worked out in all form degrees $k<n-1$, where $n$ is the spacetime dimension. It is shown that this cohomology is finite-dimensional and completely generated by the forms dual to the field strengths. The gauge invariant characteristic cohomology is also computed. The results are extended to interacting $p$-form gauge theories with gauge invariant interactions. Implications for the BRST cohomology are mentioned.Comment: Latex file, no figures, 44 page

Theory of the Lorentz force flowmeter

A Lorentz force flowmeter is a device for the contactless measurement of flow rates in electrically conducting fluids. It is based on the measurement of a force on a magnet system that acts upon the flow. We formulate the theory of the Lorentz force flowmeter which connects the measured force to the unknown flow rate. We first apply the theory to three specific cases, namely (i) pipe flow exposed to a longitudinal magnetic field, (ii) pipe flow under the influence of a transverse magnetic field and (iii) interaction of a localized distribution of magnetic material with a uniformly moving sheet of metal. These examples provide the key scaling laws of the method and illustrate how the force depends on the shape of the velocity profile and the presence of turbulent fluctuations in the flow. Moreover, we formulate the general kinematic theory which holds for arbitrary distributions of magnetic material or electric currents and for any velocity distribution and which provides a rational framework for the prediction of the sensitivity of Lorentz force flowmeters in laboratory experiments and in industrial practice.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58171/2/njp7_8_299.pd