Hidden Uq (sl(2)) Uq (sl(2)) Quantum Group Symmetry in Two Dimensional Gravity

In a previous paper, the quantum-group-covariant chiral vertex operators in the spin 1/2 representation were shown to act, by braiding with the other covariant primaries, as generators of the well known U_q(sl(2)) quantum group symmetry (for a single screening charge). Here, this structure is transformed to the Bloch wave/Coulomb gas operator basis, thereby establishing for the first time its quantum group symmetry properties. A U_q(sl(2)) U_q(sl(2)) symmetry of a novel type emerges: The two Cartan-generator eigenvalues are specified by the choice of matrix element (Vermamodules); the two Casimir eigenvalues are equal and specified by the Virasoro weight of the vertex operator considered; the co-product is defined with a matching condition dictated by the Hilbert space structure of the operator product. This hidden symmetry possesses a novel Hopf-like structure compatible with these conditions. At roots of unity it gives the right truncation. Its (non-linear) connection with the U_q(sl(2)) previously discussed is disentangled. Received: 25 April 1996/Accepted: 20 July 1996     

Non-classical configurations in Euclidean field theory as minima of constrained systems

We study a systematic method of applying the semiclassical approximation to Euclidean field theory. First, we extract generalized collective coordinates which are not in general zero modes. We then apply the semiclassical approximation to the other degrees of freedom by minimizing the action with constraints. Hence we are using configurations which are not classical solutions of the original system. After Gaussian integration we are left with a truncated system, involving only the collective coordinates, with non-trivial dynamics. In particular, this is a clear-cut way to introduce multi-instanton or meron-type configurations. The collective coordinates should be chosen such that their dynamics are a good approximation to the original system for the physical phenomenon considered; a familiar concept in other branches of physics with many degrees of freedom. The formalism leads naturally to the introduction of dynamics in an extra time evolution; in particular cases, we show that this is a very powerful tool. In this paper, we only discuss general ideas and formalisms. Specific applications are postponed to to later publications.     

Kinematically complete study of dissociative ionization of by ion impact

We present a kinematically complete study of dissociative ionization of D(2) by 13.6 MeV/u S(15+) ions. The experiment allows us to unravel the competing mechanisms, namely, direct single ionization, autoionization of doubly excited states, ionization excitation, and double ionization, and to analyze the corresponding electron angular distribution from fixed-in-space molecules. The conclusions are supported by theoretical calculations in which the correlated motion of all electrons and nuclei and the interferences between them are described from first principles.     

Scattering of wavepackets by a flat plate in the vicinity of a turbulent jet

We present an investigation of the acoustic scattering due to the presence of a flat plate in the vicinity of a turbulent subsonic jet. Experiments have been performed to measure changes in the velocity and sound fields for Mach numbers ranging from 0.4 to 0.6, and for distances between the plate and the jet axis ranging from 1 to 2 jet diameters. Results show only very slight changes in the mean flow induced by the plate, and no differences in the velocity fluctuation amplitudes on the jet centreline, suggesting that wave-packet models derived for jets without installation effects may be representative of the installed case, at least for the jet–plate distances considered here. The acoustic results, on the other hand, include a significant increase in the low-frequency sound radiation, and phase opposition between the shielded and unshielded sides of the plate. There is an exponential decay of the scattered sound with increasing jet–plate distance, suggesting that low-frequency radiation is due to the scattering of evanescent hydrodynamic wavepackets in the jet near field. To model this phenomenon, we calculate sound generation from wave-packet sources in two ways: on one hand we use a tailored Green?s function that accounts for the presence of a semi-infinite, rigid flat plate; and, on the other, we solve numerically the Helmholtz equation, with boundary conditions representative of a finite flat plate, using a fast multipole boundary element method. In agreement with the experimental measurements, numerical calculations capture the phase opposition between shielded and unshielded sides, and the scattered sound depends exponentially on the position of the plate. This exponential dependence is related to non-compact effects associated with wavepackets, as compact sources would lead to an algebraic dependence. Acoustic pressure directivities computed for the finite and semi-infinite flat plates agree well where acoustic reflection and diffraction from the trailing edge of the plates are concerned. However, additional diffraction effects associated with the leading and lateral edges of the finite plate, and which take the form of multiple lobes in the directivity, are illustrated by the comparison. As the plate dimensions are increased, i.e. the Helmholtz number is increased, the solution approaches that obtained for the semi-infinite plate.     

Thermogravimetric analysis of single-walled carbon nanotubes synthesized by induction thermal plasma

A standard procedure for thermogravimetric analysis (TG) of carbonaceous materials including single-walled carbon nanotubes was developed based on a statistical design to precisely study the effect of three main TG parameters: temperature ramp (TR, °C), initial mass (IM) of the sample (mg), and the rate of flowing gas (sccm) on the TG results. In addition, the effect of sampling including sample morphology and moisture content on TG were studied. The results of statistical design clearly showed that TG was affected by these three parameters and particularly by IM and TR. Interestingly, it was observed that the TG results are affected insufficiently by the sample morphology and low moisture content. This study also confirmed the potential of TG combined with high-resolution scanning electron microscopy to be a simple and straightforward method for purity evaluation of SWCNT-containing samples with a complex TG behavior such as those of induction thermal plasma grown. A complementary study on nano-metric catalysts indicated that these types of materials enable to gain or loss mass in an oxidative ambient during TG. A mass loss of 6% and a mass gain of 23% were observed for pure nano-metric yttrium oxide and nickel, respectively. A simple calculation showed a total mass gain of 1?wt% particularly by the catalysts in the SWCNT sample during TG.     

Subsonic jet aeroacoustics: associating experiment,modelling and simulation

An overview of jet noise research is presented wherein the principal movements in the field are traced since its beginnings. Particular attention is paid to the evolution of our understanding of what we call a “source mechanism” in free shear flows; to the theoretical, experimental and numerical studies which have nurtured this understanding; and to the currently unresolved conceptual difficulties which render analysis of experimental and numerical data so difficult. As it is clear that accelerated progress in this field of research can be made possible by a more effective synergy between the theoretical, experimental and numerical disciplines—one which draws in particular on the impressive recent progress in experimental and numerical techniques—we endeavour to elucidate the various “source” characteristics identified by these different means of study; the points on which the studies agree or disagree, and the significance of such accord or discord; and, the new analysis possibilities which can now be realised by effectively associating experiment, modelling and simulation.

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Experimental study and Monte Carlo simulation of the correlation between electron emission and stopping power for swift proton impact on amorphous carbon target

It is usually well accepted that for swift protons, the induced backward and forward electron emission yield is proportional to the projectile electronic stopping power. This was observed in particular for thin amorphous carbon foils. However, this law was established from a non extensive set of experimental data and somewhat confirmed by rough macroscopic theories. We then developed a standard Monte Carlo simulation to predict the yield dependence on proton energy [0.5–10 MeV] and for a wide range of foil thickness. After evaluating the reliability of this simulation, we showed and explained why the law of proportionality cannot generally hold for forward electron emission. In particular, the ratio between forward yield and stopping power generally depends on foil thickness and proton energy. We performed a new experiment that confirmed our theoretical predictions. Received 9 March 2001