Existence and multiplicity for elliptic problems with quadratic growth in the gradient

We show that a class of divergence-form elliptic problems with quadratic growth in the gradient and non-coercive zero order terms are solvable, under essentially optimal hypotheses on the coefficients in the equation. In addition, we prove that the solutions are in general not unique. The case where the zero order term has the opposite sign was already intensively studied and the uniqueness is the rule.Comment: To appear in Comm. PD

Particle-hole excitations in the interacting boson model; 4, the U(5)-SU(3) coupling

In the extended interacting boson model (EIBM) both particle- and hole-like bosons are incorporated to encompass multi-particle-multi-hole excitations at and near to closed shells.We apply the group theoretical concepts of the EIBM to the particular case of two coexisting systems in the same nucleus exhibiting a U(5) (for the regular configurations) and an SU(3) symmetry (for the intruder configurations).Besides the description of ``global'' symmetry aspects in terms of I-spin , also the very specific local mixing effects characteristic for the U(5)-SU(3) symmetry coupling are studied.The model is applied to the Po isotopes and a comparison with a morerealistic calculation is made

Intruder bands and configuration mixing in the lead isotopes

A three-configuration mixing calculation is performed in the context of the interacting boson model with the aim to describe recently observed collective bands built on low-lying $0^+$ states in neutron-deficient lead isotopes. The configurations that are included correspond to the regular, spherical states as well as two-particle two-hole and four-particle four-hole excitations across the Z=82 shell gap.Comment: 20 pages, 4 figures, accepted by PRC, reference added for section 1 in this revised versio

Many-body-QED perturbation theory: Connection to the Bethe-Salpeter equation

The connection between many-body theory (MBPT)--in perturbative and non-perturbative form--and quantum-electrodynamics (QED) is reviewed for systems of two fermions in an external field. The treatment is mainly based upon the recently developed covariant-evolution-operator method for QED calculations [Lindgren et al. Phys. Rep. 389, 161 (2004)], which has a structure quite akin to that of many-body perturbation theory. At the same time this procedure is closely connected to the S-matrix and the Green's-function formalisms and can therefore serve as a bridge between various approaches. It is demonstrated that the MBPT-QED scheme, when carried to all orders, leads to a Schroedinger-like equation, equivalent to the Bethe-Salpeter (BS) equation. A Bloch equation in commutator form that can be used for an "extended" or quasi-degenerate model space is derived. It has the same relation to the BS equation as has the standard Bloch equation to the ordinary Schroedinger equation and can be used to generate a perturbation expansion compatible with the BS equation also for a quasi-degenerate model space.Comment: Submitted to Canadian J of Physic

Assessment of alternative divertor configurations as an exhaust solution for DEMO

Plasma exhaust has been identified as a major challenge towards the realisation of magnetic confinement fusion. To mitigate the risk that the single null divertor (SND) with a high radiation fraction in the scrape-of-layer (SOL) adopted for ITER will not extrapolate to a DEMO reactor, the EUROfusion consortium is assessing potential benefits and engineering challenges of alternative divertor configurations. Alternative configurations that could be readily adopted in a DEMO design include the X divertor (XD), the Super-X divertor (SXD), the Snowflake divertor (SFD) and the double null divertor (DND). The flux flaring towards the divertor target of the XD is limited by the minimum grazing angle at the target set by gaps and misalignments. The characteristic increase of the target radius in the SXD is a trade-off with the increased TF coil volume, but, ultimately, also limited by forces onto coils. Engineering constraints also limit XD and SXD characteristics to the outer divertor leg with a solution for the inner leg requiring up-down symmetric configurations. Capital cost increases with respect to a SND configuration are largest for SXD and SFD, which require both significantly more poloidal field coil conductors and in the case of the SXD also more toroidal field coil conductors. Boundary models with increasing degrees of complexity have been used to predict the beneficial effect of the alternative configurations on exhaust performance. While all alternative configurations should decrease the power that must be radiated in the outer divertor, only the DND and possibly the SFD also ease the radiation requirements in the inner divertor. These decreases of the radiation requirements are however expected to be small making the ability of alternative divertors to increase divertor radiation without excessive core performance degradation their main advantage. Initial 2D fluid modeling of argon seeding in XD and SFD configurations indicate such advantages over the SND, while results for SXD and DND are still pending. Additional improvements, expected from increased turbulence in the low poloidal field region of the SFD also remain to be verified. A more precise comparison with the SND as well as absolute quantitative predictions for all configurations requires more complete physics models that are currently only being developed

Large-scale Observations of a Subauroral Polarization Stream by Midlatitude SuperDARN Radars: Instantaneous Longitudinal Velocity Variations

We present simultaneous measurements of flow velocities inside a subauroral polarization stream (SAPS) made by six midlatitude high-frequency SuperDARN radars. The instantaneous observations cover three hours of universal time and six hours of magnetic local time (MLT). From velocity variations across the field-of-view of the radars we infer the local 2D flow direction at three different longitudes. We find that the local flow direction inside the SAPS channel is remarkably constant over the course of the event. The flow speed, however, shows significant temporal and spatial variations. After correcting for the radar look direction we are able to accurately determine the dependence of the SAPS velocity on magnetic local time. We find that the SAPS velocity variation with magnetic local time is best described by an exponential function. The average velocity at 00 MLT was 1.2 km/s and it decreased with a spatial e-folding scale of two hours of MLT toward the dawn sector. We speculate that the longitudinal distribution of pressure gradients in the ring current is responsible for this dependence and find these observations in good agreement with results from ring current models. Using TEC measurements we find that the high westward velocities of the SAPS are - as expected - located in a region of low TEC values, indicating low ionospheric conductivities

Event shapes in e+e- annihilation and deep inelastic scattering

This article reviews the status of event-shape studies in e+e- annihilation and DIS. It includes discussions of perturbative calculations, of various approaches to modelling hadronisation and of comparisons to data.Comment: Invited topical review for J.Phys.G; 40 pages; revised version corrects some nomenclatur