86 research outputs found
A model for microinstability destabilization and enhanced transport in the presence of shielded 3-D magnetic perturbations
A mechanism is presented that suggests shielded 3-D magnetic perturbations
can destabilize microinstabilities and enhance the associated anomalous
transport. Using local 3-D equilibrium theory, shaped tokamak equilibria with
small 3-D deformations are constructed. In the vicinity of rational magnetic
surfaces, the infinite-n ideal MHD ballooning stability boundary is strongly
perturbed by the 3-D modulations of the local magnetic shear associated with
the presence of nearresonant Pfirsch-Schluter currents. These currents are
driven by 3-D components of the magnetic field spectrum even when there is no
resonant radial component. The infinite-n ideal ballooning stability boundary
is often used as a proxy for the onset of virulent kinetic ballooning modes
(KBM) and associated stiff transport. These results suggest that the achievable
pressure gradient may be lowered in the vicinity of low order rational surfaces
when 3-D magnetic perturbations are applied. This mechanism may provide an
explanation for the observed reduction in the peak pressure gradient at the top
of the edge pedestal during experiments where edge localized modes have been
completely suppressed by applied 3-D magnetic fields
Direct Instantons in QCD Nucleon Sum Rules
We study the role of direct (i.e. small-scale) instantons in QCD correlation
functions for the nucleon. They generate sizeable, nonperturbative corrections
to the conventional operator product expansion, which improve the quality of
both QCD nucleon sum rules and cure the long-standing stability problem, in
particular, of the chirally odd sum-rule.Comment: 10 pages, UMD PP#93-17
Geometrothermodynamics of black holes
The thermodynamics of black holes is reformulated within the context of the
recently developed formalism of geometrothermodynamics. This reformulation is
shown to be invariant with respect to Legendre transformations, and to allow
several equivalent representations. Legendre invariance allows us to explain a
series of contradictory results known in the literature from the use of
Weinhold's and Ruppeiner's thermodynamic metrics for black holes. For the
Reissner-Nordstr\"om black hole the geometry of the space of equilibrium states
is curved, showing a non trivial thermodynamic interaction, and the curvature
contains information about critical points and phase transitions. On the
contrary, for the Kerr black hole the geometry is flat and does not explain its
phase transition structure.Comment: Revised version, to be published in Gen.Rel.Grav.(Mashhoon's
Festschrift
An interpretation for the entropy of a black hole
We investigate the meaning of the entropy carried away by Hawking radiations
from a black hole. We propose that the entropy for a black hole measures the
uncertainty of the information about the black hole forming matter's
precollapsed configurations, self-collapsed configurations, and inter-collapsed
configurations. We find that gravitational wave or gravitational radiation
alone cannot carry all information about the processes of black hole
coalescence and collapse, while the total information locked in the hole could
be carried away completely by Hawking radiation as tunneling
Soft Photons in Hadron-Hadron Collisions: Synchrotron Radiation from the QCD Vacuum?
We discuss the production of soft photons in high energy hadron-hadron
collisions. We present a model where quarks and antiquarks in the hadrons emit
``synchrotron light'' when being deflected by the chromomagnetic fields of the
QCD vacuum, which we assume to have a nonperturbative structure. This gives a
source of prompt soft photons with frequencies in the c.m.
system of the collision in addition to hadronic bremsstrahlung. In comparing
the frequency spectrum and rate of ``synchrotron'' photons to experimental
results we find some supporting evidence for their existence. We make an
exclusive--inclusive connection argument to deduce from the ``synchrotron''
effect a behaviour of the neutron electric formfactor proportional
to for . We find this to be consistent with
available data. In our view, soft photon production in high energy
hadron-hadron and lepton-hadron collisions as well as the behaviour of
electromagnetic hadron formfactors for low are thus sensitive probes of
the nonperturbative structure of the QCD vacuum.Comment: Heidelberg preprint HD-THEP-94-36, 31 pages, LaTeX + ZJCITE.sty
(included), 12 figures appended as uuencoded compressed ps-fil
Thermodynamics of Reissner-Nordstrom-anti-de Sitter black holes in the grand canonical ensemble
The thermodynamical properties of the Reissner-Nordstr\"om-anti-de Sitter
black hole in the grand canonical ensemble are investigated using York's
formalism. The black hole is enclosed in a cavity with finite radius where the
temperature and electrostatic potential are fixed. The boundary conditions
allow us to compute the relevant thermodynamical quantities, e.g. thermal
energy, entropy and charge. The stability conditions imply that there are
thermodynamically stable black hole solutions, under certain conditions.
Instantons with negative heat capacity are also found.Comment: 15 pages, 9 figures, Revtex. Published version. Changes: figures
added to tex
Geometrothermodynamics of five dimensional black holes in Einstein-Gauss-Bonnet-theory
We investigate the thermodynamic properties of 5D static and spherically
symmetric black holes in (i) Einstein-Maxwell-Gauss-Bonnet theory, (ii)
Einstein-Maxwell-Gauss-Bonnet theory with negative cosmological constant, and
in (iii) Einstein-Yang-Mills-Gauss-Bonnet theory. To formulate the
thermodynamics of these black holes we use the Bekenstein-Hawking entropy
relation and, alternatively, a modified entropy formula which follows from the
first law of thermodynamics of black holes. The results of both approaches are
not equivalent. Using the formalism of geometrothermodynamics, we introduce in
the manifold of equilibrium states a Legendre invariant metric for each black
hole and for each thermodynamic approach, and show that the thermodynamic
curvature diverges at those points where the temperature vanishes and the heat
capacity diverges.Comment: New sections added, references adde
Black hole thermodynamical entropy
As early as 1902, Gibbs pointed out that systems whose partition function
diverges, e.g. gravitation, lie outside the validity of the Boltzmann-Gibbs
(BG) theory. Consistently, since the pioneering Bekenstein-Hawking results,
physically meaningful evidence (e.g., the holographic principle) has
accumulated that the BG entropy of a black hole is
proportional to its area ( being a characteristic linear length), and
not to its volume . Similarly it exists the \emph{area law}, so named
because, for a wide class of strongly quantum-entangled -dimensional
systems, is proportional to if , and to if
, instead of being proportional to (). These results
violate the extensivity of the thermodynamical entropy of a -dimensional
system. This thermodynamical inconsistency disappears if we realize that the
thermodynamical entropy of such nonstandard systems is \emph{not} to be
identified with the BG {\it additive} entropy but with appropriately
generalized {\it nonadditive} entropies. Indeed, the celebrated usefulness of
the BG entropy is founded on hypothesis such as relatively weak probabilistic
correlations (and their connections to ergodicity, which by no means can be
assumed as a general rule of nature). Here we introduce a generalized entropy
which, for the Schwarzschild black hole and the area law, can solve the
thermodynamic puzzle.Comment: 7 pages, 2 figures. Accepted for publication in EPJ
- âŠ