303 research outputs found
Extending the Use of HTS to Feeders in Superconducting Magnet Systems
Following the successful adoption of high temperature superconductors (HTS) in over a thousand current leads that will feed 3 MA from warm to cold in the Large Hadron Collider (LHC), the use of HTS has been generally accepted as suitable technology for the design of efficient leads feeding cryo-magnets. We now consider the extension of the technology to the interconnection of strings of superconducting magnets and their connection to feed-boxes through which the excitation current is fed. It is proposed to use HTS material for this application instead of low-temperature superconductor or normal-conducting material. The implications of adopting this technology are discussed with regard to the choice of materials, highlighting the differences with more conventional schemes. Examples are given of how this approach could be applied to the consolidation and upgrade of the LHC
Diffusion due to the Beam-Beam Interaction and Fluctuating Fields in Hadron Colliders
Random fluctuations in the tune, beam offsets and beam size in the presence
of the beam-beam interaction are shown to lead to significant particle
diffusion and emittance growth in hadron colliders. We find that far from
resonances high frequency noise causes the most diffusion while near resonances
low frequency noise is responsible for the large emittance growth observed.
Comparison of different fluctuations shows that offset fluctuations between the
beams causes the largest diffusion for particles in the beam core.Comment: 5 pages, 3 postscript figure
Geometric Cone Surfaces and (2+1)- Gravity coupled to Particles
We introduce the (2+1)-spacetimes with compact space of genus g and with r
gravitating particles which arise by ``Minkowskian suspensions of flat or
hyperbolic cone surfaces'', by ``distinguished deformations'' of hyperbolic
suspensions and by ``patchworking'' of suspensions. Similarly to the
matter-free case, these spacetimes have nice properties with respect to the
canonical Cosmological Time Function. When the values of the masses are
sufficiently large and the cone points are suitably spaced, the distinguished
deformations of hyperbolic suspensions determine a relevant open subset of the
full parameter space; this open subset is homeomorphic to the product of an
Euclidean space of dimension 6g-6+2r with an open subset of the Teichm\"uller
Space of Riemann surfaces of genus g with r punctures. By patchworking of
suspensions one can produce examples of spacetimes which are not distinguished
deformations of any hyperbolic suspensions, although they have the same masses;
in fact, we will guess that they belong to different connected components of
the parameter space.Comment: 14 pages Late
Geometry and observables in (2+1)-gravity
We review the geometrical properties of vacuum spacetimes in (2+1)-gravity
with vanishing cosmological constant. We explain how these spacetimes are
characterised as quotients of their universal cover by holonomies. We explain
how this description can be used to clarify the geometrical interpretation of
the fundamental physical variables of the theory, holonomies and Wilson loops.
In particular, we discuss the role of Wilson loop observables as the generators
of the two fundamental transformations that change the geometry of
(2+1)-spacetimes, grafting and earthquake. We explain how these variables can
be determined from realistic measurements by an observer in the spacetime.Comment: Talk given at 2nd School and Workshop on Quantum Gravity and Quantum
Geometry (Corfu, September 13-20 2009); 10 pages, 13 eps figure
The Torus Universe in the Polygon Approach to 2+1-Dimensional Gravity
In this paper we describe the matter-free toroidal spacetime in 't Hooft's
polygon approach to 2+1-dimensional gravity (i.e. we consider the case without
any particles present). Contrary to earlier results in the literature we find
that it is not possible to describe the torus by just one polygon but we need
at least two polygons. We also show that the constraint algebra of the polygons
closes.Comment: 18 pages Latex, 13 eps-figure
Collisions of particles in locally AdS spacetimes II Moduli of globally hyperbolic spaces
We investigate 3-dimensional globally hyperbolic AdS manifolds containing
"particles", i.e., cone singularities of angles less than along a
time-like graph . To each such space we associate a graph and a finite
family of pairs of hyperbolic surfaces with cone singularities. We show that
this data is sufficient to recover the space locally (i.e., in the neighborhood
of a fixed metric). This is a partial extension of a result of Mess for
non-singular globally hyperbolic AdS manifolds.Comment: 29 pages, 3 figures. v2: 41 pages, improved exposition. To appear,
Comm. Math. Phys. arXiv admin note: text overlap with arXiv:0905.182
Collisions of particles in locally AdS spacetimes I. Local description and global examples
We investigate 3-dimensional globally hyperbolic AdS manifolds containing
"particles", i.e., cone singularities along a graph . We impose
physically relevant conditions on the cone singularities, e.g. positivity of
mass (angle less than on time-like singular segments). We construct
examples of such manifolds, describe the cone singularities that can arise and
the way they can interact (the local geometry near the vertices of ).
We then adapt to this setting some notions like global hyperbolicity which are
natural for Lorentz manifolds, and construct some examples of globally
hyperbolic AdS manifolds with interacting particles.Comment: This is a rewritten version of the first part of arxiv:0905.1823.
That preprint was too long and contained two types of results, so we sliced
it in two. This is the first part. Some sections have been completely
rewritten so as to be more readable, at the cost of slightly less general
statements. Others parts have been notably improved to increase readabilit
Dimension of the Torelli group for Out(F_n)
Let T_n be the kernel of the natural map from Out(F_n) to GL(n,Z). We use
combinatorial Morse theory to prove that T_n has an Eilenberg-MacLane space
which is (2n-4)-dimensional and that H_{2n-4}(T_n,Z) is not finitely generated
(n at least 3). In particular, this recovers the result of Krstic-McCool that
T_3 is not finitely presented. We also give a new proof of the fact, due to
Magnus, that T_n is finitely generated.Comment: 27 pages, 9 figure
Polygon model from first order gravity
The gauge fixed polygon model of 2+1 gravity with zero cosmological constant
and arbitrary number of spinless point particles is reconstructed from the
first order formalism of the theory in terms of the triad and the spin
connection. The induced symplectic structure is calculated and shown to agree
with the canonical one in terms of the variables.Comment: 20 pages, presentation improved, typos correcte
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