Zoll Metrics, Branched Covers, and Holomorphic Disks

We strengthen our previous results regarding the moduli spaces of Zoll metrics and Zoll projective structures on S^2. In particular, we describe a concrete, open condition which suffices to guarantee that a totally real embedding of RP^2 in CP_2 arises from a unique Zoll projective structure on the 2-sphere. Our methods ultimately reflect the special role such structures play in the initial value problem for the 3-dimensional Lorentzian Einstein-Weyl equations.Comment: 21 pages, LaTeX2

The Einstein-Weyl Equations, Scattering Maps, and Holomorphic Disks

We show that conformally compact, globally hyperbolic, Lorentzian Einstein-Weyl 3-manifolds are in natural one-to-one correspondence with orientation-reversing diffeomorphisms of the 2-sphere. The proof hinges on a holomorphic-disk analog of Hitchin's mini-twistor correspondence.Comment: 11 pages, LaTeX2e. Revised version strengthens result and completes proo

Zoll Manifolds and Complex Surfaces

We classify compact surfaces with torsion-free affine connections for which every geodesic is a simple closed curve. In the process, we obtain completely new proofs of all the major results concerning the Riemannian case. In contrast to previous work, our approach is twistor-theoretic, and depends fundamentally on the fact that, up to biholomorphism, there is only one complex structure on CP2

Nonlinear Gravitons, Null Geodesics, and Holomorphic Disks

We develop a global twistor correspondence for pseudo-Riemannian conformal structures of signature (++--) with self-dual Weyl curvature. Near the conformal class of the standard indefinite product metric on S^2 x S^2, there is an infinite-dimensional moduli space of such conformal structures, and each of these has the surprising global property that its null geodesics are all periodic. Each such conformal structure arises from a family of holomorphic disks in CP_3 with boundary on some totally real embedding of RP^3 into CP_3. An interesting sub-class of these conformal structures are represented by scalar-flat indefinite K\"ahler metrics, and our methods give particularly sharp results in this more restrictive setting.Comment: 56 pages, LaTeX2

Cooling with Superfluid Helium

The technical properties of helium II ('superfluid' helium) are presented in view of its applications to the cooling of superconducting devices, particularly in particle accelerators. Cooling schemes are discussed in terms of heat transfer performance and limitations. Large-capacity refrigeration techniques below 2 K are reviewed, with regard to thermodynamic cycles as well as process machinery. Examples drawn from existing or planned projects illustrate the presentation. Keywords: superfluid helium, cryogenicsComment: 24 pages, contribution to the CAS-CERN Accelerator School: Superconductivity for Accelerators, Erice, Italy, 24 April - 4 May 2013, edited by R. Baile

IGR J11215-5952: a hard X-ray transient displaying recurrent outbursts

The hard X-ray source IGRJ11215-5952 has been discovered with INTEGRAL during a short outburst in 2005 and proposed as a new member of the class of supergiant fast X-ray transients. We analysed INTEGRAL public observations of the source field in order to search for previous outbursts from this transient, not reported in literature.Our results are based on a systematic re-analysis of INTEGRAL archival observations, using the latest analysis software and instrument calibrations. We report the discovery of two previously unnoticed outbursts, spaced by intervals of ~330 days, that occurred in July 2003 and May 2004. The 5-100keV spectrum is well described by a cut-off power law, with a photon index of 0.5, and a cut-off energy ~15-20keV, typical of High Mass X-ray Binaries hosting a neutron star. A 5-100keV luminosity of 3E36 erg/s has been derived (assuming 6.2kpc, the distance of the likely optical counterpart). The 5-100keV spectral properties, the recurrent nature of the outbursts,together with the reduced error region containing the blue supergiant star HD306414,support the hypothesis that IGRJ11215-5952 is a member of the class of the Supergiant Fast X-ray Transients.Comment: 4 pages, 4 figures, accepted for publication in Astronomy and Astrophysics Letter

Towards Cost-To-Performance Optimisation of Large Superfluid Helium Refrigeration Systems

The field range of superconducting devices may be extended by lowering their operating temperature, using superfluid helium refrigeration systems which have to deliver working pressures down to 1.6 kPa. The corresponding pressure ratio can be produced by integral cold compression or using a combination of cold compressors in series together with "warm" compressors at room temperature. The optimisation of such a system depends on the number, arrangement and characteristics of cold and warm machines as well as on the operating scenario and turndown capability. The aim of this paper is to compare relative investment and operating costs of different superfluid helium cryogenic systems, with the aim of optimising their cost-to-performance ratio within the constraints of their operating scenario

Development of large-capacity refrigeration at 1.8 K for the Large Hadron Collider at CERN

CERN, the European Laboratory for Particle Physics, is working towards the construction of the Large Hadron Collider (LHC), a high-energy, high-luminosity particle accelerator and collider [1] of 26.7 km circumference, due to start producing frontier physics, by bringing into collision intense proton and ion beams with centre-of-mass energies in the TeV-per-constituent range, at the beginning of the next century. The key technology for achieving this ambitious scientific goal at economically acceptable cost is the use of high-field superconducting magnets using Nb-Ti conductor operating in superfluid helium [2]. To maintain the some 25 km of bending and focusing magnets at their operating temperature of 1.9 K, the LHC cryogenic system will have to produce an unprecedented total refrigeration capacity of about 20 kW at 1.8 K, in eight cryogenic plants distributed around the machine circumference [3]. This has requested the undertaking of an industrial development programme, in the form of a collaboration between CERN and CEA, France, for investigating specific machinery, i.e. very-low pressure cryogenic heat exchangers, volumetric and hydrodynamic compressors, as well as practical and efficient thermodynamic cycles. We report on the aims lines of action and present progress of this ongoing programme

Supercritical Helium Cooling of the LHC Beam Screens

The cold mass of the LHC superconducting magnets, operating in pressurised superfluid helium at 1.9 K, must be shielded from the dynamic heat loads induced by the circulating particle beams, by means of beam screens maintained at higher temperature. The beam screens are cooled between 5 and 20 K by forced flow of weakly supercritical helium, a solution which avoids two-phase flow in the long, narr ow cooling channels, but still presents a potential risk of thermohydraulic instabilities. This problem has been studied by theoretical modelling and experiments performed on a full-scale dedicated te st loop

Performance Assessment of Industrial Prototype Cryogenic Helium Compressors for the Large Hadron Collider

In order to develop the technology of large-capacity refrigeration at superfluid helium temperature, essential for the LHC project, CERN has procured from industry three prototype single-stage hydrody namic cryogenic helium compressors, based on different construction choices, and tested them in the laboratory. After recalling the common functional specification, as well as the main design features of the three machines, we present comparative performance results, and draw conclusions as concerns future full-scale machines for the LHC