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A slender string design for deep-sea neutrino telescopes

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A slender string design for deep-sea neutrino telescopes
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  A slender string design for deep-sea neutrino telescopes E. Heine a , E. Berbee a , R. de Boer a , H. Boer Rookhuizen a , J. Hogenbirk a , H. Kok a , P. Kooijman a,b,c ,A. Korporaal a , S. Mos a , G. Mul a , H. Peek a , P. Timmer a , E. de Wolf  a,b,  a Nikhef, Science Park 105, 1098 XG Amsterdam, The Netherlands b University of Amsterdam, Institute for High Energy Physics, Science Park 904, 1098 XH Amsterdam, The Netherlands c University of Utrecht, Institute for Subatomic Physics, Princetonplein 5, 3584 CC Utrecht, The Netherlands On behalf of the KM3NeT consortium a r t i c l e i n f o Available online 12 May 2010 Keywords: Neutrino telescopePhotomultiplierDeep-sea technologyOptical communicationPBOF cableDeployment a b s t r a c t In the context of the Design Study of the KM3NeT consortium for a new-generation deep-sea telescopein the Mediterranean Sea, several designs for vertical structures with optical sensors have beendeveloped. We will present a design of a slender string with a single optical modules at regular intervalsand a flexible electro-optical backbone cable with break-outs for each optical module. The design of thebackbone cable allows for point-to-point communication between the optical sensors and theelectronics on shore. The flexible slender string will be relatively easy to deploy using compactdeployment methods, thus allowing for a timely realisation of the instrumentation of a sea-watervolume larger than one cubic kilometre. &  2010 Elsevier B.V. All rights reserved. 1. Introduction A design of a slender string detection unit for a multi-km 3 deep-sea neutrino detector is presented. The detection unit isbased on the ‘Cabled Design’ described in the KM3NeT CDR  [1]and builds on the experience of earlier neutrino projects and otherdeep-sea scientific projects [2]. The detection unit consists of 20Multi-PMT optical modules [3] held by strings at a verticalspacing of 40m. An electro-optical cable runs the length of thedetection unit. The strings are attached to a network of branchcables on the sea floor. The branch cables are connected via junction boxes (PJB) and two main electro-optical cables (MEOC)to the shore. The string is designed to be deployed to depths of upto 5000m and to receive no maintenance for the 15 year lifetimeof the experiment.The neutrino detector consists of 640 of these strings. They aredistributed over an area of about 9.2km 2 of the sea bed. 2. General description The detection unit is a slender flexible string, derived from theANTARES detector, with modifications in order to meet therequirements of cost reduction in construction and deploymentwhile attaining the required reliability and retaining the detectorsensitivity. The design of the string allows for efficient production,easy transport and compact deployment of multiple stringsduring the same sea operation. This makes a construction timeof four years possible. The strings (Fig. 1(a)) consist of 20 opticalmodules spaced at a distance of 40m. The optical modules arebuilt of 17-in. glass spheres housing the optical sensors (Fig. 1(b)).The optical modules are connected to two Dyneema s cablesrunning in parallel the length of the unit. The ropes have adiameter of 4mm and provide the mechanical strength of theunit. They can sustain the combined force of buoyancy of thestring and the drag of the sea current. To ensure resistance againsttorque, the two ropes will be connected by small braces made of composite material placed half way between the optical modules.Buoyancy at the top of the string is provided by a number of empty glass spheres. All glass spheres are equipped with spring-loaded titanium collars with a cleat system to allow forconnection to the Dyneema ropes (see Fig. 1(c)). 3. Vertical structure A flexible electro-optical cable (VEOC) with a diameter of  o 10mm contains copper conductors to provide the power to theopticalmodules and opticalfibresfor data transport from the opticalmodule to the shore. A fibre-optic multiplexer is housed in a glasssphere roughly half way up the string. The fibre-optic readoutsystem is described in a contribution to this conference [4]. Contents lists available at ScienceDirectjournal homepage: www.elsevier.com/locate/nima Nuclear Instruments and Methods inPhysics Research A 0168-9002/$-see front matter  &  2010 Elsevier B.V. All rights reserved.doi:10.1016/j.nima.2010.04.142  Corresponding author at: Nikhef, Science Park 105, 1098 XG Amsterdam, TheNetherlands. Tel.: +31205925123; fax: +31205925155. E-mail address:  e.dewolf@nikhef.nl (E. de Wolf).Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S136–S138  The hydrodynamic behaviour of the slender string has beensimulated using two different programmes for mooring designsand dynamics [5,6]. The displacement of the top of the string is87m for a sea current speed of 30cm/s. For speeds below 10cm/sthat occur more than 90% of the time at the ANTARES site, thedisplacement is less than 10m.The design of the vertical electro-optical cable has a breakout ateach optical module. Two conductors provide power for the opticalmoduleandrunfromaconnectoratthefootofthestringoverthefulllength of the cable with drop and pass connections at each opticalmodule. Two optical fibres run from the node in the branch cable atthe seafloor to a master module located approximately at the centreof the string. From the master module individual bidirectional fibresrun to each optical module in a star network topology. The mastermodule is connected by penetrators to the cable to create a pressure-free housing for a dense wavelength division multiplexing unit. Tofacilitate the breaking out of the fibres at each optical module, apressure-compensated oil-filled cable will be used. This means thatthe optical fibres operate at the local hydrostatic pressure. Thissystem has been prototyped in cooperation with SEACON s . The oilthatwillbe used is the recentlydevelopedSiemensMidel750oilthathas a density very close to that seawater. In addition it isenvironmentally safe. Pressure equalisation will be done at the baseof the string. In this way the cable is kept at slight overpressure withrespect to the surrounding seawater. Fig. 1(d) shows prototype of thecable that was successfully tested.The breakout terminates in a deep-sea connector. Thisconnector has a the special feature that it incorporates a DC/DCconverter that transforms the 400V of the VEOC down to the 10Vrequired by the optical module. This affords the possibility tocreate a galvanic separation between the VEOC and the opticalmodule. In this way corrosion will not create a water penetrationinto the VEOC, should the optical module be flooded. 4. Network  The strings are attached to a network of branch cables on the seafloor. The branch cables are connected via junction boxes and themain electro-optical cables (MEOC) to the shore. Fig. 2 shows thelayout of the electrical power system. Electrical power is provided ata 10kV DC level to the junction box via six copper wires with across-section of 16mm 2 in the 100km long main electro-opticalcable. In the junction box the power is transformed to 400V DC.From the junction box the branches are feed with 400V DC fromboth sides via four 130m long copper wires with a cross-section of 16mm 2 . Via two 860m long copper wires in the vertical electro-optical cable with a cross-section of 1mm 2 , the electrical power istransported to the optical module. In the optical module the 400V isconverted to several low voltages asked for by the local electronics.The overall efficiency will be in the order of 75% from the mains onshore to the electronics in the optical module.By optimising the dissipation in the optical module the totalpower sent from shore will be in the order of 100kW, inclusive Fig. 1.  (a) Artist’s impression of the slender string; (b) multi-PMT optical module;(c) detail of the collar of the optical module; and (d) prototype Vertical Electro-Optical Cable (VEOC). Fig. 2.  Layout of the electrical power system. E. Heine et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S136–S138 S137  30kW for the earth and sea science nodes in the KM3NeT researchfacility.In older deep-sea networks power over large distances istransported by a single pole and sea return. Two wire systemsbecome more common to avoid chemical influences on theenvironment due to the use of electrodes and to minimise theinfluence of magnetic fields on the biotope. Therefore, the MEOCis equipped with two electrical conductors in this concept. 5. Deployment Many strings can be deployed in a single sea campaign. Thestring is wound on a spherical ‘launcher’ (see Fig. 3). This launcheris deployed to the seafloor. Once positioned on the sea floor it isreleased and rises through its buoyancy, unrolling the string inthe process. Once the launcher has freed itself from the string itrises to the surface where it is recovered for reuse in subsequentdeployments. The deployment method has been testedsuccessfully.  Acknowledgements This work is part of the research programme of the ‘Stichtingvoor Fundamenteel Onderzoek der Materie (FOM)’, which isfinancially supported by the ‘Nederlandse Organisatie voorWetenschappelijk Onderzoek (NWO)’ and through the EU-fundedFP6 KM3NeT Design Study Contract no. 011937. The collaborationwith the ‘Nationaal Instituut Onderzoek der Zee (NIOZ)’ has asignificant contribution to the knowledge of sea techniques. References [1] KM3NeT, Conceptual Design Report for a Deep-Sea Research Infrastructure inthe Mediterranean Sea Incorporating a Very Large Volume Neutrino Telescope,2008.[2] AMANDA, IceCube  / http://www.icecube.wisc.edu S ; ANTARES  / http://www.antares.in2p3.fr S ; DUMAND  / http://www.hawaii.edu/dumand S ; NEMO / http://nemoweb.lns.infn.it S ; NESTOR   / http://nestor.noa.gr S .[3] P. Kooijman, et al., A new generation optical module for deep-sea neutrinotelescopes, in: VLVnT09 Proceedings, Athens, Greece, Nucl. Instr. and Meth. A,2009.[4] J. Hogenbirk, et al., A photonic readout and data acquisition system for deep-sea neutrino telescopes, in: VLVnT09 Proceedings, Athens, Greece, Nucl. Instr.and Meth. A, 2009.[5] T. Hillebrand, Presentation on the KM3NeT General Meeting, Athens, Greece,April 2009.[6] R. Dewey, University of Victoria, BC, Canada Design Simulation TechnologiesInc., URL:  / http://www.design-simulation.com S . Fig. 3.  Deployment launcher on deck before deployment. E. Heine et al. / Nuclear Instruments and Methods in Physics Research A 626-627 (2011) S136–S138  S138
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