ELECTRODYNAMICS OF SUPERCONDUCTING CABLES IN ACCELERATOR MAGNETS

Transcription

1 ELECTRODYNAMICS OF SUPERCONDUCTING CABLES IN ACCELERATOR MAGNETS

2 Publication of this thesis has been financially supported by Holec Ridderkerk, The Netherlands CIP-GEGEVENS KONINKLIJKE BIBLIOTHEEK, DEN HAAG Verweij, Arjan Peter Electrodynamics of superconducting cables in accelerator magnets / Arjan Peter Verweij. - [S.l. : s.n.]. - I11. Proefschrift Universiteit Twente Enschede. - Met lit. opg. - Met samenvatting in het Nederlands. ISBN Trefw.: supergeleiding / versnellers / electrodynamica. Eerste Uitgave 1995 Druk: Universiteit Twente A.P. Verweij 1995

3 ELECTRODYNAMICS OF SUPERCONDUCTING CABLES IN ACCELERATOR MAGNETS PROEFSCHRIFT ter verkrijging van de graad van doctor aan de Universiteit Twente, op gezag van de rector magnificus, prof. dr. Th.J.A. Popma, volgens besluit van het College voor Promoties in het openbaar te verdedigen op vrijdag 15 september 1995 te uur. door Arjan Peter Verweij geboren op 7 januari 1968 te Arnhem

4 Dit proefschrift is goedgekeurd door: prof. dr. C. Daum (promotor) en dr. ir. H.H.J. ten Kate (assistent-promotor).

5 Preface The work described in this thesis has been carried out partially in the Low Temperature group at the University of Twente and partially in the AT-MA group at CERN, in the framework of a collaboration agreement between CERN, the University of Twente, STW and NIKHEF. I would like to thank all members of both groups for their help and the pleasant atmosphere during the day. Also the support of those who took care of the helium and the cryogenics of the magnets, often in the evening and in the weekend, is acknowledged. I appreciated the help of the following persons in particular: Kees Daum and Herman ten Kate for their general support and many useful comments on the concept text, Daniel Leroy, Luc Oberli, David Richter, Andrjez Siemko, Peter Sievers, Louis Walckiers and Rob Wolf for their interest, advice and many fruitful discussions, Stephan Russenschuck, for providing various field maps, Christian Giloux, Guillaume Gerin and Benoît Geroudet for measuring many magnets in Block 4 and SM18, Lars Eriksson, Andy Hofstede, Marijn Oomen, Bart Sachse and Maarten Bouwhuis for performing many experiments at the university, John Baxter and Joyce Moore for correcting my English language. Finally, I would like to thank my family and friends for their interest and encouragement, our sporting activities on clay, grass and snow, and other distractions in the evenings and weekends. Enschede, September 1995 Arjan Verweij

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7 Contents 1. General introduction Introduction to the electrodynamic properties of superconducting cables and magnets Superconducting magnets Accelerator magnets Scope of the thesis Multistrand cables and magnets Magnetic field in the aperture of a magnet Magnet characteristics Magnet designs Cold mass Pre-compression of the coils Field and force distributions Superconductor Quenching and protection Operating procedure Beam losses Survey of the model magnets Strand and cable characteristics The I C (B, T ) relation Conclusions Losses in strands Introduction Hysteresis loss The J C -B relation Interfilament coupling currents Interfilament time constants Conclusions Interstrand coupling currents Introduction Network model of a Rutherford-type cable Contact resistances R a and R c Weak excitation Steady-state calculations Step response calculations Strong excitation 80

8 4.6 Cables with R c varying across the cable width Cables with &B varying across the cable width Cables of finite length with constant &B Stacked cables Influence of transverse pressure on R c Introduction Theoretical model for the calorimetric method Theoretical model for the electrical method Experimental set-up Results and discussion Conclusions Boundary-induced coupling currents Introduction Cables with insulated strands Simulating BICCs Cables exposed to a &B -step Characteristic BICC pattern Magnitude and characteristic length of BICCs under steady-state conditions Characteristic time of BICCs Propagation velocity of BICCs Arbitrary &B -distributions Cable that are partially exposed to &B &B -steps with R c >> R a Non-uniform R c -distributions Experimental observation of BICCs in a 1.3 m long cable Introduction Experimental set-up Results and discussion Conclusions Coupling-current losses in accelerator dipole magnets Introduction Loss components in magnets Hysteresis loss Interfilament coupling loss Interstrand coupling loss Losses in the connections and the wedges Total loss Measuring losses of a magnet during ramping Experimentally determined R c -values of LHC magnets Conclusions 162

9 7. Coupling-current induced field distortions Introduction Calculating coupling-current induced field distortions Field B if in dipole magnets Field B is in dipole magnets Field B bi in dipole magnets Experimental methods to determine B cc Experimental results of field B cc in LHC dipole magnets m long CE1 magnet m long EL2 magnet m long AN2 magnet m long AN3 magnet Evaluation of fields B is and B bi Conclusions Ramp-rate limitation of dipole magnets Introduction Calculation of the RRL of magnets Influence of IFCCs on the RRL Influence of ISCCs on the RRL Influence of BICCs on the RRL Influence of the ISCL on the RRL Discussion Influence of BICCs on the RRL in LHC dipole magnets Estimate of the temperature increase of the cable due to power losses in the coil Negative field-sweep rates Conclusions General conclusions and recommendations Modelling of coupling currents in multistrand cables Restrictions of the contact resistances Controlling and measuring contact resistances Effect of coupling currents in other magnets 230 References 231 Nomenclature 239 Summary (in Dutch) 245

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