HIGH FIELD SPLIT PAIR MAGNET SYSTEM FOR HORIZONTAL AND VERTICAL FIELD OPERATION

Transcription

1 HIGH FIELD SPLIT PAIR MAGNET SYSTEM FOR HORIZONTAL AND VERTICAL FIELD OPERATION P. Jarvis To cite this version: P. Jarvis. HIGH FIELD SPLIT PAIR MAGNET SYSTEM FOR HORIZONTAL AND VER- TICAL FIELD OPERATION. Journal de Physique Colloques, 1984, 45 (C1), pp.c1-803-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C1, suppl6ment au no 1, Tome 45, janvier 1984 page C1-803 HIGH FIELD SPLIT PAIR MAGNET SYSTEM FOR HORIZONTAL AND VERTICAL FIELD OPERATI ON P. Jarvis INTRODUCTION Cryogenic Consultants Limited, Metrostore BuiZding, 231 me Vale, Acton, London W3 7QS, U.K. Rdsumd - L'aimant, qui est construit utilisant l'alliage Nb3Sn filamentaire et l'alliage NbTi, fournit des champs jusqu'l 10 Tesla h 4,2 K, et 13 fen@tres d'accss radial h 90 degrds. Le systpme complet est extrsmement compact et il utilise un rdservoir mont6 1 45" de l'axe du champ, de telle sorte que, sans casser le vide, le systpme peut tourner de manisre Zi fournir un champ magndtique soit horizontal soit vertical. Abstract - The magnet, which is constructed using filamentary Nb3Sn and %Ti, provides fields of up to 10 Tesla at 4.2K and has 3x90 degrees radial access windows. The complete system is extremely compact and features a helium reservoir mounted at 45' to the field axis so that, without breaking vacuum, the system can be rotated to provide either a horizontal or vertical magnetic field. Many applications of superconducting magnets, particularly those involving particle beams, require access to th? magnetic field both along the field axis and radially. This necessitates the use of a split pair magnet and, until recently, magnets of even modest gap have been limited to fields of around 7 Tesla at 4.2K using filamentary NbTi or have been constructed using pancakes of NbsSn tape conductor /I/. The present systemisusedforcarrying out ion beam studies of hyperfine interactions in magnetic fields of up to 10 Tesla at Hahn-Meitner-Institute in Berlin. Owing to their extremely large size, & tape magnet would have been completely impractical for both the laboratory and the in-beam experiments planned. This paper gives design and constructional details of a high field split pair magnet system which can be operated in both vertical and horizontal field modes. During the development of a reliable 10 Tesla magnet,ageneralpurpose data acquisition system was built; we describe its construction and some operating software to indicate how the use of a monitoring system is essential in order to understand the behaviour of complex superconducting magnet systems. Finally, we give some construction and performance data for the complete cryomagnetic system. MAGNET DESIGN AND CONSTRUCTION The advent of high current density filamentary NbsSn has enabled us to provide an extremely compact design for a 10 Tesla (4.2K) split pair magnet system. To maximise current density, we chose an unstabilised 10,000 filament Nb3Sn composite of 0.7mm diameter (Vacuumschmelze) for the inner coil sections. The use of the expensive copper stabilised Nb3Sn composites can only be justified in much larger magnets where the dictates of protection and/or stabilisation may render it essential. Compact laboratory solenoids with operating parameters of 13 Tesla (2K) and 40mm bore can also be readily built using unstabilised Nb3Sn conductors. This design has 6 coil sections connected in series, with three sections synnnetrically arranged on each side of the mid-plane. A high resistance persistent mode switch is fitted across the complete coil assembly. The four outer coils were tight wound using standard filamentary NbTi conductors with glass fibre insulation (Cryogenic Consultants Ltd's UK patent no ). The inner Nb3Sn coils were constructed using the "wind before react" technique /2/. The Nb3Sn windings are fully insulated so that rapid ramping of the complete system would be possible. All winding mandrels were removed after vacuum impregnation with epoxy resin. Article published online by EDP Sciences and available at

3 CI-804 JOURNAL DE PHYSIQUE The six coils are assembled in a solid aluminium alloy former with three radial windows each with 90 degrees angular access and a slant split which increases from 21mm at the bore (Figure 1). There are 3x30 degree support pillars I. 12. Fig Table 1 : WINDING DATA Coil No. g A2 DL DR - NJ -- Conductor (Id ( 1 ( 1 (G-3 Cu:S/C ( t z ) NbsSn a NbTi :l) b NbTi :l ) 3a NbTi :l) b NbTi :l) A1 is inner radius; A2 is outer radius; DL is inner edge relative to mid-plane; DR is outer edge; NJ is number of turns. Nominally identical windings are on other side of split. Forces are calculated for 10 Tesla field. These pillars contain all interconnections and also support the large attractive forces (Table 1) between the magnet halves. Between the pillars the former provides negligible support so the coils are designed to be self-supporting and to accept the large bending stresses without degradation. Standard bending and torsion analysis, using a circular beam approximation, shows that the shear, torsional and bearing stresses contribute negligibly to the maximum principal stress and, although the bending stress is large, it is dominated by the tensile hoop stress. Whilst the

4 stress values are well below stress-induced degradation levels, they do necessitate that all coils behave as monolithic, uniform entities, placing stringent requirements on the vacuum impregnation and the bonding to individual wires. INITIAL TESTS When first assembled, the system performed poorly. Training was slow and, even after Q15 quenches, the system was operating at only %8 Tesla, well below the design short sample field of 10.5 Tesla. After thermal cycling to room temperature, the performance usually degraded significantly and a number of training quenches were required to restore 8T operation. DATA ACQUISITION SYSTEM To study quenching behaviour, we have constructed a data acquisition system using a small 280-based business microcomputer and a precision 12 bit AID module with onboard counterltimer plugged into the SlOO bus. This provides an extremely flexible system for the capture and processing of analogue waveforms with 8 differential input channels. The software sequentially scans the coil section voltages at a rate of approximately 15kHz and continuously writes the voltage data into the available blocks of the main 64k memory. Once memory is full, data collection is continued by overwriting the oldest data held. This process operates in a continuous loop. After detection of a voltage exceeding a preset threshold trigger, data collection continues for a preset time and is then automatically stored on disc for subsequent analysis. The data comprises a record of all coil voltages for 20.5 second; typically half before the trigger event and half afterwards. The data acquisition system generates information on the quench process by defining, for example, which coil initiated the quench and the time constant for current decay. In addition, during energisation, several rapid transients were observed. The number of these events increased as the quench current approached and, in a typical energisation to quench, as many as twenty events were observed. Typical peak coil voltages were 2V and the total duration of an event was ".]Oms. These events are associated with the movement of the coils under the large forces and indicate the necessity for an efficient pre-loading clamping system to minimise coil disturbance during either thermal ( K) or magnetic cycling (0-10 Tesla). By changing the clamping system, we significantly influenced the occurence of these transient events and varied the training behaviour. A variety of clamping systems were studied and the final clamping arrangement produces excellent magnet performance. After the first assembly of this revised system, the system was cooled and was energised to 10 Tesla with no training quenches and no transient events. The coil sections were unchanged, so the success is solely due to the revised clamping system design. CRYOSTAT CONSTRUCTION To achieve the possibility for field rotation, the system has a small 5 litre liquid helium reservoir which is mounted at 45' to the field axis. The magnet, which is housed in a vacuum-tight container x 259mm high) formed by epoxy bonding close-fitting aluminium covers to the main magnet former, is surrounded by a slotted radiation shield which is continuously cooled by a flow of liquid nitrogen. By eliminating a bulky nitrogen reservoir, the system dimensions are minimised. Liquid nitrogen is drawn, by a small diaphragm pump, from a storage bessel into the shield cooling loop. Nitrogen usage (<I litre/hour) can be optimised by controlling the pump from a temperature monitor (DTG 210) which utilises a RhFe resistance thermometer mounted on the shield. The magnet and shield assembly are suspended in an aluminium alloy outer vacuum case by a series of axial and radial tie rods manufactured from titanium and stainless steel. These tie rods accurately position the magnet and the change in position of the field centre on rotation from horizontal to vertical field geometry is less than 0.25m. The outer casing has three, demountable, 90 degree windows with an overall window diameter of 360mm. Close access into the high field zone parallel to the field is also possible since the overall height of the main container is only 400mm.

5 Cl-806 JOURNAL DE PHYSIQUE FINAL PERFORMANCE DATA The magnet has been energised several times to 10 Tesla in both horizontal and vertical field directions without training. If the system is induced to quench at 10 Tesla - this has been carried out both by letting the helium run out and by letting helium gas into the main cryostat vacuum - one or two training quenches are sometimes required to return the magnet to its full 10 Tesla operation. The magnet is capable of fast ramping and has been energised to 10 Tesla in 25 minutes and has been de-energised from 10 Tesla in 15 minutes without quench. It has also been operated for an extended period in persistent mode at 10.2 Tesla using the automatic refill system. Thercryomagnetic system itself has a helium consumption of less than 500cc/hour. When using the automatic refill system, the total system helium loss (cryostat, storage vessel, transfer, and some additional losses for magnet energisation) is approximately 1 litrelhour. Figure 2 : System in horizontal field orientation This split pair system provides a combination of high field, small size, large radial access and flexibility of use, which is currently unique. Its successful construction and testing opens the way for the development of many other high field laboratory systems, for example, for neutron diffraction studies. ACKNOWLEDGEMENTS We wish to express our sincere thanks to the staff of Hahn-Meitner-Institute, in particular to Dr. H.H. Bertschat and Dr. W. Semmler, for their stimulating interest throughout a long development programme. We wish to also thanlc our many colleagues and co-workers at Cryogenic Consultants Ltd. who have each made significant contributions to the success of this project, particularly John Mellors, Jeremy Good, Keith White and Chris Fearnley. REFERENCES 1. Murray F.S., Tanski J.V., Damian D.A., and Markiewicz W.D., IEEE Trans on Mag, Mag-17, (1981) Weisse H.J., Wilhelm M., Wohlleben K., and Springer E., IEEE Trans on Mag, Mag-17, (1981) 1628