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Submitted on 1 Jan 1984
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SUPERCONDUCTING HEXAPOLE MAGNET FOR ECR ION BEAM SOURCE
M. Dickens, J. Good
To cite this version:
M. Dickens, J. Good. SUPERCONDUCTING HEXAPOLE MAGNET FOR ECR ION BEAM SOURCE. Journal de Physique Colloques, 1984, 45 (C1), pp.C1-799-C1-801.
�10.1051/jphyscol:19841163�. �jpa-00223637�
JOURNAL DE PHYSIQUE
Colloque C l , suppldment au no 1, Tome 45, janvier 1984 page C1-799
SUPERCONDUCTING HEXAPOLE MAGNET FOR ECR I O N BEAM SOURCE
M.H. Dickens and J.A. Good
Cryogenic ConsuZtants Limited, Undon, u.K.
RQsumQ - On ddcrit un aimant supraconducteur hexapole pour le confine- ment du plasma de la radiation cyclotron Qlectronique avec un champ de
1 Tesla.
Abstract - Description of a superconducting hexapole cryomagnet for ECR plasma confinement with I Tesla plasma field.
In this paper we describe a superconducting hexapole 'magnetic bottle' developed by Cryogenic Consultants Ltd., for plasma confinement in a large scale ECR ion beam source. The complete source named ECREVIS is now operational at the CYCLONE iso- chronous cyclotron at Louvain-la-Neuve. The initial results show that remarkably high intensities of high charge state ions are achieved / I / .
The early small ECR sources such as the SUPERMAFIOS / 2 / used a combination of perm- anent hexapole and water cooled copper pinch-coils. The latter required several megawatts of power, so that plasma volumes were limited by economical considerations.
In the fully superconducting system which is the subject of this paper a very large plasma confinement volume of about 100 litres is provided by the 1.5 metre long x 0.4 metre diameter superconducting hexapole winding consisting of 6-racetrack coils clamped together with two superconducting pinch coils, one at each end of the hexa- pole, and has allowed the effects of scaling on ion beam intensity and charge state distribution to be investigated for the first time.
In common with all non-solenoidal superconducting magnet systems, the interactive forces between the various parts of the coil system present particular design problems. In our case these forces are typically a few Tonnes, and require a care- fully designed support structure in order to avoid quenching due to coil movements.
To enhance stability, a high copper to superconductor ratio of 5:1 was chosen, and with full field operation, a large margin in field and current was allowed compared with the short sample performance of the wire. This is illustrated in Figure 1.
The method of manufacture of all coils followed our standard techniques / 3 / of dry winding, followed by vacuum impregnation with epoxy resin. The coils were wound on aluminium formers and after casting were removed from the formers as complete coils.
The racetrack coils were cryogenically tested before assembly.
A total of 12 coils formed the assembly - the six hexapolar racetracks, the two pinch field solenoids, and four smaller central solenoids. These coils were clamped in an assembly of stainless steel hoops which provided the basic alignment and structural support for the hexapole.
A 5 Tonne interactive force occurs between each curved end of the racetrack coils and the adjacent section of the solenoid circumscribing the ends of the hexapole coils.
These forces are alternately repulsive and attractive. The repulsive forces were restrained by an arched C-clamp structure in which the elasticity of the clamps ensured precompression of greater than 5 Tonnes'at 4.2K. Because of the requirement for continuous operation of the system for many months, a low loss cryostat was constructed with radiation shields cooled by a mechanical cryocooler. The radiation shield temperature of 60K and 15K resulted in the extremely low boil off given in the table. This boil off was dominated by the heat load from the current leads.
As the full field is not always required provision was made for 250 and 450 Amp current operation by having one set of leads demountable.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19841163
CI-800 JOURNAL DE PHYSIQUE
On the basis of measured helium consumption with the 450 Amp current leads installed, we estimate that consumption of less than 100cc/hour would be achieved with the
current leads removed corresponding to a hold time of greater than 6 months. This demonstrates the effectiveness with which the cryocooler is used. The system is one of the first examples of a commercially built magnet system to successfully incorp- orate this helium-saver technique.
The photograph shows the aluminium cryostat, with cryocooler mounted at 45', and a purpose-built 450 Amp power supply.
The experience of operating the system has been very good, with all performance specifications satisfied or exceeded. The magnet is extremely stable, and operates reliably at full field. At low fields the system is cryostatically stable and supports half-field supercurrent at 4.2'-5'~, when completely uncovered with liquid helium.
We wish to thank Yves Jongen, Guido Ryckewaert and their co-workers at UCL, Louvain- la-Neuve, who inspired the project and collaborated with us throughout the construct- ion phase. We also wish to thank A. Murray, C. Fearnley, and K. White, our colleag- ues at Cryogenic Consultants Ltd., who also contributed to the success of the project.
HEXAPOLE MAGNET SYSTEM - Technical Specification
-
Superconducting Wire Type
Diameter before insulation Copper-to-superconductor ratio Number of filaments
Diameter of filaments
Critical current guaranteed at 3 Tesla, 4.2K
Insulation
Diameter after insulation Hexapole Coil Data
Hexapole.diameter Hdxapole length
Number of turns per coil Current at full field Coil shape
Field at plasma wall (r=17.5cm) Max-field on conductor
Intrinsically stabilised filamentary NbTi alloy in copper matrix
1.88mm 5 : 1 672 30 microns
1000 Amps
Heavy formvar enamel 1.97mm * 20 micron
0.46m 3.2m 552 450 A
Racetrack on cylinder
Tilted heads to keep constant turn length 1 Tesla
%
3 Tesla
Solenoid Coil Data Diameter
Number of turns Switch
2
9: 700z
Loou
ZOO
0.6m
Main solenoids - 727 and 800 4 Small solenoids - 104 each
persistent mode switch decay less than 10-~/hr
1 2 3 4 5 6 7 8
~ I E L B
