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Design and construction of a superconducting magnet system for an Electron Cyclotron Resonance (ECR) ion source
ICEC 15 Proceedings
Design and Construction of a Superconducting Magnet System For an Electron Cyclotron Resonance (ECR) Ion Source Peter Seyfert*, Paul Briand*, Giovanni Ciavola***, Santo Gammino***, Grrard Melin*, Robert Lagnier*, Marcello Losasso**, Antonella Menicatti**, Roberto Penco** * Drpartement de Recherche Fondamentale sur la Matirre Condensre, Centre d'Etudes Nuclraires de Grenoble, 38054 Grenoble Cedex 9, France ** Magnets and Special Products Division, ANSALDO GIE, Via N. Lorenzi 8, 16152 Genova, Italy *** Laboratorio Nazionale del Sud, Istituto Nazionale di Fisica Nucleare, PP.TT.25, 95100 Catania, Italy The cyclotron accelerator at Laboratorio Nazionale del Sud in Catania, Italy will be equipped with a high-performance ECR ion source. This source which is presently under construction comprises a so-called Minimum-B magnetic bottle of 130 mm diameter and 480 mm length. The specified magnetic field values are 2.7 T, 1.6 T and 0.37 T respectively for the axial component at the two ends and at the center. The transverse field component is required to reach 1.4 T at the cylindrical boundary. The complex field configuration is produced by means of a compound superconducting coil system whose essential parts are an approximately 800 mm long sextupole and three coaxial solenoids surrounding the sextupole. All windings will be made from NbTi composite conductors and impregnated with epoxy resins. Bath cooling at 4.5 K for all coils will be provided by a warm-bore type cryostat with horizontal axis. The paper presents design and construction of the coil system, its support structure and the cryostat. INTRODUCTION Electron cyclotron resonance (ECR) ion sources are, at present, the leading source of highly charged continuous beams of positive ions. They are being used for research in all major atomic and high-energy physics laboratories in the world. The principle they work on is the ionization of gas molecules by hot electrons in a magnetically confined volume. The magnetic field configuration used/'or confinement is the so-called minimum-B bottle. This is a sort of three-dimensional magnetic well in which the absolute magnitude of the field IBI increases in every direction outwards from the centre. Minimum-B bottles for ECR ion sources are currently achieved by superimposing axial magnetic mirrors produced by at least two solenoids and a transverse well produced by a sextupole. Up to now, most ECR ion sources have used a combination of water-cooled copper coils for the axial mirrors and a rare-earth permanent magnet sextupole. The peak field of that kind of systems is limited to values of about 1 T. The performance of ECR ion sources, both in terms of extracted ion current and in terms of the atomic number of obtainable highly charged ions, is expected to rise further as the confinement field rises above 1 T. This explains the growing interest for superconducting magnet systems. An early high-field ECR ion source using superconducting magnets has been built by T. Antaya et al [1]. A high-performance superconducting-magnet ECR ion source, called SERSE, is presently under construction and is to be connected to the cyclotron accelerator at Laboratorio Nazionale del Sud, Catania, Italy in 1996. A comprehensive account of the project has been given elsewhere [2]. SERSE MAGNET DESIGN An outline of the SERSE magnet system is shown in Figure 1, with a summary of the main design parameters in Table 1. The useful volume of the minimum-B bottle has a cylindrical shape and is entirely contained in a metallic vessel, the plasma chamber, which has access to room temperature at both ends. Sextupole The sextupole transverse field is zero on the axis and is designed to reach 1.4 T at the side walls of the plasma chamber. Since the sextupole field scales with the square of the distance from the axis, the
Cryogenics 1994 Vo134 ICEC Supplement
663
ICEC 15 Proceedings
sextupole coils are positioned as close to the plasma chamber as possible to minimize the field on the winding. Table 1 SERSE magnet system design parameters Plasma chamber bore diameter Axial length of sextupole field Transverse field at plasma chamber wall Axial distance between the two mirrors Mirror peak fields Field minimum between the two mirrors
130 mm 550 mm 1.4 T 480 mm 2.7 T and 1.6 T 0.4 T
The sextupole is built up from six fiat race-track coils. Flat race-track coils are relatively easy to wind and are therefore preferred to the curved saddle coils typically used in accelerator dipoles. The stronger deviations from the ideal sextupole field produced by race-track coils is not expected to affect the operation of the ion source. Table 2 shows a summary of the main coil and wire parameters. Monolithic NbTi conductors will be used throughout the whole magnet system and all coils will be impregnated with epoxy resin. Particular attention has been paid to the mechanical behaviour of the sextupole coils where the straight, winding sections are unable to support the magnetic forces by themselves. These coils are therefore entirely encased in a supporting cylinder and the considerable tangential bursting forces (420 kN on one straight section) and radial compression forces (44 kN on one straight section) exerted by the coils will be reacted against this structure. As shown by a three-dimensional stress analysis done for the windings and their support structure, the largest stress values in the windings occur at the end of the straight sections. Peak values of 55 MPa for compression and 32 MPa for shear have been found there. They are close to (especially for shear), but do not exceed, the limiting values of strength in epoxy composite. The supporting cylinder is made of a high strength aluminium alloy. As a result of the difference in thermal contraction between this alloy and the winding a compressional load will appear on cool-down and firmly hold the winding in place. Table 2 Coil and superconducting wire parameters
stored energy, kJ current density, A mm-2 maximum operating current, A peak field on winding, T bore diameter, mm Operating temperature, K temperature safety margin, K type of winding conductor type bare size, mm Cu:Sc ratio critical current at 4.2 K, 5 T, A
sextupote
solenoid 1
solenoid 2
solenoid 3
100 208 120 6.4 194
202 113 5.0
150 (total) -87 -49 3.6 340
154 88 4.8
t.13
4.5 1.2 2.4 2.1 vacuum impregnation by epoxy resin
Q 0.7
NbTi monolithic 0.86 x 0.47
1.35
1.8
54O
295
Axial mirrors Three short solenoids surround the sextupole and produce a double axial mirror (see Table 1) centered on the somewhat longer sextupole field~ The field of the central solenoid is in opposite direction to the field of the two main mirror coils. The result is a deeper magnetic well and a steeper axial field gradient, a feature that was found to improve the performance of ion sources. The solenoids will be wound on three separate aluminium bobbins and blocked at appropriate positions on the outside of the supporting cylinder. All three solenoid coils will be fitted with circumferential clamps made of aluminium. This helps to reduce the internal stresses in the windings which would otherwise be dangerously high. Details of the coils and the conductors used are shown in Table 2. 664 Cryogenics 1994Vol 34 ICEC Supplement
ICEC 15 Proceedings
CRYOSTAT A special warm-bore type of cryostat with horizontal axis has been designed for the superconducting magnet system. Its central aperture tube provides room enough to house the plasma chamber of the ECR ion source. The external walls of the cryostat are made of sof~ ferromagnetic material so that they serve as a magnetic shield. The compound coil system, enclosed in a helium vessel, is mounted inside the vacuum insulated cryostat by means of a low heat-loss suspension system. In radial direction, the load is mainly due to the coil weight (~ 450 kg), while in axial direction, the coils exert a net magnetic force of 23 kN on the ferromagnetic end flanges of the cryostat. The radial position of the coil system can be adjusted in order to align the magnetic axis with the geometrical axis of the central aperture tube. Cooling of the coils is obtained by immersion in a bath of boiling liquid helium. All coils have been designed with a temperature safety margin of at least 1 K with respect to a nominal bath temperature of 4.5 K. A total of six vapour-cooled current leads connect the coil system to an external power supply unit and permit to set the operating currents of the sextupole and all three solenoids independently. The helium vessel is surrounded by a 80 K thermal radiation shield. Liquid helium consumption of the cryostat is expected to be 3.7 1 h-' with all coils at full operating current. Continuous operation on a 24 hours per day basis has been anticipated when designing the cryogenic system for automatic supply of liquid helium and nitrogen to the cryostat. STATUS OF THE PROJECT A close collaboration has been set up between the future user and several groups having special experience in the technologies involved in the project: generation of hot plasma by ECR combined with extraction of charged ions, superconducting magnets, custom-built helium cryostats. Since superconducting magnet systems are novel components in ECR ion sources a thorough design study has been devoted to that aspect. The result, which is briefly described in the present paper, is believed to be the best possible compromise today between the often conflicting requirements of performance and reliability. The project entered the construction phase in October, 1993. The complete ion source system will first be assembled and tested at CEN Grenoble before being transferred to LNS Catania in the summer of 1996. REFERENCES 1
Antaya, T., Zeller, A.F., Moskalik, J.M., Blosser, H.G., Nolen, J.A., Harrison, K.A. Magnetic Structure For a Superconducting Variable Frequency Electron Cyclotron Resonance Ion Source _IEEE Trans. Magn. (1989) MAG-25 1671-1675
2
Ciavola, G., Gammino, S., Briand, P., Rev. Sci. Instr.(1994) 65 1057-1059 solenoid 1
solenoid2
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383
The
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: 120 .. . . . .143 . .i ~i-80..-:
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Seyfert, P.,
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Figure 1 Upper half of an axial section through the SERSE magnet system. For the sake of clarity only one race-track coil is shown. All dimensions are in millimetres.
Cryogenics 1994 Vol 34 ICEC S u p p l e m e n t
665
Design and Construction of a Superconducting Magnet System For an Electron Cyclotron Resonance (ECR) Ion Source Peter Seyfert*, Paul Briand*, Giovanni Ciavola***, Santo Gammino***, Grrard Melin*, Robert Lagnier*, Marcello Losasso**, Antonella Menicatti**, Roberto Penco** * Drpartement de Recherche Fondamentale sur la Matirre Condensre, Centre d'Etudes Nuclraires de Grenoble, 38054 Grenoble Cedex 9, France ** Magnets and Special Products Division, ANSALDO GIE, Via N. Lorenzi 8, 16152 Genova, Italy *** Laboratorio Nazionale del Sud, Istituto Nazionale di Fisica Nucleare, PP.TT.25, 95100 Catania, Italy The cyclotron accelerator at Laboratorio Nazionale del Sud in Catania, Italy will be equipped with a high-performance ECR ion source. This source which is presently under construction comprises a so-called Minimum-B magnetic bottle of 130 mm diameter and 480 mm length. The specified magnetic field values are 2.7 T, 1.6 T and 0.37 T respectively for the axial component at the two ends and at the center. The transverse field component is required to reach 1.4 T at the cylindrical boundary. The complex field configuration is produced by means of a compound superconducting coil system whose essential parts are an approximately 800 mm long sextupole and three coaxial solenoids surrounding the sextupole. All windings will be made from NbTi composite conductors and impregnated with epoxy resins. Bath cooling at 4.5 K for all coils will be provided by a warm-bore type cryostat with horizontal axis. The paper presents design and construction of the coil system, its support structure and the cryostat. INTRODUCTION Electron cyclotron resonance (ECR) ion sources are, at present, the leading source of highly charged continuous beams of positive ions. They are being used for research in all major atomic and high-energy physics laboratories in the world. The principle they work on is the ionization of gas molecules by hot electrons in a magnetically confined volume. The magnetic field configuration used/'or confinement is the so-called minimum-B bottle. This is a sort of three-dimensional magnetic well in which the absolute magnitude of the field IBI increases in every direction outwards from the centre. Minimum-B bottles for ECR ion sources are currently achieved by superimposing axial magnetic mirrors produced by at least two solenoids and a transverse well produced by a sextupole. Up to now, most ECR ion sources have used a combination of water-cooled copper coils for the axial mirrors and a rare-earth permanent magnet sextupole. The peak field of that kind of systems is limited to values of about 1 T. The performance of ECR ion sources, both in terms of extracted ion current and in terms of the atomic number of obtainable highly charged ions, is expected to rise further as the confinement field rises above 1 T. This explains the growing interest for superconducting magnet systems. An early high-field ECR ion source using superconducting magnets has been built by T. Antaya et al [1]. A high-performance superconducting-magnet ECR ion source, called SERSE, is presently under construction and is to be connected to the cyclotron accelerator at Laboratorio Nazionale del Sud, Catania, Italy in 1996. A comprehensive account of the project has been given elsewhere [2]. SERSE MAGNET DESIGN An outline of the SERSE magnet system is shown in Figure 1, with a summary of the main design parameters in Table 1. The useful volume of the minimum-B bottle has a cylindrical shape and is entirely contained in a metallic vessel, the plasma chamber, which has access to room temperature at both ends. Sextupole The sextupole transverse field is zero on the axis and is designed to reach 1.4 T at the side walls of the plasma chamber. Since the sextupole field scales with the square of the distance from the axis, the
Cryogenics 1994 Vo134 ICEC Supplement
663
ICEC 15 Proceedings
sextupole coils are positioned as close to the plasma chamber as possible to minimize the field on the winding. Table 1 SERSE magnet system design parameters Plasma chamber bore diameter Axial length of sextupole field Transverse field at plasma chamber wall Axial distance between the two mirrors Mirror peak fields Field minimum between the two mirrors
130 mm 550 mm 1.4 T 480 mm 2.7 T and 1.6 T 0.4 T
The sextupole is built up from six fiat race-track coils. Flat race-track coils are relatively easy to wind and are therefore preferred to the curved saddle coils typically used in accelerator dipoles. The stronger deviations from the ideal sextupole field produced by race-track coils is not expected to affect the operation of the ion source. Table 2 shows a summary of the main coil and wire parameters. Monolithic NbTi conductors will be used throughout the whole magnet system and all coils will be impregnated with epoxy resin. Particular attention has been paid to the mechanical behaviour of the sextupole coils where the straight, winding sections are unable to support the magnetic forces by themselves. These coils are therefore entirely encased in a supporting cylinder and the considerable tangential bursting forces (420 kN on one straight section) and radial compression forces (44 kN on one straight section) exerted by the coils will be reacted against this structure. As shown by a three-dimensional stress analysis done for the windings and their support structure, the largest stress values in the windings occur at the end of the straight sections. Peak values of 55 MPa for compression and 32 MPa for shear have been found there. They are close to (especially for shear), but do not exceed, the limiting values of strength in epoxy composite. The supporting cylinder is made of a high strength aluminium alloy. As a result of the difference in thermal contraction between this alloy and the winding a compressional load will appear on cool-down and firmly hold the winding in place. Table 2 Coil and superconducting wire parameters
stored energy, kJ current density, A mm-2 maximum operating current, A peak field on winding, T bore diameter, mm Operating temperature, K temperature safety margin, K type of winding conductor type bare size, mm Cu:Sc ratio critical current at 4.2 K, 5 T, A
sextupote
solenoid 1
solenoid 2
solenoid 3
100 208 120 6.4 194
202 113 5.0
150 (total) -87 -49 3.6 340
154 88 4.8
t.13
4.5 1.2 2.4 2.1 vacuum impregnation by epoxy resin
Q 0.7
NbTi monolithic 0.86 x 0.47
1.35
1.8
54O
295
Axial mirrors Three short solenoids surround the sextupole and produce a double axial mirror (see Table 1) centered on the somewhat longer sextupole field~ The field of the central solenoid is in opposite direction to the field of the two main mirror coils. The result is a deeper magnetic well and a steeper axial field gradient, a feature that was found to improve the performance of ion sources. The solenoids will be wound on three separate aluminium bobbins and blocked at appropriate positions on the outside of the supporting cylinder. All three solenoid coils will be fitted with circumferential clamps made of aluminium. This helps to reduce the internal stresses in the windings which would otherwise be dangerously high. Details of the coils and the conductors used are shown in Table 2. 664 Cryogenics 1994Vol 34 ICEC Supplement
ICEC 15 Proceedings
CRYOSTAT A special warm-bore type of cryostat with horizontal axis has been designed for the superconducting magnet system. Its central aperture tube provides room enough to house the plasma chamber of the ECR ion source. The external walls of the cryostat are made of sof~ ferromagnetic material so that they serve as a magnetic shield. The compound coil system, enclosed in a helium vessel, is mounted inside the vacuum insulated cryostat by means of a low heat-loss suspension system. In radial direction, the load is mainly due to the coil weight (~ 450 kg), while in axial direction, the coils exert a net magnetic force of 23 kN on the ferromagnetic end flanges of the cryostat. The radial position of the coil system can be adjusted in order to align the magnetic axis with the geometrical axis of the central aperture tube. Cooling of the coils is obtained by immersion in a bath of boiling liquid helium. All coils have been designed with a temperature safety margin of at least 1 K with respect to a nominal bath temperature of 4.5 K. A total of six vapour-cooled current leads connect the coil system to an external power supply unit and permit to set the operating currents of the sextupole and all three solenoids independently. The helium vessel is surrounded by a 80 K thermal radiation shield. Liquid helium consumption of the cryostat is expected to be 3.7 1 h-' with all coils at full operating current. Continuous operation on a 24 hours per day basis has been anticipated when designing the cryogenic system for automatic supply of liquid helium and nitrogen to the cryostat. STATUS OF THE PROJECT A close collaboration has been set up between the future user and several groups having special experience in the technologies involved in the project: generation of hot plasma by ECR combined with extraction of charged ions, superconducting magnets, custom-built helium cryostats. Since superconducting magnet systems are novel components in ECR ion sources a thorough design study has been devoted to that aspect. The result, which is briefly described in the present paper, is believed to be the best possible compromise today between the often conflicting requirements of performance and reliability. The project entered the construction phase in October, 1993. The complete ion source system will first be assembled and tested at CEN Grenoble before being transferred to LNS Catania in the summer of 1996. REFERENCES 1
Antaya, T., Zeller, A.F., Moskalik, J.M., Blosser, H.G., Nolen, J.A., Harrison, K.A. Magnetic Structure For a Superconducting Variable Frequency Electron Cyclotron Resonance Ion Source _IEEE Trans. Magn. (1989) MAG-25 1671-1675
2
Ciavola, G., Gammino, S., Briand, P., Rev. Sci. Instr.(1994) 65 1057-1059 solenoid 1
solenoid2
\
I
.
.
.
.
.
.
.
.
I
I
.
i
I,
.
.
.
.
.
.
.
.
.
383
The
SERSE
Project
.
.
.
.
.
.
.
.
.
.
.
.
.
:
: 120 .. . . . .143 . .i ~i-80..-:
L6_
Seyfert, P.,
I
I "
.
,,
and
solenoid3
I
N' J '°'T ° _i'il :
/
Melin, P.
:
i:
I
224
i 97
383
Figure 1 Upper half of an axial section through the SERSE magnet system. For the sake of clarity only one race-track coil is shown. All dimensions are in millimetres.
Cryogenics 1994 Vol 34 ICEC S u p p l e m e n t
665