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Superconducting high-field three-pole wigglers at Budker INP
Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Superconducting high-"eld three-pole wigglers at Budker INP M. Fedurin, G. Kulipanov, N. Mezentsev, V. Shkaruba* Budker Institute of Nuclear Physics, SB RAS, Lavrentiev ave.11, 630090 Novosibirsk, Russia
Abstract The problem of creation of high-"eld superconducting wigglers has been attracting rising attention of many synchrotron radiation centers. Several high-"eld three-pole wigglers have been produced at Budker INP in the last few years. Some of the new wigglers are now under fabrication. The main parameters and properties of three-pole high-"eld superconducting wigglers created at BINP are described in the article. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 41.85.Lc; 85.25.Ly; 07.55.Db Keywords: Superconducting wiggler; Magnetic "eld; Cryostat; Synchrotron radiation
1. Introduction The e!orts of many SR centers is now aimed at using high-"eld superconducting wigglers. The rising interest in superconducting wigglers (`wave length shiftersa) may be explained by the possibility of shifting radiation spectrum of SR sources already built to the range of shorter wavelengths. It expands signi"cantly the possibilities of the existing sources and makes it possible to realize low-cost experiments with hard photons at installations with relatively low energy. Besides this, installation of superconducting wigglers into storage rings allows improvement of `#exibilitya of experiments with SR by fast change of magnetic "eld value and, consequently, of spectrum of irradiated photons. In the last few years Budker INP has been actively developing the manufacture of high-"eld three* Corresponding author: Tel.: #7-3832-394976; fax: #73832-342163. E-mail address: [email protected] (V. Shkaruba).
pole superconducting wigglers. The "rst wiggler of this type was made in 1995 for 2 GeV PLS storage ring (Korea) [1]. In 1998 activity was aimed at a simultaneous creation of three superconducting wigglers for synchrotron radiation centers LSUCAMD (USA) [2,3], Spring-8 (Japan) [4], BESSY-II (Germany). Table 1 presents the main features of these wigglers.
2. Main principles of the wiggler design One of the main peculiarities of all the high-"eld three-pole wigglers which were made at BINP is that only the central pole of the magnet has a highlevel "eld. As for the side poles, they are destined only for restoration of the closed orbit, which was disturbed by the central pole. In this case choice of the side pole length as well as the length of the magnetic system as a whole is determined by the necessary value of the "rst magnetic "eld integral along the wiggler longitudinal axis. In Fig. 1 one
0168-9002/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 5 0 - 0
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Table 1 The main parameters of high-"eld three-pole superconducting wigglers Storage ring
CAMD-LSU
BESSY-II
SPRING-8
Location Status Maximum "eld at central pole (required/reached), (T) Field at side pole (T) Magnetic length (mm) Pole gap, mm Stored energy (kJ) Aperture of vacuum chamber: Vertical (mm) Horizontal, mm Weight of cooled parts (kG) Orbit deviation, mm Orbit angle (mm) Consumption of liquid helium (1/h) Total radiation power (kW) Electron energy (GeV) Beam current (mA)
Louisiana (USA) Operation, 1998 7.0 (7.55)
Berlin (Germany) Fabrication, 1998 7.0 (7.54)
Himeji (Japan) Fabrication, 1998 10 (10.14)
1.5 972 51 140
1.5 972 52 140
1.9 1042 40 500
32 90 1000 0 (20) $85 1.5
32 90 1000 0 (20) $65 0
20 90 1000 7 $25 0
2.7 1.5 200
13 2.0 500
100 8.0 100
can see the distribution of magnetic "eld along the longitudinal axis as an example of the wiggler for BESSY-II. It should be noted that the side pole "eld level selected is as small as possible } approximately 20% of that of the central pole. It allows one to separate radiation of the central and side poles by spectrum, thus signi"cantly reducing
Fig. 1. Distribution of the wiggler magnetic "eld and electron beam orbit along wiggler BESSY-II straight section.
contribution of the so-called `second sourcea. Fig. 2 presents angular distribution of #ux of di!erent energy photons, the 7 T wiggler being installed at BESSY-II. Fig. 3 presents spectral #uxes of photons irradiated from the wiggler central poles, wigglers being "tted to the storage rings of CAMD}LSU, BESSY-II, and SPring-8. One of the inconvenient factors arising when standard three-pole wigglers are used is the horizontal deviation of the equilibrium electron orbit at wiggler central pole and thus shift of the radiation point. To eliminate this e!ect and to "x the radiation point one can use a "ve-pole wiggler. However, it would lead to enlargement of the magnet `colda part, increase helium consumption and complicate design of the magnet as well as of the whole cryostat. That is why when the 7 T wigglers were "tted to the sources of CAMD-LSU and BESSY-II, a more simple scheme was used. One can see from Fig. 1 that due to two `warma (normal conductivity) correctors with a "eld of 0.5 T, placed at both the ends of the straight section, the electron beam orbit is always geometrically "xed to the center of the wiggler. In doing so, it is possible to
M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
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Fig. 2. Angular distribution of the photon #ux of 7 T wiggler for BESSY-II.
change the wiggler "eld without shifting the radiation point. Here, of course, the "rst "eld integral along the wiggler axis is not equal to zero because in this case the integral is to be made zero over the whole straight section, including the correction magnets.
3. Magnet system The wiggler magnet system (as one can see from Fig. 4) is two halves of an iron yoke with three superconducting dipoles which are located above and below of the vacuum chamber. The yoke is not only a mechanical base of the wiggler structure but also provides an increase of magnetic induction at the median plane and magnetic #ux closure. One of the distinctive features of the wigglers being described is the unconventional scheme of bandaging of superconducting windings inside the
iron yoke. This scheme is realized with the help of two pairs of wedges of a material with low heat extension factor (e.g. invar). The magnet is cooled down to the liquid helium temperature, due to di!erent thermal contraction of the used material, the superconducting winding being `crimpeda from the iron yoke side through the invar wedges. Such a scheme makes it possible to precisely normalize the required pressure to the winding. The key element of the wiggler is the high-"eld superconducting central pole. To obtain maximum "eld it is necessary to place the superconducting windings in space and to select currents in them so that the "eld level on the winding does not exceed the critical value at the required maximum "eld value in the orbit. The "eld critical value is determined by the superconducting wires used. In particular, for the production of central windings for wigglers with "eld not exceeding 8 T, a Nb}Ti superconducting wire may be used. All sections of
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Fig. 3. Spectral #uxes of photon irradiated from the wigglers on the storage rings of CAMD-LSU, BESSY-II and SPring-8.
the wiggler central poles for LSU-CAMD and BESSY-II were reeled up from Nb}Ti wire with diameter of 0.87 mm, which has a critical current of 360 A in a 7 T "eld. To obtain higher magnetic "elds, one requires to use a Nb SN superconduct ing wire with a higher critical current. A rectangu-
Fig. 4. Magnetic system of 7 T wiggler for LSU-CAMD before assembling.
lar Nb Sn wire of 1.45;0.85 mm in size was used for the manufacture of inner section of the central pole for the 10 T wiggler for Spring-8. The load curves of the used superconducting wires are presented in Fig. 5. In the same "gure one can see the `operation pointsa that describe values of magnetic "elds and currents in the points of the winding that are critical from the quench standpoint. Due to the fact that in the wiggler the electron beam undergoes substantial orbit deviations in the horizontal plane, good uniformity of magnetic "eld in the transversal direction is one of the most important requirements. The central pole magnetic "eld is formed as a superposition of the "elds created by the iron core, on which the superconducting windings are reeled up, and by the windings themselves. The iron core contribution to the resulting "eld is about 1.5 T. To form a transversal uniformity of the "eld, the central pole is stretched in this direction. Especially stringent requirements are imposed on the quality of "eld of the BESSY-II wiggler since this will be used for calibration
M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Fig. 5. The load curves of the superconducting wires and `operation pointsa of the wigglers windings.
experiments in the PTB-laboratory. To additionally improve transversal uniformity, we used shimming by means of putting thin iron plate on the central poles [5]. It permitted us to bring the transversal uniformity up to 10\ at a 7 T "eld. To simulate the uniformity and to select the central pole shape we used 3D MASTAC code [6]. It should be noted that 3D computations of iron magnet systems are complicated signi"cantly by the e!ects associated with saturation of the iron yoke. Computation of the yoke of wiggler was
55
made with 3D MASTAC code also. The outer areas of the yoke do not reach the saturation level, which indicates very low level of stray magnetic "elds around the wiggler. It excludes the in#uence of the wiggler "eld upon elements around the storage ring. Interaction of stray "elds of the wiggler with the surrounding iron is extremely small, which helps the wiggler to be stable in space and signi"cantly simpli"es the scheme of supporting the wiggler inside the cryostat. For a more e$cient use of the superconducting wires, the central pole winding is divided into several sections. Table 2 presents results of the sectioning of the central windings. To feed the multisection superconducting windings we suggested an electric scheme with two independent DC power supplies. Fig. 6 presents the feeding circuit for the 10 T wiggler for Spring-8. Such a feeding circuit has the following advantages: 1. Each section of the multi-section windings is fed by optimal current; 2. A three-current combination is used, only two sources being available; 3. The magnet system is fed with the help of only two pairs of current leads, which reduces signi"cantly the incoming heat to the liquid helium cryostat. 4. We obtain a possibility of easy control of wiggler "eld "rst integral. For any "eld level, currents of both the sources can be selected so as to make the wiggler "eld "rst integral go to zero. For `zeroinga the "rst integral
Table 2 Parameters of the superconducting windings Wiggler
LSU-CAMD, BESSY-II
Spring-8
Numer of winding Material of winding Dimension of wire, mm
1 Nb}Ti d"0.87 (0.92)
2 Nb}Ti d"0.87 (0.92)
Number of turns per winding Number of turns per layer Number of layers Current in wire (A) Square of winding (mm) Density of current (A/mm) Field at winding (T)
959 68.69 14 101 64.5;11.5 130.6 7.72
2192 68.69 32 253 64.5;26 330.7 6.54
1 Nb Sn 1.45;0.85 (1.65;1.1) 1008 42 24 198 74.3;27.6 97.3 10.4
2 Nb}Ti d"0.87 (0.92)
3 Nb}Ti d"0.87 (0.92)
2448 77.76 32 96 74.3;26 121.6 8.28
1836 77.76 24 294 74.3;19.5 372.6 5.9
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
4. Cryostat
Fig. 6. Feeding circuit of 10 T wiggler for SPring-8.
we used the method of a strained current wire, which behaves as the electron beam in a magnetic "eld does [7]. In doing so, accuracy of the `zeroinga is determined only by possibilities of the power supplies and by accuracy of determination of the measurement wire coordinate in space. The "eld second integral value is determined only by precision of manufacture of the wiggler magnet system.
The wiggler magnet system is placed in the cryostat with liquid helium. Its vacuum chamber of the room temperature is connected to a vacuum chamber of a straight section of a storage ring. Geometry of output of wiggler radiation is selected so that the radiation certainly does not touch the walls of the wiggler inner chamber. It permits signi"cant simpli"cation of the chamber design since the use of water-cooled radiation absorbers is abondoned. Thus all the radiation output outside the wiggler and its super#uous part is absorbed in the vacuum chamber of the straight section. Fig. 7 presents the design of the cryostats meant for the wigglers of BESSY-II and SPring-8. To cool two thermal screens of 80 and 20 K temperature, a two-stage cooling machine is used. Two liquid helium recondensers are used for long-term operation, the cryostat not being "lled up with helium. The cryostat is equipped with numerous sensors for thermal monitoring in the course of operation. Feeding current was inputted through 2 pairs of highemperature superconductive current leads. The
Fig. 7. The view of the cryostat of wigglers for BESSY-II and SPring-8.
M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
magnet system is equipped with persistent keys, which permit the use of the wiggler in the `closed currenta mode.
5. Tests and training of the magnet system Usually quenches are provoked by local heat emissions in the winding thickness that are caused by micro-movements of conductors and by cracking of epoxy resin in the superconducting winding. Besides this, every new quench takes place at a higher "eld level, i.e. we meet the so-called training of the superconducting winding. Such training processes for wigglers for BESSY-II and SPring-8 are presented in Fig. 8. Current in the windings reaches 90% of current of a short sample of the superconducting wires used. The inner section of the central pole of the 10 T wiggler for Spring-8 was manufactured using Nb Sn superconducting wire, which can degrade at a quench. That is why, for the purpose of inner section protection, currents in the windings are distributed in such a way that two outer Nb-Ti sections are closer to critical state and they are quenched "rst. During quenching the voltages arising across the wiggler windings were recorded. The process of transition of the windings from the superconducting state to the normal one takes about 50 ms. Duration of dumping of current is equal to 0.5 s and maximum voltage between current leads is not more than 250 V. The protection system allows
Fig. 8. Quench history of maximum magnetic "eld of the wigglers for BESSY-II and SPring-8 versus quench number.
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output of about 30% of the stored energy from the cryostat. The other 70% is absorbed on the inside of the windings and by the iron yoke. Repeated quenches without damage of the windings prove their high reliability both to overheat destruction and to electrical discharge. To make the superconducting winding protection more secure, one is supposed to shunt each winding with a circuit of a diode and resistor. This additional protection system, placed outside the liquid helium, allows signi"cant reduction of helium evaporation during quenching. To obtain the wiggler magnetic "eld map both at ambient temperature and in liquid helium, several measurement methods were used, including those with the use of integrating coils [8] and Hall probes. In both the methods the sensors were calibrated with the help of NMR. Several NMR sensors are installed at the wiggler poles for the purpose of stabilization of magnetic "eld level by feedback, with a precision not worse than 10\.
6. Conclusion The design of the high-"eld three-pole wigglers described has much in common. However, these wigglers do not duplicate each other but have their own distinctive features, de"ned by speci"c requirements. For instance, in the 7 T wigglers for CAMDLSU and BESSY-II `warma correctors were used for "xing the radiation point at the wiggler center, which makes it possible to conduct experiments with SR at any "eld level. High uniformity in the transversal direction as well as long-term stability of "eld at a level of 10\ due to application of NMR distinguishes the BESSY-II wiggler. This allows one to use it as a standard for calibration experiments at the PTB-laboratory after installation on the storage ring in 2000. The next wiggler (for SPring-8) is a key element for the slow positron source under construction. The superconducting wiggler with a uniquely high magnetic "eld of 10.14 T being "tted to the storage ring with the electron beam extreme energy of 8 GeV, we obtain a possibility of generation of a slow positron beam with high intensity. To
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
create this wiggler, a technology of manufacture of high-"eld superconducting Nb Sn windings of the `race-tracka type was developed and tested successfully. Wiggler commissioning in the SPring-8 site is planned for the beginning of 2000. It should be noted that design of the cryostats for the BESSY-II and Spring-8 wigglers used a number of high-technology novelties that improve their parameters signi"cantly. Among these are application of low-temperature cooling machines instead of liquid nitrogen for cooling the thermal screens, high-temperature superconductive current leads, persistent keys, and recondensers of liquid helium.
References [1] A.V. Grudiev, V.K. Djurba, G.N. Kulipanov, V.B. Khlestov, N.A. Mezentsev, S.I. Ruvinsky, V.A. Shkaruba, S.V. Sukhanov, P.D. Vobly, Y.M. Koo, D.E. Kim, Y.U. Sohn, Nucl. Instr. and Meth. A 359 (1995) 101.
[2] V.M. Borovikov, V.K. Djurba, M.G. Fedurin, G.N. Kulipanov, O.A. Lee, N.A. Mezentsev, V.A. Shkaruba, B. Craft, V. Saile, Nucl. Instr. and Meth. A 405 (1998) 208. [3] V. Borovikov, B. Craft, M. Fedurin, V. Jurba, V. Khlestov, G. Kulipanov, O. Li, N. Mezentsev, V. Sail, V. Shkaruba, J. Synchrotron Radiat. 5 (Part 3) (1998) 440. [4] A. Ando, S. Datacute, M. Fedurin, M. Hara, H. Kamitsubo, A. Kiselev, G. Kulipanov, N. Kumagai, N. Mezentsev, Y. Miyahara, T. Nakamura, H. Ohkuma, V. Shkaruba, A. Skrinsky, K. Soutome, M. Takao, H. Tanaka, J. Synchrotron Radiat. 5 (Part 3) (1998) 360. [5] M. Fedurin, N. Mezentsev, Nucl. Instr. and Meth. A 448 (2000) 59, These Proceedings. [6] A. Grudiev, M. Rojak, E. Shurina, Yu. Soloveychik, M. Tiunov, P. Vobly, MASTAC } new code for solving three-dimentional nonlinear magnetostatic problems. Proceedings of the IEEE Particle Acceleration Conference, 1995. [7] E. Bekhtenev, E. Dementiev, M.G. Fedurin, N.A. Mezentsev, V.A. Shkaruba, P.D. Vobly, Nucl. Instr and Meth. A 405 (1998) 214. [8] V. Borovikov, M. Fedurin, A. Kerginsky, M. Kuzin, N. Mezentsev, V. Shkaruba, J. Synchrotron Radiat. 5 (Part 3) (1998) 382.
Superconducting high-"eld three-pole wigglers at Budker INP M. Fedurin, G. Kulipanov, N. Mezentsev, V. Shkaruba* Budker Institute of Nuclear Physics, SB RAS, Lavrentiev ave.11, 630090 Novosibirsk, Russia
Abstract The problem of creation of high-"eld superconducting wigglers has been attracting rising attention of many synchrotron radiation centers. Several high-"eld three-pole wigglers have been produced at Budker INP in the last few years. Some of the new wigglers are now under fabrication. The main parameters and properties of three-pole high-"eld superconducting wigglers created at BINP are described in the article. 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 41.85.Lc; 85.25.Ly; 07.55.Db Keywords: Superconducting wiggler; Magnetic "eld; Cryostat; Synchrotron radiation
1. Introduction The e!orts of many SR centers is now aimed at using high-"eld superconducting wigglers. The rising interest in superconducting wigglers (`wave length shiftersa) may be explained by the possibility of shifting radiation spectrum of SR sources already built to the range of shorter wavelengths. It expands signi"cantly the possibilities of the existing sources and makes it possible to realize low-cost experiments with hard photons at installations with relatively low energy. Besides this, installation of superconducting wigglers into storage rings allows improvement of `#exibilitya of experiments with SR by fast change of magnetic "eld value and, consequently, of spectrum of irradiated photons. In the last few years Budker INP has been actively developing the manufacture of high-"eld three* Corresponding author: Tel.: #7-3832-394976; fax: #73832-342163. E-mail address: [email protected] (V. Shkaruba).
pole superconducting wigglers. The "rst wiggler of this type was made in 1995 for 2 GeV PLS storage ring (Korea) [1]. In 1998 activity was aimed at a simultaneous creation of three superconducting wigglers for synchrotron radiation centers LSUCAMD (USA) [2,3], Spring-8 (Japan) [4], BESSY-II (Germany). Table 1 presents the main features of these wigglers.
2. Main principles of the wiggler design One of the main peculiarities of all the high-"eld three-pole wigglers which were made at BINP is that only the central pole of the magnet has a highlevel "eld. As for the side poles, they are destined only for restoration of the closed orbit, which was disturbed by the central pole. In this case choice of the side pole length as well as the length of the magnetic system as a whole is determined by the necessary value of the "rst magnetic "eld integral along the wiggler longitudinal axis. In Fig. 1 one
0168-9002/00/$ - see front matter 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 0 7 5 0 - 0
SECTION II.
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Table 1 The main parameters of high-"eld three-pole superconducting wigglers Storage ring
CAMD-LSU
BESSY-II
SPRING-8
Location Status Maximum "eld at central pole (required/reached), (T) Field at side pole (T) Magnetic length (mm) Pole gap, mm Stored energy (kJ) Aperture of vacuum chamber: Vertical (mm) Horizontal, mm Weight of cooled parts (kG) Orbit deviation, mm Orbit angle (mm) Consumption of liquid helium (1/h) Total radiation power (kW) Electron energy (GeV) Beam current (mA)
Louisiana (USA) Operation, 1998 7.0 (7.55)
Berlin (Germany) Fabrication, 1998 7.0 (7.54)
Himeji (Japan) Fabrication, 1998 10 (10.14)
1.5 972 51 140
1.5 972 52 140
1.9 1042 40 500
32 90 1000 0 (20) $85 1.5
32 90 1000 0 (20) $65 0
20 90 1000 7 $25 0
2.7 1.5 200
13 2.0 500
100 8.0 100
can see the distribution of magnetic "eld along the longitudinal axis as an example of the wiggler for BESSY-II. It should be noted that the side pole "eld level selected is as small as possible } approximately 20% of that of the central pole. It allows one to separate radiation of the central and side poles by spectrum, thus signi"cantly reducing
Fig. 1. Distribution of the wiggler magnetic "eld and electron beam orbit along wiggler BESSY-II straight section.
contribution of the so-called `second sourcea. Fig. 2 presents angular distribution of #ux of di!erent energy photons, the 7 T wiggler being installed at BESSY-II. Fig. 3 presents spectral #uxes of photons irradiated from the wiggler central poles, wigglers being "tted to the storage rings of CAMD}LSU, BESSY-II, and SPring-8. One of the inconvenient factors arising when standard three-pole wigglers are used is the horizontal deviation of the equilibrium electron orbit at wiggler central pole and thus shift of the radiation point. To eliminate this e!ect and to "x the radiation point one can use a "ve-pole wiggler. However, it would lead to enlargement of the magnet `colda part, increase helium consumption and complicate design of the magnet as well as of the whole cryostat. That is why when the 7 T wigglers were "tted to the sources of CAMD-LSU and BESSY-II, a more simple scheme was used. One can see from Fig. 1 that due to two `warma (normal conductivity) correctors with a "eld of 0.5 T, placed at both the ends of the straight section, the electron beam orbit is always geometrically "xed to the center of the wiggler. In doing so, it is possible to
M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
53
Fig. 2. Angular distribution of the photon #ux of 7 T wiggler for BESSY-II.
change the wiggler "eld without shifting the radiation point. Here, of course, the "rst "eld integral along the wiggler axis is not equal to zero because in this case the integral is to be made zero over the whole straight section, including the correction magnets.
3. Magnet system The wiggler magnet system (as one can see from Fig. 4) is two halves of an iron yoke with three superconducting dipoles which are located above and below of the vacuum chamber. The yoke is not only a mechanical base of the wiggler structure but also provides an increase of magnetic induction at the median plane and magnetic #ux closure. One of the distinctive features of the wigglers being described is the unconventional scheme of bandaging of superconducting windings inside the
iron yoke. This scheme is realized with the help of two pairs of wedges of a material with low heat extension factor (e.g. invar). The magnet is cooled down to the liquid helium temperature, due to di!erent thermal contraction of the used material, the superconducting winding being `crimpeda from the iron yoke side through the invar wedges. Such a scheme makes it possible to precisely normalize the required pressure to the winding. The key element of the wiggler is the high-"eld superconducting central pole. To obtain maximum "eld it is necessary to place the superconducting windings in space and to select currents in them so that the "eld level on the winding does not exceed the critical value at the required maximum "eld value in the orbit. The "eld critical value is determined by the superconducting wires used. In particular, for the production of central windings for wigglers with "eld not exceeding 8 T, a Nb}Ti superconducting wire may be used. All sections of
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M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Fig. 3. Spectral #uxes of photon irradiated from the wigglers on the storage rings of CAMD-LSU, BESSY-II and SPring-8.
the wiggler central poles for LSU-CAMD and BESSY-II were reeled up from Nb}Ti wire with diameter of 0.87 mm, which has a critical current of 360 A in a 7 T "eld. To obtain higher magnetic "elds, one requires to use a Nb SN superconduct ing wire with a higher critical current. A rectangu-
Fig. 4. Magnetic system of 7 T wiggler for LSU-CAMD before assembling.
lar Nb Sn wire of 1.45;0.85 mm in size was used for the manufacture of inner section of the central pole for the 10 T wiggler for Spring-8. The load curves of the used superconducting wires are presented in Fig. 5. In the same "gure one can see the `operation pointsa that describe values of magnetic "elds and currents in the points of the winding that are critical from the quench standpoint. Due to the fact that in the wiggler the electron beam undergoes substantial orbit deviations in the horizontal plane, good uniformity of magnetic "eld in the transversal direction is one of the most important requirements. The central pole magnetic "eld is formed as a superposition of the "elds created by the iron core, on which the superconducting windings are reeled up, and by the windings themselves. The iron core contribution to the resulting "eld is about 1.5 T. To form a transversal uniformity of the "eld, the central pole is stretched in this direction. Especially stringent requirements are imposed on the quality of "eld of the BESSY-II wiggler since this will be used for calibration
M. Fedurin et al. / Nuclear Instruments and Methods in Physics Research A 448 (2000) 51}58
Fig. 5. The load curves of the superconducting wires and `operation pointsa of the wigglers windings.
experiments in the PTB-laboratory. To additionally improve transversal uniformity, we used shimming by means of putting thin iron plate on the central poles [5]. It permitted us to bring the transversal uniformity up to 10\ at a 7 T "eld. To simulate the uniformity and to select the central pole shape we used 3D MASTAC code [6]. It should be noted that 3D computations of iron magnet systems are complicated signi"cantly by the e!ects associated with saturation of the iron yoke. Computation of the yoke of wiggler was
55
made with 3D MASTAC code also. The outer areas of the yoke do not reach the saturation level, which indicates very low level of stray magnetic "elds around the wiggler. It excludes the in#uence of the wiggler "eld upon elements around the storage ring. Interaction of stray "elds of the wiggler with the surrounding iron is extremely small, which helps the wiggler to be stable in space and signi"cantly simpli"es the scheme of supporting the wiggler inside the cryostat. For a more e$cient use of the superconducting wires, the central pole winding is divided into several sections. Table 2 presents results of the sectioning of the central windings. To feed the multisection superconducting windings we suggested an electric scheme with two independent DC power supplies. Fig. 6 presents the feeding circuit for the 10 T wiggler for Spring-8. Such a feeding circuit has the following advantages: 1. Each section of the multi-section windings is fed by optimal current; 2. A three-current combination is used, only two sources being available; 3. The magnet system is fed with the help of only two pairs of current leads, which reduces signi"cantly the incoming heat to the liquid helium cryostat. 4. We obtain a possibility of easy control of wiggler "eld "rst integral. For any "eld level, currents of both the sources can be selected so as to make the wiggler "eld "rst integral go to zero. For `zeroinga the "rst integral
Table 2 Parameters of the superconducting windings Wiggler
LSU-CAMD, BESSY-II
Spring-8
Numer of winding Material of winding Dimension of wire, mm
1 Nb}Ti d"0.87 (0.92)
2 Nb}Ti d"0.87 (0.92)
Number of turns per winding Number of turns per layer Number of layers Current in wire (A) Square of winding (mm) Density of current (A/mm) Field at winding (T)
959 68.69 14 101 64.5;11.5 130.6 7.72
2192 68.69 32 253 64.5;26 330.7 6.54
1 Nb Sn 1.45;0.85 (1.65;1.1) 1008 42 24 198 74.3;27.6 97.3 10.4
2 Nb}Ti d"0.87 (0.92)
3 Nb}Ti d"0.87 (0.92)
2448 77.76 32 96 74.3;26 121.6 8.28
1836 77.76 24 294 74.3;19.5 372.6 5.9
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4. Cryostat
Fig. 6. Feeding circuit of 10 T wiggler for SPring-8.
we used the method of a strained current wire, which behaves as the electron beam in a magnetic "eld does [7]. In doing so, accuracy of the `zeroinga is determined only by possibilities of the power supplies and by accuracy of determination of the measurement wire coordinate in space. The "eld second integral value is determined only by precision of manufacture of the wiggler magnet system.
The wiggler magnet system is placed in the cryostat with liquid helium. Its vacuum chamber of the room temperature is connected to a vacuum chamber of a straight section of a storage ring. Geometry of output of wiggler radiation is selected so that the radiation certainly does not touch the walls of the wiggler inner chamber. It permits signi"cant simpli"cation of the chamber design since the use of water-cooled radiation absorbers is abondoned. Thus all the radiation output outside the wiggler and its super#uous part is absorbed in the vacuum chamber of the straight section. Fig. 7 presents the design of the cryostats meant for the wigglers of BESSY-II and SPring-8. To cool two thermal screens of 80 and 20 K temperature, a two-stage cooling machine is used. Two liquid helium recondensers are used for long-term operation, the cryostat not being "lled up with helium. The cryostat is equipped with numerous sensors for thermal monitoring in the course of operation. Feeding current was inputted through 2 pairs of highemperature superconductive current leads. The
Fig. 7. The view of the cryostat of wigglers for BESSY-II and SPring-8.
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magnet system is equipped with persistent keys, which permit the use of the wiggler in the `closed currenta mode.
5. Tests and training of the magnet system Usually quenches are provoked by local heat emissions in the winding thickness that are caused by micro-movements of conductors and by cracking of epoxy resin in the superconducting winding. Besides this, every new quench takes place at a higher "eld level, i.e. we meet the so-called training of the superconducting winding. Such training processes for wigglers for BESSY-II and SPring-8 are presented in Fig. 8. Current in the windings reaches 90% of current of a short sample of the superconducting wires used. The inner section of the central pole of the 10 T wiggler for Spring-8 was manufactured using Nb Sn superconducting wire, which can degrade at a quench. That is why, for the purpose of inner section protection, currents in the windings are distributed in such a way that two outer Nb-Ti sections are closer to critical state and they are quenched "rst. During quenching the voltages arising across the wiggler windings were recorded. The process of transition of the windings from the superconducting state to the normal one takes about 50 ms. Duration of dumping of current is equal to 0.5 s and maximum voltage between current leads is not more than 250 V. The protection system allows
Fig. 8. Quench history of maximum magnetic "eld of the wigglers for BESSY-II and SPring-8 versus quench number.
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output of about 30% of the stored energy from the cryostat. The other 70% is absorbed on the inside of the windings and by the iron yoke. Repeated quenches without damage of the windings prove their high reliability both to overheat destruction and to electrical discharge. To make the superconducting winding protection more secure, one is supposed to shunt each winding with a circuit of a diode and resistor. This additional protection system, placed outside the liquid helium, allows signi"cant reduction of helium evaporation during quenching. To obtain the wiggler magnetic "eld map both at ambient temperature and in liquid helium, several measurement methods were used, including those with the use of integrating coils [8] and Hall probes. In both the methods the sensors were calibrated with the help of NMR. Several NMR sensors are installed at the wiggler poles for the purpose of stabilization of magnetic "eld level by feedback, with a precision not worse than 10\.
6. Conclusion The design of the high-"eld three-pole wigglers described has much in common. However, these wigglers do not duplicate each other but have their own distinctive features, de"ned by speci"c requirements. For instance, in the 7 T wigglers for CAMDLSU and BESSY-II `warma correctors were used for "xing the radiation point at the wiggler center, which makes it possible to conduct experiments with SR at any "eld level. High uniformity in the transversal direction as well as long-term stability of "eld at a level of 10\ due to application of NMR distinguishes the BESSY-II wiggler. This allows one to use it as a standard for calibration experiments at the PTB-laboratory after installation on the storage ring in 2000. The next wiggler (for SPring-8) is a key element for the slow positron source under construction. The superconducting wiggler with a uniquely high magnetic "eld of 10.14 T being "tted to the storage ring with the electron beam extreme energy of 8 GeV, we obtain a possibility of generation of a slow positron beam with high intensity. To
SECTION II.
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create this wiggler, a technology of manufacture of high-"eld superconducting Nb Sn windings of the `race-tracka type was developed and tested successfully. Wiggler commissioning in the SPring-8 site is planned for the beginning of 2000. It should be noted that design of the cryostats for the BESSY-II and Spring-8 wigglers used a number of high-technology novelties that improve their parameters signi"cantly. Among these are application of low-temperature cooling machines instead of liquid nitrogen for cooling the thermal screens, high-temperature superconductive current leads, persistent keys, and recondensers of liquid helium.
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[2] V.M. Borovikov, V.K. Djurba, M.G. Fedurin, G.N. Kulipanov, O.A. Lee, N.A. Mezentsev, V.A. Shkaruba, B. Craft, V. Saile, Nucl. Instr. and Meth. A 405 (1998) 208. [3] V. Borovikov, B. Craft, M. Fedurin, V. Jurba, V. Khlestov, G. Kulipanov, O. Li, N. Mezentsev, V. Sail, V. Shkaruba, J. Synchrotron Radiat. 5 (Part 3) (1998) 440. [4] A. Ando, S. Datacute, M. Fedurin, M. Hara, H. Kamitsubo, A. Kiselev, G. Kulipanov, N. Kumagai, N. Mezentsev, Y. Miyahara, T. Nakamura, H. Ohkuma, V. Shkaruba, A. Skrinsky, K. Soutome, M. Takao, H. Tanaka, J. Synchrotron Radiat. 5 (Part 3) (1998) 360. [5] M. Fedurin, N. Mezentsev, Nucl. Instr. and Meth. A 448 (2000) 59, These Proceedings. [6] A. Grudiev, M. Rojak, E. Shurina, Yu. Soloveychik, M. Tiunov, P. Vobly, MASTAC } new code for solving three-dimentional nonlinear magnetostatic problems. Proceedings of the IEEE Particle Acceleration Conference, 1995. [7] E. Bekhtenev, E. Dementiev, M.G. Fedurin, N.A. Mezentsev, V.A. Shkaruba, P.D. Vobly, Nucl. Instr and Meth. A 405 (1998) 214. [8] V. Borovikov, M. Fedurin, A. Kerginsky, M. Kuzin, N. Mezentsev, V. Shkaruba, J. Synchrotron Radiat. 5 (Part 3) (1998) 382.