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The high field magnet laboratory of Grenoble
Journal of Magnetism and Magnetic Materials 11 (1979) 308-316 © North-Holland Publishing Company
THE HIGH FIELD MAGNET LABORATORY OF GRENOBLE
J.C. PICOCHE, P. RUB Service National des Champs Intenses, CNRS, Grenoble, France
and H.-J. SCHNEIDER-MUNTAU Hochfeld.Magnetlabor des Max-Planck-Instituts fiir Festk6rperforschung, Grenoble, France
Received 18 September 1978
A description of the high field magnet laboratory of Grenoble is given. The 10 MWpower supply, the 400 m3/h cooling water circuit and the associated technical installations are presented in detail. We describe also the water-cooled high field magnets (up to 20 T) and the superconducting magnets (14 T) of the laboratory and give some data about the utilisation of the technical installations.
1. Introduction
Hochfeld-Magnetlabor (HML) and up to 20 visitors come every month to perform experiments in high magnetic fields. In accordance with the subject of this conference we limit the description of the SNCI and the HML to the part which is concerned with the generation and utilisation of high magnetic fields with resistive and superconducting magnets.
The "Service National des Champs Intenses" (SNCI), a laboratory of the Centre National de la Recherche~ Scientifique (CNRS), was founded in 1971 with the aim to create a service institute for the research of matter subjected to extreme conditions, as high magnetic fields, low temperatures and/or high pressures. In 1972 the CNRS and the Max Planck-Gesellschaft initiated a joint agreement for the enlargement and common exploitation of the SNCI. From this cooperation and the generous support of the Volkswagenstiftung the available electrical power was doubled to 10 MW, a measure which made the high field magnet laboratory of Grenoble competitive with other existing magnet laboratories. The CNRS and the Max Planck-GeseUschaft also committed themselves to place the scientific equipment and the technical installations at the disposal of all scientists, i.e. the access is not limited to scientists from these two organisations. Today 40 French and German scientistis, engineers and technicians are working continuously and in close cooperation at the SNCI and the
2. The technical installations
The building, consisting of three floors, houses in the ground floor the magnet cells, in the first floor the laboratories and work shops and in the second floor the offices, the conference room, library and the calculator room. The machine hall with the four power supplies of 2.5 MW each and the machinery for the water cooling of the magnets adjoints the building providing for very short connections for water and current [ 1]. The electrical power and the cooling water are distributed in the basement and fed from there directly to the six magnet cells with their seven magnet sites located in the ground floor. Altogether a space of about 800 m 2 is available 308
309
J.C. Picoche et al. / Grenoble high field magnet
for experiments in high magnetic fields, comprising the section "low temperatures" with its three superconducting magnets and the three laboratories in the first floor (two superconducting magnets). 2.1. The power supplies
The total installed power of 10 MW is generated in four identical power supplies each giving 7500 A at 335 V. They can be connected in parallel, therefore either two 5 MW magnets (up to 15 T) or one 10 MW magnet (20 T) can be run at the same time. Each power supply is fed independently from a 15 kV high power line via a transformer which generates two three-phase systems with a phase shift of 15 degrees by two different secondary windings, one in star, one in delta (fig. 1). The alternate current passes two bridge rectifiers with silicon controlled rectifiers and a balance inductance Lo. The fundamental frequency of the current is therefore 600 Hz. It is reduced by the following L] C 1 f'dter with a resonance frequency of 30 Hz. The harmonics, generated by the SCRs are damped by inductances in front of the SCRs, by a special arrangement of the zig-zag windings and double shielding of the transformers. The voltage in this first part of the power supply is stabilized in a slow control loop ( 0 - 1 0 0 Hz)
The voltage at point A is sensored and commands the time of switching of the SCRs in such a way that voltage fluctuations are reduced to a value of A U / U ~< 1 × lO -a. Two additional amplifiers quarantee equal current in the two bridge rectifiers. To obtain a higher stability an active filter was added. The alternate component of the voltage as measured at point B is amplified and fed in with opposite phase via transformer L2 compensating any deviations u p to +5 X l0 -6 in the frequency range from 50 Hz to 10 kHz. The shunt voltage (IV at 7500 A) is used as current control, the difference between the shunt voltage and the stabilized variable reference voltage being amplified and fed into the two voltage regulation circuits. The current rise time from 0 to maximum is approximately one minute. The magnet current can also be controlled and/or modulated by an external voltage ( 0 - 1 0 V). We measured the long time stability of the magnetic field with a proton NMR system [2] and found +5 X 10 -6 over one day with an integration time of two seconds and ---1 X 10 - s with an integration time of 100 ms. The ripple is essentially independent of the field value, having an amplitude of 20/aT. In addition, there are also fluctuations of +100/aT in the frequency range from 0.1 to 1 Hz.
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Generation of o
six phase system
RI
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~ U
-~10.3
50Hz-10kHz
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~ _.5.10-6
Fig. 1. Basic circuit diagram of a 2.5 MWpower supply.
310
Z C. Picoche et al. / Grenoble high field magnet
2.2. The cooling circuit
The magnets are cooled by deionized water of high resistivity in a dosed loop circuit, which itself is cooled with river water via a heat exchanger (fig. 2). A demineralizer keeps the resistivity to about 5 M~2 cm. The cooling circuit and also the flow rate and the pressure drop in the magnets (25 bar) are designed in such a way that the cooling power of one pump corresponds to the electrical power of one power supply. A 5 MW magnet uses two power supplies and consequently two pumps with 100 ma/h each, a 10 MW magnet uses all four of them. A constant water pressure of 27 bar igtherefore maintained in the cooling circuit and excellent cooling conditions are assured. Two of the three 10 MW magnets are equipped with additional valves allowing the inner coils to be used alone. Filters with 200 pan mesh aperture are installed in front of the magnets to avoid obstruction of the cooling holes of the Bitter stacks and one 100/am filter is installed in front of the heat exchanger to eliminate
those pieces and particles which originate from burnt-through stacks or broken parts. A 5/am bypass £flter was added for the extraction of smaller particles. The shunts of the power supplies are cooled with well water because it changes its temperature only by a few degrees during the year and therefore no thermostatic regulation of the water flow is required. 2.3. The computer control
Because of the high costs of a Bitter stack, we have devoted much care to the question of how to increase the longevity of our magnets and we have introduced with success a few modifications [2]. The best measure to prevent the burning-through of a stack is of course switch off the magnet as soon as any irregularities of the resistance or the magnetic field can be seen and to replace the damaged discs. The check of the resistance of the magnet before or after its daily run is of limited help only. Our experience has shown that a stack with a few short
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,
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J.C. Picoche et al. / Grenoble high field magnet
circuited plates can still be used without any further risk if the power level is not too high. At maximum power, however, the short circuit propagates through the whole stack in less than a second. A fast and sensitive system is therefore required that detects a short circuit between two discs, i.e. 1% of the whole stack, in less than a second. Our detection system follows this philosophy. The computer (PDP 1 l/20) scans the voltage, the current, the inlet and outlet water temperature of each magnet coil three times a second, calculates the reduced resistance and compares it with the calibration value at the corresponding power. Any deviation which exceeds a preset limit, releases an alarm.
3. Resistive magnets The magnet laboratory has at present seven magnet sites at its disposal which may be equipped with different coils or coil combinations, as shown on fig. 3.
Fig. 3. The magnet sites of the high field magnet laboratory.
311
The magnets (table 1) consist of one or two coils, each using two power supplies with a rated power of together 5 MW. The single coil magnets nos. 1 to 4 are therefore called 5 MW magnets, the two coil magnets nos. 5 to 7 10 MWmagnets, although they use considerably less power than 5 or 10 MW, respectively. Our magnets cover a wide range from 6 T in 284 mm to 20 T in 50 mm, both horizontal and vertical. We distinguish between maximum and nominal field, the first being the design value, up to which the magnet is run only for test purposes. In the daily runs the magnets are limited to their nominal fields as listed in table 1. All coils but one are Bitter magnets, stacked in the original Bitter version which gives an equal force distribution around the circumference. Coil no. 7int is an entirely new designed polyhelix magnet, built to test the feasibility and reliability of this new technology. It consists of 12 helices, between 3.14 and 3.86 mm thick and 130 or 132 mm long. The resistances of the helices were futed in such a way that a radial constant stress current distribution was achieved [7]. Four helices are always connected in series, therefore 5000 A pass each chain. The magnet is calculated to generate 14.5 T with 4.8 MW in a volume of only 2.25 liter, i.e., more than 2 MW are dissipated in one liter of coil volume. The overall power density is increased by a factor of five compared to our Bitter magnets (fig. 4). In spite of this small volume the heat flux remains rather modest, being only 4.3 to 5.4 W/mm*. Special care was devoted to the lay-out of the hydraulic parameters of the coil. Practically no vibrations or noise is observed. Since the roughness of the helix surfaces is about 10% of the distance between them and because of the water velocity of 20 m/s, an extreme high degree of turbulence is achieved and an optimum heat transfer is assured. The average copper temperature lies between 60 and 70°C. In a series of tests, the magnet was first operated alone up to 13.4 T and performed to full satisfaction. Tests at higher fields were not possible because the power supplies were limited to about 92% of their full current rating. As second test the polyhelix magnet was run with the background field of a Bitter magnet. With the same current limitation a total field of 20.4 T was achieved, the outer magnet contributing 7 T. However, the currents in the three helix chains showed
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313
Fig. 4. The size of a polyhelix magnet compared to a Bitter magnet. Both coils generate the same induction (14.5 T) with practically the same power (4.8 and 4.5 MW, resp.) in the same bore of 56 mm.
some fluctuations and derivations from the computed values. After the dismantling of the magnet we found that some of the current feeds had detached due to the high Lorentz forces and made contact between three helices. A reinforced version is designed presently and will be tested soon. A common feature of all our Bitter magnets is their high space factor. It results from the high water pressure, requiring only small cooling holes and giving relatively low copper temperatures, the silver plating of the discs and the therefore reduced contact surfaces and the rather thin insulators. For this reason our magnets compare favorably with those of other magnet laboratories. As basis for comparison we take the criterion of efficiency, derived from the well known Fabry relation which we define as: c*
= B ( d / w ) l/2 .
It relates the magnetic induction B(T) generated, the power I¢(MW) consumed and the room temperature bore diameter d (cm). The efficiency factor G* reflects all the decisions of the magnet designer, as dimensions, coil shape, current distribution, insulator, mechanical support structure, conductor material, etc. Fig. 5 gives the efficiency of the best magnets of some magnet laboratories. It shows the growing difficulties encountered in building magnets to produce higher fields.
4. Rate of utilisation of the technical installations The high field magnet laboratory is now running in the seventh year with its full power of 10 MW [ 1 - 4 ] . Table 2 gives a listing of the hours of use of our 6 magnet sites in the last years, reflecting in the
,1.6'. Picoehe et al. / Grenoble high fieM magnet
314
G Ill
tr. l
18-
17
Table 2 Hours of use of different magnet sites
A B C G N W
ANU Canberra HMFABrounschweig FBNMLCambridge HML+SNCIGrenoble NRL Washington IL Wroclaw
• x
h y b r i d insert (3OT total f i e l d ) p u l s e d magnet (t0s}
Year
Magnet site
Total
1
2
3
4
5
253 127 282 132 646 256
150 781 1184 1489 1215 1613
0 0 0 111 108 0 1166 1079 20 1100 821 394 1233 382 602 1230 455 988
6
--8N
1972 1973 1974 1975 1976 1977
~N 16
15-
0 0 30 205 336 430
403 1127 3761 4141 4414 4972
14" w
w
15
20
steadily growing d e m a n d for m a g n e t time. Especially fields a b o v e 15 T ( m a g n e t sites nos. 5 a n d 6) s h o w a very s t r o n g rate o f increase. In 1977 t h e m a g n e t s have c o n s u m e d a t o t a l energy o f 3.9 GWh. O n t h e average 7 8 0 k W h were c o n s u m e d p e r h o u r o f m a g n e t time. T h e average electrical runn i n g costs o f o n e h o u r o f m a g n e t t i m e , c o o l i n g pumps and infrastructure included, amounted to FFR 197.- (US $ 45.-).
13
12
11
1(3 10
w
25
30
B [ T]
Fig. 5. The efficiency factor of water cooled magnets versus magnetic induction.
T a k i n g i n t o a c c o u n t the t i m e w h e r e t h e installat i o n s were s w i t c h e d o f f because o f m a i n t e n a n c e , repairs a n d h o l i d a y s , a rate o f 72% is c a l c u l a t e d , i.e. the l a b o r a t o r y runs m o r e t h a n 17 h o u r s a day.
Table 3 The superconducting magnets of the high field magnet laboratory of Grenoble Magnet
Dimensions (mm) 2a I
2a 2
Magnetic field (T) at 2b 4.2K
30
170
Homogeneity
200
Inductance (H)
Remarks
2K
9.3
1
Stored energy (K)
13.9
-
2X10 - 3 over ±20 mm
2.5
Nb3Sn tape
8.1
NbTi wire
350
180
350
340
2
40
194
200
4.6 11.9
13.0
2×10 -3 over -+20 mm
120
3
50
140
200
9.0
11.5
1 X10 - 3 over -+25 mm
80
4
23
230
210
10.3
-
7 X 10 - 4 in 1 cm DSV
5
79
140
230
7.5
-
10 - s in 1 cm DSV
12
Nb3Sn tape
10.5
NbTi MF, used with 10 mK dilution refrigeratoJ Nb3Sn tape, split coil with ~ 1 5 - 4 3 mm conical radial access
41
31
50 mm horizontal room temperature bore
J.C. Picoehe et al. I Grenoble high field magnet 5. Superconducting magnets The superconducting magnets of the laboratory as listed in table 3 are 'also at the disposal of scientific visitors. The first three magnets were built in our laboratory by Mr. VaUier. They were designed for magnetisation measurements and offer therefore a high homogeneity along the axis. The first magnet, built in 1971, generates 13.9 T and consists of two coils, the outer being wound from NbTi wire of 1.5 X 1.5 mm 2, the inner coil from Nb3Sn tape [5]. The second magnet was constructed with Nb3Sn tape from IGC who also provided technical support. For this magnet a cryostat was designed which allows the coil to operate at 2 K at atmospheric pressure [6]. The field increases from 11.9 T at 4.2 K to 13 T at 2 K. For NbTi conductors the field increase is much more important: magnet no. 3 wound from MF NbTi generates 9 T at 4.2 K and 11.5 T at 2 K, corresponding to a magnetic induction of 12 T at the wire. The magnet forms part of a dilution refrigerator for magnetisation measurements at 10 mK. The last two magnets are commercial magnets; the 10.3 T split coil has rather large opening angles of 12 and 20 degrees respectively and serves for optical measurements. The 7.5 T magnet has a 50 mm room temperature bore for preparatory experiments at lower fields.
315
We also want to increase further the efficiency of our Bitter magnets by choosing extreme hard, but unalloyed copper [2] as conductor material. The question of better insulator foils will have to be examined. Also the application of current distributions which vary along the z axis (Gaume distribution) will be considered in the future. A very promising technique to increase the efficiency are polyhelix magnets [8] and we await with impatience the results of further tests concerning the reliability of our first magnet. Fig. 6 shows the magnetic induction which can be generated for a given power for different outer coil diameters and a 5 cm room temperature bore. The straight lines represent the Fabry relation, the optimized shape and radial current distribution and the reduced stress result in N [MW] !
100
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50.
.
As the demand for higher fields is permanently growing we rind ourselves confronted with the fact that the field limit of 20 T in 5 cm, given by our present coils, will soon have to be pushed higher. Modifications of our magnets and new stacks are planned to increase both field and efficiency. The final goal of 30 T in 5 cm room temperature bore shall be achieved in 1982 with a hybrid magnet, a cooperation of CNRS, GfK and MPG. In detail the following projects have been started: We are designing at present a stack with reduced inner diameter and which will be used as inner coil in our vertical 10 MW magnet. The total field is expected to be 22 T in a 44 mm room temperature bore.
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Fig. 6. The power required to generate the magnetic induction B with polyhelix magnetsfor different outer radii a 2 and inner radius al = 28 mm, correspondingto a 50 mm diameter room temperature bore. For comparison the power requirements of a Bitter magnet with a 2 = 280 mm and three existing magnets with the same inner bore are given (the 15 T and 20 T magnets of Grenoble and the 30 T 30 MW pulsed magnet of Canberra, the inner coil of which is a polyhelix magnet).
316
J.C. Picoche et al. / Grenoble high field magnet
higher G-factors and therefore increased efficiency compared to Bitter magnets. Bigger coil volumes as they are required for very high power magnets raise also the efficiency and therefore the attainable fields. A reduced inner diameter also increases the maximum field. A room temperature bore of half the diameter as shown in fig. 6 adds about 5 T to the maximum field. But we have to state that also polyhelix magnets show excessive power requirements for stationary fields above 35 T and quasi-stationary fields may be possible up to 60 T. In the field of superconducting magnets the 2 K technology at atmospheric pressure will be further pursued. It allows one to build magnets with increased magnetic induction at the expense of a cryostat of somewhat increased complexity. A 10.5 T, 30 cm cold bore, MF NbTi magnet together with its 1.8 K cryostat is under construction in cooperation with the CENG to examine the feasibility of this technology for large magnets. We hope that the results of this magnet will show the superiority of this cooling mode and that together with the experience gained from the very compact polyhelix magnets, essential indications and information will be obtained for the construction of the 30 T hybrid magnet.
Acknowledgements The design and construction of the magnets described and the smooth and troublefree running
of the magnets and of the technical installations with their high rate of utilisation was only possible thanks to the strong efforts of our engineers and technicians. We want to thank therefore Messieurs Dresler, Faivre, Geoffray, Giroutru, Hackbarth, Morettini, Pey, Schmid and Spitznagel for their excellent workmanship and support.
References [1] H.-J. Schneider-Muntau, Proc. Conf. Application of High Magnetic Fields in Semiconductor Physics, Wiirzburg, 1974, p. 120. [2] P. Rub, M. Lombardi and H.-J. Schneider-Muntau, Proc. 6th Int. Conf. Magnet Technology, Bratislava, 1977, p.406. [3] P. Rub, J.C. Picoche and H.-J. Schneider-Muntau, Physique sous Champs Magn6tiques Intenses (Colloq. Int. CNRS) Grenoble, 1974, p. 149. [4] P. Rub, J.C. Picoche and H.-J. Schneider-Muntau, Proc. 5th Int. Conf. Magnet Technology, Rome, 1975, p. 402. [5] J.C. Vallier, Proc. 4th Int. Conf. Magnet Technology, Brookhaven, 1972, p. 233. [6] G. Bon Mardion, G. Claudet and J.C. Vallier, Proc. 6th Int. Cryogenic Engineering Conf., Grenoble, 1976, p. 159. [7] H.-J. Schneider-Muntau and P. Rub, Proc. 5th Int. Conf. Magnet Technology, Rome, 1975, p. 398. [8] H.-J. Schneider-Muntau and P. Rub, in Physics sous Champs Magn6tiques Intenses (Colloq. Int. CNRS) Grenoble, 1974, p. 161.
THE HIGH FIELD MAGNET LABORATORY OF GRENOBLE
J.C. PICOCHE, P. RUB Service National des Champs Intenses, CNRS, Grenoble, France
and H.-J. SCHNEIDER-MUNTAU Hochfeld.Magnetlabor des Max-Planck-Instituts fiir Festk6rperforschung, Grenoble, France
Received 18 September 1978
A description of the high field magnet laboratory of Grenoble is given. The 10 MWpower supply, the 400 m3/h cooling water circuit and the associated technical installations are presented in detail. We describe also the water-cooled high field magnets (up to 20 T) and the superconducting magnets (14 T) of the laboratory and give some data about the utilisation of the technical installations.
1. Introduction
Hochfeld-Magnetlabor (HML) and up to 20 visitors come every month to perform experiments in high magnetic fields. In accordance with the subject of this conference we limit the description of the SNCI and the HML to the part which is concerned with the generation and utilisation of high magnetic fields with resistive and superconducting magnets.
The "Service National des Champs Intenses" (SNCI), a laboratory of the Centre National de la Recherche~ Scientifique (CNRS), was founded in 1971 with the aim to create a service institute for the research of matter subjected to extreme conditions, as high magnetic fields, low temperatures and/or high pressures. In 1972 the CNRS and the Max Planck-Gesellschaft initiated a joint agreement for the enlargement and common exploitation of the SNCI. From this cooperation and the generous support of the Volkswagenstiftung the available electrical power was doubled to 10 MW, a measure which made the high field magnet laboratory of Grenoble competitive with other existing magnet laboratories. The CNRS and the Max Planck-GeseUschaft also committed themselves to place the scientific equipment and the technical installations at the disposal of all scientists, i.e. the access is not limited to scientists from these two organisations. Today 40 French and German scientistis, engineers and technicians are working continuously and in close cooperation at the SNCI and the
2. The technical installations
The building, consisting of three floors, houses in the ground floor the magnet cells, in the first floor the laboratories and work shops and in the second floor the offices, the conference room, library and the calculator room. The machine hall with the four power supplies of 2.5 MW each and the machinery for the water cooling of the magnets adjoints the building providing for very short connections for water and current [ 1]. The electrical power and the cooling water are distributed in the basement and fed from there directly to the six magnet cells with their seven magnet sites located in the ground floor. Altogether a space of about 800 m 2 is available 308
309
J.C. Picoche et al. / Grenoble high field magnet
for experiments in high magnetic fields, comprising the section "low temperatures" with its three superconducting magnets and the three laboratories in the first floor (two superconducting magnets). 2.1. The power supplies
The total installed power of 10 MW is generated in four identical power supplies each giving 7500 A at 335 V. They can be connected in parallel, therefore either two 5 MW magnets (up to 15 T) or one 10 MW magnet (20 T) can be run at the same time. Each power supply is fed independently from a 15 kV high power line via a transformer which generates two three-phase systems with a phase shift of 15 degrees by two different secondary windings, one in star, one in delta (fig. 1). The alternate current passes two bridge rectifiers with silicon controlled rectifiers and a balance inductance Lo. The fundamental frequency of the current is therefore 600 Hz. It is reduced by the following L] C 1 f'dter with a resonance frequency of 30 Hz. The harmonics, generated by the SCRs are damped by inductances in front of the SCRs, by a special arrangement of the zig-zag windings and double shielding of the transformers. The voltage in this first part of the power supply is stabilized in a slow control loop ( 0 - 1 0 0 Hz)
The voltage at point A is sensored and commands the time of switching of the SCRs in such a way that voltage fluctuations are reduced to a value of A U / U ~< 1 × lO -a. Two additional amplifiers quarantee equal current in the two bridge rectifiers. To obtain a higher stability an active filter was added. The alternate component of the voltage as measured at point B is amplified and fed in with opposite phase via transformer L2 compensating any deviations u p to +5 X l0 -6 in the frequency range from 50 Hz to 10 kHz. The shunt voltage (IV at 7500 A) is used as current control, the difference between the shunt voltage and the stabilized variable reference voltage being amplified and fed into the two voltage regulation circuits. The current rise time from 0 to maximum is approximately one minute. The magnet current can also be controlled and/or modulated by an external voltage ( 0 - 1 0 V). We measured the long time stability of the magnetic field with a proton NMR system [2] and found +5 X 10 -6 over one day with an integration time of two seconds and ---1 X 10 - s with an integration time of 100 ms. The ripple is essentially independent of the field value, having an amplitude of 20/aT. In addition, there are also fluctuations of +100/aT in the frequency range from 0.1 to 1 Hz.
220kV 40
MIA
I C ÷
L2
f
Generation of o
six phase system
RI
~
I-
~-~
Rectification Passive filter Active filter Shunt Current control Current equilibration fr : 30 Hz Voltage stabilization C u r r e n t Voltageinput for Protecting diode stabilization remotecontrol and current modulation Current stabilization Voltage stabilization 0-100 Hz
~ U
-~10.3
50Hz-10kHz
~
~ _.5.10-6
Fig. 1. Basic circuit diagram of a 2.5 MWpower supply.
310
Z C. Picoche et al. / Grenoble high field magnet
2.2. The cooling circuit
The magnets are cooled by deionized water of high resistivity in a dosed loop circuit, which itself is cooled with river water via a heat exchanger (fig. 2). A demineralizer keeps the resistivity to about 5 M~2 cm. The cooling circuit and also the flow rate and the pressure drop in the magnets (25 bar) are designed in such a way that the cooling power of one pump corresponds to the electrical power of one power supply. A 5 MW magnet uses two power supplies and consequently two pumps with 100 ma/h each, a 10 MW magnet uses all four of them. A constant water pressure of 27 bar igtherefore maintained in the cooling circuit and excellent cooling conditions are assured. Two of the three 10 MW magnets are equipped with additional valves allowing the inner coils to be used alone. Filters with 200 pan mesh aperture are installed in front of the magnets to avoid obstruction of the cooling holes of the Bitter stacks and one 100/am filter is installed in front of the heat exchanger to eliminate
those pieces and particles which originate from burnt-through stacks or broken parts. A 5/am bypass £flter was added for the extraction of smaller particles. The shunts of the power supplies are cooled with well water because it changes its temperature only by a few degrees during the year and therefore no thermostatic regulation of the water flow is required. 2.3. The computer control
Because of the high costs of a Bitter stack, we have devoted much care to the question of how to increase the longevity of our magnets and we have introduced with success a few modifications [2]. The best measure to prevent the burning-through of a stack is of course switch off the magnet as soon as any irregularities of the resistance or the magnetic field can be seen and to replace the damaged discs. The check of the resistance of the magnet before or after its daily run is of limited help only. Our experience has shown that a stack with a few short
OemineroHzer
Reservoir
Town Woter
River
3bar6OOm , Pumps
~oo~m
~J Exchanger
m
Power Supphes
200mh
M gne,s 200 m3/h 200 m3/h
,
200 mS/h
- ~ , ~ - ~~
~oom~/h
~
200 mVh
200 m3/h - " 1
200 m3/h zt~vmI.I~N~200 m31h .~I " --.I
[] Fig. 2. Cooling water flow diagram.
I>
i-~
J.C. Picoche et al. / Grenoble high field magnet
circuited plates can still be used without any further risk if the power level is not too high. At maximum power, however, the short circuit propagates through the whole stack in less than a second. A fast and sensitive system is therefore required that detects a short circuit between two discs, i.e. 1% of the whole stack, in less than a second. Our detection system follows this philosophy. The computer (PDP 1 l/20) scans the voltage, the current, the inlet and outlet water temperature of each magnet coil three times a second, calculates the reduced resistance and compares it with the calibration value at the corresponding power. Any deviation which exceeds a preset limit, releases an alarm.
3. Resistive magnets The magnet laboratory has at present seven magnet sites at its disposal which may be equipped with different coils or coil combinations, as shown on fig. 3.
Fig. 3. The magnet sites of the high field magnet laboratory.
311
The magnets (table 1) consist of one or two coils, each using two power supplies with a rated power of together 5 MW. The single coil magnets nos. 1 to 4 are therefore called 5 MW magnets, the two coil magnets nos. 5 to 7 10 MWmagnets, although they use considerably less power than 5 or 10 MW, respectively. Our magnets cover a wide range from 6 T in 284 mm to 20 T in 50 mm, both horizontal and vertical. We distinguish between maximum and nominal field, the first being the design value, up to which the magnet is run only for test purposes. In the daily runs the magnets are limited to their nominal fields as listed in table 1. All coils but one are Bitter magnets, stacked in the original Bitter version which gives an equal force distribution around the circumference. Coil no. 7int is an entirely new designed polyhelix magnet, built to test the feasibility and reliability of this new technology. It consists of 12 helices, between 3.14 and 3.86 mm thick and 130 or 132 mm long. The resistances of the helices were futed in such a way that a radial constant stress current distribution was achieved [7]. Four helices are always connected in series, therefore 5000 A pass each chain. The magnet is calculated to generate 14.5 T with 4.8 MW in a volume of only 2.25 liter, i.e., more than 2 MW are dissipated in one liter of coil volume. The overall power density is increased by a factor of five compared to our Bitter magnets (fig. 4). In spite of this small volume the heat flux remains rather modest, being only 4.3 to 5.4 W/mm*. Special care was devoted to the lay-out of the hydraulic parameters of the coil. Practically no vibrations or noise is observed. Since the roughness of the helix surfaces is about 10% of the distance between them and because of the water velocity of 20 m/s, an extreme high degree of turbulence is achieved and an optimum heat transfer is assured. The average copper temperature lies between 60 and 70°C. In a series of tests, the magnet was first operated alone up to 13.4 T and performed to full satisfaction. Tests at higher fields were not possible because the power supplies were limited to about 92% of their full current rating. As second test the polyhelix magnet was run with the background field of a Bitter magnet. With the same current limitation a total field of 20.4 T was achieved, the outer magnet contributing 7 T. However, the currents in the three helix chains showed
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Fig. 4. The size of a polyhelix magnet compared to a Bitter magnet. Both coils generate the same induction (14.5 T) with practically the same power (4.8 and 4.5 MW, resp.) in the same bore of 56 mm.
some fluctuations and derivations from the computed values. After the dismantling of the magnet we found that some of the current feeds had detached due to the high Lorentz forces and made contact between three helices. A reinforced version is designed presently and will be tested soon. A common feature of all our Bitter magnets is their high space factor. It results from the high water pressure, requiring only small cooling holes and giving relatively low copper temperatures, the silver plating of the discs and the therefore reduced contact surfaces and the rather thin insulators. For this reason our magnets compare favorably with those of other magnet laboratories. As basis for comparison we take the criterion of efficiency, derived from the well known Fabry relation which we define as: c*
= B ( d / w ) l/2 .
It relates the magnetic induction B(T) generated, the power I¢(MW) consumed and the room temperature bore diameter d (cm). The efficiency factor G* reflects all the decisions of the magnet designer, as dimensions, coil shape, current distribution, insulator, mechanical support structure, conductor material, etc. Fig. 5 gives the efficiency of the best magnets of some magnet laboratories. It shows the growing difficulties encountered in building magnets to produce higher fields.
4. Rate of utilisation of the technical installations The high field magnet laboratory is now running in the seventh year with its full power of 10 MW [ 1 - 4 ] . Table 2 gives a listing of the hours of use of our 6 magnet sites in the last years, reflecting in the
,1.6'. Picoehe et al. / Grenoble high fieM magnet
314
G Ill
tr. l
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17
Table 2 Hours of use of different magnet sites
A B C G N W
ANU Canberra HMFABrounschweig FBNMLCambridge HML+SNCIGrenoble NRL Washington IL Wroclaw
• x
h y b r i d insert (3OT total f i e l d ) p u l s e d magnet (t0s}
Year
Magnet site
Total
1
2
3
4
5
253 127 282 132 646 256
150 781 1184 1489 1215 1613
0 0 0 111 108 0 1166 1079 20 1100 821 394 1233 382 602 1230 455 988
6
--8N
1972 1973 1974 1975 1976 1977
~N 16
15-
0 0 30 205 336 430
403 1127 3761 4141 4414 4972
14" w
w
15
20
steadily growing d e m a n d for m a g n e t time. Especially fields a b o v e 15 T ( m a g n e t sites nos. 5 a n d 6) s h o w a very s t r o n g rate o f increase. In 1977 t h e m a g n e t s have c o n s u m e d a t o t a l energy o f 3.9 GWh. O n t h e average 7 8 0 k W h were c o n s u m e d p e r h o u r o f m a g n e t time. T h e average electrical runn i n g costs o f o n e h o u r o f m a g n e t t i m e , c o o l i n g pumps and infrastructure included, amounted to FFR 197.- (US $ 45.-).
13
12
11
1(3 10
w
25
30
B [ T]
Fig. 5. The efficiency factor of water cooled magnets versus magnetic induction.
T a k i n g i n t o a c c o u n t the t i m e w h e r e t h e installat i o n s were s w i t c h e d o f f because o f m a i n t e n a n c e , repairs a n d h o l i d a y s , a rate o f 72% is c a l c u l a t e d , i.e. the l a b o r a t o r y runs m o r e t h a n 17 h o u r s a day.
Table 3 The superconducting magnets of the high field magnet laboratory of Grenoble Magnet
Dimensions (mm) 2a I
2a 2
Magnetic field (T) at 2b 4.2K
30
170
Homogeneity
200
Inductance (H)
Remarks
2K
9.3
1
Stored energy (K)
13.9
-
2X10 - 3 over ±20 mm
2.5
Nb3Sn tape
8.1
NbTi wire
350
180
350
340
2
40
194
200
4.6 11.9
13.0
2×10 -3 over -+20 mm
120
3
50
140
200
9.0
11.5
1 X10 - 3 over -+25 mm
80
4
23
230
210
10.3
-
7 X 10 - 4 in 1 cm DSV
5
79
140
230
7.5
-
10 - s in 1 cm DSV
12
Nb3Sn tape
10.5
NbTi MF, used with 10 mK dilution refrigeratoJ Nb3Sn tape, split coil with ~ 1 5 - 4 3 mm conical radial access
41
31
50 mm horizontal room temperature bore
J.C. Picoehe et al. I Grenoble high field magnet 5. Superconducting magnets The superconducting magnets of the laboratory as listed in table 3 are 'also at the disposal of scientific visitors. The first three magnets were built in our laboratory by Mr. VaUier. They were designed for magnetisation measurements and offer therefore a high homogeneity along the axis. The first magnet, built in 1971, generates 13.9 T and consists of two coils, the outer being wound from NbTi wire of 1.5 X 1.5 mm 2, the inner coil from Nb3Sn tape [5]. The second magnet was constructed with Nb3Sn tape from IGC who also provided technical support. For this magnet a cryostat was designed which allows the coil to operate at 2 K at atmospheric pressure [6]. The field increases from 11.9 T at 4.2 K to 13 T at 2 K. For NbTi conductors the field increase is much more important: magnet no. 3 wound from MF NbTi generates 9 T at 4.2 K and 11.5 T at 2 K, corresponding to a magnetic induction of 12 T at the wire. The magnet forms part of a dilution refrigerator for magnetisation measurements at 10 mK. The last two magnets are commercial magnets; the 10.3 T split coil has rather large opening angles of 12 and 20 degrees respectively and serves for optical measurements. The 7.5 T magnet has a 50 mm room temperature bore for preparatory experiments at lower fields.
315
We also want to increase further the efficiency of our Bitter magnets by choosing extreme hard, but unalloyed copper [2] as conductor material. The question of better insulator foils will have to be examined. Also the application of current distributions which vary along the z axis (Gaume distribution) will be considered in the future. A very promising technique to increase the efficiency are polyhelix magnets [8] and we await with impatience the results of further tests concerning the reliability of our first magnet. Fig. 6 shows the magnetic induction which can be generated for a given power for different outer coil diameters and a 5 cm room temperature bore. The straight lines represent the Fabry relation, the optimized shape and radial current distribution and the reduced stress result in N [MW] !
100
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/ !. ! !
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50.
.
As the demand for higher fields is permanently growing we rind ourselves confronted with the fact that the field limit of 20 T in 5 cm, given by our present coils, will soon have to be pushed higher. Modifications of our magnets and new stacks are planned to increase both field and efficiency. The final goal of 30 T in 5 cm room temperature bore shall be achieved in 1982 with a hybrid magnet, a cooperation of CNRS, GfK and MPG. In detail the following projects have been started: We are designing at present a stack with reduced inner diameter and which will be used as inner coil in our vertical 10 MW magnet. The total field is expected to be 22 T in a 44 mm room temperature bore.
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316
J.C. Picoche et al. / Grenoble high field magnet
higher G-factors and therefore increased efficiency compared to Bitter magnets. Bigger coil volumes as they are required for very high power magnets raise also the efficiency and therefore the attainable fields. A reduced inner diameter also increases the maximum field. A room temperature bore of half the diameter as shown in fig. 6 adds about 5 T to the maximum field. But we have to state that also polyhelix magnets show excessive power requirements for stationary fields above 35 T and quasi-stationary fields may be possible up to 60 T. In the field of superconducting magnets the 2 K technology at atmospheric pressure will be further pursued. It allows one to build magnets with increased magnetic induction at the expense of a cryostat of somewhat increased complexity. A 10.5 T, 30 cm cold bore, MF NbTi magnet together with its 1.8 K cryostat is under construction in cooperation with the CENG to examine the feasibility of this technology for large magnets. We hope that the results of this magnet will show the superiority of this cooling mode and that together with the experience gained from the very compact polyhelix magnets, essential indications and information will be obtained for the construction of the 30 T hybrid magnet.
Acknowledgements The design and construction of the magnets described and the smooth and troublefree running
of the magnets and of the technical installations with their high rate of utilisation was only possible thanks to the strong efforts of our engineers and technicians. We want to thank therefore Messieurs Dresler, Faivre, Geoffray, Giroutru, Hackbarth, Morettini, Pey, Schmid and Spitznagel for their excellent workmanship and support.
References [1] H.-J. Schneider-Muntau, Proc. Conf. Application of High Magnetic Fields in Semiconductor Physics, Wiirzburg, 1974, p. 120. [2] P. Rub, M. Lombardi and H.-J. Schneider-Muntau, Proc. 6th Int. Conf. Magnet Technology, Bratislava, 1977, p.406. [3] P. Rub, J.C. Picoche and H.-J. Schneider-Muntau, Physique sous Champs Magn6tiques Intenses (Colloq. Int. CNRS) Grenoble, 1974, p. 149. [4] P. Rub, J.C. Picoche and H.-J. Schneider-Muntau, Proc. 5th Int. Conf. Magnet Technology, Rome, 1975, p. 402. [5] J.C. Vallier, Proc. 4th Int. Conf. Magnet Technology, Brookhaven, 1972, p. 233. [6] G. Bon Mardion, G. Claudet and J.C. Vallier, Proc. 6th Int. Cryogenic Engineering Conf., Grenoble, 1976, p. 159. [7] H.-J. Schneider-Muntau and P. Rub, Proc. 5th Int. Conf. Magnet Technology, Rome, 1975, p. 398. [8] H.-J. Schneider-Muntau and P. Rub, in Physics sous Champs Magn6tiques Intenses (Colloq. Int. CNRS) Grenoble, 1974, p. 161.