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Design of superconducting magnets for full scale MHD generators
This paper describes the results o f conceptual design studies and preliminary design work carried out relative to full-scale superconducting magnets for base-load size magnetohydrodynamic (MHD) generators. Conceptual layouts and design data were prepared for 6 T magnets o f alternate configurations (circular-saddle coil and rectangular saddle coil) and for 5 T and 7 T variations. The major characteristics of the various designs are summarized and compared. Problem areas revealed during the design effort are identified and specific recommendations for future investigations and R & D effort in support of large MHD magnet technology are made.
Design of superconducting magnets for full scale MHD generators A.M. Hatch, J. Zar and F.E. Becker
Fossil-fuel fired magnetohydrodynamic (MHD) electric power generating systems whose potential overall efficiencies greatly exceed those of present-day base-load central station power plants, are now undergoing intensive development with the objective of reaching the commercial-scale demonstration stage within a decade. ~ Very large superconducting magnets are an essential part of base-load MHD generators and must be considered among the critical components. In terms of stored energy, these magnets will be about two orders of magnitude larger than the largest superconducting MHD-type magnets existing today. While the basic materials and science needed for superconducting MHD magnets are currently available, more quantitative design information, additional engineering experience and further development in manufacturing technology are needed before very large MHD magnet construction can be undertaken with assurance of meeting performance reliability and economic objectives. 2
Conceptual design A set of rQtuirements including MHD channel size, desired field strength and related items were established at the outset to serve as a basis for all designs. The requiremets assumed open-cycle coal-fired base-load type generators. The channel sizes and field strengths specified for the primary base-load magnet design are shown in Fig. 1, together with a computer developed magnetic field prof'de for one of the actual magnet designs prepared. It was decided that 7 T and 5 T variations of the basic 6 T design should also be investigated. The conceptual design work included investiagtion of two variations of the saddle coil geometry, namely the circular-
p
This paper describes the results of a programme consisting of conceptual design studies and the preparation of preliminary designs and engineering data for full scale superconducting magnets for base-load MHD power generators. The major purposes in developing these designs were as follows
2.25 mdio (199m sq)
I
Worm
?bore
4.75 m di0 (4.2m sq)
1
1.35 m ~ ' ~ 16 m ~tive l e n g t h ~ | sq
•
(ST ~ - - ~ . ~ TOlerancet5%
1 To evaluate the state-of-the-art of large MHD magnet technology.
3.5T
2 To identify specific problems both technical and manufacturing that must be solved before full scale base-load magnets can be designed and built with assurance of meeting their requirements.
Inlet
Distancealongchannel
Exif
3 To develop recommendations for R & D programmes that should be undertaken before construction of such large magnets can begin. ~- 4
The project was divided into two phases, conceptual design and preliminary design. This paper will outline the work done in both phases and summarize the results. The authors are with the Avco Everett Research Laboratory Inc. Everett, Massachusetts USA. The work reported was supported by the United States Energy Research and Development Administration. It is a part of an MHD Magnet Development Programme managed for ERDA by the Francis Bitter National Magnet Laboratory Massachusetts Institute of Technology. Paper received 19 October 1977.
CRYOGENICS
. FEBRUARY
1978
no
._~ 2 ta-
O
b
-4
O 4 8 12 16 Distance alongchannel axis , m
20
Fig. 1 a--Design requirements, base-load scale MHD magnet. b--also shown is typical calculated curve of field vs distance along axis for preliminary design magnet (circular saddle BL6-p7)
67
~
B
o
~
e
d
~
o
t
e
cross-over
a
strengths. The designs prepared were conservative in approach, using mainly today's technology and emphasizing practical, economical construction. All were based on the use of copper-stabilized NbTi conductor operating at relatively low current densities ( < 3750 A cm -2) with windings cJesigned for full cryostatic stability. Fig. 3 shows plan views and end views of conceptual designs of circularsaddle and rectangular-saddle 6 T base-load size magnets. Fig. 4 shows cross-sections of the alternative designs at the plane of the channel inlet, indicating the arrangement of structure and windings. Both designs have modular (subdivided) windings, with coils held in substructures which are stacked to form the complete winding. In both designs, the substructure and the superstructure (major structural members such as the girders) are of aluminium alloy. This material was chosen because its physical properties are suitable for the service, it is considered less expensive than stainless steel for structures of the type shown and also because the relative coefficients of contraction of the aluminium alloy structure and of the conductor are such that the winding will be in compression after cool-down, a desirable feature.
b Fig. 2 Alternate winding geometries, a--circular saddle coil, tapered, with intermediate cross-over b--rectangular saddO coil, tape red
saddle and the rectangular saddle. Fig. 2 shows these two geometries. In the circular-saddle design, the required sloping field profile was achieved by both divergence of the winding from inlet to exit end and the use of intermediate crossovers. In the rectangular-saddle design, the sloping field profde was achieved by divergence of the winding and also fanning out of the conductor layers toward the exit end so that intermediate crossovers were not needed. A series of conceptual layouts and engineering calculations were made for base-load size magnets of the two alternative geometries mentioned and for magnets with various field
Endturn
crn..o.
Winding
"""7
Girders Tension (above plot., and below] /
/
?
Radiation LiauldHe shield / c°htu'ner
In the circular-saddle design, the winding modules are semicircular in cross-section and are stacked around a circular cross-section core tube. Circular ring girders of I-beam section surround the winding and support the modules against the outward acting magnetic forces. The vacuum jacket (dewar outer wall) is circular in cross-section. In the rectangular-saddle design, the winding modules are fiat plates turned out at the ends. They are stacked around a rectangular aperture and are held in place against outward magnetic forces by solid rectangular cross-section girders or beams on the top and bottom held together by tension plates connecting them on each side of the channel aperture. The vacuum jacket is rectangular. Table I lists engineering data for the various conceptual designs prepared, including ampere turns, major dimen-
End turn cross-over
Ringgirders (I-beamtype}?
,\
,,
/\ \
i
9'~m
/ \
A
~
Radiation LiquidHe _ shield ~ container
Intermediate cross-overs ~ \
': "
'' "
//
-
1
|125rndia
12am
1.2m typ
I~
2r~sm
-I
Planview
Vacuum jacket
J"
24.5m Planview
--
Winding.m o d u l e s ~ _ _ _
10.7m
End view
Rectangularsaddle Fig. 3
68
Ringgirders
Endview (Inletend -partialsect A-A ) Circularsadd!e
Plan views and end views of conceptual design 6T base-load magnets, circular-saddle type and rectangular saddle type
CRYOGENICS.
FEBRUARY
1978
Mechanical girder nt (typ) Outer shell Winding modules (half shells) Winding regions Core tube ;irder De~oilof mechanical girder joint (rect. saddle)
Girder (solid plate)
f Helium container
,'nsionplates ding modules plates tyrned at ends j lding regions
Core tube Vacuum Outer vOCuum walt
! Rectangular saddle
Fig. 4 Comparison o f circular and rectangular-saddle designs, cross-sections at plane of channel inlet. Arrows F m show direction of principal magnetic forces exerted on girders
sions, estimated weights and stored energy. It is of interest that the rectangular-saddle 6 T design uses more conductor and is considerably heavier than the circular-saddle 6 T design. However the rectangular design is advantageous in that cooling passages are better oriented for venting and component manufacture may be easier due to the use of primarily fiat plate components instead of the formed (conical) members characteristic of the circular-saddle design.
Preliminary design The alternate 6 T base-load designs, circular and rectangular were upgraded during the preliminary design phase and more
careful design calculations were made. Particular attention was given to the design of mechanical joints between the major components, so that sub-assemblies requiring extensive welding could be pre-fabricated at manufacturing facilities and then transported to the plant site where they would be assembled into the complete magnet with a minimum of on-site fabrication and welding. Investigation of the manufacturing aspects of major components for both designs indicated that both were feasible from the manufacturing standpoint with perhaps some prefabrication advantage for rectangular-saddle components since they did not require as much welding and forming. However when on-site assembly and mechanical joining aspects were investigated, it was found that the rectangularsaddle design involved more difficult and complicated joining arrangements and it was therefore decided that the circular-saddle design was preferable because its substantially lower material cost and less difficult assembly more than offset the slightly easier pre-fabrication characteristic of the rectangular design. Work on the circular-saddle design was therefore carried forward in more detail than that of the rectangular design. Significant engineering data and features of the finalized 6 T circular base-load design will now be summarized. Table 2 lists the characteristics of the design including mechanical, cryogenic and electrical data. Fig. 5 shows the type of conductor proposed and the winding configuration, including insulation and cooling passages within the winding module. Fig. 6 shows an assembly layout of the 6 T base-load circular-design. The liquid helium container directly surrounds the winding modules and its outer wall is at the inside surface of the ring girders. Thus the ring girders are in the vacuum space and the amount of liquid helium holdup is kept to a minimum. A vacuum jacket of circular cross-section encloses the assembly. Lowheat-leak support posts of titanium alloy, designed to carry the weight of the magnet plus the magnetic force interaction which may exist between the magnet and adjacent steel equipment are provided between the cold mass and the vacuum jacket, together with struts of the same material
Table 1. Engineering data for MHD magnet conceptual designs Nominal field, inlet
T
6
6
5
7
Type of saddle
-
Circular
Rectangular
Circular
Circular
Winding build
m
0.9
0.9
0.7
1.4
Average current density (overall winding)
A cm -2
1270
1270
1300
1000
Ampere turns (front)
106 NI
34.7
39.0
27.4
47.0
Stored energy
10eJ
5500
7600
3500
10700
Maximum field in winding
T
8.1
8.1
7.2
8.1
Major dimensions* Length
m
24.5
26.5
24.5
27.4
m
12.5 dia.
12.7x 10.7
11.2 dia.
14.8 dia.
Weight of conductor
103 kg
410
700
322
751
Total weight of magnet
103 kg
2380
2800
1400
5000
Exit end
*--outside vacuum jacket
CRYOGENICS. FEBRUARY 1978
69
Table 2. Design characteristics o f 6 T base-load M H D magnet (Preliminary design - Circular saddle type)
Dimensions and Magnetic Fields Active length Magnetic field on axis: Start of active length End of active length (from calculated field profile, Fig. 1) Field variation in channel cross-section Overall length outside vacuum jacket Warm bore diameter: Inlet end of vacuum jacket End of active length Distance from inlet end of vacuum jacket to start of active length Outside diameter of vacuum jacket*
m
16.0
T T
6.0 3.4
m
-+ 3% 25.0
m m
2.25 4.8
m m
4.15 12.5
103 kg 103 kg 103 kg 103 kg
460 1960 820 230
Weights Conductor Major structure (rectangular girder design) Sub-structure, insulation, etc. Vacuum jacket and radiation shield
3470
Total
Winding Details (grading winding, 3 regions) Region Maximum field in region Current density (overall) in region Current density in conductor Conductor width Conductor thickness Ratio, copper to superconductor
-
A
B
C
T A cm-2 A cm :'2 cm cm -
8.1 1180 3030 3.48 1.43 15
7.2 1270 3270 3.23 1.43 22.5
5.5 1400 3610 2.92 1.43 32
Winding Overall Design current in conductor Ampere turns, inlet end Total length of conductor Inductance Stored energy Packing factor (average) Cooling, passage width Percent cooled surface, conductor Heat flux, all current in copper Ratio helium to conductor volumes Total liquid helium in winding
A
14,500
-
37 x 106
m H mj cm
litres
12.6 x 104 57 6100 0.37 0.34 39% 0.4 0.4 23,000
Mn m -2 Mn m -2 Mn m -2
83 (12,000 psi) 179 (26,000 psi) 83 (12,000 psi)
-
w cm -2 -
Design Stresses (maximum) Conductor Major structure Electrical insulation (compression) *-not including external stiffeners
70
CRYOGENICS. FEBRUARY 1978
to carry transverse and longitudinal loads. A thermal radiation shield operating at 80 K with helium gas tracer tubes is interposed between the cold winding and structure and the room-temperature vacuum jacket walls.
1.57 mm die plain copper wire (typ) Solder
Copper core
953mm
•
:C
254mm
-
The more detailed attention in Phase 2 to structure fabrication techniques and material properties resulted in a considerable increase in total weight over that estimated for the conceptual design. The largest increase was due to a change from I-beam cross-section to solid cross-section ring girders, found necessary because the I-beam girder required extensive welding in manufacture, causing mechanical properties of the final weldment to fall below those required for this highly stressed component.
' '
1.57 Into dia superconducting wire (typ). Copper to superconductor ratio, 2:1
~ i Gravity
Windingsupportshell AI. alloy
The net continuous refrigeration requirement of the magnet is estimated to be 330 W at liquid helium temperature and 2000 W at 80 K.
Groovefor coolant Winding
b
Protection in the event of inadvertant initiation of quench such as may occur due to low liquid helium level or loss of vacuum is accomplished by the opening of circuit breakers that will disconnect the power supply and cause the magnet to start dumping its stored energy into emergency dump resistors connected directly across the terminals outside the magnet enclosure. Calculations indicate that upon actuation of the dumping system, induced currents in the aluminium substructure within the magnet will cause rapid heating and fast boil-off of the liquid helium within the winding. The helium coolant will leave the magnet via the emergency vent stack shown in Fig. 6. As a result, the entire winding is expected to transfer quickly to the normal state, and will absorb much of the stored magnetic energy internally with only moderate temperature rise. This is an advantage since the stored energy of the magnet as a whole will be absorbed quickly enough to prevent overheating and damage to local regions of conductor in which the transition to normal first started, while maximum voltage to ground during quench is expected to be less than 500 V.
Supportshell
"~'
Directionof coolant flow
~ Insulatingspacer strips Turn to turn insulation \, ,Conductor '~ Groove forcoolant
,\,
\
Insulatingspacer strips
d Coo!ing
passage
Fig, 5 Conceptual detail design for winding, circular saddle magnet, a-Typical conductor b--winding module c--detail from b d--detail showing cooling passages and insulating strips
Vacuum Jacket. Ring girder (typi 27 required Stiffner Girder (typ) ~ seat(typ.)
/
Vacuum - insulated line
Radiation shield AI. alloy
/
Alternate design
(I-beam) ring. girders. Vent ¢stack ~ Emergency ~- blow- out
disc
/
_
Multi - layer insulation
-Radiation ~ ,~Vacuum shield ~I Electrical and ~/ / '/ Fcryogenic feed ~ / / ~ . ' throughs
_
~ II ~ FF
~making Access openings for [I~!J~.~_L~,t power connections
T
Channel g i r d e r / / ~ End of c o i l s - J ~ Winding modules t-~ (structural shells 2.25 with windings installed) r
Coil container
Core tube
~--Bore tube Radiation shield .... l
4.84 m I dia
I0.1 m die Mounting --. foot
Mounting foot
Vacuum jacket
HII57 Fig. 6
Vacuum pump connection
.......
25m
Low heat leak support struts
Assembly layout of preliminary design 6 T base-load circular-saddle coil magnet
CRYOGENICS. FEBRUARY 1978
71
Conclusions and recommendations No technological obstacles were encountered in designing the series of magnets discussed here. However, this does not mean that base-load magnets can be designed and built immediately, with assurance of successful operation. The scale-up in size from today's magnets is very great; therefore further investigation and refinement of designs particularly from the manufacturing standpoint is essential. R & D work as mentioned later herein must be carried out to prepare for the successful design and construction of base-load magnets in the future. So far as could be determined by preliminary discussions with large equipment manufacturers, all base-load magnet components are manufacturable although for some com ponents, special manufacturing equipment not presently available must be designed and built. Also, equipment for winding the size and type of coils required is not available and will need to be developed and proven. A critical item is the matter of motion and friction heating within the winding during charging, and the relation of this phenomenon to coil stability. There is insufficient quantitative data available on this subject, where very large windings and possibly large motions are concerned. The preliminary designs discussed here used conservative stability criteria based on experience with the sizes of magnets existing today. However, these criteria cannot safely be extrapolated to very large windings without substantiation by experimental investigations taking into account the effect of mechanical motion. Items of R & D effort that appear appropriate for carrying out in the future relative to large MHD magnets include the following: 1 The design and practice of laboratory tests to determine the effect of motion and friction heating on the
72
stability of representative sections of winding of the type proposed for large MHD magnets. 2 The collection and testing of sample quantities of conductor of the type selected for base-load magnets. 3 The development and laboratory testing of joints (splices) of types suitable for use with the conductor planned for large base-load magnets. 4 Further analyses and experiments conducted to prove the feasibility of the protective scheme proposed herein, involving inductive heating of aluminium alloy winding forms as a means of rapidly driving the entire winding normal. 5 The early design, fabrication and laboratory testing of full scale or near-full scale critical components such as conductor and sections of winding, for the purpose of obtaining in advance manufacturing experience and performance data on components of the design and materials proposed for full scale MHD magnets. 6 The development of techniques for predicting at the design stage stresses and relative movements within windings and associated structure during cool-down and charging, taking into account friction at conductor, insulation and structure interfaces.
References 1 Jackson,W.D. Zygielbaum P.S. Open cycle MHD power generation, status and engineeringdevelopmentapproach, Proceedings of the AmericanPower Conference Vol 37 Illinois Institute of Technology TechnologyCenter, Chicago Illinois ( 1975) 2 Montgomery,D.B.,Williams,J.E.C. The Technology Basefor Large MHD SuperconductingMagnets Proceedingsof the 15th Symposium EngineeringAspects of Magnetohydrodynamics. The Universityof Pennsylvania(May 24-26 1976)
CRYOGENICS. FEBRUARY 1978
Design of superconducting magnets for full scale MHD generators A.M. Hatch, J. Zar and F.E. Becker
Fossil-fuel fired magnetohydrodynamic (MHD) electric power generating systems whose potential overall efficiencies greatly exceed those of present-day base-load central station power plants, are now undergoing intensive development with the objective of reaching the commercial-scale demonstration stage within a decade. ~ Very large superconducting magnets are an essential part of base-load MHD generators and must be considered among the critical components. In terms of stored energy, these magnets will be about two orders of magnitude larger than the largest superconducting MHD-type magnets existing today. While the basic materials and science needed for superconducting MHD magnets are currently available, more quantitative design information, additional engineering experience and further development in manufacturing technology are needed before very large MHD magnet construction can be undertaken with assurance of meeting performance reliability and economic objectives. 2
Conceptual design A set of rQtuirements including MHD channel size, desired field strength and related items were established at the outset to serve as a basis for all designs. The requiremets assumed open-cycle coal-fired base-load type generators. The channel sizes and field strengths specified for the primary base-load magnet design are shown in Fig. 1, together with a computer developed magnetic field prof'de for one of the actual magnet designs prepared. It was decided that 7 T and 5 T variations of the basic 6 T design should also be investigated. The conceptual design work included investiagtion of two variations of the saddle coil geometry, namely the circular-
p
This paper describes the results of a programme consisting of conceptual design studies and the preparation of preliminary designs and engineering data for full scale superconducting magnets for base-load MHD power generators. The major purposes in developing these designs were as follows
2.25 mdio (199m sq)
I
Worm
?bore
4.75 m di0 (4.2m sq)
1
1.35 m ~ ' ~ 16 m ~tive l e n g t h ~ | sq
•
(ST ~ - - ~ . ~ TOlerancet5%
1 To evaluate the state-of-the-art of large MHD magnet technology.
3.5T
2 To identify specific problems both technical and manufacturing that must be solved before full scale base-load magnets can be designed and built with assurance of meeting their requirements.
Inlet
Distancealongchannel
Exif
3 To develop recommendations for R & D programmes that should be undertaken before construction of such large magnets can begin. ~- 4
The project was divided into two phases, conceptual design and preliminary design. This paper will outline the work done in both phases and summarize the results. The authors are with the Avco Everett Research Laboratory Inc. Everett, Massachusetts USA. The work reported was supported by the United States Energy Research and Development Administration. It is a part of an MHD Magnet Development Programme managed for ERDA by the Francis Bitter National Magnet Laboratory Massachusetts Institute of Technology. Paper received 19 October 1977.
CRYOGENICS
. FEBRUARY
1978
no
._~ 2 ta-
O
b
-4
O 4 8 12 16 Distance alongchannel axis , m
20
Fig. 1 a--Design requirements, base-load scale MHD magnet. b--also shown is typical calculated curve of field vs distance along axis for preliminary design magnet (circular saddle BL6-p7)
67
~
B
o
~
e
d
~
o
t
e
cross-over
a
strengths. The designs prepared were conservative in approach, using mainly today's technology and emphasizing practical, economical construction. All were based on the use of copper-stabilized NbTi conductor operating at relatively low current densities ( < 3750 A cm -2) with windings cJesigned for full cryostatic stability. Fig. 3 shows plan views and end views of conceptual designs of circularsaddle and rectangular-saddle 6 T base-load size magnets. Fig. 4 shows cross-sections of the alternative designs at the plane of the channel inlet, indicating the arrangement of structure and windings. Both designs have modular (subdivided) windings, with coils held in substructures which are stacked to form the complete winding. In both designs, the substructure and the superstructure (major structural members such as the girders) are of aluminium alloy. This material was chosen because its physical properties are suitable for the service, it is considered less expensive than stainless steel for structures of the type shown and also because the relative coefficients of contraction of the aluminium alloy structure and of the conductor are such that the winding will be in compression after cool-down, a desirable feature.
b Fig. 2 Alternate winding geometries, a--circular saddle coil, tapered, with intermediate cross-over b--rectangular saddO coil, tape red
saddle and the rectangular saddle. Fig. 2 shows these two geometries. In the circular-saddle design, the required sloping field profile was achieved by both divergence of the winding from inlet to exit end and the use of intermediate crossovers. In the rectangular-saddle design, the sloping field profde was achieved by divergence of the winding and also fanning out of the conductor layers toward the exit end so that intermediate crossovers were not needed. A series of conceptual layouts and engineering calculations were made for base-load size magnets of the two alternative geometries mentioned and for magnets with various field
Endturn
crn..o.
Winding
"""7
Girders Tension (above plot., and below] /
/
?
Radiation LiauldHe shield / c°htu'ner
In the circular-saddle design, the winding modules are semicircular in cross-section and are stacked around a circular cross-section core tube. Circular ring girders of I-beam section surround the winding and support the modules against the outward acting magnetic forces. The vacuum jacket (dewar outer wall) is circular in cross-section. In the rectangular-saddle design, the winding modules are fiat plates turned out at the ends. They are stacked around a rectangular aperture and are held in place against outward magnetic forces by solid rectangular cross-section girders or beams on the top and bottom held together by tension plates connecting them on each side of the channel aperture. The vacuum jacket is rectangular. Table I lists engineering data for the various conceptual designs prepared, including ampere turns, major dimen-
End turn cross-over
Ringgirders (I-beamtype}?
,\
,,
/\ \
i
9'~m
/ \
A
~
Radiation LiquidHe _ shield ~ container
Intermediate cross-overs ~ \
': "
'' "
//
-
1
|125rndia
12am
1.2m typ
I~
2r~sm
-I
Planview
Vacuum jacket
J"
24.5m Planview
--
Winding.m o d u l e s ~ _ _ _
10.7m
End view
Rectangularsaddle Fig. 3
68
Ringgirders
Endview (Inletend -partialsect A-A ) Circularsadd!e
Plan views and end views of conceptual design 6T base-load magnets, circular-saddle type and rectangular saddle type
CRYOGENICS.
FEBRUARY
1978
Mechanical girder nt (typ) Outer shell Winding modules (half shells) Winding regions Core tube ;irder De~oilof mechanical girder joint (rect. saddle)
Girder (solid plate)
f Helium container
,'nsionplates ding modules plates tyrned at ends j lding regions
Core tube Vacuum Outer vOCuum walt
! Rectangular saddle
Fig. 4 Comparison o f circular and rectangular-saddle designs, cross-sections at plane of channel inlet. Arrows F m show direction of principal magnetic forces exerted on girders
sions, estimated weights and stored energy. It is of interest that the rectangular-saddle 6 T design uses more conductor and is considerably heavier than the circular-saddle 6 T design. However the rectangular design is advantageous in that cooling passages are better oriented for venting and component manufacture may be easier due to the use of primarily fiat plate components instead of the formed (conical) members characteristic of the circular-saddle design.
Preliminary design The alternate 6 T base-load designs, circular and rectangular were upgraded during the preliminary design phase and more
careful design calculations were made. Particular attention was given to the design of mechanical joints between the major components, so that sub-assemblies requiring extensive welding could be pre-fabricated at manufacturing facilities and then transported to the plant site where they would be assembled into the complete magnet with a minimum of on-site fabrication and welding. Investigation of the manufacturing aspects of major components for both designs indicated that both were feasible from the manufacturing standpoint with perhaps some prefabrication advantage for rectangular-saddle components since they did not require as much welding and forming. However when on-site assembly and mechanical joining aspects were investigated, it was found that the rectangularsaddle design involved more difficult and complicated joining arrangements and it was therefore decided that the circular-saddle design was preferable because its substantially lower material cost and less difficult assembly more than offset the slightly easier pre-fabrication characteristic of the rectangular design. Work on the circular-saddle design was therefore carried forward in more detail than that of the rectangular design. Significant engineering data and features of the finalized 6 T circular base-load design will now be summarized. Table 2 lists the characteristics of the design including mechanical, cryogenic and electrical data. Fig. 5 shows the type of conductor proposed and the winding configuration, including insulation and cooling passages within the winding module. Fig. 6 shows an assembly layout of the 6 T base-load circular-design. The liquid helium container directly surrounds the winding modules and its outer wall is at the inside surface of the ring girders. Thus the ring girders are in the vacuum space and the amount of liquid helium holdup is kept to a minimum. A vacuum jacket of circular cross-section encloses the assembly. Lowheat-leak support posts of titanium alloy, designed to carry the weight of the magnet plus the magnetic force interaction which may exist between the magnet and adjacent steel equipment are provided between the cold mass and the vacuum jacket, together with struts of the same material
Table 1. Engineering data for MHD magnet conceptual designs Nominal field, inlet
T
6
6
5
7
Type of saddle
-
Circular
Rectangular
Circular
Circular
Winding build
m
0.9
0.9
0.7
1.4
Average current density (overall winding)
A cm -2
1270
1270
1300
1000
Ampere turns (front)
106 NI
34.7
39.0
27.4
47.0
Stored energy
10eJ
5500
7600
3500
10700
Maximum field in winding
T
8.1
8.1
7.2
8.1
Major dimensions* Length
m
24.5
26.5
24.5
27.4
m
12.5 dia.
12.7x 10.7
11.2 dia.
14.8 dia.
Weight of conductor
103 kg
410
700
322
751
Total weight of magnet
103 kg
2380
2800
1400
5000
Exit end
*--outside vacuum jacket
CRYOGENICS. FEBRUARY 1978
69
Table 2. Design characteristics o f 6 T base-load M H D magnet (Preliminary design - Circular saddle type)
Dimensions and Magnetic Fields Active length Magnetic field on axis: Start of active length End of active length (from calculated field profile, Fig. 1) Field variation in channel cross-section Overall length outside vacuum jacket Warm bore diameter: Inlet end of vacuum jacket End of active length Distance from inlet end of vacuum jacket to start of active length Outside diameter of vacuum jacket*
m
16.0
T T
6.0 3.4
m
-+ 3% 25.0
m m
2.25 4.8
m m
4.15 12.5
103 kg 103 kg 103 kg 103 kg
460 1960 820 230
Weights Conductor Major structure (rectangular girder design) Sub-structure, insulation, etc. Vacuum jacket and radiation shield
3470
Total
Winding Details (grading winding, 3 regions) Region Maximum field in region Current density (overall) in region Current density in conductor Conductor width Conductor thickness Ratio, copper to superconductor
-
A
B
C
T A cm-2 A cm :'2 cm cm -
8.1 1180 3030 3.48 1.43 15
7.2 1270 3270 3.23 1.43 22.5
5.5 1400 3610 2.92 1.43 32
Winding Overall Design current in conductor Ampere turns, inlet end Total length of conductor Inductance Stored energy Packing factor (average) Cooling, passage width Percent cooled surface, conductor Heat flux, all current in copper Ratio helium to conductor volumes Total liquid helium in winding
A
14,500
-
37 x 106
m H mj cm
litres
12.6 x 104 57 6100 0.37 0.34 39% 0.4 0.4 23,000
Mn m -2 Mn m -2 Mn m -2
83 (12,000 psi) 179 (26,000 psi) 83 (12,000 psi)
-
w cm -2 -
Design Stresses (maximum) Conductor Major structure Electrical insulation (compression) *-not including external stiffeners
70
CRYOGENICS. FEBRUARY 1978
to carry transverse and longitudinal loads. A thermal radiation shield operating at 80 K with helium gas tracer tubes is interposed between the cold winding and structure and the room-temperature vacuum jacket walls.
1.57 mm die plain copper wire (typ) Solder
Copper core
953mm
•
:C
254mm
-
The more detailed attention in Phase 2 to structure fabrication techniques and material properties resulted in a considerable increase in total weight over that estimated for the conceptual design. The largest increase was due to a change from I-beam cross-section to solid cross-section ring girders, found necessary because the I-beam girder required extensive welding in manufacture, causing mechanical properties of the final weldment to fall below those required for this highly stressed component.
' '
1.57 Into dia superconducting wire (typ). Copper to superconductor ratio, 2:1
~ i Gravity
Windingsupportshell AI. alloy
The net continuous refrigeration requirement of the magnet is estimated to be 330 W at liquid helium temperature and 2000 W at 80 K.
Groovefor coolant Winding
b
Protection in the event of inadvertant initiation of quench such as may occur due to low liquid helium level or loss of vacuum is accomplished by the opening of circuit breakers that will disconnect the power supply and cause the magnet to start dumping its stored energy into emergency dump resistors connected directly across the terminals outside the magnet enclosure. Calculations indicate that upon actuation of the dumping system, induced currents in the aluminium substructure within the magnet will cause rapid heating and fast boil-off of the liquid helium within the winding. The helium coolant will leave the magnet via the emergency vent stack shown in Fig. 6. As a result, the entire winding is expected to transfer quickly to the normal state, and will absorb much of the stored magnetic energy internally with only moderate temperature rise. This is an advantage since the stored energy of the magnet as a whole will be absorbed quickly enough to prevent overheating and damage to local regions of conductor in which the transition to normal first started, while maximum voltage to ground during quench is expected to be less than 500 V.
Supportshell
"~'
Directionof coolant flow
~ Insulatingspacer strips Turn to turn insulation \, ,Conductor '~ Groove forcoolant
,\,
\
Insulatingspacer strips
d Coo!ing
passage
Fig, 5 Conceptual detail design for winding, circular saddle magnet, a-Typical conductor b--winding module c--detail from b d--detail showing cooling passages and insulating strips
Vacuum Jacket. Ring girder (typi 27 required Stiffner Girder (typ) ~ seat(typ.)
/
Vacuum - insulated line
Radiation shield AI. alloy
/
Alternate design
(I-beam) ring. girders. Vent ¢stack ~ Emergency ~- blow- out
disc
/
_
Multi - layer insulation
-Radiation ~ ,~Vacuum shield ~I Electrical and ~/ / '/ Fcryogenic feed ~ / / ~ . ' throughs
_
~ II ~ FF
~making Access openings for [I~!J~.~_L~,t power connections
T
Channel g i r d e r / / ~ End of c o i l s - J ~ Winding modules t-~ (structural shells 2.25 with windings installed) r
Coil container
Core tube
~--Bore tube Radiation shield .... l
4.84 m I dia
I0.1 m die Mounting --. foot
Mounting foot
Vacuum jacket
HII57 Fig. 6
Vacuum pump connection
.......
25m
Low heat leak support struts
Assembly layout of preliminary design 6 T base-load circular-saddle coil magnet
CRYOGENICS. FEBRUARY 1978
71
Conclusions and recommendations No technological obstacles were encountered in designing the series of magnets discussed here. However, this does not mean that base-load magnets can be designed and built immediately, with assurance of successful operation. The scale-up in size from today's magnets is very great; therefore further investigation and refinement of designs particularly from the manufacturing standpoint is essential. R & D work as mentioned later herein must be carried out to prepare for the successful design and construction of base-load magnets in the future. So far as could be determined by preliminary discussions with large equipment manufacturers, all base-load magnet components are manufacturable although for some com ponents, special manufacturing equipment not presently available must be designed and built. Also, equipment for winding the size and type of coils required is not available and will need to be developed and proven. A critical item is the matter of motion and friction heating within the winding during charging, and the relation of this phenomenon to coil stability. There is insufficient quantitative data available on this subject, where very large windings and possibly large motions are concerned. The preliminary designs discussed here used conservative stability criteria based on experience with the sizes of magnets existing today. However, these criteria cannot safely be extrapolated to very large windings without substantiation by experimental investigations taking into account the effect of mechanical motion. Items of R & D effort that appear appropriate for carrying out in the future relative to large MHD magnets include the following: 1 The design and practice of laboratory tests to determine the effect of motion and friction heating on the
72
stability of representative sections of winding of the type proposed for large MHD magnets. 2 The collection and testing of sample quantities of conductor of the type selected for base-load magnets. 3 The development and laboratory testing of joints (splices) of types suitable for use with the conductor planned for large base-load magnets. 4 Further analyses and experiments conducted to prove the feasibility of the protective scheme proposed herein, involving inductive heating of aluminium alloy winding forms as a means of rapidly driving the entire winding normal. 5 The early design, fabrication and laboratory testing of full scale or near-full scale critical components such as conductor and sections of winding, for the purpose of obtaining in advance manufacturing experience and performance data on components of the design and materials proposed for full scale MHD magnets. 6 The development of techniques for predicting at the design stage stresses and relative movements within windings and associated structure during cool-down and charging, taking into account friction at conductor, insulation and structure interfaces.
References 1 Jackson,W.D. Zygielbaum P.S. Open cycle MHD power generation, status and engineeringdevelopmentapproach, Proceedings of the AmericanPower Conference Vol 37 Illinois Institute of Technology TechnologyCenter, Chicago Illinois ( 1975) 2 Montgomery,D.B.,Williams,J.E.C. The Technology Basefor Large MHD SuperconductingMagnets Proceedingsof the 15th Symposium EngineeringAspects of Magnetohydrodynamics. The Universityof Pennsylvania(May 24-26 1976)
CRYOGENICS. FEBRUARY 1978