Mirror advanced reactor superconducting magnet set design

Nuclear Engineering and Design/Fusion 3 (1986) 151-172 North-Holland, Amsterdam

151

MIRROR ADVANCED REACTOR SUPERCONDUCTING

MAGNET SET DESIGN

J e r o m e F. P A R M E R , R o b e r t W. B A L D I , K e n L. A G A R W A L , R i c h a r d A. S U T T O N a n d Mark W. LIGGETT General Dynamics Space Systems Division, P.O. Box 85377, San Diego, CA 92138, USA

Received 30 April 1985

Major magnetic field requirements of MARS are derived from basic physics and reactor design consideration. A superconducting magnet set has been conceptually designed for a tandem mirror fusion power reactor. The work, sponsored by the Lawrence Livermore National Laboratory, was a part of their Mirror Advanced Reactor Study (MARS).

1. General Requirements Major magnetic field requirements of MARS are derived from basic physics and reactor design consideration. A superconducting magnet set has been conceptually designed for a tandem mirror fusion power reactor. The work, sponsored by the Lawrence Livermore National Laboratory, was a part of their Mirror Advanced Reactor Study (MARS). Plasma fusion power output and wall loading limitations determine major parameters of the central cell, including its dimension, the axial field of 4.7 T, and a maximum field ripple of 6%. The inner radius (2.1 m) of the central cell magnet encloses the blanket and shield system protecting the magnets for 24 full-power years (FPY) while minimizing total capital plus operating cost of the magnets and their cryogenic system. 1.1. Magnet set characteristics

The central cell magnet spacing allows removal of blanket modules through service stations located between magnet pairs. Magnet width and clearance are such that the on-axis tipple is less than the maximum allowable. The peak field of 7.2 T occurring at the inner radius of the windings is a result of central field magnet dimensions and winding grades. Central cell coils use state-of-the-art NbTi with pool boiling helium technology. The choke coils' 0.6 m inner bore and 24 T field are selected for cost and power optimization. In particular, the 24-T peak field is chosen to be as high as possible, requiting only a modest, near-term development of coil

technology. Choke coil design is based on Nb3Sn, He II superconducting solenoid technology and on MZC copper, radiation-resistant, normal resistive coils. Magnetic configuration of transition, anchor, plug, and recircularizer coils magnetic field values and magnetic field curvatures to enhance plasma equilibrium; reduce radial transport; and be compatible with M H D stability to achieve a high /3 ha the central cell. High field levels (fig. 1) ha end cells require superconductor technology somewhat more advanced than used ha MFTF-B to achieve the 10-T peak fields in end cell magnets with ductile superconductor. NbTi superconductor is selected with LHe II coolant at 1.8 K. Forces due to the intense fields are supported by thick coil casing and strongback support structures integrated to allow space for radiation shields, neutral beam penetrations, and scheduled maintenance. Because of stringent space requirements imposed on end cell coils by the magnetic configuration, coils are enclosed with the plasma and the superconducting magnets ha a common vacuum. 1.2. Operative environments

Superconducting and resistive magnets are subjected to large stresses, neutron and gamma-ray irradiation with broad energy spectra, high magnetic fields, and thermal cycling from 1.8 to 300 K. Power required for refrigeration necessary to remove nuclear heating must be less than some small fraction of total power produced by the reactor. Also, performance degradation by radiation should be sufficiently small to ensure operathag margins over the life of the reactor.

0 1 6 7 - 8 9 9 x / 8 6 / $ 0 3 . 5 0 © Elsevier Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)

152

J.F. Partner et al. / M A R S magnets

1.3. Magnet materials

Specifications for materials used are based on Section VIII of the A S M E Boiler and Pressure Vessel Code. Table 1 lists major component materials. These materials are compatible with the operating environment but, in case of spinel and M Z C copper alloy, effects of long-term neutron fluence are unknown. 1.3.1. Properties and use limitation o f materials Of all magnet materials, 304L, 304LN, and 316L are least affected by the operating environment. Most affected are the M Z C copper alloy conductor and spinel

(MgAI204) insulator. Both are used in the choke resistive insert. These components face an unattenuated plasma neutron source. Superconducting magnets are shielded and are designed to last the life of the reactor. The most radiation-sensitive part of the superconducting magnet is the organic electrical insulator. N b T i and Nb3Sn are also affected by radiation. Some important physical, mechanical, and radiation-related properties of these materials are described below. Copper Alloy Conductor - A m a x - M Z C alloy is an oxygen-free copper containing controlled quantities of magnesium (0.04%), zirconium (0.15%), and chromium

Table 1 Materials selection list for magnet components Magnet 1. Center-cell

Part

Material

Condition

304LN 304L Fiberglass-polyimide Tissuglass a Aluminum 316L NbTi OFHC copper Fiberglass-polyimide

Annealed Annealed Cured

(i) Structure (ii) Insulation (iii) Conductor

304L Spinel (MgA1204) MZC copper alloy

(iv) Coolant (i) Magnet case (ii) Vacuum vessel (iii) Radiation shield (iv) Superconductor Grade I Grade II Grade III (v) Stabilizer

Deionized water 304LN 304L 316L

Annealed Coldworked & aged Annealed Annealed Annealed

(i) Magnet case (ii) Vacuum vessel (iii) Magnet support (iv) External insulation (v) (vi) (vii) (viii)

2. Choke-coll (A) Resistive

(B) Superconducting

Radiation shield Superconductor Stabilizer Electric insulator

(vi) Insulation (vii) Magnet supports (viii) External insulation 3. C-shape coils

a

(i) Magnet case (ii) Radiation shield (iii) Superconductor (iv) Stabilizer (v) Insulation (vi) Magnet supports

Trade name for felted quartz fiber.

Annealed ½ Hard Cured

Nb3Sn(Ti ) Nb3Sn or (Ti) NbTi Composite of OFHC Cu reinforced with Nitronic 40 Fiberglass-polyimide Same as for center cell Same as for center cell

Optimized Optimized

304LN 316L NbTi OFHC Cu Fiberglass-polyimide Same as for center cell magnets

Annealed Annealed

Cured

½ Hard Cured

153

J.F. Partner et al. / M A R S magnets

o

-

2°ri

°

A 20Ii

15--

<

10 ! -

8

.°

5 0

[

60

!

I

65

I

70

I

75

I

I

80

I

85

90

95

100

I

105

110

Axial distance (m)

Fig. 1. Axial magnetic field of MARS versus axial distance.

(0.80%). It is a precipitation-hardenable material that derives strength by cold work followed by aging. Me-hanical yield strengths up to 620 MPa (90 ksi) and ~iectrical conductivity up to 80% of International Annealed Copper Standard (IACS) can be developed. MZC retains 83% of its room temperature tensile, and yield strengths at 200°C. Its electrical resistivity increases, following neutron irradiation, due to two mechanisms, production of: (1) defects, and (2) transmutation products (Ni, Zn, and Co). Impurity-induced resistivity increases linearly with neutron fluence. At lower neutron fluences, most induced defects anneal out at 300 K, leaving a residual resistivity of 0.08 #12cm. Electrical resistivity changes due to transmutation products can be estimated using specific change values for transmutation products. The neutron irradiation effects on mechanical properties is not known. Spinel (MgAI204), Ceramic Insulation - Most sintered ceramics have high compression strength. The rationale behind selection of spinel (MgAI204) lies in its: (1) low swelling property, (2) freedom from micro-cracking, and

(3) increase in strength following neutron irradiation. Single crystals of spinel have no swelling when irradiated to a neutron fluence of 2.1 × 2022 n / c m 2 ( E , > 0.1 MeV). Sintered polycrystals of spinel undergo a 0.8% density change under similar irradiation (c.f. alumina > 4%). OFHC Copper Stabilizer - Oxygen-free, high-conductivity copper comes in two grades, CDA 10100 (with 99.99% Cu minimum) and CDA 10200 (with 99.95% Cu + Ag minimum). The OFHC copper is used in annealed as well as in work-hardened conditions produced by cold drawing. Fibeqglass-Reinforced Polyimides - They are an order of mangitude more radiation resistant than epoxies. Radiation dose varies from 1.4 x 109 rad in the centercell magnets to about 2 x 1030 rad in the anchor yinyang magnets. Fiberglass-reinforced polymides have been irradiated at 5 K to a gamma dose of 1 x 101° rad and tested at 77 K after room temperature warm-up. Polymides retain 65% of unirradiated flexural strength [1]. In a recent experiment [2] thin discs of fiberglass-reinforced polyimide (Spaulrad-S) were irradiated to 4 x

154

J.F. Parmer et al. / M A R S magnets 1.1-//

I

1.0

'

'

' ' ''"I

,

,

,

, , ,

2.0

0.8

0.90.~5 ~

11.8.0 1.4

~Nb3Sn\

1.2

--..,..~,.,.. NbTi~

o.,-

Central field Maximum axial field ripple Winding inner radius Clearance between coils Center-to-center spacing of coils Individual vacuum vessels Compatibility of coil supports with blanket removal scheme and blanket coolant piping Design conformity to ASME Pressure Vessel Code Design life

1.6

\

0.7

8 0.6

Table 2 Design requirements of MARS central cell

2.2

. . . . . . . . . .

g

--

0.3

0.6

0.1

0.2

0:-//

I

,

,

,

, tJ,t[

,

,

,~N

Fig. 2. Reduced critical temperature and reduced critical current as a function of neutron fluence. 1011 rad at 325 K and, when tested in compression, did not fail up to 2753 MPa (399 ksi) loads. Superconductor NbTi and Nb3Sn - NbTi is an alloy having good radiation resistance. The critical parame1.4 ~

I

I

I

24FPY

ters of NbTi are not affected by gamma-ray irradiation. There is a slight decrease ( < 5%) in the critical temperature, To, of NbTi when irradiated to a fast neutron fluence of 4 × 1019 n//cm2. The critical current density of the optimized NbTi superconductor at first decreases; then, at high neutron fluences, levels off at 80% of the unirradiated value. Isochronal annealing at 300 K of the irradiated specimens produces a 70% recovery of Jc (critical current density) in NbTi. Nb3Sn in an ordered intermetaUic compound. Both neutrons and gamma rays affect the ordered structure

0 1020

, ,,,

1018 1019 Neutron fluence (n/cm2, E > Q.1 Mev)

4.7 T 5.5% 2.1 m 1.85 m 3.16 m

I

1.2

1

E

E

1.0-

.=" 10-1

0.6 -

=

\

0.0_25

.=_

10-2

T^.o* . . . .

C

.~ QI. Q "0

0.4-

l

~

"

10-3 0.2 -

1,8

"I-

Design radius

I

[

1.9

2.0

2.1

]

I

2.2

2.3

Magnet Inner Radius (m)

Fig. 3. Life cycle cost of central cell magnets - Radiationshield plus cryogenic system of MARS versus inner radius of the central cell magnets.

155

J.F. Parmer et a L / M A R S magnets

and there is a monotonous decrease of T~ with increasing radiation dose. The critical current of NbaSn first increases, reaching a maximum around 4 × 10 ]8 n / c m 2 fluence, and then drops sharply. The radiation behavior of both superconductors is shown in fig. 2.

1: Central cell magnets

The central cell of M A R S incorporates 42 superconducting solenoids. The magnets employ NbTi conductors stabilized with 1 / 2 - h a r d copper to operate at a peak of 7.2 T. The unconditionally cryostable conductor is cooled with LHe I at 4.5 K. The central cell magnets

ipor-cooled lead (2) rvice stack Helium vessel inner shell (5.0 thk)

r,,~-

I I I .

I !

Clearance line for blanket removal F Removable cover / for lifting lug (2)

LN 2 radiation shield 53 ° 2: ol

r

~

t

185 \

,'

~.

/ \

- Helium vessel outer shell (5.0 thk) note 6

/.

/

&, S \

.

45

o Vacuum vessel outer shell (2.5 thk) note 5 reinforcement (7.5 thk) 4 pl.

Lateral support note 3 ~-Vertical support (2 pl) note 2

Notes: 1. All dimensions in centimeters. 2. Material - Polyimide fiberglass. 3. Material - Polyimide fiberglass & 304 L. 4. Minimum clearance between adjacent coils for blanket installation/removal.

Fig. 4. Central cell.

A_~

Roller supportdetails not shown

5. Material -- 304 L.

6. Material - 304 LN. 7. Stabilizer - OFHC, 1/2 HD, s/c NbTi. 8. Coil cooled by LHe I.

J.F. Partner et aL / MARS magnets

156

are 55% of the magnet weight of MARS. The peak field of 7.2 T allows use of conventional superconducting magnet technology, thereby contributing to the magnet reliability required to operate 24 full-power years. Radiation protection of the magnets is achieved by locating the power blanket and shield between the plasma and the magnet inner radius. Radiation-resistant polyimide insulation is used.

2.1. Design requirements Table 2 summarizes the magnet requirements. The inner radius of the winding is chosen to minimize the total life cycle cost of the magnet, radiation shield and cryogenic system. Neutronic and economic data were used to perform the study. Fig. 3 shows results. Smaller radii lead to a thinner radiation shield, thereby increasing total cost because of larger cryogenic system needed to remove heat deposited by neutron and gamma radiation in the windings. At larger radii, the magnets and radiation shield cost more. However, this cost increase is offset by the cost of refrigerators. The selected inner radius is 2.1 m.

2.2. Baseline design The baseline design is shown in fig. 4. As shown in the figure, a coil consists of a winding pack, insulation a coil case, an LN2-cooled thermal radiation shield, ~. vacuum vessel, and a service stack. The two coils at each end of the central cell have a reduced current density to partially offset buildup of central field from the highfield choke coil. The winding shown has an inner radius of 2.10 m. The width of the winding pack (0.774 m) is derived as a result of the minimum clear space needed between coils to remove blanket modules. The central field generated by a single coil is 2.74 T. The field of all coils combined is 4.7 T with a ripple of 5.5%. End life dpa in the Cu stabilizer is 9 × 10 -5 and conductor stability margin (inner layer) is 1.58, where the margin is defined as the ratio of the maximum allowable to the actual stabilizer heat flux. Table 3 Choke coil design requirements

Total field

3. Choke coil

Located at Z = q: 67.85 m, between the central cell and transition coil, are the choke coils. Fig. 5 shows general arrangement of the coil and emphasizes its two independent coils, a salient feature. This hybrid arrangement is required by lack of technology for a realistic 24-T superconductor material. Therefore, a resistive insert coil is used in conjunction with a superconducting coil to create a 24 T on-axis field. The choke coil is the most advanced design challenge of all magnets in the MARS reactor, and accordingly, more effort has been devoted to defining a workable design than for the other coils. The outer solenoid employs NbaSn superconductor doped with titanium. The conductor is cooled with LHe designed to operate for 3.2 FPY with a neutron wall loading of 2 M W / m 2 at the inner bore wall.

3.1. Design requirements Table 3 summarizes the design requirements of the choke coil. Peak field at the conductor of the superconducting solenoid was limited to 16.1 T so that Nb3Sn(Ti)-LHe 11-1.8 K technology is practical. The resistive insert coil incorporates design features such as internal conductor cooling and uses radiationresistant insulation to extend to lifetime of the coil in the radiation environment. Operational life is limited to 3.2 FPY by insulation swelling of 3% by volume. The resistive coil is replaced during power blanket scheduled maintenance.

3.2. Superconducting background coil

Axial Field Components Field due to central cell & end cell coils Field due to the superconducting coil Field due to the resistive insert

The coil is divided into three graded sections with current densities choosen to keep the conductor combined stresses below 193 MPa. 1/2-hard copper was chosen as the stabilizer. Grading is accomplished by reducing the conductor size keeping the current constant. To do this, the conductors are layer wound and connected in series. Cryostability takes into account 24 FPY of radiation damage in the stabilizer. This indicates no magnet annealing is required for the full life of the reactor.

3.2.1. Choice of cooling 1.3 Tesla 13.4 Tesla 9.3 Tesla 24.0 Tesla

Initially, both LHe I and LHe II were considered. LHe I because of proven cryogenic technology and LHe II because it allows operation with higher current densities. Preliminary estimates of maximum design heat

J.F. Parmer et al. / MARS magnets End cell vacuum vessel and bulkhead structure

Central cell Solenoid-

Chokeooi,

,

157

60cm square t0nsion column pn

n

Vacuum 7

Intercoil struts rotated 53 ° from

vessel

'

winding

nding [

pack-~ [ ~

drive mechani:

Nuclear radiation shield Reflects.

Background coil winding pack

Vacuum seal r~

Choke coil interface sl

Thermal ~

/radiation

pack

L___

Ill

Detail A

"

-Polyimide compression column rotated 53 ° from true location I •Resistive / insert coil ]

V:tuum gate /

Engaged vacuum seal.

Central cell

Transition c o i l / / ~ T r a n s i t i o n winding pack / coil

Fig. 5. Choke and adjacent coils.

fluxes were 0.25 W / c m 2 for LHe I and 2.0 W / c m 2 for LHe II. Because maximum cryostable current density is proportional to the square root of heat flux, LHe II allows a current density 2.8 times higher than that of LHe I. However, in later estimates, LHe II heat flux wa.s reduced to 1.0 W / c m 2. There the current density using LHe II cooling can be twice the value over that obtained using LHe I cooling. The greater current density results in a smaller, less expensive coil. This benefit outweighs the cryogenic plant capital and operating cost penalties of LHe II. 3.2.2. Choice of conductor Two types of conductors were considered for coil winding. Initially, the conductor was stabilized with half-hard copper and reinforced with 304-LN stainless steel. NbTi was used in the two outer regions of graded winding and Nb3Sn and NbaSn doped with titanium were used in the inner and innermost regions of the coil. Maximum stresses were 193 MPa in the copper and 445

MPa in the steel. Maximum elongation was 0.2%. Because of large coil mass and large cost of the first approach, a second conductor type was considered to reduce total coil mass. The conductor employs Nitronic 40 steel-reinforced soft copper for the stabilizer. Maximum elongation is limited to 0.355 and stresses are 90 MPa in the copper and 676 MPa in the steel. This approach led to substantial reduction of total coil mass (about a factor of two) and was adopted for baseline design. 3.2.3. Background coil baseline design The general arrangement of the background coil is given in fig. 6. In the design of the conductors for this coil, use is made of a technology [4] which utilizes high-strength steel reinforced conductors. It is assumed at the outset that the coolant would have to be liquid Helium II at 1.8 K. The peak neutron fluence of 1.1 x 1018n/cm2 is found to have no adverse affect on the superconductor, which for grade I was selected to be

158

J.F. Partner et al. / M A R S magnets

the TASKA study [3], (3) internally cooled uniform current-density coils, and (4) externally cooled uniform-current-density coils. Our initial choice was the externally cooled, urliform-current-density coil because of its high achievable current density that results in a much smaller encircling superconducting coil. The modified Kelvin design, while using less power, requires much larger diameters and axial space.

Nb3Sn:Ti (Niobium Tin doped with Titanium). The peak field of the inner radius of the superconducting coil increases from 15.8 T to 16.1 T when the resistive insert in deenergized. 17 T is used as the maximum design value so one can operate the superconducting coil whether or not the resistive insert is energized. Winding - The inner bore radius of the coil is chosen to be as small as possible and yet leave sufficient room for the insert coil. The selected radius is 1.40 m. Fig. 6 shows details of conductors for the three graded sections. Because of the rapidly diminishing field with increasing radius, the superconductor material varies with each grade. For grade I, the superconductor is Nb3Sn:Ti; for grade II, NbaSn is used; and for grade III, NbTi can be used. In all cases, the copper stabilizer is OFHC.

3.3.2. Resistive insert coil baseline design

The resistive insert coil is shown on fig. 7. The coil is internally water cooled and consumes approximately 40 MW of ohmic power plus 0.4 MW pumping power. The baseline design has evolved through efforts to minimize both ohmic heating and pumping power, maintain reliability (3 FPY life goal) and keep the size small. The coil design is built around an internally cooled conductor. The conductors form a double pancake that is the basic unit of the coil; the number of double pancakes determines the length of the coil. Each double pancake is machined from a plate of heat-treated and cold worked MZC copper having a working stress

3.3. Resistive insert coil 3.3.1. Coil alternatives

Several coil types were considered: (1) Bitter coils, (2) modified Kelvin distribution coils as exemplified by

Vapc Sen

ield

I thk)

I I- -

I0 thk) 3 Lat~ sup~ Not~

I

Z-67.85

Fig. 6. Background coil.

-

J.F. Parmer et al. / MARS magnets

of 331 MPa. Three parallel conductors are machined spirally down each face of the plate (conductor height is varied to keep stress uniform) and the plate is machined with a narrow cut that divides it equally but leaves the inner conductors joined. The machining approach eliminates brazing which would degrade the strength of the copper. The conductors are also grooved to accept a thin-wall, soldered-in-place copper tube covered with a spiral plate that matches the conductor. The copper tube carries cooling water and, at the inner radius, passed through the first layer to the second layer. For each double pancake, the water and the electrical current travel in parallel along the conductors. Starting at the outside of the first layer, the water/current spiral inward to the inner layer, transfer axially to the second layer and spiral, in the same sense as the first layer, radially outward. The cooling water makes one pass through a double pancake and is carded away in a manifold. The electrical current transfers from the outside conductor of the second layer to the outside conductor of the first layer of the adjacent double pancake and so passes in series through the coil. The hoop forces in the conductors are reacted within each double pancake. At the inner layer, continuity between the two pancakes is ensured since they are machined from the same plate. At the outer layer, insulated shear pins connect the two pancakes. In fact, the shear pins are carried through all the double pancake and become tie bolts used to hold the magnet assembly together. The coil consists of 17 double pancakes.

159

uration. The radiation transport problem is modeled as an infinite cylinder. Discrete ordinates calculations are carried out to investigate the various responses of interest. In the normal coils, there is concern with both electrical and mechanical degradation of the ceramic insulation and the electrical resistivity of the copper conductor, primarily due to transmutations. Problems of concern in the S / C magnet are the dose to the insulators (after the 24 FPY design lifetime of the reactor), heating in the S / C magnet, and atomic displacement (dpa) rate in the copper stabilizer of the conductor. 4. End cell coils

End cell coils provide proper magnetic configuration for MHD stabilization and electron plugging of open ends of the mirror magnetic geometry. The coil system is made of a number of C-shaped coils and one solenoid. These coils include the transition coil, two yin-yang pairs (i.e., anchor and plug coils), and the recircularizer coil set (which includes the C coil and the solenoid). 4.1. Design requirements

The most stringent requirements for the magnet system are severe space constraints imposed by the magnetic configuration and the high peak field at the windings. 4.2. Design alternatives

3.3.3. Insulation The inner bore of the resistive inset is not shielded from radiation from the plasma. It was realized that any attempt to shield the insert would increase not only its size, cost and power consumption, but the size and cost of .the background coil as well. The material selected for insulation in this coil is spinel (AI2MgO4). Although the baseline design will accept more swelling in the insulation, there are so many unknowns concerning a high-field resistive magnet life that we are reluctant to predict a life greater than the 3.2 FPY. (This happens to approximate the blanket lifetime which calls for reactor shutdown to replace blanket modules, so the resistive insert could be replaced when the adjacent blanket module is replaced.)

Initially, the end cell of MARS consisted of transition and anchor transformation coils. The transition coil provided a double ellipse transformation of the flux bundle from the choke coil into the yin-yang anchor. As confinement physics evolved, the magnet configuration was changed to that of the present baseline design. The modification resulted in more stringent space requirements for the windings and higher peak fields at the conductor. The peak field was increased to 10.7 Tesla and coil technology was changed to N b T i - L H e 11-1.8 K. This approach has the advantage of utilizing the high heat transfer capability of LHe II. Therefore, the conductor design, originally of the type used in MFTF-B, was replaced with a simpler and less expensive conductor of rectangular cross section.

3. 4. Choke coil: radiation shielding

4.3. Baseline design

The choke coil region present difficult shield problems to designers. Fig. 8 illustrates the geometric config-

The C-shaped end cell coils share much in common. The mean minor radius of each is 1.10 m and the mean

160

J.F. Parmer et al. / M A R S magnets

radial forces. Since the radial forces were in opposition, the problem was solved by allowing the cases to butt against each other so that the radial forces could react against each other by' putting the cases in pure compression. In fact, this concept was carried through for axial support as well. A feature common to all the C-shaped coils is that

major radius is either 2.50 or 3.25 m. The cross-section of the winding packs is 0.4 by 1.60 m; the current densities fall between 2.6 x 107 and 3.0 × 107 A//m2, and the peak fields range from 8.7 T to 10.7 T. The proximity of the C-shaped coils along the Z-axis created a problem in that there was insufficient room for structural cases to allow each coil to react its own 110r clearance - A

2.5-304L shell

No. 1 conductor water inlet manifold upper pancakes;-

105.0r

Insulating bushing-Wp

Note 7

/ /

Insulator-typ No. 1 conductor ~

120 ° I 120 °

water exit m a n i f o l d - I. lower oancakes-~ I,

Note 4

\\ \

10 cm No. 2 conductor water inlet manifoldupper pancakes )

~A

Fig. 7. Resistive coil insert.

".

Water f l o w ~ :

161

J.F. Parmer et aL / M A R S magnets

they will have a winding case within the structural case. This is necessary as the capacity of the coil winding machines cannot support the additional weight of the case which is three to four times the weight of the winding. The winding case will also become the helium containment vessel. One could never hope to match the surfaces of the winding and structural cases, so a gap is

!

L

purposely left which contains an inflatable stainless steel bladder. After placing the winding and its case in the structural case, the bladder is pumped full with liquid polyirnide and then cured so that the gap is completely filled with a material capable of transmitting compressive loads. Another feature common to all the C-shaped coils is

,,38 "P-1

~

1

~

4, . p ~~-Conductor

No. 3

Conductor No. 2 ' ~ ~ ¢ ~ - I M M wall tube 0.64 W p ~ ~ J ~ typcu

t

~~

~',,----"~I,..--Conductor No. I

Conductor heights ~ ~ ~ . . . . vary with radius ~ L-~:~:~~ j ' - / y p l c a ! cover ~ ~;5~5rr I(. rasteRers IL ' I Double pancake Section B-B -~ "~. Typical double pancake ! ~ 7 P-~122.0 •I /o /.~ " ~ ~ ~ / - 13.o l _l -_- - ~ 1 2 •o \ II

--

/16.0

~.Sym~

/

~

~-'--Z... / ~

/

t

/

'

3 .0 2 .0

10i.5

--~

Insulating~~, bushing-typ 50 E~dplate, 304L

99.0

I

,

! View A-A

:::i:e

c

:.0/~.

8.

0.45 - ~ Section C-C Note 6

i Resistive insert coil

Notes:

/-No. 2

1. All dimensions in centimeters except as noted. 2. Conductor materiaI-MZC copper, 48 ksi working stress. 3. Each double pancake machined from a single ~. of MZC copper. 4. Water transfer port from upper to lower pancake. 5. Each double pancake consists of three parallel conductors, each separately water cooled. Water path length per conductor per double pancake 28.0 maters, manifold to manifold. e~- Upper :] pancake, typ 6. This Cu spacer connects electrically the lower of a double pancake beneath to create a serieselectrical path for each ----Lower conductor. 48 P pancake, typ 7. Fill atl voids with crushed spinel & adhesive. Note 6 8. Conductor current 32,211 A. conductor water inlet manifoldupper pancakes

~ 1 . ~~

I

Fig. 7. Resistive coil insert (continued).

162

J.F. Parmer et al. / M A R S magnets

for a beam, a tie plate approach was used. The transition coil also reacts the sum of the axial forces of the anchor, plug, and recircularizer coils. The forces of the transition coil are then reacted in the endcell vessel. The current density in the winding pack is 2.6 × 107 A / m 2 and the peak fields are 8.8 and 9.5 T in the major and minor radii, respectively.

the trace cooling of the outer structural case with LHe I. Fig. 9 shows a typical cross-section where the trace cooling is applied. Cooling the case in this manner reduces the LHe II cryoplant load and also eliminates need for a LHe I-cooled thermal radiation shield surrounding the structural case (a LNz-cooled shield is still required). The recircularizer solenoid has a smaller size 1.1 m mean radius and a 0.50 m by 0.50 m peak dimension; its current density is 2.0 by 107 A / m 2 and the peak field is 6.46 T. The conductor has a monolith NbTi superconductor soldered into a stabilizer of OFHC 1/2-hard copper. The coolant for the solenoid is LHe I.

4. 3.2. Anchor yin-yang coil

The anchor yin-yang and the plug yin-yang basically differ only in the value of the mean major radius: 2.50 m for the anchor and 3.25 m for the plug. Since the plug coil represented the greater challenge due to its larger forces and structure size, a design was created for it which is applicable (by scaling down) to the anchor coil as well. The following discussion covers both.

4.3.1. Transition coil

This is the first C-shaped coil following the choke coil. It has a mean major radius of 3.25 m and is a single coil. Fig. 10 shows the transition coil. The figure also shows the background choke coil, highlighting the lack of room between the coil windings for case structure. The structural case on the outside of the minor radius region presented a problem due to the proximity of the background choke coil. Normally, a deep-section beam would be used to help react the forces which try to spread open the lobes; but with no space available

Section I

4.3.3. Plug yin-yang coil

The plug yin-yang pair has a mean major radius of 3.25 m. A combined structural drawing of the pair is given in fig. 11. The only space limitations on the structure occur where the major outer radii are in close proximity to those of the anchor coils and the recircularizer coil. As discussed previously, in these areas the coil cases are allowed to butt against each other and thus react forces in direct compression.

Section II

I

•

I

=

I 1

V . V . & cryostat

~.4"C'sl~iel¢l ,~ con'clu~ors/water iines ..... Normal

o

I 65

66

I

I

I

~k~,~.'x'~\\\\N~,~'x'~\ Pb - shield \\

i

67

68

Z(m) Fig. 8. Geometrical configuration of the choke coil region.

69

70

163

J.F. Parmer et al. / MARS magnets

Stainlesssteelcase(6 inchthick avg.)-~ \ 4.5K .

.

.

.

.

.

.

T Bladderwall Polyimide filler

I~

t iii

Ca

I"

/-Total bladder thickness -0.5 in.

t~

iiiiii:1"8 :i::"KsiS~d~t~tree, jacket(1inchthic~k )~ Pol~imidegroundinsulltion(:4 inchthick ~)

Fig. 9. Trace cooling of structural case with LHe I reduce LHe 11 cryoplant load.

Whereas the transition (recircularizer - c ) coil uses a single strongback, the plug (anchor) coils use a doublestrongback design. Each pair of strongbacks are each connected by two tie-plates. The total load in a strongback is approximately 460 MN, which accounts for the massive size of the structure and the fact that it weighs 3.28 times that of the winding. 460 MN can be equated to the weight of a good sized battleship!. As with the transition coil, maximum use is made of fiat plate to simplify fabrication. The path for neutral beam injection cuts through one of the tie plates; however, sufficient reinforcement was added to compensate for the cutout. The same limitation on structural case thickness for the inside faces of the major radius lobes (to allow a maximum amount of radiation shielding) is applicable here as with the transition coil. Lorentz forces are directed away from these faces. The current density in the winding for the inner plug coil is 2.6 by 1 0 7 A / m 2, and for the outer coil it is 2.854 by 1 0 7 A / m 2. The peak fields are on the inner plug: 9.2 T at the inner major radius and 9.8 T at the minor radius. On the outer plug, values are 9.7 T at the inner major radius and 9.8 T at the inner minor radius. By .comparison, the current density in both anchor coils is 2.6 by 107 A / m 2. The peak fields at the inner minor

radius; and on the outer anchor, 9.7 T at the inner major radius and 10.7 T at the inner minor radius. 4.3. 4. Recircularizer C-shaped coil This coil differs from the others in that its sweep angle in the major radius is 140 degrees, whereas all the other coils have an angle of 140 degrees. The current density is 2.946 × 107 A/m2; the mean major radius is 2.50 m, and the peak fields are 9.8 T in the inner major radius and 9.5 T in the inner minor radius. Case structure design is similar to that of the transition coil so no new figure is presented. This coil butts against the plug yin-yang pair and transmits its axial force to those coils. 4.3.5. Recircularizing solenoid coil This is the last coil at each end of the reactor. Its mean radius is 1.1 m and the winding cross-section is 0.5 by 0.5 m with a current density of 2 x 107 A / m 2. The peak field in this coil is 6.46 T and, with 0.2 m of shielding, the radiation damage in the copper and insulation is negligible. For this coil, the maximum hoop stress is 146 MPa. We have selected the same conductor type as used in the yin-yangs. The conductor uses NbTi for the superconductor and pool boiling liquid Helium I for the coolant. Fig. 12 gives some details of the conductor. Using 1/2-hard copper for the stabilizer, the

164

J.F. Parmer et al. / M A R S magnets

conductor heat flux is 0.22 W / c m 2, which indicates that the conductor is unconditionally stable. The structural case for the solenoid is simple. The coil's axial force is transmitted to the recircularizer

C-shaped coil. This force is relatively low at 15.2 × 106 N. Because radiation damage is negligible, annealing of the coil is not required.

rface ice

S~

JA ~2.25

=

i

Z-67.85 ~

1.85 ~ 1 . 2 0 . .

_1I

-

.

.

.

.

.

.

.

.

.

.

.

:i

~

See detail B Sym

eL

a S°

\\

", .... i

I; h

2.35

i

3.20

Chokecoil 3.75 Transition coil Fig. 10. Transition coil.

rs

I

---+Z

J.F.. Parmer et al. / M A R S magnets 4. 4. Drift p u m p coils

The baseline design is shown in fig. 13. The coil consists of a single loop of water-cooled conductor supported at the service end by the vacuum vessel wall and at the loop end by radiation shielding. There are

.--7. ........

four coils at each end cell. They are used in pairs facing each other on opposite sides of the plasma fan. The coils must be placed near the plasma so they are fitted into slots in the radiation shielding. The design makes use of the fact that, for a high frequency voltage, current travels mainly in the skin of a

---

i

---, ...............

"-~

0.2 cm ,~ Note 1 1 ~

4.50 ~'

~--- "- --- "-'2 "--"-'-'-; "- = '- _ _"

View A

Notes: 1. All dimensions in meters except where noted. 2. Winding pack 0.40 X 1.60. 0.2 cm--~ k x Current 3. Servicestack & radiation shield omitted. Detail B 6440 amps 4. Support attachments omitted. 5. Concept based on forces given in Nuclear radiation shield "~ Livermore memo LLNL-MARS-83-009 DTD Feb 22, 1983, as amended. uglas) 6. Z-Iocetion 71.30. Llnsi.de I~ ~ NL 2 Radiati_onshield 7. Plate thicknesses have not been optimized. casing I ~ J . ~ l n s u l a t i o n (Tisuglas) ~ ' ~ S t r u c t u r a l casing 8. Plate material 304 LN. i-Outside ~ - P o l y i m i d e filled bladder 9. Superconductor material NbTi. 15 cm ~Winding casing 10. Stabilizer material. ~X-Ground insulation x-Winding pack 40 X 160 cm 11. Material polyimide fiberglass. "Typical cross section Winding pack casings & thermal shields

Fig. 10. Transition coil (continued).

165

166

J.F. Parmer et al. / M A R S magnets

conductor. A copper pipe is used with two longitudinal flanges to get a large surface area, structural strength, and a tube to carry the coolant. The flanges additionally serve to support weight of shielding they carry (this is to replace shielding lost in making the coil slot) and to key the coil to the magnet shielding. Another feature of the

coil is that it is coaxial in the regions near the vacuum vessel wall. To meet this requirement, the return leg of the coil is a hollow rectangle into which is fitted the basic conductor. De-ionized coolant water is used. Due to the coils' proximity to the plasma, radiation damage will be high and coil life relatively short (no service life

I;

\,,'i: r:',;!: i:

I: I

Neutral beam window~

I 2::

'

:

. . . . . . . . . .

:!];

- _-

Z:-:-::_--g.'::1122::222

See detail

E~

V J

~0.28 2 pl ,

-H .....

~

'\i~:

B

:

'-"~A

ii

/ Conductors

75

:-:-/2:7; ]1-'"~-'----~'~"~:--~ ~ 0.786 --Z=93.15

0.786 t

L.--Z_ ......

----:----ZZ£

/'.j 80 Fig. 11. Plug yin-yang coil.

A-A

Section

0.30 2 I

~0.4(

-----Z

L- 0.40typ

= 93,95

1 6~ typ

detail E

J.F. Parmer et aL

calculations were made). The ohmic power dissipated in the pair of Transition/Anchor coils is 1.54 MW; the corresponding loss for the Anchor/Plug coils is 0.2 MW.

~

_

=

i!', :

::~

~ ii.,,

.............. ~ !: .,.i 111

i / "i

i/"i

:1

r i~

!i

'!!

/

167

M A R S magnets

5. Magnet power, protection, and control The design approach to the magnet power protection and control is conventional. Present and near-term tech-

\\

.! i . J_ Ill ,,,'II~,I I I!,,,"IlJ !IH ~:~ . . . . .i. .---tI• I,............, , . .-i:i--: 1..-,: . . . - 4 - i ~.!-:,-,-"dl/ ..... , . . , Ii: :. if! I !! 1 :i:

r...-

i!

!

View

IIMI t~ I

i...."ii

B-B

i!/

F! j

SectionC-C . . . .

~ 2-in. poanels ~ s u g l a s ) ~hn/51e;~i~id-~[,~ -,~ LN2 radiationshield ....... I ~ . . r . . ~ Insulation (Tisugles) e 7"~in~ ~ ~ Structural casing

2.7cm 1.0 2.7

10.0

~\~o,yimi~t.ed~,adder 1.5 15 c m - , , ~ . . - . - ~-.\ ~ _ Windingcasing 2.5 Typical crosssectio1~ ~ 1o0 Winding pack casings Ground insulation Note 9 ~ & thermalshields -Winding pack40 X 160 cm Notes. 1. All dimensionsin meters. 2. Windingpack0.40 X 1.60. Note 3. Serviceslacks& radiationshieldomitted. Note 4. Supportattachmentsomitted. 5. Conceptbasedon forcesgivenin Livermore memo LLNL-MARS-83-009DTD 1.905 Feb 22, 1983,asamended. 6. Neutralbeamsare limited to injection in XZ & YZ planes. O.~i~ ~~Note 9 7. Superconductor materialNbTi. Current 8. StabilizermaterialOFHCCu half hard. Inner coil 6440 amp.s Detail E 0.z ~ Outercoil 7069 amps 9. MaterialPolyimidefiberglass. cm I ~ 1 Section D-D

~

Fig. 11. Plug yin-yang coil (continued).

168

J.F. Partner et al. / M A R S magnets

nology has been used to establish the necessary coil circuits. Design assumptions are: • Coils will be monitored for normal zones and dumped to an external resistor should a zone appear. • Maximum dump voltages of 1000 volts across coil terminals and 500 volts between coil and ground are used to prevent electrical breakdown of gaseous helium. • If one coil quenches, coil protection circuitry will cause all coils to be discharged to prevent overcurrent due to coil intercoupling (12%). • A charge time of 24 h has been used for all superconducting coils. Table 4 has listed all the MARS coils for purposes of comparison. All are superconducting except the resistive choke coil and the eight drift pump coils. A number of the coils are designed for the same operating current. They therefore can be connected in series. A simplified schematic is provided in fig. 14 showing how a series of coils with the same operating current may be connected. Circuit breakers (CBs) are used to interrupt the current between each coil, permitting stored

_

energy to be discharged into its associated dump resistor (R). Note the use of internal voltage taps in the coils for detecting normal zones with quench detectors. 5.1. Power supplies

To reduce the number of power supplies all coils with the same operating current have been connected in series. Table 5 lists all power supplies. Six power supplies support the 58 superconducting coils. The power supplies are centrally located at one end of the reactor and the bus losses are small. The duty factor of these units must be considered to be unity because the charging time is 24 h. Conversion efficiency for thysistor supplies in the 500 kVA range is approximately 92%. Sizing has been estimated from current industry information for water-cooled supplies in the 500 kVA range. A value of 0.029 ma/kVA and 43 kg/kVA has been used. Inverting-type supplies are selected so stored coil energy is returned to the power grid during a normal discharge. This is much simpler than providing

_

0.50

l_ 1

0.85

L

--...

x17

1.883 cm

"~ ~-

0.2 cm

I = 4960 A View A

Fig. 12. Recircularizingsolenoid coil.

169

J.F. Parmer et al. / M A R S magnets

Table 4 MARS coil list

Description

Current Conductor density current (amps) Quantity (AJcm2)

Central cell

42

Choke, S/C

2

Choke, M2 Transition Anchor Plug (inner) Plug (outer) Recirculation (C-coil) Recirculation (sol.) Drift pump (47 kHz) Drift pump (450 kHz)

2 2 4 2 2 2

956 1,206 1,460 1,235 1,445 1,946 DNA 2,600 2,600 2,600 2,854 2,946

2

Inductance Total Stored No. of (current sh.) inductance energy turns (u H) (H) (GJ)

5,830

2,060

9.69

41.1

0.7

.7,866

6,796

2.07

95.7

2.96

96,600 6,526 6,526 6,526 6,910 7,129

120 2,543 2,651 2,605 2,710 2,650

DNA 7.88 6.38 8.37 8.37 5.3

0.0346 51.0 48.1 56.8 61.6 37..4

DNA 1.09 1.02 1.21 1.47 0.95

2,000

TBD

1,000

TBD

4

DNA

18,000RMS

1

3.4/pair

4

DNA

970 RMS

1

3.4/pair

slow dissipation resistors such as those used for emergency discharge. Approximately nine different power supply protection interlocks are required for each power supply and are monitored by the central processing unit.

cuum vesselwallfsingle.tur I coil

Fig. 13. Drift pump coils.

3.58

TBD

The resistive choke coils require a very large power input continuously, (80 MW total of delivered power at 92% efficiency requires an input of nearly 87 MW). Due to the large currents, it is assumed that a separate power supply will be required at each end of the reactor. Size

J.F.. Parmer et aL / M A R S magnets

170 Table 5 MARS power supply list Pwr supply no.

1

Quantity Output (in series) (volts) (amps)

Coils Central Cells

42

Power (kva)

Size (m) (H X W × D)

Weight (kg)

125

6,000

750

2.1 X 3,2 × 3.2

32,300

40

6,500

260 2.1 × 1.9 X 1.9

11,200

20

7,000

140

1.8 X 1.5 X 1.5

6,030

15 20

7,200 8,000

108 160

1.8 × 1.3 × 1.3 1.8 × 1.6 × 1.6

3,130 6,900

3

Transition Anchor Plug (inner) Plug (outer)

2 4 2 2

4 5

Recircul. C-Coil Choke, S/C

2 2

6

End Solenoid

2

5

7

Choke coil (res.)

1

380

96,000 36,500

4 × 16 X 16

1,578,100

8

Choke coil (res.)

1

380

96,000 36,500

4 X 16 X 16

1,578,100

9

Drift pump (47 khz)

2

37 kv

10

Drift pump (450 khz)

2

19 kv

11

Drift pump (47 khz)

1

37 kv

12

Drift pump (450 khz)

2

19 kv

2

2,000

10 1.8 × 0.4 X 0.4

18 ka

430

9,700

2.1 X 12 × 12

418,000

276

1.8 X 2.1 × 2.1

11,868

18 ka

9,700

2.1 × 12 × 12

418,000

970 a

276

1.8 × 2.1 × 2.1

11,868

970 a

and weight estimates are tentative because of large size of the power supply and the specific power supply technology. The drift pump probe power sources are lowfrequency power oscillators requiring approximately 36 kA of 47 kHz R F and almost 2 kA of 450 kHz R F .

Combined dissipated power for the drift pumps is 20 MW. In summary, the majority of the power supply requirements for the MARS coils are available in current technology.

Table 6 Superconducting coil dump resistors

Coil

Quantity

Time Dump Energy const, value (GJ) L/R (sec) (volts) 0.7

Water (I)

Size (m) (H × W × D) 2.1 X 2.1 X 2.1

Central cell

42

240

1,000

6,100

Choke, S/C Transition Anchor Plug (inner)

2 2 4 2

2.96 1.09 1.02 1.21

1,675 333 314 371

1,000 1,000 1,000 1,000

40,000 6,100 6,100 6,100

Plug (outer) Recirculation C-coil Recirculation solenoid

2 2

1.47 0.95

426 267

1,000 1,000

6,100 6,100

2

TBD

2.1 2.1 2.1 2.1

× × × ×

4.9 2.1 2.1 2.1

× × × ×

Temp Weight rise (kg) (*C) 6,985

25

4.9 40,823 2.1 6,985 2.1 6,985 2.1 6,985

18 39 37 44

2.1 × 2.1 X 2.1 2.1 × 2.1 x 2.1

6,985 6,985

Note: One dump resistor design with adjustable resistance taps has been used for 54 coils. A second larger concept is used for the S/C choke coil.

53 34

171

J.F. Parmer et al. / M A R S magnets

Inverting power supply

R1

R2

RN CBN

CB1 ~

Voltage taps

o--o

Voltage taps

Central process controller

Voltage taps

•{15

Analog 30 Discrete bits (per coil)

~)perator CRT & control

Fig. 14. Typical simplified superconducting coil schematic. 5.2. Protection control and instrumentation

A central control computer system will perform the two-fold task of power supply control (with resulting reactor field control) and simultaneous monitoring of all coil protection functions. The power supply control is a relatively small task requiring the input of current ramp rates and final set points. Communications between the central controller and power supplies normally uses fiber optic links since large RF fields will be present in the MARS reactor vault. The larger task of monitoring power supply and coil performance for system protection functions will require most of the central control processor capacity. Functions monitored for each coil will include helium gas flow in vapor cooled leads; lead temperatures; LHe level in the dewars; cryostat vacuum; nine power supply functions; quench detector status, and dump resistor status. For each coil, these items will total 15 analog measurements and 30

discrete bits monitored continuously. A predetermined program will provide a decision as to continue operation, normal ramp down, or emergency energy dump after each survey of the input data. A quench detector is used continuously. Typically, five voltage taps are utilized in a balanced bridge input to an isolation amplifier to detect the voltage drop caused by a developing "normal" zone in a superconducting coil. The bridge is balanced to eliminate the voltages produced during routine coil charging. Sensitivities of 5 mV have been accomplished with these techniques. The outputs from the quench detectors are generally hardwired to the circuit breakers. Circuits breakers used are available from switch gear manufacturers. Typical sizes are 2.5 m high, 0.6 m wide, and 1.0 m deep and weighing approximately 320 kg. Separate breakers are required for each superconducting coil. The control center for the operator interface will use

172

J.F. Parmer et al. / M A R S magnets

color cathode ray tube (CRT) displays for protection systems status. Bar graphs using color for alarm display will provide information to the operator. Either keypad/push button or "touch screen" control can be provided. 5. 3. Energy dump resistors

Emergency dump of the stored energy in the superconducting coils might be required. This could be caused either by operator decision or a protection system response such as a normal zone in the coil conductor. Table 6 provides a list of all the MARS dump resistors. Since large stored energies are involved, water bath types have been planned. The dump resistors must be located adjacent to the reactor coils and their associated circuit breakers. A possible alternative approach would be a single, long water pool with each resistive elemen! being installed adjacent to its coil. 5.4. Power bus

Practical sizes for interconnecting power bus has been demonstrated to be 1100 MCM per 1000 A. Oper-

ating temperature for the bus is 38°C in a 20°C environment. But losses are not considered too large to position the S / C power supplies at one end of the reactor, particularly with the series connected central cells. The resistive choke coil, however, has a very large current of 96 000 A and its power supply should be at each end of the reactor. Consideration should be given to using superconducting buses to reduce losses.

References [1] R.R. Coltman, Jr., and C.E. Klabunde, Mechanical strength of low-temperature-irradiated polyimide: A five-to tenfold improvement in dose-resistanceover epoxies, J. Nucl. Mater. 103&104 (1981) 717-722 [2] R.E. Schmunk, Irradiation and testing of Spaulrad-S for fusion magnet applications, EG&G Idaho, Inc., Idaho Falls, Idaho 83145, Report No. EGG-FT-6277 (May 1983). [3] TASKA, University of Wisconsin, UWFDM-500. [4] R. Hoard, D. Cornish, R. Scanlan et al., Advanced HighField Coil Designs: 20 Tesla, Lawrence Livermore National Laboratory, University of California, Livermore, California 94550.