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Review Article

An Overview and Status of the NHMFL 45-T Hybrid Project

J. R. MILLER, S. W. VAN SCIVER and H. -J. SCHNEIDER-MUNTAU

National High Magnetic Field Laboratory, 1800E. Paul Dirac Drive, Tallahassee, FL 32310, U.S.A.

(Received November 24, 1995)

Synopsis:

The 45-T Hybrid Magnet System represents an important commitment for the new National

High Magnetic Field Laboratory, and when completed, it will establish a new frontier for steady

magnetic fields for research. This system will use a resistive insert to produce 31T on axis,

with the remaining 14T contributed by a superconducting outsert with 616-mm warm bore. The

superconducting outsert will be capable of providing over 15T on axis when operated alone.

Progress on the design, development, and fabrication of the system has been steady. Several

critical components and major subsystems have been completed and are now being tested in pre

paration for the overall system integration.Keywords: high-field magnets, superconducting magnets, resistive magnets, cryogenics, cable

in-conduit conductors

1. Introduction

With the founding of the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida, a commitment was made to install a hybrid magnet system capable of

producing 45T and higher. Initially, competing concepts were developed for this system by teams from the NHMFL and the Francis Bitter National Magnet Laboratory

(FNML). Down selection was made in consultation with a committee of experts following a Conceptual Design Review in February 1992.1) From that time, NHMFL and FBNML have collaborated on the design, development, and manufacture of the NHMFL 45-T Hybrid Magnet System.2) The most basic requirements of the system are that it be:

● Versatile-Produce 45T in a 32-mm bore

in the near term using a 24-MW insert,

upgradable to 50T and higher with fu

ture, higher-power inserts

● Reliable-Stable under all "normal" op

erating conditions, with a 10-year life

time for the outsert and 600 hours or

2,000 charge/discharge cycles for the insert (Normal conditions for the outsert include the possibility of a sudden trip of the insert power supply or a failure of the insert magnet)

● User Friendly-Maximize access, utility,

and availability by minimizing obstruc

tions at top of the magnet and by providing for rapid charging

With the intent to enhance our ability to meet these requirements, some important fea

tures that were included in the system design

are as follows:● A superconducting outsert composed of

3 sucoils, each with a different conductor grade (2 containing Nb3Sn and 1 with NbTi), allowing the overall current density to be enhanced and the costs to be restrained

● Cable-in-conuit-conductors (CICC) cooled

internally by static He ‡U at 1.8K in the

superconducting outsert, providing im

portant advantages for transient stability,

ac loss, distributed structure, helium con

tainment, and electrical insulation

● Outsert subcoils assembled as a unit in-

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side a strong, leak-tight vessel and positioned inside the magnet cryostat atop a

stiff, metallic, heat-stationed support columm capable of safely transmitting fault

loads to the cryostat base

Fig. 1 Cross-sectional view of the insert and outsert magnets of the 45-T Hybrid showing their

interconnection through the insert housing and outsert cryostat.

● The top of the magnet cryostat kept

clear by concentrating key cryogenic com

ponents and system interfaces in a supply cryostat, offset from the magnet cryostat

and connected to its lower part by a

services duct● The resistive-insert housing attached to

the base of the cryostat with all electrical

and water connections underneath, also

maintaining the clear space at the top of

the magnet system.

● Maintenmce access to the resistive mag

net by removal of the top cover of the

housing only

These and other features are illustrated in

the cross-sectional view in Fig. 1. Some gen

eral specifications of the magnet system that

were developed from the basic requirements

and design criteria are listed in Table 1.For describing the 45-T Hybrid System in

more detail, it is helpful to consider three main parts: the resistive insert system (including the electrical-power and cooling systems for the magnet, the magnet housing, and the magnet itself), the superconducting outsert system (including the cryogenic system, the magnet, and its power/protection system), and the site for the system (including user access and general instrumentation and controls).

2. Resistive Insert System

To meet the basic requirements within the

general specifications, the resistive-insert manet must routinely produce 31T on axis within a 32-mm clear bore while immersed in the background field (14T on axis) from the superconducting outsert. We expect the minimum operating time between faults in the coils to be 600h, with at least 2,000 chargedischarge cycles. We also envision several

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alternative coil designs being employed over

the life of the system, but initially, the insert

design will be based on a 24-MW consump

tion electrical power.3-5) In the future, designs

will take advantage of available power in

the 32-40MW range for producing higher

field.

Table 1 General specifications of the 45-T Hybrid Magnet System.

2.1 Resistive Magnet Power SuppliesThe power supplies for the resistive insert,

also available by remote switching for power

ing all NHMFL resistive magnets, have been installed and operable since the Fall of 1993.6)

Briefly, their specification are as follows:● 4 completely independent unit

● Each with a steady-state rating of 17kA

and 500V (32MW total)

● Each with a 1-hour overload capacity of

20kA and 500V (40MV total)

● Specifications on current:

Precision-10ppmAccuracy-1,000ppmRipple and noise-10ppm (dc-500Hz)

Ramp rate-500A/s (increasing or decreasing)

Line sensitivity-100ppm for 10% variation

Load sensitivity-100ppm for 30% variation

Maximum fault current-30kA (output voltage zeroed in 10ms)

2.2 Resistive Magnet Cooling SystemSimilarly to the power supplies, the cooling

water system for the resistive insert (and other NHMFL resistive magnets) is also fully operational and meets the following specifications:

● Demineralized water supPlied at 7℃,

100kΩ ・m, gas free and with low vibra

tlon

● 4 pumps, each providing up to 100l/s

with 3-MPa head (up to 4-MPa head at flows below 40l/s)

● 4 chil1ers, each with 7-MW cooling ca

parity

● Chilled water storage of approximately

4,000m3 (～60MW・h)

2.3 Resistive Magnet HousingThe resistive insert magnet will be con

tained inside a steel housing that accurately

positions it in the bore of the superconducting outsert cryostat and is sufficiently strong to contain or transmit all normal and fault loads

safely and without contacting the cryostat

bore tube. As pictured in Fig. 1, the housing features concentric shells of 304L steel for

containing the cooling water, directing its flow and transmitting loads to the base plate

of the magnet cryostat.

With 32-mm-bore inserts contributing 31T and considering the fault current limitations

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of our power supplies, the maximum credible

fault loads were derived to be approximately

±3.5MN axiaily directed or 100kN radially

directed. Consequently, it was decided to set the requirements for our first housing accordingly, postponing for now the possible need for a stronger (and more costly) housing for higher-field, larger-bore inserts. As designed, the first housing has a transverse stiffness of approximately 130kN/mm, which is large compared to the magnetic decentering force factor of approximately 10kN/mm, and analysis shows that all normal and fault loads will be safely contained. Fabrication and factory testing of the housing are expected to be complete in January 1996.

2.4 Resistive Insert MagnetInsert magnet designs with either radial or

axial cooling can be accommodated in the housing and both options have been studied.3,4) Presently, the most detailed design has been

produced for a radially cooled option, which would comprise 3 structurally graded coils sets.7) Each coil set would contain multiple zones with the material selection in each zone based on a trade study considering strength, conductivity, and cost. The conductor alloys to be used are CuAg, CuBe, CuZr, and pure Cu. Materials have been procured sufficient to manufacture 2 complete radially cooled insert magnets and a spare set of plates for the innermost coil.

3. Suerconducting Outsert System

3.1 Cryogenic SystemThe cryogenic system for the supercon

ducting outsert is designed for safe, reliable, and economical operation while maintaining the magnet temperature at 1.8 K continuously for long periods. This system has been described in detail in a recent article for Teion Kogaku,8) so only the key features are reviewed here.

The overall cryogenic system is based on a ref rigerator/liquefier system that contains two identical compressor/cold-box sets, model 1630s manufactured and installed by Process Systems International. Each unit is designed

to run independently or in tandem, the intent

being to use a single unit for normal opera

tion, with the other as backup, and two units

combined for initial cooldown or recovery

from an upset. The system is designed for

three modes of operation. The performance

specifications of a single unit in each mode

is described in Table 2. In tests and during

operation as a liquefier over the past year,

both units have routinely exceeded these spec

ifications. The refrigerator/liquefier system

also includes two identical pumping systems,

each capable of approximately 500l/s at 13

mbar. These operate in closed-loop fashion

with the system and provide the drive for

the He ‡U refrigeration components in the

cryostat.

Table 2 Performance specifications for a single ref rigerator/liquefier unit

(with LN2 cooling).

The cryostat requirement to provide a 1.8K environment for the magnet must be met within the performance specifications of a single ref rigerator/liquefier unit. A budget of a single unit's capacity among the various estimated heat loads in the cryostat during normal, full-current operation is given in Table 3 (to be compared with Mode-2 operation in Table 2). Tests to precisely measure these loads and bring them within budget are in progress.

In Table 3, the higher loss components in each category are generally linked either to the 10-kA operating current selected for the outsert (simplifying protection while retaining high winding-pack current densities) or to the use of a steel, heat-stationed support column to handle the potentially extreme loads occurring during an insert fault. Consistently with plans to accommodate higher-power and larger-bore inserts in the future, the support column in the magnet cryostat was designed

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for axially directed fault loads of ±6MN or

a transversely directed load of 300kN. The

decision to use a steel column greatly sim

plified the cryostat design task.

Table 3 Cryogenic heat-load budget for the superconducting

outsert cryostat.

Solid conduction plus He ‡U superleak.

3.2 uperconducting Outsert MagnetA detailed list of parameters describing the

superconucting outsert magnet is given in Table 4. Note that winding-pack current densities, field contributions, and total stored energies are given for two levels of current. The first, 10kA, is the normal value, appro

priate for operation in combination with the resistive insert. Future higher-power and larger-bore inserts could conceivably induce overcurrents nearly 10% greater than the normal current. Our choice to accommodate such events without quench became one of

the most important design drivers for the system. The result is a large-bore supeconducting magnet design capable of greater than 15.6T when operated without the insert. The maximum field at the winding in this case will be nearly 17T.

The conductors used in three outsert coils are similar in appearance to the illustration in Fig. 2. Details of their compositions and dimensions are given in Table 5, The Nb3Sn wires were fabricated y Teledyne Wah Chang and the NbTi wires y Oxford Superconductor Technology. All cabling was done y New England Electric Wire. The jacketing of conductors for all three coils was contracted to the industrial team of Intermagnetics General Corporation (IGC) and Gibson

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Tube, Inc.

Table 4 Parameters describing the superconducting outsert magnet.

a Normal operation of outsert combined with insert , b Upset following an insert trip or operation of outsert alone.

Fig. 2 A typical CICC for one of the supercon

ducting outsert coils.

Illustrated schematically in Fig. 3 is the

production conductor-jacketing machinery for continuously seam-welding a tube around the outside of a cable and forming it into the desired shape and void fraction. As described elsewhere, by the end of the 45-T Hybrid Project, the IGC/Gibson team will have fab

ricated more than 10km of CICC (including dummy conductor for development activities) with this machinery.9) Fundamentally, the length of a single piece of CICC that can e

jacketed on this equipment is limited only by the initial length of the cable, since pieces of steel strip for the jacket can be welded end to end to achieve any length. A "scarf" weld is used for joining the lengths of strip, which causes the weld to curl around the tube in helical fashion during forming (see Fig. 2), improving the reliability of the joint and giving it a more favorable orientation relative to loads on the conductor during magnet operation.

The two Nb3Sn coils will e fabricated by the insulate-wind-react-impregnate technique. Layer winding is practically essential for these coils to avoid the loss of space between coils that would be needed for joints.10-12) Coil A will be wound from a single length of conductor, but two length, with a layer-layer

joint between, will be used for Coil B. Fabrication of Coil C will combine pancake winding and the wind-insulate-impregnate tech

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nique.13)

Table 5 Compositions and dimensions of conductors for the three superconducting outsert coils.

Fig. 3 Schematic illustration of the CICC production line.

All conductor for coil C has been com

pleted and a novel "Nautilus" winding technique for forming it into pancakes has been developed.14) Forming the individual pancakes is now in process now with the cleaning, insulating, impregnating, and terminating processes to follow.

Each step of the fabrication process for the "real" coils A and B is being preceded by the development and practice of that process on a "dummy" coil that has attributes of both A and . The dummy coil has been insulated, wound, and heat treated and is

presently being prepared for impregnation. The conductor cables for the real coils A and B have been completed and are ready for

jacketing.3.3 Outsert Power/Protection SystemThe outsert power/protection system was

designed primarily to ensure that: the magnet's energy can e safely extracted in the

event of a quench, the outsert's contribution to field ripple will be negligible in comparison to the insert's, the ramping time to or from maximum field can be significantly less than 1h, and the field direction can be easily reversed. A schematic presentation of the major components of the system is shown in Fig. 4, including some details of the component specifications. This part of the system has been completely installed and is undergoing performance tests.

The magnet design is based on the assumption that an emergency discharge can be initiated within 1s after creation of a non-recovering normal zone. The identificaton of a

quenched zone will depend primarily on the observation of a compensated voltage in excess of a predetermined threshold for a specified duration. Compensation to suppress the reactive components will be achieved by com

paring voltage signals across neighboring layers or pancakes according to the following

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expression:

Vcomp=|2Vi-(Vi-1+Vi+1)|,where the indices indicate the total voltages across individual layers or pancakes. This technique allows the 6 voltage signals from coil A to be reduced to 2, the 7 voltage signals from coil B to be reduced to 3, and the 29 voltage signals from coil C to be reduced to 10. An analysis of what we consider the worst case for inductive voltage-the sudden trip of a large-bore insert capable of inducting 10% overcurrent in the outsert-indicates that even the combinations giving poorest compensation will fall below 100mV within approximately 0.5s of the event. An analysis of quench development in typical situations suggests that the resistive voltage across a normal zone will rise above 100mV in less time.15)

Fig. 4 Schematic showing major components of the outsert power/protection system and including brief specifications for each. PS: Alpha Scientific do poWer supply rated at ±25V, 11kA, 3V p-p voltage

ripple, 10ppm current ripple, 100ppm regulation/stability, 100ppm precision/accuracy of the current setpoint, and 5kV isolation of the secondary. SR: Reversing switch supplied by Alpha Scientific. BKR1/2: Two sets of current interrupters or breakers manufactured by Secheron, each set comprising 3 identical individual breakers, and each of those rated for 4kA, 4kV (7kV transient), and 50ms opening time. RD: Emergency discharge resistor manufactured by Microelettrica Scientifica, designed to absorb safely at least 121MJ within 4s, resistance taps provided in 7 increments of approximately 10% from 0.27 to 0.60 ohms (0.45 ohms will be the initial choice), 5-kV isolation. ZFCT : Zero-Flux Current Transducer manufactured by Holec and incorporated into the Alpha Scientific

power-supply control system. Lo: The superconducting outsert, which presents an inductance of approximately 2H to the power system and stores approximately 118MJ at 11kA.

4. Site

The 45-T Hybrid system will be installed

in a cell with floor area roughly 15•~15m2.

The ceiling in that cell is approximately 13m

above the main floor: however, the magnet

cryostat has been installed at a position well

above the main floor to allow the resistive

insert housing to be installed or removed

from underneath. To allow easy access by

users to the top of the magnet system, a floor

has been constructed flush with the top of

the magnet cryostat.

The user-floor level is 7m above the cell

floor, leaving approximately 6-m clearance

below the ceiling. The space available on the

user floor is illustrated in photograph of

Fig. 5. Access into the bore of the magnet

cryostat is visible at middle-right in the photo

while the top of the supply cryostat, where

the various system interface connections are

made, is visible in the foreground. Controls

for the system and user instrumentation will

be located just out of view near the lower

left of the photo. A stairway between floors

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can be seen to the left of the photo and space allocated for a service elevator is visible in the background, just left of center. The entire area in Fig. 5 is covered by a 20-ton crane.

Fig. 5 A view of the user floor area above the magnet cryostat for the 45-T Hybrid system .

A general system of instrumentation and controls (the GI & C) will give an operator control of the cryogenics, superconducting outsert magnet, and resistive insert magnet. The GI & C uses desktop computers, virtual instrument software, and both commercial and custom interface hardware. The same distributed control system that is used to monitor and control the laboratory's other resistive magnets is also used for most controls of the resistive insert magnet. A few additional inputs have been added to the standard resistive magnet protection system to handle coordination with the operation of the superconducting outsert magnets. Each major subsystem's operator interface permits variation of parameters within preset ranges, but the computer retains the capability to take actions required for protection of personnel and equipment when conditions require them. For example, the QDM & C system will always monitor critical functions of the outsert magnet system and either irrevocably command

a ramp down or an emergency discharge as warranted by the particular situation.

5. Summary

The NHMFL 45-T Hybrid will establish a new frontier for steady magnetic fields for research. Progress on the design, development, and fabrication of this system has been steady. Several components and subsystems have been completed and are now being tested in preparation for the overall system integration.

Research supported by the State of Florida and the National Science Foundation through NSF Cooperative Grant Number DML 9016241.

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(1994) 21925) B. J. Gao, et al.: IEEE Trans. Magn. 30 (1994)

2196

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6) H. Boenig, et al.: IEEE Trans. Magn. 30 (1994) 1774

7) R. J. Weggel, et al.: "Final design of a 24MW radially cooled insert for a 45-T Hybrid system," the 1995 Int. Conf. on Magnet Technology, Tampere, Finland, June (1995)

8) S. W. Van Sciver, et al.: Teion Kogaku 31

(1995) 29) J. R. Miller, et al.: the 1995 Cryogenic Engi

neering Conf. Columbus, OH; Adv. Cryog. Eng. July (1995) submitted for publication

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11) T. A. Painter, et al.: IEEE Trans. Magn. 30

(1994) 220412) A. Deverone, et al.: "Tension layer winding

of cable-in-conduit conductor," the 1995 Int. Conf. on Magnet Technology, Tampere, Finland, June (1995)

13) J. E. C. Williams, et al.: IEEE Trans. Magn. 30 (1994) 1633

14) J. E. C. Williams, et al.: "Nautilus: A method to wind disk coils," the 1995 Int. Conf. on Magnet Technology, Tampere, Finland, June

(1995)15) M. B. Gorbounov, et al.: the 1995 Cryogenic

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