CRUCIAL ISSUES FOR THE DESIGN OF FUSION MAGNETS Electrical insulation

The very high level of neutron and gamma radiation at the vacuum vessel of a fusion reactor must be screened to limit the nuclear heat load and the radiation damage at the wind­ing components. The size of the shield (up to 1 m thick in the ITER project) may have a substantial impact on the size and cost of the superconducting magnets. After screening, the ra­diation damage on the metallic components of a supercon­ducting coil (steel, copper, NbTi and Nb3Sn), is not critical and partly recovers (e. g., for copper) upon warming up at

LCT-WH (1981)

Polo (1987)

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DPC-EX (1988)

US-DPC (1988)

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Toshiba (1983)

W7-X (1996)

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LHD-OV (1994)

Figure 10. Selection of cable-in-conduit super­conductors, drawn to the same scale. The strands of Polo, LHD-OV and W7-X are NbTi, all the other are Nb3Sn strands. The jacket material is steel except for ITER (Incoloy) and W7-X (alumi­num alloy).

Figure 11. Winding tool with 13 numerically controlled axes for the helical coils of the LHD (courtesy of K, Takahata, NIFS).

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room temperature. The actual weak link for radiation damage is the organic fraction of the electrical insulation. In potted windings, the glass-epoxy is broadly used, either as laminates or prepreg wraps or vacuum-impregnated fabrics, to bond to­gether the winding turns and to provide the required dielec­tric strength. The neutron and gamma act on the long molecu­lar chain of the resin, irreversibly breaking the atomic links. The mechanical strength of the composite, mostly the shear strength, is affected and macroscopic cracking may occur un­der operating loads, eventually leading to a short circuit be­tween winding sections. To limit this risk, the magnets must be designed to have low stress in the insulation, that is, limit the risk of crack propagation. Another design approach is to separate the mechanical and electrical functions, for example, including a redundant electrical insulation layer, either inter­leaved or overlapped to the glass-epoxy, to stop the crack propagation in the resin. The free radicals originated from the broken organic polymers are chemically active and evolve into gaseous molecules. The most severe consequence of radiation induced chemical reactions at 4 K is the accumulation of fro­zen gas bubbles (mostly hydrogen). Upon warming-up of large windings, the internal pressure of the evolved gas increases dramatically due to the little permeation and may lead even­tually to swelling in the insulation (20). A possible cure against postirradiation hazards of organic insulation is to re­duce the resin volume fraction and select the resin composi­tion to minimize the gas evolution rate. On the other hand, there is a broad reluctance to start an expensive and time­consuming task for the industrial development of innovative insulation systems, which will be actually needed only when a fusion reactor will work at full power on a time scale of several years. The full replacement of organic insulation sys­tems by ceramic materials with adequate mechanical proper­ties may be the ultimate, long-term goal to solve the issue of the electrical insulation in the heavily irradiated fusion magnets.

Quench Protection

In case of quench, the huge amount of energy stored in a fu­sion magnet must be actively dumped in an outer resistor. If a quench fails to be detected, the ohmic power locally dissi­pated in the slowly expanding normal zone is sufficient within one minute or less to melt the conductor and start a chain of serious failures (vacuum break, electric arc, mechanical col­lapse). A number of quench detectors have been developed and are currently applied in superconducting magnets, from the easy ones (voltage balance of different winding sections, monitoring of outlet mass flow rate) to the most sophisticated, including the laser interference on optical fibers used as dis­tributed thermometer, transmission, and reflection of super high-frequency waves in the coolant channel, acoustic emis­sion, magnetization change at the normal zone (21). However, a redundant and intrusive instrumentation is not welcome in a fusion reactor, as it may increase the risk of leaks and insu­lation failure, due to the large number of feedthrough re­quired. Whatever the quench detector is, the ultimate ques­tion always arises: What happens if the active quench protection fails? The design approach for an actual fusion magnet (i. e., not for an experimental device) will need to offer both a reliable and robust quench-protection system and a conductor/magnet layout that intrinsically limits the damage in case of failure of the protection system, for example, en­hancing the quench propagation and the enthalpy at interme­diate temperature.

Cost Optimization

Figure 12. The OV poloidal field coil of the LHD (courtesy of K. Takahata, NIFS).

In several applications of the superconducting magnets (e. g., accelerators, detectors, high-field magnets, prototypes), the achievement of the technical goal is the main care of the de­
signer, while the cost of the device does not play a major role. However, after completion of the demonstration phase for the fusion magnets, the cost optimization will be a key issue for the commercial success of fusion. On one side, the behavior of the superconductor needs to be mastered by the designer (e. g., ac loss, stability, mechanical properties), in order to set the design margins at a safe but realistic level and make effective use of the expensive superconductors. On the other hand, the choice of the manufacturing methods and tooling may have a very strong impact on the cost of the coil and should be in­cluded as a driving factor in the design. Two examples are given to show how a design choice may affect the cost.

A high electrical conductivity material (stabilizer) needs to be added to the superconductor cross-section, to allow effec­tive current-sharing and fast recovery for small thermal dis­turbances. The required stabilizer cross-section may be much larger than the superconductor. In cable-in-conduit conduc­tors, the straight choice is to equally distribute the stabilizer cross-section in each superconducting strand, specifying a high Cu : non-Cu ratio. However, the cost of the Nb3Sn strand is independent of the copper ratio. If the designer masters the mechanism of the current-sharing among strands and knows the operating values of the interstrand resistance, he or she may select a much smaller Cu : non-Cu ratio in the Nb3Sn strand and add extra copper wires in the strand bundle. Keeping the same superconductor cross-section, that is, with­out affecting the operating margins, the amount of Nb3Sn strand can be significantly reduced with a large cost saving.

A Nb3Sn conductor needs a heat treatment at 650°C to 700°C to form the brittle intermetallic composite by solid — state diffusion. If the designer does not master heat resistant electrical insulation systems, he or she will conservatively choose to first heat-treat the conductor and then insulate it and wind in the final shape (e. g., react and wind or wind and react and transfer methods). As the Nb3Sn after heat treat­ment is degraded for permanent deformation as large as 0.2% to 0.3%, the handling for post-heat treatment insulation and final assembly requires sophisticated tooling and continuous adjustment (e. g., shimming of each turn) to achieve the re­quired tolerance with minimum strain on the conductor. If a reliable insulation system is selected, compatible with the heat treatment procedure, the coil can be wound in the final form and to the final tolerance before the heat treatment (wind and react method), saving the cost of a large number of tools and manufacturing steps and avoiding the risk associ­ated to the post-heat-treatment handling.

Risk and Quality Assurance

Large superconducting magnets are usually unique items for which a thorough quality assurance program cannot be conve­niently established in advance, as is the case for series pro­duction, due to the lack of iterative improvements in the man­ufacturing procedures. The global acceptance tests of the magnets tend to replace the quality assessment of the individ­ual procedures, achievable only on the basis of a broad statis­tical database. However, an individual acceptance test of a fusion magnets cannot reproduce all the actual operating con­ditions, for example, mechanical load and peak field from other coils, nuclear radiation, mechanical and thermal cy­cling. Moreover, a global acceptance test should not be pushed to the failure limit, that is, the operating margin cannot be assessed. The lack of confidence may push the designer in a circle of overconservative choices, for example, assuming min­imum performance for material properties, welds, assembly tolerances, which adversely affect the cost and the effective­ness of the design. To avoid this trend, the designer should identify the critical area for quality assurance and select a low-risk design and procedure. For example, the resistance of a joint between conductor sections cannot be checked during the manufacture. In this case, the designer should aim for a joint layout where the resistance performance is only margin­ally affected by incorrect assembly procedure.

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