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ITER's central solenoid

-Paul Libeyre

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Magnet experts from the ITER Organization and the Japanese, European and US Domestic Agencies met at Naka from 9-11 March to carry out an intermediate design review of the central solenoid, the "backbone" of ITER's magnet system.

This review was chaired by Elwyn Baynham, formerly of the Rutherford Appleton Laboratory, UK, and hosted by the Japanese Energy Agency to assess the status of the central solenoid's baseline design and provide a benchmark to assess future changes. The overall conclusion is that the solenoid's design, including its six identical modules, is accepted by the review panel, although within the scope of this review it was not possible to fully assess the baseline design tolerances against the plasma physics requirements.

The review panel identified a number of top-level design issues which should be addressed as priority items by the ITER Organization design team. The first one is the conductor jacket material, submitted to high stress levels and cyclic loading. Two options were presented: one using JK2LB, a high-manganese steel specially developed by the Japanese Domestic Agency, and one using a commercially sourced high-grade stainless steel (316LN).

The review concluded that whereas it is possible to use the first as the jacket material owing to the already-existing database for stainless steel, a thorough qualification and materials mechanical property database would need to be developed for the second one.

Another issue is the material to be used for the precompression structure that holds the six coil modules together: if JK2LB is to be used as conductor jacket material, standard commercially available grades of stainless steel can be used for the structure. But if 316LN is to be used as jacket material, either high-performance grade stainless steel or aluminium plates would be required to meet the requirements in terms of yield strength and fracture toughness for the structure. Nevertheless, the review concluded that the low fracture toughness achievable with aluminium alloys would increase the risk in terms of fatigue life.

A third issue considered by the review is tolerances. One analysis presented a requirement of 33 mm as a clearance between the central solenoid and the toroidal field coil straight leg. The review recommended that this analysis be extended to include all manufacturing steps. The review also recommended that a detailed review should be made by the ITER Organization of the central solenoid and toroidal field envelopes and interfaces as part of the completion of the baseline design.

Two options are also being considered for quench detection, one using pick-up coils and the other relying on voltage taps. Studies are underway to evaluate the effectiveness of quench detection methods. Detecting a quench reasonably early in order to avoid increase of temperature in the coil above 150 K requires a sensitivity threshold of 200 mV, which was considered by the review as marginal given the high voltages applied to the central solenoid coils. The review panel also pointed out that the temperature margin of 0.7 K was not large for a system of this size and complexity. (The temperature margin is the amount the temperature of the superconductor needs to be increased for a voltage of 10microV/m to appear - i.e., it is on the verge of losing its superconductivity at which point a quench starts.)

Concerning manufacture, the panel draw the attention on the challenging concept of circular winding using joggles to change from one turn to next compared to the more conventional winding along a spiral providing a continuous variation of curvature. The circular winding with short transition from turn to turn has been adopted in the baseline design to maximize the available flux but the review suggested that the trade-off analysis between circular winding and spiral winding should be documented as part of the baseline design completion.

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