Speaker
N. Mitchell
(on behalf of the ITER Central Team)
Description
The magnet system is one of the critical core components of the ITER magnets, defining the machine capabilities to form and drive 15MA 500MW nuclear plasmas for 100s of seconds. The magnets, the largest superconducting magnet system ever built with 50GJ of stored energy, are also technologically highly advanced components using large composite Nb3Sn 4-6K force flow cooled conductors that also, in order to maximise plasma performance and minimise cost, stretch current manufacturing technology to its limits. They work at the highest possible electrical (20-30kV), mechanical (primary stresses up to 600MPa) and superconducting performance consistent with very safe and very reliable operation over the life of the machine.
The transition from the design phase to the manufacturing phase of the magnets has required tight integration and occasionally several iterations between the design and the results of manufacturing development. This is particularly the case when qualification tests on as-manufactured components reveal smaller margins than anticipated and the original design or manufacturing route (or both) have to be modified, often leading to knock-on effects on other parts of the magnets or, in a tightly packed machine like ITER, other components (and vice-versa). For example, we have encountered significant issues on Nb3Sn filament breakage under magnetic loads that have had to be resolved (for schedule reasons) by special development and test programs executed within the ongoing manufacturing, leading to the selection of an improved multi-stage cable design. Early issues with the flatness of the radial plates that form the TF coil double pancake ‘units’ were resolved by adjustments in the sequencing of welding and machining steps, achieving a flatness (on a product of dimensions 7mx10mx0.1m) of better than 1mm. The development of current leads using High Temperature Superconductor technology, supported by technology transfer from CERN, required development in brazing, very accurate machining (tolerances of 1/100 mm) and high voltage pre-impregnated insulation to achieve a successful result.
The magnets, and the associated feeder system, are now entering the final manufacturing stages. Almost all prototyping and qualification work is completed, the sub-components such as the conductors are nearly fully manufactured (over 80% complete), some of the coil production lines are nearly fully ‘loaded’ and in several cases the ‘first of a kind’ final winding packs are nearing completion. For example, the winding of over half of the 18 TF coils is completed and about one quarter of the total double pancake units have undergone the reaction heat treatment to form the Nb3Sn. Although we can still expect a few manufacturing non-conformities to occur, we can now be confident to detect and correct these without schedule impact. The first component deliveries to the site start in about 18 months and become an avalanche within 3 years: assembling these components, keeping up with the delivery rate is the next challenge.
In this paper we review the main manufacturing difficulties that have been overcome and summarise the present production status of the magnets.
Co-authors
A. Devred
(on behalf of the ITER Central Team, Domestic Agency and Supplier Magnet Groups, France)
N. Mitchell
(on behalf of the ITER Central Team, Domestic Agency and Supplier Magnet Groups, France)
