P. Libeyre

The ITER Magnet System

Procurement of the ITER magnets is due to start at the end of 2007/early 2008, with the launch of the longest lead time items, the conductor and the TF coil windings. The base design for procurement was established in 2001, and the build up of the Cadarache ITER team has been accompanied by a review of the most critical, or controversial, features of the 2001 design. At the same time, an urgent R&D program has been launched to complete the necessary verification of the design solutions that are proposed. In this paper an overview will be presented of the main design features and drivers, and some of the recent issues and R&D results will be summarized.

The ITER Magnets: Design and Construction Status

The ITER magnet procurement is now well underway. The magnet systems consist of 4 superconducting coil sets (toroidal field (TF), poloidal field (PF), central solenoid (CS) and correction coils (CC)) which use both NbTi and Nb3Sn-based conductors. The magnets sit at the core of the ITER machine and are tightly integrated with each other and the main vacuum vessel. The total weight of the system is about 10000 t, of which about 500 t are strands and 250 t, NbTi. The reaction of the magnetic forces is a delicate balance that requires tight control of tolerances and the use of high-strength, fatigue-resistance steel forgings. Integration and support of the coils and their supplies, while maintaining the necessary tolerances and clearance gaps, have been completed in steps, the last being the inclusion of the feeder systems. Twenty-one procurement agreements have now been signed with 6 of the ITER Domestic Agencies for all of the magnets together with the supporting feeder subsystems. All of them except one (for the CS coils) are so-called Build to Print agreements where the IO provides the detailed design including full three-dimensional CAD models. The production of the first components is underway (about 175 t of strand was finished by July 2011) and manufacturing prototypes of TF coil components are being completed. The paper will present a design overview and the manufacturing status.

Overview of the ITER Correction Coils Design

The Correction Coils (CC) of the ITER Tokamak are developed to reduce the range of magnetic error fields created by imperfections in the location and geometry of the other coils used to confine, heat, and shape the plasma. The proposed system consists of three sets of 6 coils each, located at the top (TCC), side (SCC) and bottom (BCC) of the Tokamak device and using a NbTi cable-in-conduit superconducting conductor (CICC). Within each set, the coils are connected in pairs to produce a toroidal field to reduce the most troublesome, lower order, poloidal mode number fields (m = 1,2,3) in order to operate below the locked mode threshold. The conductor is designed to operate up to 6 T. The winding uses pancakes of one-in-hand conductor (quadpancakes for SCC, octopancakes for TCC and BCC), thus avoiding internal joints. The winding-pack is enclosed inside a 20 mm thick stainless steel casing. The coils are supported by rigid connections to the Toroidal Field (TF) coils. The structural design of the CC is mainly driven by the allowable fatigue stress levels in the conductor jacket, in the case material and in the glass-polyimide electrical insulation system. The boundary conditions on the CC are imposed by the TF coils deformation and the electromagnetic interactions with the PF coils system. The thermo-hydraulic and electrical performance of the CICC is also addressed.

Electrical Design Requirements on the ITER Coils

The ITER coils are all designed using superconducting conductors with a high current carrying capability. Plasma operation and control requires fast variation of the currents in the pulsed coils inducing more than 10 kV during normal operation on the coil terminals. The power supplies are designed to limit the high voltages, but under short conditions, voltages may rise above 20 kV in certain coils. Moreover, the acceptance tests of the coils must be carried out at higher voltages, up to 28 kV for the CS coils. The electrical design philosophy of the coils includes recovery options in the event of electrical faults internal to the coils.

Preparation for the ITER Central Solenoid Conductor Manufacturing

The ITER Central Solenoid (CS) conductor is composed of 576 superconducting strands and 288 Cu strands assembled together into a multistage cable and protected by a circle-in-square jacket with the outer dimension of 49 mm × 49 mm. In R&D to prepare for the ITER CS conductor manufacturing, mechanical tests of jacket, welding tests and manufacturing of 181-m long dummy conductor have been performed. In this paper, the R&D activities are presented, showing that as a result of this R&D, the CS conductor manufacturing technologies have been preliminary defined to start the procurement of the CS conductor.

Progress of the ITER Correction Coils in China

The ITER Correction Coils (CC) include three sets of six coils each, distributed symmetrically around the tokamak to correct error fields. Each pair of coils, located on opposite sides of the tokamak, is series connected with polarity to produce asymmetric fields. The manufacturing of these superconducting coils is undergoing qualification of the main fabrication processes: winding into multiple pancakes, welding helium inlet/outlet on the conductor jacket, turn and ground insulation, vacuum pressure impregnation, inserting into an austenitic stainless steel case, enclosure welding, and assembling the terminal service box. It has been proceeding by an intense phase of R&D, trials tests, and final adjustment of the tooling. This paper mainly describes the progress in ASIPP for the CC manufacturing process before and on qualification phase and the status of corresponding equipment which are ordered or designed for each process. Some test results for the key component and procedure are also presented.

Progress in Production and Qualification of Stainless Steel Jacket Material for the Conductor of the ITER Central Solenoid

When energized, the ITER Central Solenoid coils experience large pulsed electromagnetic forces that the conductor jacket itself must withstand. The conductor jacket consists of circle-in-square extruded and drawn austenitic stainless steel pipes. The qualification of the production of stainless steel jacket material was carried out on jackets manufactured in both a very low carbon AISI 316LN grade and a high Mn-bearing austenitic stainless steel, called JK2LB. Two different suppliers provided fully representative batch productions of both grades. Extensive metallurgical and dimensional metrology examinations were carried out at different steps of the processing, starting from the forged billets used as semifinished products to be engaged in the extrusion process, to the solution annealed jackets in their final shape. A specific method of Phased Array Ultrasonic Testing (PAUT) was developed and successfully applied for the non-destructive examination of the different jacket productions. PAUT sectorial scan inspections were carried out with probes traveling on the outer surface of the section, allowing almost 95% of the volume to be examined despite the complex geometry of the jacket.

Research on Manufacture and Enclosure Welding of ITER Correction Coils Cases

Extensive research and analysis has illustrated that there are field errors existed on ITER magnet system due to the misalignment of the coils and winding deviations from the nominal shape. To compensate the errors, correction coils (CCs) are developed and employed on ITER. The CCs consist of 6 top CCs (TCC), 6 bottom CCs (BCC) and 6 side CCs (SCC), arranged toroidally around the machine inside the PF coils and mounted on the TF coils. The CC case provides structural reinforcement to the winding packs. It is made of 316LN stainless steel and has a thickness of 20 mm. The main characteristics of the case are small section (~240 mm × 147 mm), large dimensions (~7 m) over wide angle (~60°) and large bending radius (~8 m). During the manufacture, the SCC case is divided into two L-shaped parts and the B/TCC case is a U-shaped part and a flat cover plate. Both the L-shaped part and the U-shaped part are obtained by respectively welding L-shaped sub-parts and U-shaped sub-parts which are manufactured by extrusion. During the final enclosure welding of the case, the fiber laser multi-pass welding technique with filler wire is proposed as a manufacture route due to its little deformation and narrow heat affected zone. The configuration of the enclosure welding machine, the tests for welding process parameters and the welding procedures are discussed in the paper.

An Optimized Central Solenoid for ITER

The Central Solenoid (CS) of the ITER tokamak has to provide the flux variation needed to induce the plasma current and to shape the field lines in the divertor region. It is designed as a stack of 6 identical coils, independently power supplied. Repulsing forces arising between the coils during a scenario are withstood by a precompression structure installed around the coils. Studies were carried out to simplify the winding manufacture, to optimize the precompression structure and procedure, to optimize the stack assembly of the 6 coils and the assembly of the central solenoid inside the tokamak which allows withdrawal from the machine, while meeting the ITER design criteria and in particular the Magnet Structural Design Criteria (static and fatigue).

ITER Central Solenoid design

The Central Solenoid (CS) is a critical component in the ITER tokamak providing plasma current drive and shaping. The CS final design is being completed at the US ITER Project Office (USIPO) in Oak Ridge, TN under a Procurement Arrangement with the ITER Organization (IO). Key design decisions have been made and CAD models and drawings developed. Interfaces have been established. An extensive R&D program has been completed. Analyses have been conducted to verify the design meets requirements. Design documentation is being completed in anticipation of a Final Design Review in the fall of 2013. The paper describes the key features of the CS final design.