Byung Su Lim

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.

Performance Analysis of the ITER Poloidal Field Coil Conductors

Recent design changes to the ITER Poloidal Field (PF) coils and PF conductor layout have been implemented to give a greater operational window for low li (self-inductance) plasmas during burn, extend the operating window for plasmas with currents above 15 MA, and improve the plasma vertical stability control. In addition, the PF and CS (Central Solenoid) operating window has been updated leading to higher currents and peak fields in some of the PF coils. Altogether, this results in higher heat generation in the PF conductors due to AC losses. To confirm that the range of heat loads is acceptable for the PF conductors, a time-dependent thermo-hydraulic analysis of the PF coils has been performed with a model based on the GANDALF code. The deposited heat due to AC losses in the conductors, thermal radiation, thermal conduction, and nuclear heating are given as input data. The results show a moderate impact on minimum temperature margin of the PF conductors during the baseline scenario, stemming from the design modifications.

Development of the ITER PF Coils

The ITER Poloidal Field (PF) magnet system consists of six coils. Niobium-Titanium (NbTi) is used as superconducting material and cable-in-conduit conductor(CICC) type are used as a conductor. All coils are fabricated by stacking 6 to 9 double-pancakes wound by two-in-hand winding scheme. The six PF coils (PF1 to PF6) are attached to the Toroidal Field (TF) coil cases through flexible plates or sliding supports to allow small radial and vertical displacements. The outer diameters of the coils vary between 8 m and 24 m. Since the PF coil system provides magnetic field for plasma shaping and position control together with the Central Solenoid (CS) coil, it needs to operate in fast pulse mode, leading to induced voltages of up to 14 kV on the coil terminals during operation.

Reliability Considerations for the ITER Poloidal Field Coils

Since it is practically impossible to remove the Poloidal Field (PF) coils from the assembled ITER (International Thermonuclear Experimental Reactor) tokamak without major interruption in operation, the design of these coils shall provide their high reliability under high voltage operation. The design of the coil insulation relies on a separation of functions: mechanical function of the load transmission, performed by glass-fiber impregnated with epoxy resin on one side, and the independent electrical barrier made of polyimide tapes on the other side. Numerical simulation has shown that the maximum electrical field in the coil is lower than 4 kV/mm, which is taken as the design criterion for the PF insulator system. In case of a single insulation failure in a coil, its functionality can be recovered by installing a so-called jumper to by-pass the faulty double pancake. The design of the jumpers and their installation procedure are described.

Design of the ITER PF Coil Joints

The ITER poloidal field magnet system consists of 6 pulsed coils with a diameter ranging from 8 m to 24 m and a weight of up to 400 t. The coils are made of independent modules connected to each other by joints. This paper summarizes the design constraints, including manufacturability and repair procedure, and the layout of the joints based on the “twin-box” concept. It particularly focuses on the structure of the joint and its support since the joints will operate at the maximum current of 55 kA in a 2 T magnetic field, and there- fore should be able to withstand high electromagnetic forces. In this article, we present the main results of the finite-element structural analyses of the joint. Finally, we summarize the main results of the electrical model developed to determine the losses (AC and transport current) and current sharing in the joint.

Analysis of Heat Load, Current Margin and Current Nonuniformity in ITER PF Coil Joints

The poloidal field (PF) magnet system of the International Thermonuclear Experimental Reactor (ITER) consists of six pulsed coils. Each coil comprises independent modules connected to each other through “shaking hands” joints. In the paper the results of the analysis of the electro-magnetic and thermal performance of the joints during the ITER 15 MA plasma scenario are presented. Of special concern is the radial magnetic field component that is particularly high close to the upper and lower edges of the coils. Moreover, the orientation of the joints in the PF coils is such as to give scope to large current loops between the two cables, which could potentially reduce the temperature and current margins to critical levels. The study has been carried out with the code JackPot-AC, which has been recently upgraded to allow a strand-level detailed analysis of lap-type joints.

Approaching ITER PF Coil Manufacturing

The ITER poloidal field (PF) coil system provides a magnetic field for plasma shaping and position control together with the central solenoid coils; it needs to operate in a fast pulse mode, leading to induced voltages of up to 14 kV on the coil terminals during operation. The PF magnet system consists of six coils. The cable-in-conduit conductors with niobium-titanium (NbTi) superconducting material are used in the coils. All coils are fabricated by stacking six to nine double pancakes wound by two-in-hand winding scheme. The six PF coils (PF1 to PF6) are attached to the toroidal field coil cases through the flexible plates or sliding supports to allow small radial and vertical displacements. The PF coils will be procured by the European and Russian domestic agencies under separate procurement arrangements. To accelerate the PF6 coil schedule, which is one of the critical paths for the ITER schedule, a cooperation agreement has been placed between F4E and ASIPP in China in October 2013 with the CN-DA support. Before starting the manufacturing of the coil, the component qualification has been started, such as the 3 × 3 conductor mock-up, turn insulation, and helium inlet with the dummy conductors. Corresponding mechanical and electric tests were carried out at room temperature and 77 K. The PF dummy double pancake is also wound to demonstrate the winding. This paper presents the updated design for manufacturing of components. Their fabrication methods are also described. This paper concludes with a summary state report on PF1 dummy winding.

Assembly of the Poloidal Field Coils in the ITER Tokamak

The ITER poloidal field (PF) magnet system is a set of six circular coils attached to the periphery of the toroidal field coil (TFC) structure. While the manufacturing of the PF coils is being launched in three different countries, the ITER Organization (IO) is looking ahead at assembly of PF coils in the tokamak. The PF coils can be grouped in three pairs with respect to their similarities in design and attachment to the TFC, which leads to preparation of different strategies and tooling for the transportation and assembly. The procedures shall also consider the integration, assembly sequence, and environment of the whole tokamak. For example, the two lower coils PF6 and PF5 must be brought in storage position in the tokamak pit and stay there until all the TFCs are assembled, before being attached to them. This paper presents the assembly scenario of the PF coils, including logistics, analysis of assembly tolerances, aligning strategy, and detailing the particularities of each PF coil, from the moment they are delivered to IO until they are connected to the feeder, waiting for the commissioning of the machine.