AC loss and stability of superconducting cables


TF cross section   Artist view ITER

Research financially supported by the ITER Organisation. ITER logo  

The cabled superconductors to be applied in ITER have to withstand enormous electromagnetic forces and fast ramping magnet fields. Important research on this subject is done at the EMS group for more than a decade. The effort led to discovery of the remarkable change in AC loss with cyclic loading, inevitable knowledge for fusion magnets.

Creating reliable and economic operating superconducting magnets needs basic understanding of materials science, its fundamental phenomena and a multi-physics approach. Facing the great challenge of the complex interaction of all individual aspects, e.g. current and potential distribution, lattice strain, thermal instabilities, material diffusion and interaction that can occur in composite cabled superconductors, asks for an integrated multidisciplinary approach. One of the key ingredients is the Twente Cryogenic Cable Press, developed to study AC loss, interstrand resistance and cable deformation under ITER relevant loading conditions in terms of temperature, applied magnet field alteration, force and number of load cycles. Besides delivering important new scientific results, the facility also serves for qualification tests of ITER conductors and the Press AC loss results are the exclusive basis for AC loss analysis and stability of the entire ITER magnet system.

ITER Magnets and conductors

The impressive and unprecedented ITER Magnet System comprises 18 superconducting Toroidal Field (TF) and 6 Poloidal Field (PF) coils, a Central Solenoid (CS), and a set of Correction coils (CC) that magnetically confine, shape and control the plasma inside the Vacuum Vessel. The power of the magnetic fields required to confine the plasma in the ITER Vacuum Vessel is extreme. For maximum efficiency and to limit energy consumption, ITER uses superconducting magnets that lose their resistance when cooled down to very low temperatures. The TF and PF coils lie between the Vacuum Vessel and the Cryostat, where they are cooled (at 4 K) and shielded from the heat generating neutrons of the fusion reaction taking place at a temperature of 150 million K (see illustration below). The TF coils will weigh 6540 tons total and the strands have a total length of 80,000 kilometers; besides the Vacuum Vessel, they are the biggest components of the ITER machine. The superconducting magnets (CS and TF) are designed to achieve operation at high magnetic field (13 Tesla) and large current (up to about 70 kA). The superconductors are special alloys made of Niobium and Tin (Nb3Sn). The PF and the CC coils use a different, Niobium-Titanium (NbTi) alloy.

The coils will be made of Cable-In-Conduit superconductors, in which a bundle of superconducting strands is cabled together and cooled by flowing Helium, and contained in a structural steel jacket (see photo below).

Artist view of ITER magnet system   Cross section and layout of ITER cable
The ITER fusion reactor with superconducting coils and plasma (left), a full-size ITER superconductor from the Central Solenoid Magnets with almost 1000 superconducting and copper strands of 0.8 mm diameter (middle) and the twisted structure of the cable elements (right). 

Characteristics of ITER superconductors

The open nature of the cable bundles in the sizeable Cable-In-Conduit-Conductors for ITER containing more than thousand strands, have a void fraction of around 30 % to be filled with helium, which is needed for sufficient cooling capacity preserving stable behavior.

The maximum values of temperature, electrical current and magnetic field are interdependent and the pictures below show the critical surface for a superconductor, which is the boundary between superconductivity and normal resistivity in 3 dimensional space. The applied field (B), the temperature (T) and the current density (J) must be maintained below the critical surface in order to retain superconductivity (see plots below). In other words, if the operating temperature is about 5 K and the transition at 7 K, the margin left is 2 K.

   critical surface NbTi Nb3Sn
The critical surface for a superconductor, which is the boundary between superconductivity and normal resistivity in 3 dimensional space, applied field (B), temperature (T) and current density (J).  

When conditions in a localized region of a superconductor exceed the critical surface, that area will start to conduct resistively producing local heating. The resulting heat and current transfer will start a cascade effect and result in a rapid transition of the whole superconductor to the normal state (quenching).

The electromagnetic AC loss of ITER Nb3Sn and NbTi CICCs, related to changing magnetic field depending on the plasma scenario, is in this manner crucial for their stability as it leads to a higher temperature of the conductor. The overall lifetime characteristics in terms of AC loss, interstrand contact resistance, cable compression, changes in transverse stiffness and mechanical losses are experimentally evaluated on various NbTi and Nb3Sn conductors in the Twente Press. This is particularly critical for application in fusion reactors like ITER, because the number of load cycles can be up to 60,000.

Presently the unique test site is also used for ITER conductor qualification testing. The large cables are subjected to the magnet operation conditions with huge mechanical forces, transient magnet field and cryogenic temperatures (4.2 K) proving mainly their quality in terms of AC loss and thus sufficient temperature margin. The AC loss data are essential for the thermal analysis of the entire ITER magnet system as these results are the only adequate data available for the ITER magnet system.

Twente Press
The unique Twente Cryogenic Cable Press for simulation of the ITER magnet operating conditions up to 70,000 A in 12 T background magnet field (left).

Plot stress - strain Press sample   Increase AC loss PF CICC
On top the results of the mechanical characterization and below the drastic increase of the AC loss (and heating by power dissipation) with the number of load cycles in the ITER Poloidal Field Conductor.  

Sustainable energy

Nuclear fusion is seen as one of the answers to the worldwide energy issues: it is clean, safe and sustainable and does only produce short-living radioactive waste. The energy is generated from melting together light and heavy atomic nuclei, within a plasma at extremely high temperature. Nuclear fusion is the energy source of the sun and the stars. Compared to fossil fuels, this source of energy is inexhaustible. Superconducting magnets will be almost one third of the building costs of the ITER Tokamak, to be built in Cadarache in France; this experimental reactor aims at delivering 500 Megawatt by nuclear fusion. November 2006, the participating countries have signed the contracts for building this reactor. Parallel to this, the G8 countries have placed nuclear fusion high on their agendas as a sustainable way of generating energy.

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