The Toroidal Field Model Coil Project (L- 2)

The construction and test of the TFMC is one of the seven large R & D projects of the ITER EDA. The TFMC has been conceptually designed by the EURATOM Associations. A consortium of European companies, called AGAN, consisting of Ansaldo Superconduttori (I), Alstom (F), Accel (D) and Babcock Noell Nuclear GmbH (D), have performed the engineering design of the coil and manufactured the TFMC under the management of the EFDA-CSU Garching. The TFMC was tested at the TOSKA facility of the Forschungszentrum Karlsruhe (Germany). The final assembly of the test rig and its installation into the TOSKA facility was a joint action of the AGAN consortium and the Forschungszentrum Karlsruhe.
The TFMC embedded in an inter-coil structure (ICS) is tested in two phases. In phase 1 the TFMC is tested alone in its self-field to determine the magnetic, hydraulic and mechanical behavior of the coil. In the more complex phase 2 the TFMC has been tested together with the EURATOM LCT coil delivering a background field and causing large ITER relevant stresses in the TFMC to test the mechanical integrity of the construction and to explore its operational limits. The TFMC, completed and delivered to the Forschungszentrum Karlsruhe in January 2001, was assembled with the ICS and integrated into the TOSKA facility till june 2001. The test phase 1 started in June 2001. The test phase 2 was completed in December 2002.

The TFMC main project objectives are to:

• Demonstrate the manufacturing feasibility of very large Nb3Sn coils as needed for ITER
• Achieve the required industrial tolerances
• Benchmark QA and inspection methods
• Test the coil to its operational limits

Guidelines for Layout and Design

Figure 1:The BB1 connections to the coil
conductor terminals

The TFMC and its test arrangement are designed to be representative for the ITER TF coils in respect of layout and electrical and mechanical stresses. The design of the TFMC took over as many as possible features of the ITER TF coils on a scale nearly 1:1. Only the overall dimensions had to be chosen in a way that the TFMC, assembled together with the already existing LCT coil, could fit into the TOSKA facility.

Layout and Manufacture of Toroidal Field Model Coil (TFMC)

Figure 2:The TFMC and the
ICS supported by the
auxiliary structure lifted for
insertion into TOSKA

The main components of the TFMC are the conductor, the radial plates that form the double pancakes (DP) and the coil case. The circular Nb3Sn cable in conduit conductor lengths were wound into the grooves of reaction moulds, then submitted to a reaction heat treatment at 650 °C for 200 h. After that the reacted conductor had to be transferred into the spiral grooves of the austenitic 316LN radial plates, being insulated at the same time with interleaved glass-Kapton tapes. This has required the development of new manufacturing methods including the related tooling. A 1.3 mm thick combined Glass-Kapton insulation has been applied to the DP surface and impregnated with DGEBA epoxy resin. Five double pancakes were stacked, insulated and impregnated to form the winding pack. Key components of the TFMC are the inter-pancake joints, which were designed and made to have very low resistances (~1 to 2 n Ohm). The winding pack is embedded in a stainless steel case.

The TFMC and ICS are equipped with a large number of sensors for protection, control and diagnostic. Each sensor has an identification number linked to the calibration data.

A quality assurance plan, according to ISO 9001, had to be followed stricktly during the manufacture of the TFMC and the ICS. Sub-components have been rigorously tested before proceeding to the next phase. In order to reduce the installation time, the application of industrial type quality procedures has been introduced for all trial and final assembly work. Deviations and deficiencies have been detected in an early stage allowing rectification. To avoid tolerance "build up" during assembly CNC measuring machines and laser tracker systems were used to ensure the required accuracy. Their use has allowed for comparison of "as built" and "as designed" components to assess installation tolerances.

The results of the project

Figure 3: 

After having achieved, on the 19th July 2001, an operating current of 57 kA, the power supply control system had to be readjusted. On the 23rd July the test program was resumed and 69.3 kA were reached on the 24th July. This value is already above the nominal design current of the toroidal field coils of the ITER machine.

On the 25th July the TFMC was ramped up to the maximum current of 80 kA allowed by the facility, corresponding to a maximum magnetic field of nearly 8 T (see graph).

The behavior of the superconductor and the joint resistances are as expected, as well as the temperature increase in the structures during fast ramp down and safety discharge. The NbTi busbars carried 80 kA at a maximum magnetic field of about 3 T and at a cryogenic temperature of about 5 K. At that field level the inter-busbar joints behaved better than expected. These results are a preliminary confirmation that the design of the ITER Poloidal Field coils is viable.

Beginning of 2002 the TFMC was assembled with the LCT Coil to simulate the actual operating conditions (with transversal field and radial forces) of the Toroidal Field Coils in the ITER device. The cool down started in August 2002 and the maximum operating conditions have been achieved on 21 November 2002.

The main steps of Phase 2 testing have been:

• Repeat the operation of the TFMC alone at 80 kA
• Cycle the TFMC in presence of transversal magnetic field.
• Reach the current sharing temperature at different load case up to 80 kA in the TFMC and 16 kA in the LCT

All tests have achieved the expected results and the operating conditions of ITER have been achieved with a temperature margin of about 1.5"K.

Conclusions

With the successful completion of the TFMC it has been demonstrated that large superconducting Nb3Sn coils can be manufactured by industry. Thus the objectives of this project are fulfilled. All tests have achieved the expected results and the operating conditions of ITER have been confirmed within a reasonable temperature margin.

Figure 4:The winding pack of the TFMC
lifted after impregnation

Figure 5:The 117 t heavy test
configuration (ITER TFModel Coil with
the EURATOM LCT coil and intercoil
structure) on the way to the TOSKA
vessel