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Coping with extreme stress

-Juan Knaster, ITER Magnet Division

In order to cope with the extreme electromagnetic forces, pre-compression rings (red) will be installed. (Click to view larger version...)
In order to cope with the extreme electromagnetic forces, pre-compression rings (red) will be installed.
ITER will be the biggest tokamak built so far. The electromagnetic forces acting on the machine structure during operation will be huge -- to the order of several hundred Meganewtons (MN). In order to cope with the out-of-plane forces caused by electromagnetic interaction between the Toroidal Field (TF) coil current and the magnetic field generated by the Poloidal Field (PF) coils, pre-compression rings will be installed to pre-compress the poloidal keys at the top and bottom of the inner straight leg of the TF coils with a centripetal force of 32MN. This will suppress any gap opening and greatly reduce any cyclic fatigue stresses.

Mockup rings have been stressed to rupture in a testing machine which presents 18 hydraulic independent pulling holders providing a stress distribution in the rings similar to the one the 18 TF coils will induce during operation. (Click to view larger version...)
Mockup rings have been stressed to rupture in a testing machine which presents 18 hydraulic independent pulling holders providing a stress distribution in the rings similar to the one the 18 TF coils will induce during operation.
But what material would hamper the induced currents and withstand the high loads? A thorough study of possible alternatives was carried out at the Ente per le Nuove Tecnologie, l'Energia e l'Ambiente (ENEA) in Frascati, Italy, and the most suitable material turned out to be S2 glass-fiber/epoxy composite. More recently, a mockup compression ring one fifth the original size was manufactured following vacuum pressure impregnation techniques. By using this technique, it is possible to reach over 60% of glass content in the composite which results in a stiffness of over 50 Gigapascale (GPa): four times lower than that of steel which would accommodate machine settling effects.

Recently, a mockup compression ring one-fifth the original size was manufactured following vacuum pressure impregnation techniques. (Click to view larger version...)
Recently, a mockup compression ring one-fifth the original size was manufactured following vacuum pressure impregnation techniques.
On 28 January this year, a rupture test was carried out reaching an amazing 1400 Megapascal (MPa) before breaking in a testing facility specifically prepared with this aim. This is more than a factor 3 higher than the ITER design value of 440MPa. A large-scale testing facility is now foreseen to be built for the qualification of the final rings. The Procurement Arrangement is scheduled to be signed this year.


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