Ignitor

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Design

Physics

The machine design is characterized by a complete structural integration of its major components (toroidal field system, poloidal field system, central post, C-clamps and plasma chamber). A "split" central solenoid (see section magnetic fields) provides the flexibility to produce the expected sequence of plasma equilibrium configurations during the plasma current and pressure rise [1].
The adopted design of the machine magnets is aimed at having a stress distribution as close as possible to an isotropic distribution in the most critical areas.
ANSALDO has carried out extensive mechanical design and structural analyses for all the main components of the machine core, the so-called Load Assembly. 3D modeling of the components that mechanically interact with each other have been performed. In parallel, 2D/3D drawings of each individual component have been produced (Dassault Systems CATIA V software).
In particular, the machine Central Post, the  Central Solenoid, the Shape and Equilibrium coils, the Plasma Chamber and supported First Wall, the surrounding mechanical structures, the Vacuum Cryostat and supported polyethylene-boron composite sheets for neutron shielding have been structurally analyzed and mechanically verified to confirm they can withstand both the normal and the off-normal operating loads for the maximum current scenario at 11 MA/13 T. Also the structural analysis of other scenarios concerning plasma configurations of the double X-point type and of the refrigeration plant (the latter performed by the LINDE Kryotechnik AG) have been carried out. Results have demonstrated the feasibility of the machine from the structural point of view.
A new configuration for the cooling channels in the toroidal magnet that minimizes the distance between the region (adjacent to the central solenoid) where the maximum temperature is produced and the region reached by the coolant has been designed. For the most extreme operation parameters of the toroidal coils (13 T), the cooling time of 5 hours is reduced by a factor two, for the reduced scenarios (6-7 MA/9 T) cooling times under 1 hour are permitted by this new solution.
The Plasma Chamber is a key component that has to sustain the electromagnetic and thermal loads due to plasma off–normal events, assuring the required degree of high vacuum. Moreover, it plays a role in an effective stabilization of MHD modes near the plasma boundary, given its closeness to the plasma. According to the present design, the Plasma Chamber is made of Inconel 625 for its excellent mechanical properties. In fact, Inconel (often referred to in English as "Inco"), which is a family of austenitic nickel-chromium-based superalloys (the name is a trademark of Special Metals Corporation), is a corrosion resistant material well suited for service in extreme environments subjected to pressure and heat. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack.
The computational investigation of the toroidal distribution of the EM loads due to halo currents and a scaling to Ignitor from the values observed in JET disruptions have confirmed the value of about 10 MN for the net horizontal force produced during an off-normal event. This has led to a significant increase of the toroidal vessel thickness up to 26 mm at the inboard region, 36 mm at the top/bottom regions where the halo currents flow during vertical disruptions, and 52 mm at the outboard region where twelve equatorial ports are attached.
A High Performance Plasma Parameter Control (H3PC) is aimed at avoiding/mitigating disruptions by simultaneously regulating a number of plasma parameters (e.g. a relevant safety factor, the internal inductance, the plasma density relative to the density limit, the status of MHD activities) that can play the role of disruption precursors.
The ICRH antenna design is being carried out by Politecnico di Torino in collaboration with the Oak Ridge National Laboratories. A full size prototype of
the 25 Ω (actual value) Vacuum Transmission Line (the part of the antenna that transfers the power into the plasma and separates the vacuum of the plasma chamber from the outside port flange) has been manufactured and tested to verify the antenna design and the associated installation procedures.
High voltage tests, at different vacuum and pressure conditions, have been carried out to verify the capability of the system to withstand the high voltages (
~ 12 kV) and electric fields (~ 5 kV/cm) associated with the maximum RF power in the presence of a strong mismatch with the plasma load.
A further important aspect of the Ignitor design is the adoption of the MgB
2 (Magnesium Diboride) superconducting material for the largest poloidal coils (P14, diameter of about 5 m). These coils will be cryocooled at the operating temperature of 10-15 K and are designed to carry 34,7 kA, with a magnetic field, at the coils location, in the range 4 to 5 T. These requirements can be met by commercially available MgB2 strands. The construction of a scaled (1:3) version of the strand is planned [2].


1) F. Bombarda et al, Braz. J. Phys. 34, 1786 (2004)
2) B. Coppi et al, Overview Paper OV/P-02, Proceedings of the 24 th IAEA Fusion Energy Conference, San Diego, US, 8-13/10/2012



 
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