11.6.1 Fabrication of the TF Coil Case in HT-7U
The TF coil cases in HT-7U device are 316LN stainless steel plate welded-up structures with full penetration welds. It’s structure is shown on fig.1. These case structures that encase each TF coil are used as the primary structure for the total superconducting magnet system. All poloidal field (PF) coils and cold-mass support system attachments are made to the TF coil-case primary structure. Shape of cross-sections of the TF coil-case is a “keystone” for the straight leg and a rectangle for the rest parts. The four walls thickness of the coil cases is not equal. The steel plate thick of inboard leg (nose) is 60mm.The 16 TF coil case inboard legs are wedged together and wedging loads are reacted by hoop compression yielded from centering force. The steel plate thick of outboardleg is 40mm. The steel plate thick of two sideboards is not equal thick of two sideboards is not equal, 25mm for the wedged parts and 30mm for the rests parts. The TF coil case is made of two halves with welding structures. The inter-coil structure is also a welded-up structure from 316LN stainless steel plate, thickness of which is 20mm. There are ribs between inner plate and outer plate. The inter-coil structure also reacts the overturning torque experienced by the TF coils. Because of the disruption of Plasma during operating, a Significant amount of heat energy is desosited in the TF case. In order to reinforce cool-down of TF case, fourteen cooling flat tube (meterial: 316LN) channels will put into the four wall of TF case with grooves.
Fig.1 The structure of TF case in HT-7U device
The overall TF case structure is a type “weld-up plate” structure, and its structure welding can be divided into two basic type: subassembly welds and closure weld at the time of final assembly. Each of these sub-assemblies contains many parts that have been fabricated by rolling and other machining of flat plate stock. Most of the welding will be done at the sub-assembly stage in a weld shop environment for better access easier make handling and stress-relieving and dimensional control. In the design of welding structure of case, we have considered the number of welded joints and fixture-setting weld positions, in order to avoid cross welded joint and putting them in high stress region. At the same time, all these welded joints are arranged and designed as down-hand welds and fillet welds.
The fourteen cooling flat tube channels will be welded with the soft solder adhering the exterior of surface of TF casing, seeing fig.2. The cross-section dimension of cooling channel is 22mm×8mm.
Fig.2 Cooling channels adhering the surface of the case are
being welded with the soft solder
When subassembly welds of the TF case are completed, the redundant metal material caused by the signification weld distortion had to be machined off by the large numerical control miller in order to acquire the suitable dimensional precision. Fig.3 shows the TF coil case is machining.
Fig.3 The TF coil case is machining.
The impregnated winding pack will be put into the 316LN stainless steel TF coil case. The basic TF coil case unit to be welded up is made up of two weld sub-assemblies, so the two circumferential welded joints will be needed to close up the TF coil case as the final welding. The closure welding is much more difficult than sub-assembly. The welding operation will require a fixture for aligning and clamping components to limit weld distortion. Furthermore, The closure welding requires that great care be taken in weld joint design, welding technique, and welding process in order to avoid overheating the ground insulation.