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Description
The cryo compatible and helium leak-tight vacuum barrier (VB) is necessary in the superconducting (SC) Tokamak, which is required to separate the vacuum between the machine cryostat and superconducting current feeder system. It is a dissimilar material joint that electrically isolates 2 kV DC voltage during the quenching of SC magnets. The current feeder system, which houses the 4.2 K vapor-cooled current lead and superconducting bus-bars for carrying electrical current up to 10 kA in a vacuum of less than 1x10-5 mbar. In the Stead State Superconducting Tokamak (SST-1) machine, the need for the replacement of failed alumina ceramic VB, the glass fiber composite VB was developed. It was a great challenge to design and fabricate a joint of metal and GFRP composite at cryogenic temperatures in contrast to the metal and ceramic joints, which insisted on the development of a high-toughness epoxy resin system for bonding. Considering the SC magnet application and stringent service needs of electrical isolation of 2 kV, helium leak tightness of 1.0x10-8 mbar-l/s at 300 K to 4.2 K temperatures and vacuum isolation, the most appropriate method selection is adhesive bonding. The epoxy resin-based dissimilar material joint in the form of a VB consists of boron-free S-glass fiber of G-10 grade, and stainless steel 316L joints were designed and fabricated using the wet filament winding process. The thermal contraction and distance between the conductors were optimized for the Paschen discharge event. The counter geometry and sharp edge surfaces have taken into account the electric field strength and dielectric breakdown phenomenon criteria. The real technical challenges were encountered, especially the differential thermal contraction variation of glass fiber, epoxy resin, and metal-induced high radial thermal residual stress in larger VB, like 5-inch-diameter joints, at low temperatures, which is prone to the development of cracks in the joints. To overcome the thermal stress and flexibility issues, cryogenic bellows were developed of 118 mm diameter and 0.25 mm thickness in three-ply construction and assembled with VB. The cryogenic bellows provide flexibility of axial 35 mm and lateral movement of 6 mm in SC hydraulic. The cryogenic VB underwent rigorous performance testing at 0–30 bar (g) pressure, a thermal cycle test at 77 K temperature and 10 kV electrical tests during each stage of development. The VB joints of different sizes were investigated for mechanical tensile breaking strength test in pull load condition. The fracture was found at the parent material location of the GFRP tube cross-section; no joints were found broken at the bonded epoxy resin section. The tensile strength test resulted 75 MPa in ½ inch metal to glass fiber VB, which is higher than the allowable bulk shear strength of GFRP (30 MPa) that is parallel to reinforced fiber and 20 MPa value of bonding shear strength of epoxy resin. The developed larger-size glass fiber epoxy-based VB overcomes the important issues of brittle failure and electrical breakdown occurrence due to the short gap between metal conductors and the short radial distance between the SC bus bar and ceramic VB. The VB were installed and validated in the SST-1 machine at operating conditions in the SC bus bar of the current feeder system. In-house-developed components save significantly high cost than imported available. It is not readily available item in the local or international market. The development has demonstrated the indigenous and ‘Make in India’ concept for a cost-effective and innovative solution which facilitated more availability as per requirement, elimination of dependency on foreign agencies and long delivery schedule of item. This component can be used for future indigenous fusion machine and electrical isolation application at low temperatures. For repeatability, acceptance and reliability, hundreds of dissimilar material joints of glass fiber and metal were fabricated; a failure rate of 5% was observed. In this work, the design, development, fabrication technique, mechanical, electrical and LN2 thermal shock performance tests and the results of cryogenic VB joints will be discussed.
| Submitters Country | India |
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