The ADITYA (R0 = 75 cm, a = 25 cm) ohmically heated circular limiter tokamak has been designed and built to produce plasma in a toroidal vacuum chamber of rectangular cross-section with 20 numbers of toroidal field coils capable of producing ~ 1.5 Tesla toroidal magnetic field at plasma centre. The ADITYA tokamak has been successfully operated for 3 decades with more than 30,000 discharges. Recently ADITYA tokamak has been upgraded into a state-of-art machine with new plasma chamber of circular cross section for divertor operation and good plasma control to support the Indian Fusion program. To minimize the up gradation cost and schedule, the existing sets of toroidal magnetic field (TF) coils, poloidal magnetic field (PF) coils and the Ohmic coils has been retained in the ADITYA-U tokamak. The TF coil of 1260 x1030 mm overall dimensions are picture frame type, made of two “C” halves with each half having 6 machined turn made of ETP copper conductor of 12.5 mm thick with inter turn insulation of GFRP as shown in figure-1. Among two halves, one is large in size (in radial direction) and another one is small in size. The two halves designed is considered to facilitate in-situ integration/dis-assembly of the plasma chamber. Two halves of the each TF coil finger joints are bolted together using 16 nos. of M20 bolts. TF coils are designed to with stand in plane and out of plane EM forces arising during operation. The maximum in plane EM forces calculated due to 50 kA current is ~0.16MN resisted by the buckling cylinder. The TF coil system is analysed to ensure the structural reliability. The TF coil finger joint’s fasteners, which connect the two C’s (large and small), are impossible to tighten due to non-availability of access, in presence of the surrounding supporting structure and wedge blocks after initial assembly of the ADITYA tokamak. After successful operation of TF coils of ADITYA tokamak close to the design parameters, the TF coils were dismantled for ADITYA upgrade. During the dis-assembly of Toroidal magnetic field (TF) coils, it was found that many TF coil’s joints were damaged and finger joints fasteners were completely loosen. The loosened bolting connections, resulted into arcing lead to TF coil damage. To avoid this happening again in ADITYA upgrade, finger joints of TF coils are designed to ensure the structural integrity of the joints without any maintenance once it is assembled. The novel mitigation technique implemented, is the use of multifunction wedge lock washers. The wedge type lock washer prevents bolt loosening caused by vibration and dynamic loads, compensates for loss of preload due to settlement and relaxations. It facilitated accurate preload, increased operational reliability while reducing the risk of unplanned production stops and accidents. The support structure as shown in figure-2 is designed and implemented along with a pair of lock washers used in each bolted joints in the TF Coil.
The damaged coils are refurbished using in-house techniques. These 20 TF coils are assembled one by one on Test Stand by joining both C’s sections and the electrical parameter testing (Resistance and Inductance) of the coils have been carried out. The measured values of all coils are within satisfactory limits and found that they are in good condition to be reused again. After successful tests, TF coils are assembled on the ADITYA-U tokamak as shown in figure-3.
The EM forces, support structure design and implementation to ensure the structural reliability of Toroidal Field magnet system of ADITYA-U tokamak is presented in this paper.
|Country or International Organization||India|
|Affiliation||Institute for Plasma Research, India|