The Electromagnetics Key Problems Of TOKAMAK Superconducting Magnet And Feeder Systems

The feeders are the lifelines of the 1TER (International Thermonuclear Experi-mental Reactor) magnet system, which connect the magnet systems located inside the main cryostat to the cryogenics, power-supply and control system interfaces outside the cryostat. The current flow through superconducting busbar, the busbars conduct high currents using a Cable in Conduit Conductor (CICC) cable design concept. The main busbar conductors (MB) in ITER magnet system provide electrical as well as cryogenic connection of Toroidal Field (TF), Central Solenoid (CS), and Poloidal Field (PF) coils to the current leads, and the corrector busbar (CB) of corrector coils (CCs). Despite the zero resistance under stationary conditions, alternating currents and alternating mag-netic fields will cause energy dissipation (AC loss) in superconductors of the busbars. AC losses will effect on safe and stable operation of the busbars, AC losses of TF, PF and CS busbars are calculated in detail in 15MA plasma current reference scenario, in this dissertation, and the temperature behavior is simulated.The local current sharing temperature Tcs is a crucial parameter for the stability of the CICC conductor. At the current sharing temperature Tcs, the operational current gets the critical current, the superconductor gets quenching, and the current is shared by the copper of NbTi strands. The current sharing temperature of the full-size sample MBCN1 and MBCN4 of main busbar is tested in SULTAN, and the current sharing temperature of the samples in SULTAN condition is simulated by 1-D Gandalf model, there is large difference between tested value of the current sharing temperature and simulated value. A new method is introduced for estimating the current sharing tem-perature in this dissertation, and the current sharing temperature of the sample MBCN1 and MBCN4 are estimated by the new method, the estimated value is approximated to the tested value. The current sharing temperature of ITER main busbars and corrector busbar in 15 MA plasma current reference scenario are estimated by new method. The superconducting busbars maybe quenched in a short circuit owing to the energy pluse caused by resistive heating, the displacement and the thermal stress. The minimum quench energy (MQE) is defined as the energy pulse, of small spatial extent and short duration applied to the conductor, which is just sufficient to initiate a quench. The MQE of the busbars in 15 MA plasma current reference scenario is simulated by 1-D Gandalf model and estimated by solving thermal balance equation in this dissertation.The toroidal field of ITER is 5.30 T at a radius of 6.20 m, it is known that the higher toroidal magnetic field in the center of the TF magnets, the better confinement of plasma. A design of high field TF magnet, which products 10.0 T field at the center of plasma, is introduced in this dissertation, while the maximum field in TF magnet equals to about 20.0 T. At the temperature of 4.2 K and field of 20.0 T, the low tem-perature superconductor (LTS) Nb3Sn cable of ITER will quench when transmit 68 kA current, however, the superconductivity property of high temperature superconductor (HTS) YBCO tapes is sutiable in this condition. The HTS and LTS hybrid magnet is in-troduced in this dissertation, YBCO HTS cable is used in high field location, and NbTi superconducting cable is used in low field location. The HTS and LTS cables are cooled by super-critical helium in hybrid magnet, and the hybrid magnet products 10.0 T field at the center of the plasma. A design of YBCO stack HTS cable is introduced in this dissertation, YBCO tapes stack is embedded into copper stabilizer, the HTS cable can transmit 68 kA current stably at the temperature of 4.2 K and the field 20.0 T. AC losses of HTS cable caused by alternating currents are simulated by COMSOL model in this dissertation, the AC losses are very high, so the HTS cable is not applicable to transmit alternating current. At the temperature of 4.2 K, the field 20.0 T and transmission 68 kA current, AC losses of the HTS cable are simulated by COMSOL model in magnet excitation and demagnetization in this dissertation, and the temperature behavior caused by AC losses is simulated by COMSOL model. And the current sharing temperature of the HTS cable is calculated in detail.A new joint is designed in this study for connecting ITER main busbar, comparing with ITER two-box joint, the new joint has smaller volume, lower resistance and lower temperature rise. Untwisted the cable, and welded the sub-cable to copper sole with pure indium. The twist pitch of sub-cable is shorter than the cable, so the length of new joint is shorter. The resistance of new joint is simualted by COMSOL model, which equals 0.1 nil and lower than the resistance of ITER tow-box joint 0.2 nΩ. The temperatuer rise of new joint and ITER joint is simualted by 1-D Gandalf model.The superconducting joints are needed in hybrid magnet to connect YBCO HTS cable and NbTi superconducting cable, a design of LTS and HTS joint is introduced in this dissertation, the NbTi cable is untwisted and the sub-cable is welded to the copper sole with pure indium, and YBCO stack is welded to the copper sole just like steps with pure indium, one step each YBCO tapes. The resistance of the LTS and HTS joint is simulated by COMSOL model. The HTS joint is needed in hybrid magnet between YBCO HTS cables, a design of HTS joint is introduced in this dissertation. In this joint, there is not copper sole, the YBCO stack is welded each other directly just like steps with pure indium, one step each YBCO tapes, the resistance of the joint is quite lower according to simualtion by COMSOL model.