Analysis and full scale test of DC circuit breaker for tokamak device JT-60

In recent large Tokamak nuclear fusion devices, the energy stored in inductance has become to be employed in order to ignite and heat plasma. The device for this purpose is operated very frequently for the sake of improving service efficiency. This has led to the necessity to obtain heavy duty, longer life DC circuit breakers. For example, in the critical plasma testing device JT-60, direct current of 92 kA must be broken at about 10 min intervals. The authors succeeded to stabilize vacuum arcs through the investigations on the method to apply a magnetic field in parallel with the arcs. This allowed to drastically increase the life and the breakable current of breakers. Thus they have developed the DC circuit breaker for JT-60 by connecting the breakers of this type in parallel, and have succeeded to break 44 kV-130 kA duty. In this paper, first the power supply circuit for a commutation coil and the DC circuit breaker, and next, the problems and experiments on parallel current sharing are described, and the DC circuit breaker made for trial and its full scale test are reported. The vacuum circuit breaker applying axial magnetic field is ideal. The key to success lies in parallel current sharing, and the unequal opening time for each parallel circuit must be minimized. Further, the balancing action to correct the unbalance in current sharing by employing a saturable reactor can also be expected. (Wakatsuki, Y.)