INVESTIGATION OF CRITICAL PARAMETERS FOR THE STABILITY OF AC MAGNETS

Testing the POLO coil (empty set = 3 m, U-N = 23 kV, I-N = 15 kA), a p rototype coil for generating the poloidal field in a tokamak, no ramp rate limitation (RRL) was observed. Such a phenomenon can be attribute d to a current imbalance amongst the cabled strands or subcables leadi ng to a lower than expected quench current; The achievement of dischar ge time constants of 2 ms and local field change rates of 240 T/s with out quench is mainly attributed to the design of the POLO conductor, a low loss conductor with a central cooling channel. Motivated by the w ell defined symmetrical structure of the conductor and winding, a mode l for calculating the current distribution in one half of the coil was developed. In superconducting magnets the current distribution during ramping depends on the electromagnetic system properties. Therefore, the model considers the complete inductance matrix of the cable and th e fact that all turns are mutually coupled. Its validity was confirmed by simulating and measuring quench propagation processes during the P OLO experiment. By means of that model a detailed analysis of principl e causes for RRL phenomena in superconducting magnets was performed. I n this paper the results of these investigations are presented. It is shown that unequal contact resistances can not be responsible for RRL in coils with parameters comparable to those of the POLO coil and that already minor geometrical disturbances in the cable structure can lea d to major and lasting imbalances in the current distribution of cable s with insulated and non-insulated strands. The simulations demonstrat e that the electrodynamics of subcable current distribution in short s ample cable experiments differ extremely from the ones in a coil, so t hat such experiments do not seem to be suited for an analysis of the c auses for RRL in superconducting magnets.