Segregated copper ratio experiment on transient stability (SeCRETS). Final Report

Two Nb3Sn, steel jacketed, cable-in-conduit conductors have been manufactured with identical non-Cu cross sections and the stabilizer either included in the Nb3Sn composite or partly segregated as copper wires. The two conductors are series connected and wound as a bifilar , single layer solenoid, assembled in the high field bore (11 T) of the SULTAN test facility. The operating current is up to 12 kA (400 A/mm2). A transverse pulsed field is applied with ΔB up to 2.7 T, field rate up to 180 T/s and field integral up to 530 T2/s. In the dc test, a good agreement is found between the Ic and the Tcs results, both correctly scaling according to the parameters derived from the strand tests. The n-value from the V-I curve is in the range of 15. The current sharing at the high field section is correlated with a local current re-distribution, observed by arrays of miniature Hall sensors, detecting the self-field around the conductor. The ac losses results in the range of 2 to 9 Hz by gas flow calorimetry indicate coupling currents constant, nτ, in the range of 1.5 ms at high field, increasing by a factor of 2 with 12 kA transport current. Loss extrapolation to 0 frequency suggests that the loss curve may be not linear outside the test range, with higher nτ at lower field rate. The calorimetric loss estimation at the fast field transient (f=15 Hz) indicates nτ ≅ 2 ms. The ITER plasma disruption transients have been reproduced by the pulsed coils. Due to the very low ac losses, no quench could be generated in either conductor even reducing the temperature margin below 0.2-0.3 K. Very large field transients, with integral above 100 T2/s, are required to quench the conductors. In that range, the conductor without segregated copper has superior performance. Due to the large interstrand resistance (very low ac losses), the segregated copper has marginal contribution to the stability. No evidence of current redistribution is observed during the field transients. No inflection due to the limiting current (calculated at h = 1000 W/Km2 is observed for either conductor in the stability curve as a function of the operating current, with dm/dt = 3.5 g/s. Computer simulations confirm that the limiting current is substantially higher than predicted. The stability, i.e. the ability to withstand large transverse field integral with 65 ms time scale, increases linearly with the mass flow rate for both conductors, in the investigated range of coolant speed, 0.3 -1.3 m/s. No evidence of induced flow is observed. The temperature margin to be allocated for the ITER field transients (plasma disruption) is marginal, <0.2 K, both with and without segregated copper. The amount of copper to be included in the strand cross section for ITER stability is by far smaller than predicted by the 'limiting current' criterion with h = 1000 W/Km2. To make effective the segregated copper for transient stability, a lower interstrand resistance is desirable, what can be safely tolerated for the transient field ac losses. (authors)