Some experiments on liquid helium heat transfer-characteristics affecting stability of superconducting magnet operation

Heat transfer from 25 μm thick, 6.36 mm wide Nb tape into boiling helium in vertical channels of 1.7 × 21 mm cross section was studied. Normal zones were initiated by heaters attached to the tape surface not in contact with the liquid. Two heat transfer effects of possible importance for superconductor stability were observed: 1. Steady normal zones enabling the measurement of localized heat transfer, and 2. Heat transport to neighboring tapes by means of the coolant. 1. Steady normal zones are those that neither grow nor decay; they occur at current densities of 13 to 17 kA/cm², corresponding to heat transfers of 0.45 to 0.8 W/cm²for the normal portion of the tape. Fluctuations of the length of the normal zone are < 0.4 mm. Comparison of measured temperature profiles with calculations yields localized heat transfer values. A heat transfer ∼3 times higher than the peak nucleate boiling value of 0.42 W/cm²(measured for brass tapes) exists in a 2 mm wide region near the normal to superconducting boundary. It is inferred that this is the region where the gaseous film covering the normal zone terminates and wetting by the boiling liquid begins. 2. In a current range where the normal zones decay again, it is observed that heater pulses (square,1 s long) larger than a critical level (0.7 W) cause normal zones to appear in tapes directly above the heater-induced normal zone, with a delay of the order of 0.1 s. Suggested explanation: The heat from pulse and induced resistive zone vaporizes the limited amount of liquid helium in contact with the zone and, before convection has time to develop, the helium becomes superheated above the critical 6 - 8 K. As it then rises normal zones are created.