Tip heater for minimum quench energy measurements on superconducting strands

Superconducting strands can be characterized by their minimum quench energy (MQE), i.e. the minimum heat pulse needed to trigger a quench in operation conditions (field, temperature, current), in the limit of a (temporally and spatially) /spl delta/-shaped disturbance. The sub-mm//spl mu/s range of perturbation space has only been achieved using the electrical graphite-paste heater technique. The present work has put this technique into practice for the strands of the LHC main magnets, which are designed to operate at 1.9 K in peak fields of up to 9 T. No way has been found yet to calibrate MQE measurements. To make relative statements on the MQE of different samples possible, the reproducibility of the measurements was emphasized. First heater prototypes did not come up to this stipulation. Finally the tip-heater configuration was found to meet the requirements. It generates a heat pulse in a thin resistive graphite paste deposit on top of a small tip that is pressed against the sample with a clamp. The clamp guarantees a maximum of exposure of the sample to the surrounding cryogen. The most striking aspect of repeated measurements on a reference sample is that in open bath conditions the MQE as a function of transport current in subcooled helium can reach hundred times the corresponding value in adiabatic conditions (i.e. with the sample potted in a low conductivity medium). This extraordinary cooling performance of superfluid helium, predicted by many has rarely been shown in superconductor stability experiments.