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 CONTROL pulse needed to trigger a quench in o peration conditions (field, temperature, current), in the limit of a (tempo rally and spatially) delta-shaped disturbance. The sub-mm/mu s range of per turbation space has only recently been achieved using the electrical graphi te-paste heater technique [1], The present work has put this technique into practice for the strands of the LHC main magnets, which are designed to op erate at 1.9K in peak fields of up to 9T [2], No way has been found yet to calibrate MQE measurements. To make relative statements on the MQE of diffe rent samples possible, the reproducibility of the measurements was emphasiz ed. First heater prototypes did not come up to this stipulation. Finally th e 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 str iking aspect of repeated measurements on a reference sample is that in open bath conditions the MQE as a function of transport current in subcooled he lium can reach hundred times the corresponding value in adiabatic condition s (i.e. with the sample potted in a low conductivity medium). This extraord inary cooling performance of superfluid helium, predicted by many (e.g. [3] ) has rarely been shown in superconductor stability experiments.