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CORNELL LABORATORY FOR ACCELERATOR-BASED SCIENCES AND EDUCATION

CLASSE NEWS | 12 Mar 2010

Temperature dependence of the superheating field in niobium measured

<noautolink>superheating.png</noautolink>
Figure: Plot of the superheating field data versus (T/Tc)2, where the sample's critical temperature was Tc=8.83 K. The green cone shows the Ginzburg-Landau prediction. The cone's width results from measurement uncertainty in a model parameter. The measured data agrees well with the model to within measurement errors..

Researchers at Cornell have recently made breakthrough measurements of the fundamental properties of the BCS superconductor Niobium, a material commonly used in microwave cavities for superconducting accelerators. Professor Matthias Liepe along with graduate student Nick Valles, has measured Niobium's superheating field in the full temperature range between 1.8K and its critical temperature. The superheating field is the maximum magnetic field up to which the Meissner state of a superconductor can exist as a metastable state. Above the superheating field, the superconductor starts to transition into the normal conducting state. They found that a simple phenomenological theory accurately (within the 10% error bars of our measurements) models the behavior of the superheating field down to temperatures of 1.8 K, which for the first time includes the region at which most superconducting RF accelerators operate (about 2K).

This research is important, because it marks the first time that scientists have been able to measure this fundamental property of Niobium over the full temperature range with certainty - made possible with the use of oscillating superleak transducers, another Cornell innovation. This critical magnetic field determines the ultimate limit for the accelerating field gradient of Niobium superconducting cavities, and suggests that for very pure Niobium, surface fields of up to 2400 Oe may be achievable, which is important for next generation accelerators such as the International Linear Collider.

A paper discussing these results is being considered for PRL-B, and a preprint is available here.