A practical fit for the critical surface of NbTi

Known expressions for the critical temperature, critical field and Pinning force in NbTi are combined into a self consistent fit formula that provides the critical current density as a function of temperature and field. The m ain advantage of such a fit is the extended validity range. Data available in literature and measurements on LHC strands are used to demonstrate the a ccuracy of the fit. The J(c) data-sets used to cover a range of field from 0 T to 9 T and temperature from 1.9 K to 9 K. The standard deviation of the fits presented is of the order of 5 % or better. This accuracy is generall y sufficient for design purposes, extrapolation and scaling of measured res ults. Better accuracy, e.g. for short sample limit prediction, can be achie ved restricting the domain of validity.