Thanks for the comments. 🙂

Yes, you are correct, but that curve was neither intended to be extrapolated beyond its 0.2 – 10 keV temperature range, nor was it to be used for scientific insight – It was only designed to be a resonably accurate fit over a limitted range of experimental values. The goal, as I stipulated it in my original post, was to find an expression for that would allow easy mathematical manipulation, focusing primarily on ease of integration. If you want a scientific model, the Bosch-Hale formulation can be used, but B-H cannot easily be integrated, and it is so complex that writing it down on paper requires almost a page, and the calculation needs to be broken into three parts prior to performance.

Unfortunately, as vansig & asymmetric_implosion correctly pointed out, the formulation is based on a stable, thermal (Maxwellian) plasma, so applying it outside of those parameters is not appropriate.