Cowan ion model

[Yurlungur]

When requesting a split equation of state, EOSPAC offers the ability to split an EOS in several ways. Singularity-eos mostly supports analytic splits through the zsplit and ideal electrons equations of state, which can be mixed and matched with SESAME tables. One additional analytic equation of state that EOSPAC supports in this context is the Cowan ion model, developed in the IONEOS model. The version used by EOSPAC is described in the manual in Section 3.2.

The model is essentially piecewise, with phase transitions around the Debye and Lindemann temperatures. The model depends also on the ionization state of the material, making it an appealing option for partially ionized gasses. We may wish to add this to singularity-eos going forward as we explore partial ionization support.