Health insurance and Carte vital

Bonjour à tous, I have a question regarding the health insurance in France. I planning to go to a University in Paris with a stipend (two-year postdoc). However the stipend won't be taxed neither in France nor in my home country (Germany). But by EU law I have to have a health insurance. I talked to my German health insurance and they require that the health insurance in France will be the public one, (as far as I know there is only one public insurance). Otherwise I might have some problems to be re-insured when returning from France. Does anyone have some experience with the voluntary health insurance in France? What are the requirements? And how much does the Carte Vital cost? Thanks a lot. All the Best, Rouven