Approximation and algebraicity in positive characteristic Hahn fields

We study the relative algebraic closure $K$ of ${\bar{\mathbb{F}}}_p((t))$ inside $\bar{\mathbb{F}}((t^{\mathbb{Q}}))$. We show that the supports of elements in $K$ have order type strictly less than ${\omega }^{\omega }$. We also recover a theorem by Rayner giving a bound to the ramification away from $p$ in the support of elements in $K$, and an analogue of Rayner’s result for the residue field. This work has applications to the decidability of the first order theory of ${\mathbb{F}}_p((t^{\mathbb{Q}}))$, and other tame fields, in the language of valued fields with a constant symbol for $t$.