\documentclass{article}
\usepackage{calc}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[scale=0.5]
\foreach \r [count=\c from 2] in {9.5,9,...,6.5}{
\pgfmathparse{\c*5}
\fill[purple!\pgfmathresult!white] (0,0) circle(\r);
}
%Fond blanc au centre
\filldraw[fill=white, draw=white] (0,0) circle(6);
\end{tikzpicture}
\end{document}\end{document}
![alt text][1]
[1]: https://texnique.fr/osqa/upfiles/Screenshot_2023-05-16_at_10.20.15.png
