7. The length of the minor axis of the ellipse $\rho=\frac{1}{2-\cos \theta}$ is $\qquad$
7. $\frac{2 \sqrt{3}}{3}$
$$
\rho(0)=a+c=1, \rho(\pi)=a-c=\frac{1}{3} \text {. }
$$

Thus $a=-\frac{2}{3}, c=\frac{1}{3} \Rightarrow b=\frac{\sqrt{3}}{3}$.
Therefore, $2 b=\frac{2 \sqrt{3}}{3}$.