Solution
= Solution
The physical filament ends when the <mass density> first reaches zero. If $x_1\simeq2.4$ is the first positive zero of the <Bessel function> $J_0$, then
$$
\boxed{
R_f=x_1a
=x_1\sqrt{\frac{K+B_0^2/(8\pi\rho_0^2)}{2\pi G}}}.
$$
The first zero must be used because continuing to the next lobe would make the density negative.