Arc length of a helix

A helix is written in a parametric form as follows,

\[ x = r \cos \phi , y = r \sin \phi , z = a \phi ,\tag{12} \]

where \(r , a > 0\) are constants and \(\phi\) varies from \(0\) to \(\infty\). Find the length of the helix as a function of \(\phi\)