Magnetic cone

A hollow cone (like a party hat) has vertex angle $2\theta$ , slant height $L$ , and surface charge density $\sigma$ . It spins around its symmetry axis with angular frequency $w$ . What is the magnetic field at the tip.

Use $\theta= 30^\circ , L = 8\text{ m}, \sigma = \dfrac2{\mu_0} , w= 18$ .