In a superconductor there is no resistance, Hence,
`L (dI)/(dt) + (d Phi)/(dt)`
So intergating `I = (Delta Phi)/(L) = (pi a^(2) B)/(L)`
because `Delta Phi = Phi_(f) - Phi_(i), Phi_(f) = pi a^(2) B, Phi_(i) = 0`
Also, the work done is `A = int xi I dt = int (d Phi)/(dt) = (1)/(2) L I^(2) = (1)/(2) (pi^(2) a^(4) B^(2))/(L)`