  We investigate the ground state current of an isolated hybrid
normal/superconducting ring (NS ring), threaded by an Aharonov-Bohm magnetic
flux. We calculate the excitation spectrum of the ring for any values of the
lengths of the normal metal and of the superconductor. We describe the
nonlinear flux dependence of the energy levels above and below the gap edge.
Using a harmonics expansion for the current, we isolate the contribution due to
these nonlinearities and we show that it vanishes for large normal segment
length. The remaining contribution is very easy to evaluate from the linearized
low energy spectrum. This decomposition allows us to recover in a controlled
way the current-flux relationships for SNS junctions and for NS rings. We also
study the crossover from persistent current to Josephson current in a
multichannel NS ring at finite temperature.
