An experimental investigation of the atomic carbon laser’s energy storage was conducted with a rotating back mirror. The high rotational speed of the mirror allowed the laser cavity to be switched to a highQconfiguration for various time delays after the discharge. This study, which combines results from experiments and computer simulations, revealed a short upper laser level lifetime of about 2 &mgr;sec. Thus, the lifetime of the upper laser state is too short to account for the delay observed (∼5 msec) in previous experiments on the atomic carbon nuclear‐pumped laser.