For a long-pulse operation of a free-electron laser driven by a recirculating electrostatic accelerator, the effects of beam loss on the operational stability of the accelerator are theoretically and experimentally analyzed. If a beam loss occurs mainly in the deceleration tube, the terminal voltage can keep constant even though the lost beam current is much higher than the charging current of the accelerator. However, the beam loss can induce an unstable operation of the accelerator such as a voltage breakdown or an increase of the beam loss with time. The accelerator instability can be the main problem disturbing the long-pulse operation of the FEL. From the analysis, a method to increase the available pulse width of the FEL is presented, which is to enlarge the capacitances between the neighboring electrodes of the deceleration tube. (C) 2003 Elsevier B.V. All rights reserved.