A prototype Long Pulse Ion Source (LPIS) was developed for the Neutral Beam Injection (NBI) system of the Korea Superconducting Tokamak Advanced Research (KSTAR) project. Beam extraction experiments of the ion source were carried out at the Neutral Beam Test Stand (NBTS) at KAERI. The second gap (the distance between the G2 grid and the G3 grid) of the accelerator was shortened from 9.0 mm to 7.0 mm and 6.0 mm as part of an effort to increase the ion beam current with low beam divergence. The performance improvement and the limitations of the LPIS with its shortened second gap are experimentally presented and discussed regarding the study of the beam characteristics. From these efforts to find the optimum beam parameters, design directions for the next accelerator are presented. (C) 2011 Elsevier B. V. All rights reserved.