A new-concept plasma grid, named frame-cooling-type plasma grid, was developed for a long pulse operation of a cesium-seeded volumeH−ion source. The frame-cooling-type plasma grid was tested using a long pulseH−ion source. The grid has a kind of bellows structure that acts as thermal insulator to keep the surface temperature at an optimum value forH−ion production. It was confirmed that the temperature was kept at over 250 °C, an optimum temperature for the cesium effect, in steady-state operations with the plasma grid cooled by room-temperature water. Using the plasma grid, a maximumH−ion beam of 40 keV, 600 mA(16 mA/cm2)for a long pulse operation of 90 s was generated successfully. ©1998 American Institute of Physics.