At JAEA, as the Japan Domestic Agency (JADA) for ITER, a MAMuG (multi-aperture multi-grid) accelerator has been developed to perform the required R&D for the ITER neutral beam (NB) system. As a result of countermeasures to handle excess heat load to the ion source by backstreaming positive ions, H− ion beam current was increased to 0.32 A (the ion current density of 140 A m−2) at a beam energy of 796 keV. This high power beam acceleration simulated the ITER operation condition maintaining the perveance (H− ion current density/beam energy3/2) of the ITER accelerator. After the high power beam operation, the pulse length was successfully extended from 0.2 to 5 s at 550 keV, which yielded a 131 mA H− ion beam as an initial test of the long pulse operation. A test of a single-aperture single-gap (SINGAP) accelerator was performed at JAEA under an ITER R&D task agreement. The objective of this test was to compare two different accelerator concepts (SINGAP and MAMuG) at the same test facility. As a result, the MAMuG accelerator was defined as the baseline design for ITER, due to advantages in its better voltage holding and less electron acceleration. In three-dimensional beam trajectory analyses, the aperture offset at the bottom of the extractor was found to be effective for compensation of beamlet deflection due to their own space charge. It has been analytically demonstrated that these compensated beamlets can be focused at a focal point by adopting the aperture offset at the final grid of the accelerator.