A 30‐kV triode accelerator has been constructed to test the design and operating characteristics of accelerators for negative hydrogen ion beams. Incorporating both a quadrupole electron trap and an octupole electron suppressor, the accelerator has been used to extract, accelerate, and focus H−ion beams of up to current densities of 20 mA cm−2. The results suggest that this accelerator design is capable of focusing across a wide range of ion current densities (0–20 mA cm−2) and acceleration voltages (15–30 kV) using the variation of a single‐gap potential to achieve the required focusing. Applying a bias voltage of up to 2 V between the beam forming electrode and the anode of the source suppresses electrons and has allowed accelerator power efficiencies of over 73% to be obtained for all ion current densities up to 20 mA cm−2so far extracted. Initial evidence that the extracted and accelerated negative ion beam may be undergoing self‐focusing is discussed.