The Kamaboko III negative ion source, which was designed and built by JAERI, Naka, Japan, is a model 25% in linear dimensions of the reference design ion source for the International Thermonuclear Experimental Reactor ITER neutral beam injectors. The ion source is a magnetic multi-pole, arc discharge, source with 12 tungsten filaments. Cesium is added to the source to enhance the extracted negative ion current density. The source is attached to a three grid accelerator plasma, extraction and acceleration grids which can produce beams of 30 keV. A target located at 1.2 m from the ion source is used to measure the accelerated current calorimetrically. Although the source has achieved the design parameters during short pulses 5 s, during long pulses 50 s the calorimetrically measured negative ion current density is, at the appropriate arc power, 60% of that required for ITER. 1 Consequently, a detailed characterization of the source has been undertaken.