The interaction between the plasma and the converter in a surface conversion negative ion source is studied. The converter consists of a porous tungsten button, through which liquid cesium diffuses towards the surface. This scheme results in a low cesium density in the discharge. Xe+and Ar+ions are used to simulate the sputtering effect of Cs+ion bombardment of the converter. The extracted H−beam energy profile and the conversion efficiency appeared to depend strongly on the heavy ion flux to the converter surface. The total H−current increases with an order of magnitude with increasing heavy ion flux. A maximum current density of 8 mA/cm2of H−ions has been extracted.