A tandem accelerator and ion implanter with improved performance is disclosed. The tandem accelerator includes a plurality of input electrodes, a plurality of output electrodes and a high voltage terminal disposed therebetween. The high voltage terminal includes a stripper tube. Neutral molecules are injected into the stripper tube, which remove electrons from the incoming negative ion beam. The resulting positive ions are accelerated toward the plurality of output electrodes. To reduce the amount of undesired positive ions that exit the stripper tube, bias electrodes is disposed at the entrance and exit of the stripper tube. The bias electrodes are biased a second voltage, greater than the first voltage applied to the terminal. The bias electrodes repel slow moving positive ions, preventing them from exiting the stripper tube and contaminating the workpiece.