Abstract
The excess energy of an ion beam from a high frequency ion source was measured by using a high frequency ion source with mesh grid. The ion source was operated using unusual electrical connections. The results suggest that the excess energy is due to the increase of plasma potential caused by energetic electrons. By the use of an electrical shielding screen, it is shown that the axial component of the electric field from the high frequency coil is only a fractional source of the excess energy.