We have discovered that the rate at which negative ions nucleate charged vortex rings in isotopically pure superfluid 4He passes through a maximum and then decreases again in very strong electric fields. This result appears to be consistent with the nucleation model proposed by Schwarz and Jang, which postulates that the initial ring appears spontaneously, surrounding the ion. A detailed quantitative comparison of theory and experiment enables us to deduce information about the hitherto quite unknown dependence of the nucleation rate upon the instantaneous velocity of the ion.