We investigated the thermo-field ion sources constructed as a low-melting metal layer deposited on the high-melting metal substrate with a sharp tip. The theoretical analysis was performed on the basis of the crater evolution model of its surface in the high electric field. We have calculated not only the stationary current-voltage plots but analysed the transitional processes, preceding the stabilisation of the ion emission and calculated the kinetic characteristics as well. Our calculation predicts an oscillative processes in ion emission. We performed experiments to examine the validity of the theory, and the agreement was found to be excellent.