Measurements of the initial ionization in a cyclotron, produced by the use of a filament, as a function of the pressure, electron emission, and dee voltage are presented. The amount of ionization is found to be too high to be simply explained by an electron passing between the region between dees only once. A theory is proposed wherein some of the electrons are caught by the changing electric field between the dees and oscillate back and forth many times during a cycle of the dee voltage. Experimental observations which conform to the theory are described.

• Received 10 July 1939

DOI:https://doi.org/10.1103/PhysRev.56.459