Energy Regulation of a 3 Mv Van de Graaff Positive Ion Accelerator by Modulation of the Ion Source Dark Current

As a result of work on a scheme to regulate the energy of the 3 Mv, Van de Graaff injector at this installation, it was found that several disturbing effects were produced upon closing the energy control feedback loop using a beam derived signal for modulation of the corona current. These undesirable effects, which are mainly a result of the long ion transit time and ion dispersion in the high pressure gas, seriously limited the manipulation and the analysis of the regulation problem. To circumvent these difficulties, it was decided to modulate the dark current (D. C. beam current) of the pulsed positive ion source by means of a variable intensity light signal transmitted to a photomultiplier tube in the high voltage terminal through a plexiglass pipe. This approach provided a rational transfer function permitting a more straight forward analysis of loop operation, a higher frequency response and greater reliability of operation all of which resulted in improved energy regulation from about ± 1.5 KV to about ± 500 V. The existing servo loop on the corona current was retained as a back-up element and it continues to handle the major portion of the slow energy regulation.