Low Voltage Injection into the Electron Linear Accelerator

Abstract

The use of low gun voltages for linac injection is, in general, impractical because the capture, or trapping, fraction of injected electrons is slight unless other especial devices are employed. For example, single gap cavities have been used as pre-accelerators, rather than velocity modulation prebunchers. Ordinarily a tapered phase velocity structure suggests itself; however, at very low voltages a reasonable disc thickness excludes any volume for the cavity. This report describes a novel use of a constant phase velocity periodic structure to capture and bunch a low voltage beam which is currently being used in low energy accelerators. A proposed additional use of this scheme is for obtaining very short pulses. Normally, high voltage diode guns have a perveance considerably lower than low voltage guns, requiring, therefore, a short high voltage pulse which is difficult to achieve owing to rise and fall time. It is suggested that the trapping scheme described above, using an achievable short pulse and low voltage gun will provide short beam pulses.