Pulse-shortening in Electron Linear Accelerators

Abstract

During the past few years several electron linear accelerators have been designed in Great Britain, France and the United States to support pulse currents in excess of 250 m.amp. for durations of 1–10 µsec. Many of these machines have failed to reach design performance due to a phenomenon which has come to be known as ‘pulse-shortening’. This effect appears as a premature termination of the output electron pulse and occurs at pulse currents in the range 300–600 m.amp. and pulse-lengths of 1 or 2 µsec. depending on the particular machine. Further increase of pulse current is possible only at the expense of pulse-length. The current pulse and the radio-frequency loading are normal until the point in time at which ‘shortening’ occurs. Beyond this point, where there is typically a 0.1/0.2 µsec. jitter of the trailing edge of the current pulse, there is little or no electron current and usually the radio-frequency pulse shows an increase somewhat above the normal loaded level but does not reach that corresponding to zero beam.