Electron Linac Instabilities

This paper attempts to summarize the present state of understanding of electron linac instabilities which result in beam loss along the accelerator. The major emphasis is placed on the type of instability which is caused by cumulative interaction of the beam with a multisection accelerator, as distinguished from the effect observed in short high-current accelerators where the instability is due to local regenerative interaction. The manifestations of the effect are first described and physical models are proposed. The properties of the HEM11 mode which is responsible for the instability are discussed in detail. Following, a short summary of the Panofsky theory is presented to illustrate various scaling laws which have been verified experimentally on the SLAC accelerator. Measured results are also compared with a detailed computer study made by R. Helm and agreement is generally found to be very good. After the discussion of beam break-up gain, a few conjectures are presented on the possible starting mechanisms of the effect. In conclusion, the program presently underway at SLAC to increase the current threshold of the instability is outlined and early results of this program are presented.