Experimental excitation and reduction of the beam breakup instability in long-pulse electron beam transport.

Abstract

The beam breakup instability (BBU) is one of the most serious problems that plague the design and operation of electron beam accelerators. The BBU results from an unstable coupling between an electron beam and a non-axisymmetric mode of the acceleration cavities. The resulting effects on the beam can range from quality degradation to total disruption. While the theory of the BBU has been advanced considerably in recent years, there have been few published attempts to reconcile theoretical prediction with experimental observation. The present work contains the results of the first experimental program whose sole objective is to investigate the cumulative BBU. This research has had the dual goal of investigating the BBU growth rates and developing techniques to reduce the severity of the instability. To accomplish the first goal of examining the BBU growth rate scalings with regard to beam current and focusing field, a microwave cavity array was designed and fabricated to excite and measure the cumulative BBU. One phase of this experiment used high Q ($\approx$1000) cavities with relatively large frequency spread $(\rm{\Delta f\over f\sb{o}} \approx 0.1\%).$ The observed TM$\sb{110}$ mode microwave growth between an upstream and a downstream cavity indicated BBU growth of 26 dB for an electron beam of kinetic energy of 750 keV, 45 A, and focused by a 1.1 kG solenoidal field. The experiments agreed well with a two-dimensional continuum theory, the agreement was worse at higher beam currents due to beam loading. The second phase experiments used lower Q ($\approx$200) cavities with relatively low frequency spread $(\rm{\Delta f\over f\sb{o}} \approx 0.03\%).$ Theory and experiment agreed well for beam currents of up to 220 A giving BBU growth rates of up to 53 dB/m. Instability growth reduction experiments using the novel technique of external cavity coupling resulted in a factor of four decrease in BBU growth when seven internal beam cavities were coupled by microwave cable to seven identical external dummy cavities. A theory invoking power sharing between the internal beam cavities and the external dummy cavities was used to explain the experimental reduction with close agreement.