BEAM-BREAKUP GROWTH AND REDUCTION EXPERIMENTS IN LONG-PULSE ELECTRON-BEAM TRANSPORT

The results of an experimental program whose sole objective is to inve stigate the cumulative beam breakup instability (BBU) in electron beam accelerators are presented. The BBU growth rate scalings are examined with regard to beam current, focusing field, cavity Q, and propagatio n distance. A microwave cavity array was designed and fabricated to ex cite and measure the cumulative BBU resulting from beam interactions w ith the deflecting TM(110) cavity mode. One phase of this experiment u sed high Q(approximate to 1000) cavities with relatively large frequen cy spread (Delta f/f(0) approximate to 0.1%). The observed TM(110) mod e microwave growth between an upstream (second) and a downstream (tent h) cavity indicated BBU growth of 26 dB for an electron beam of kineti c energy of 750 keV, 45 A, and focused by a 1.1 kG solenoidal field. A t beam currents of less than 100 A the experiments agreed well with a two-dimensional continuum theory; the agreement was worse at higher be am currents (> 100 A) due to beam loading. The second-phase experiment s used lower Q(approximate to 200) cavities with relatively low freque ncy spread (Delta f/f(0) approximate to 0.03%). Theory and experiment agreed well for beam currents up to 220 A. Distance scaling experiment s were also performed by doubling the propagation length. Instability growth reduction experiments using the technique of external cavity co upling resulted in a factor of four decrease in energy in BBU growth w hen seven internal beam cavities were coupled by microwave cable to se ven identical external dummy cavities. A theory invoking power sharing between the internal beam cavities and the external dummy cavities wa s used to explain the experimental reduction with excellent agreement using an equivalent circuit model.