Experimental reduction of electron beam breakup instability using external coupled cavities

Experiments on electron beam transport through 10 RF cavities have shown that BBU growth can be reduced by 6 dB when seven internal beam cavities are coupled by coaxial cable to seven external dummy cavities. The experiment consists of 10 brass pillbox resonant cavities immersed in a solenoidal field. The first cavity has its TM₁₁₀ mode primed at 2.5 GHz by a microwave pulse from an external magnetron. A 200 A e-beam is injected into the transport cavity system by the long pulse MELBA generator (t=0.5-1.5 μs, V=-0.7 to $0.8 MV, diode current =1-15 kA). Growth (≈36 dB) of the 2.5 GHz RF is observed between the 2nd and 10th cavities. When seven internal cavities (3rd-9th),coupled to seven identical external dummy cavities via coaxial microwave cable, the 2.5 GHz RF growth is reduced to about 30 dB average. These results are shown to be in general agreement with a model using equivalent circuits to determine degree of power sharing between cavities. Additional BBU growth experiments have been performed using 19 cavities with a propagation distance roughly twice that of the 10 cavity experiments. The experimental BBU growth has been found to scale with distance as expected using a discrete beam-mode coupled theory