<title>Experiments on the beam breakup instability in long-pulse electron beam transport through cavity systems</title>

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1407_61Experiments have been performed to investigate the beam breakup (BBU) instability in electron beams transported through rf cavity systems. Experiments have utilized two accelerators: 1) a Febetron accelerator operating with energy equals 100-300 keV, extracted current equals 10-100 (Alpha) , and pulse length equals 300 ns, and 2) the Michigan Electron Long Beam Accelerator (MELBA), with energy equals 0.6-0.9 MeV, extracted current equals 100-400 (Alpha) and pulselength equals 0.5-2 microsecond(s) . The transport system consists of 5 or 10 rf cavities with Q equals 210 +/- 30. the cavities are connected by cutoff waveguide sections, which prevent electromagnetic coupling between cavities in order to eliminate the regenerative BBU. Each cavity has a microwave probe to detect growth of e-beam emission in the non-axisymmetric TM110 mode at 2.5 GHz, corresponding to the BBU. Solenoidal magnetic fields of 1-3.5 kG are applied. Initial experimental data on the Febetron show strong (>40 dB) growth of rf at 2.5 GHz between the first and tenth cavity. MELBA data shows about 20 dB rf growth between the second and fifth cavities in a 5 cavity system. Up to 85% of the injected current is transported through the cavity system. The magnitude of BBU growth, its dependence on current and magnetic field, and the onset time to rf peaking are consistent with theory.