Experiments on the beam breakup instability in long-pulse electron beam transport through cavity systems

摘要

Abstract: Experiments 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 $EQ 100-300 keV, extracted current $EQ 10-100 $Alpha@, and pulse length $EQ 300 ns, and 2) the Michigan Electron Long Beam Accelerator (MELBA), with energy $EQ 0.6-0.9 MeV, extracted current $EQ 100-400 $Alpha and pulselength $EQ 0.5-2 $mu@s. The transport system consists of 5 or 10 rf cavities with Q $EQ 210 $POM 30. the cavities are connected by cutoff waveguide sections, which prevent electromagnetic coupling between cavities in order to eliminate the regenerative BBU. Each cavity has a microwave probe to detect growth of e-beam emission in the non-axisymmetric TM$-110$/ mode at 2.5 GHz, corresponding to the BBU. Solenoidal magnetic fields of 1-3.5 kG are applied. Initial experimental data on the Febetron show strong ($GRT@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.!