Experiments designed to investigate the beam breakup (BBU) instability have been performed using the long-pulse MELBA electron-beam generator (0.5 - 1.5 $mu@s, 0.7 - 0.8 MV, $+l$/diode $EQ 1 - 15 kA, $+l$/extracted $EQ 0.1 - 0.5 kA). The experiment consists of 10 identical pillbox cavities each containing a small microwave loop antenna designed to detect the TM$-110$/ beam breakup mode. For our cavity design the TM$-110$/ resonant frequency occurs at approximately 2.5 GHz. The cavities are connected by small diameter tubes which attenuate the RF cavity-to-cavity crosstalk. The MELBA diode and subsequent cavity system are immersed in a solenoidal magnetic field (0.8 - 3 kG). Microwaves of 2.5 GHz (1 - 4 kW), whose pulselength exceeds the beam pulse, can be injected into the initial cavity in order to prime the BBU instability. BBU instability growth is measured through the growth of 2.5 GHz RF between the first (or second) and tenth cavities. The BBU growth is compared with predictions made by beam-cavity coupled-mode theory.
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