Experiments on the excitation and coupled-cavity suppression of beam-breakup-instability in long-pulse electron beam transport

Experiments have been performed to excite and suppress the beam breakup (BBU) instability in long pulse electron beam transport through RF cavity systems. Electron beams are generated by the Michigan Electron Long Beam Accelerator (MELBA) at parameters: V = 0.7–0.8 MV, Idiode = 1–10 kA, Iinjected = 100–400 A, and pulselength= 0.5–1 μs. The transport system consists of a solenoidal magnetic field with 10 RF cavities. The TM110 mode BBU resonance frequency of the cavities is 2.5 GHz and the cavity Q is adjusted to about 200–300. Each cavity has a small coupling loop oriented to detect the RF field in the TMno mode. The cavities are separated by 6.5 cm-long sections of tubing which are cutoff to the RF frequency. The BBU instability is primed by a kW level microwave signal injected into the first cavity from a magnetron. Growth of the RF is measured between the second cavity and the last cavity. Strong growth (2–5 dB per cavity) is only observed when the RF priming signal is tuned to the exact TM110 mode resonance frequency (Δf/fo≪1%). Experiments to test techniques for suppression of the BBU instability are also being performed. The primary BBU suppression technique being investigated is cavity cross-coupling, in which 7 out of 10 internal beam-cavities are connected by cables to 7 identical cavities located externally to the e-beam. These are the first experiments which have shown a consistent reduction (about −6 dB average) in BBU microwave growth when external cavities are coupled to the internal cavity system.