BBU gain measurements on the ITS 6-MeV, 4-kA linac

The amplification of transverse beam motion by the ITS eight-cell linear induction accelerator (LIA) was measured with a 4-kA beam at frequencies between 710- and 950-MHz. The 3.75-MeV injector beam was deflected with amplitudes of similar to 1 mrad by an RF-driven tunable cavity 84 cm before the linac. The centroid-position waveforms from calibrated B-dot loops were recorded with 5-GS/s digitizers at the linac entrance, exit, and after a further free drift of 108 cm. The FFT amplitudes of the waveforms were used to compute the gains. The code LAMDA, an envelope and BBU model for the linac, was used to interpret the gain curves, using the transverse impedances of the cells as adjustable parameters. The results compare favorably with cell impedances previously measured with a two-wire TSD method.