TRANSVERSE INSTABILITIES IN THE LIL.

A part of a beam breakup (BBU) study, the authors have calculated and identified a series of transverse deflecting modes in the S band sections of the LEP injector linac (LIL). In these TW quasiconstant gradient structures, having various iris diameters from 18 to 26 mm, only pi -modes of the C band of the HEM//1//1 having negative group velocities and phase velocities and phase velocities around c will lead to cumulative BBU. These deflecting pi -modes occur at frequencies in the range from 4558 to 4290 MHz for diameters from 18 to 26 mm. Frequency variation with iris diameter is minus 15 MHz for each added millimeter for calculated and measured 0-modes and about twice that value for pi -modes. Levels for starting transverse instabilities have been determined for various accelerated beam currents with different pulse widths.