Excitation of the σ_{l_l} = 90° resonance by the cavity rf accelerating fields

Abstract In rf linacs the longitudinal focalization is done by si-nusoidal forces and at high accelerating fields zero-current phase advance per fo-cusing period σ 0l_l can be high. The nonlinear compo-nents of sinusoidal field (sextupolar, octupolar higher order components) then excite parametric resonances, including 4th-order resonance (σ l_l = 90°) when than 90°, inducing strong emittance growths acceptance reductions. As pointed out in previous papers, beam dynamics therefore complex, even space-charge are ignored. excitation analyzed before discussing additional effect forces, particular to explain why trans-verse focusing 0l_t not a relevant parameter. Examples given SPIRAL2 linac case.