Most of high power accelerator projects rely on bulk nio-bium
superconducting cavities technology. When pulsed
operation is requiered, cavities are submitted to time vary-ing
radiation pressure proportional to the square of acceler-ating
field. This excitation couples to the mechanical sys-tem
constituted of the cavity and auxilliary components,
and may excite mechanical modes at resonance. Subse-quent
deformation of the cavity induces a time-varying de-tuning.
When high accelerating gradients or low beta cav-ities
are of concern, this detuning could, if not controlled,
seriously impair RF operation because of extra power re-quired,
and add constraints on the RF stabilization system
to keep the beam quality high. We have studied the dynam-ical
behavior of SC cavities under pulsed operation using a
modal approach and coupling mechanical simulation to RF
calculations. Stiffening, mechanical modes damping and
finite stiffness of tuning system and helium vessel are in-cluded
in the numerical model. Modal Lorentz detuning
coefficients are extracted from these calculations on which
basis a simple second order system algebraic model can be
set up. |
