Beam breakup
Multi-bunch, multi-pass transverse beam break up (BBU) is
one of the challenging problems of beam dynamics in multipass
linacs and thus also in ERLs. This effect has been studied
extensively [20] including through computer simulations and
theoretically [21]. The intuitive explanation of BBU is quite
simple: When a beam bunch passes through an RF cavity with
a transverse offset, it deposits energy to some dipole HOM.
At the same time, the HOM introduces a transverse
momentum kick, which can be converted to a transverse
offset by the beam transport system. Given that the beam
bunches return to the same cavity and mode because of the
multi-pass nature of the accelerator, we have a positive
feedback loop. When the beam current exceeds a certain
threshold, Ith, this process leads to instability in the beam’s
motion and the HOM stored energy in the cavity. The effect is
enhanced by the multiplicity of the passes [21]. The beam
current threshold may be increased by several methods. The
most common are effective damping of the HOMs to reduce
their shunt impedance, and the introduction of a large spread
of the HOM frequency, by the regular manufacturing process
and possibly even by design. Other methods include
decreasing the frequency of the cavities and careful choice of
the optics functions of the beam transport system.