  The objective of this work is the evaluation of the risk of suffering a
multipactor discharge in an S-band dielectric-assist accelerating (DAA)
structure for a compact low-energy linear particle accelerator dedicated to
hadrontherapy treatments. A DAA structure consists of ultra-low loss dielectric
ciylinders and disks with irises which are periodically arranged in a metallic
enclosure, with the advantage of having an extremely high quality factor and
very high shunt impedance at room temperature, and it is therefore proposed as
a potential alternative to conventional disk-loaded copper structures. However,
it has been observed that these structures suffer from multipactor discharges.
In fact, multipactor is one of the main problems of these devices, as it limits
the maximum accelerating gradient. Because of this, the analysis of multipactor
risk in the early design steps of DAA cavities is crucial to ensure the correct
performance of the device after fabrication. In this paper, we present a
comprehensive and detailed study of multipactor in our DAA design through
numerical simulations performed with an in-house developed code based on the
Monte-Carlo method. The phenomenology of the multipactor (resonant electron
trajectories, electron flight time between impacts, etc.) is described in
detail for different values of the accelerating gradient. It has been found
that in these structures an ultra-fast non-resonant multipactor appears, which
is different from the types of multipactor theoretically studied in the
scientific literature. In addition, the effect of several low electron emission
coatings on the multipactor threshold is investigated. Furthermore, a novel
design based on the modification of the DAA cell geometry for multipactor
mitigation is introduced, which shows a significant increase in the
accelerating gradient handling capabilities of our prototype.
