  The radio-frequency (RF) system is the key element that generates electric
fields for beam acceleration. To keep the system reliable, a highly
sophisticated protection scheme is required, which also should be designed to
ensure a good balance between beam availability and machine safety. Since RF
systems are complex, incorporating high-voltage and high-power equipment, a
good portion of machine downtime typically comes from RF systems. Equipment and
component damage in RF systems results in long and expensive repairs.
Protection of RF system hardware is one of the oldest machine protection
concepts, dealing with the protection of individual high-power RF equipment
from breakdowns. As beam power increases in modern accelerators, the protection
of accelerating structures from beam-induced faults also becomes a critical
aspect of protection schemes. In this article, an overview of the RF system is
given, and selected topics of failure mechanisms and examples of protection
requirements are introduced.
