Reliability is defined as the ability of a system to serve a given function over time. The reliability of the particle accelerator is very important for user facilities, like storage ring based light sources; but it is as well an important design objective for new types of facilities as accelerator driven systems or new large scale facilities as for example the International Linear Collider \cite{Himel_2007}. An operation metrics should quantify the reliability of a particle accelerator. If the objective is to assess the improvement of a specific facility over time, then the operation metrics should be very closely related to the specific user requirements for that facility \cite{Luedeke:2014aa}. But in order to compare the reliability of different facilities one needs a common standard for the calculation of the operation metrics.
In particular beam availability is often used to compare the reliability of light sources. While these statistics are published for most light sources, few facilities do supply definitions on how these numbers are calculated. A survey of several light sources revealed \cite{L_deke_2009} that the calculation of this metrics varies considerably. The conditions under which beam is considered available are often defined in common sense terms, and even if there are formal definitions, they differ between facilities. Furthermore a large variety of failure modes is often convoluted into “beam not available”, commonly called “downtime”. Since some of these failure modes are specific to individual facilities, this further complicates the comparison of beam availabilities.
Our aim is to propose a simple, well-defined, formal operation metrics for storage ring light sources, to make the reliability of these facilities comparable. The metrics serves to clarify for each facility the beam parameter promised to the users, and the statistical data reveal, how well the standards are met. The authors are convinced that the application of this metrics will refine our ability to learn from the reliability comparison of light sources.