COMMON OPERATION METRICS FOR STORAGE RING LIGHT SOURCES
Storage ring light sources aim for a high operation reliability. Very often beam availability is used as an operation metrics to measure the reliability of the accelerator. A survey of several light sources revealed that the calculation of availability varies significantly between facilities. This prevents a useful comparison of reliability. Furthermore the beam availability does not provide inside about the reliability of beam characteristics like orbit- or beam size stability.
The authors propose a specific metrics to evaluate the reliability of storage ring light sources; a metrics that will allow a detailed and meaningful comparison across facilities. The authors are convinced that such a comparison will be useful to further optimize the reliability of storage ring light sources.
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 (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 (Lüdeke 2014). 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 (Lüdeke 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.
The definition of beam availability is important in order to compare numbers from different facilities. A survey on failure analysis in 2008 for nine light sources 1 revealed significant differences for the calculation of beam availability (Lüdeke 2009). In the following we summarise the main findings of the survey.
In many cases the beam availability calculations were determined by identifying events as “downtime” that interrupted data taking for the majority of the users. Some facilities considered “long” injector outages - causing “decaying beam” operation - to be downtime, others accounted for these events individually. Most facilities only counted beam delivery between two outages if it exceeded a minimum duration. The minimal required duration varied between 15 and 60 minutes. In cases of long beam outages, most facilities organised compensation time for the users to allow them to finish their experiments. The compensation time was also accounted for in different ways, depending on the facility: some fully subtracted the extra beam time from the downtime, whilst others ignored this extra time for the availability calculation. All light sources did record other events than beam outages, such as increased beam size or orbit problems, but no facility published statistics on these other failure modes at regular intervals.
During a discussion round at the ARW 2013 in Melbourne (Web page of the Accel...) we polled the calculation of beam availability from participants representing ten light sources 2 with the same result as the survey of 2008.
Based on this data the authors concluded that a direct comparison of accelerators reliability is currently impossible. It is in the interest of every facility, from the operators to the facility manager to be able to assess its accelerator reliability compared to other facilities. Internally this is important to support requests for upgrades and for maintenance plans. It can help to take adequate decisions if proper information to compare with other facilities is readily available. A comparison of reliability may also serve as a trigger to establish collaborations. If a particular failure mode is identified to be more frequent at some facilities, then this provides an incentive for a common project to develop more reliable solutions. As a consequence of everyone using the same common metrics, a fairer comparison may also emerge when requesting funding from supranational authorities.
Survey participants for the failure analysis questionnaire came from: APS, ESRF, SPring-8, Diamond, SOLEIL, BESSY II, Elettra, ANKA and SLS.↩
Participants of the ARW’13 presenting their calculation of beam availability came from: ALBA, Australian Synchrotron, BESSY II, Diamond, SPEAR, NSRRC, SOLEIL, Elettra, SLS and PETRA III.↩