COMMON OPERATION METRICS FOR STORAGE RING LIGHT SOURCES (V4)
A. Lüdeke, PSI, Villigen, Switzerland; M. Bieler, DESY, Hamburg, Germany; R. H. A. Farias, LNLS, Campinas, Brazil; S. Krecic, Elettra-Syncrotrone Trieste, Italy; R. Müller, HZB, Berlin, Germany; M. Pont, CELLS-ALBA, Cerdanyola del Vallès, Spain; M. Takao, JASRI/SPring-8, Japan
Storage ring light sources aim for high operational reliability. Very often beam availability is used as an operation metric to measure the reliability. A survey of several light sources reveals that the calculation of availability varies significantly between facilities. This complicates useful comparisons of reliability. Furthermore the beam availability typically does not provide insight regarding reliability of beam characteristics such as orbit- and beam size stability.
The authors propose specific metrics to evaluate the reliability of storage ring light sources; these metrics allow detailed and meaningful comparison across facilities. Such comparisons are useful to further optimize the reliability of storage ring light source facilities.
Reliability is typically defined as the ability of a system to serve a given function over time. In this sense the reliability of a particle accelerator is very important for user facilities like storage ring light sources and provides an important design objective for new large scale accelerator facilities as for example the International Linear Collider (Himel 2007). Clearly 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 metric should be closely related to specific user requirements (Lüdeke 2014). In order to compare reliability of different facilities, one needs a common standard for the calculation of operation metrics.
Commonly beam availability or “up-time” is often used to compare the reliability of light sources. While statistics are published for most light sources, few facilities supply precise definitions on how the information is calculated. A survey of several light sources revealed (Lüdeke 2009) that the calculation of this metric varies considerably. The conditions under which beam is considered ’available’ are often defined only in simple common sense terms, and even if there are formal definitions, they differ between facilities. Furthermore a large variety of failure modes is often convolved into “beam not available”, commonly referred to as “downtime”. Since many downtime failure modes are specific to individual facilities, this further complicates the comparison of beam availability.
Our aim is to provide simple, well-defined, formal operation metrics for storage ring light sources to make beam reliability at these facilities comparable. The metrics serve to clarify for each facility the beam parameters specified for the users, how the statistical data is processed and how well the standards are met. The authors make the case that the application of these metrics will refine the ability to learn from and compare reliability at storage ring of light sources.
The definition of “beam availability” is an important metric useful to compare operation at different facilities. A survey on failure analysis in 2008 at nine different light sources 11Survey participants for the failure analysis questionnaire came from: APS, ESRF, SPring-8, Diamond, SOLEIL, BESSY II, Elettra, ANKA and SLS. revealed significant differences for the calculation of beam availability (Lüdeke 2009). In many cases the beam availability calculations were determined by identifying events as “downtime” that interrupted data acquisition for the majority of the synchrotron radiation beam 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 available” between two successive 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 organized compensation time for users to finish 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 beam availability calculation. All light sources recorded events other than beam outages, such as increased beam size or orbit problems, but no facility published statistics on these 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 22Participants of the ARW’13 presenting beam availability calculations from: ALBA, Australian Synchrotron, BESSY II, Diamond, SPEAR, NSRRC, SOLEIL, Elettra, SLS and PETRA III. and obtained a similar result as the survey of 2008.
Based on this data the authors concluded that a direct comparison of accelerator reliability is currently difficult if not impossible. It is in the interest of every facility, from operations to management to be able to assess accelerator reliability as compared with other facilities. Internally this is important to support plans for upgrades and maintenance. It can also assist the decision making process when information relative to other facilities is available. Comparisons of reliability may also serve as a trigger for strong inter laboratory collaborations. If a particular failure mode is identified to be more frequent at multiple facilities, this provides incentive for common efforts to develop more reliable solutions. As a consequence of using common metrics, clear comparisons may also emerge when requesting funding from supranational authorities.
After comparing the current situation at different facilities, we discussed the advantages and disadvantages of alternate definitions for downtime and beam availability. The comparison soon revealed that local metrics for “beam availability” are often used to assess the development of the facility over time. Every change to that metric could potentially compromise the ability of comparison with historic data. In addition, local definitions of “beam availability” are often part of contracts and changes to the definiti