The two primary beam-current failure modes are easy to measure, but are not always sufficient to determine if the beam is usable for experiments or not. In this section we define a number of secondary failure modes to categorise other common problems of storage ring light sources. In contrast to the primary failure modes, most of the secondary failure modes are often not easy to measure at all facilities for all operation modes. Failure data for these modes are rarely published, and therefore a common metrics that fits all facilities is difficult to define. Therefore we give some examples of possible definitions and would consider it useful if facilities would publish data for these failure modes.
A stable orbit is a prerequisite for most experiments. A possible failure mode definition could be to record orbit deviations above 20% of the beam size. But this would require a different limit for each beam position monitor. We rather suggest a simpler definition: to require the RMS orbit distortion to stay below a nominal, facility and operation mode dependent value \(dx_{\hbox{nom}}\), \(dy_{\hbox{nom}}\).
ALBA provides to the beamlines the RMS orbit distortion in both, the horizontal and the vertical plane. The beamlines are informed whenever this deviation is larger than usual. The beam position at the photon beam position monitors is also used as a figure of merit for the orbit and beamlines are informed if the beam position at their source point is different from nominal by more than 20% in any plane. Orbit feedback outage are recorded by the operator and can be cross checked with a log-file generated by the slow orbit feedback, which registers any interruption of the feedback. Operators generate a beam incidence entry on the logbook for each feedback interruption.
BESSY II covers all “orbit-out-of-control” situations as a “Distorted-orbit” failure: if none of the orbit feedbacks is usable/active (orbit-feedback-outage) or if the RMS deviation from the nominal orbit exceeds 0.08\(\,\)mm. Typical RMS orbit deviation range between 0.00 - 0.01\(\,\)mm (installation of “Golden Orbit”, based on new BBA measurement) and 0.04 - 0.05\(\,\)mm year(s) later. Orbit-feedback outages are recorded if they lasts longer than 60 sec. Succeeding failures are counted as one if the feedback runs for less than 10 min.
Elettra has currently a long term orbit stability (2 to 5 days) of \(\pm 5\,\mu m\) maximum deviations while the short term stability (24 hours) is at 2% of the beam size (1.7\(\,\mu m\) horizontally and 1.2\(\,\mu m\) vertically). Globally the absolute RMS orbit distortion must stay below 400\(\,\mu m\) in horizontal plane and 300\(\,\mu m\) in vertical plane. Typical value are RMS x 330\(\,\mu m\) and RMS y 250\(\,\mu m\). Orbit feedback outages are recorded if they are longer than 10 sec.
LNLS-UVX records orbit distortions that exceed 10% of the beam size in any plane, measured relative to a beam-based-alignment defined “golden orbit”. The beamlines are informed about the distortion and a fault event is recorded in the logbook. The limits are vertical = 8\(\,\mu m\) and horizontal = 30\(\,\mu m\).
PETRA III has a fast orbit feedback to keep the orbit stable. If this feedback fails and certain limits for the orbit deviation are exceeded, the beam is dumped automatically to protect the machine. At the insertion devices the limits for a beam dump are 250\(\,\mu m\) deviation from the nominal orbit in the vertical plane or 500\(\,\mu m\) in the horizontal plane. A warning for the operator is issued if the beam position or the angle of the beam deviates from the nominal orbit by more than an individually given limit, which is of the order of 5\(\,\mu m\) or 0.5\(\,\mu rad\) in the vertical plane and 15\(\,\mu m\) or 2\(\,\mu rad\) in the horizontal plane. All orbit related beam dumps are investigated to find the root cause of the orbit deviation (i.e. a faulty magnet power supply or a fault in the orbit feedback).
SPring-8 stabilises closed-orbit-deviations to a sub-micron level by the orbit feedback once per second. Abrupt changes of the orbit, for example by gap changes of an insertion device, are corrected by the feedback.
The most sensitive user to the photon axis change desire variations below 1 micro radian. This corresponds to 10\(\,\mu m\) in the horizontal and 5\(\,\mu m\) in the vertical plane.
The monthly variation of the closed orbit distortion grows to almost this value in both plane. Those slow variations are ignored, and only abrupt orbit changes are noted in the logbook.
SLS does record and evaluate all outages of the fast orbit feedback which are longer than 10 seconds. If all beam position monitors (BPM) are used within the orbit feedback, then the deviation is always “zero” as long as the fast orbit feedback is running and correcting every 250\(\,\mu s\). Large transient or persistent closed orbit deviations will switch off the orbit feedback to avoid beam losses due to malfunctioning beam position monitors. Succeeding outages are counted as one if the feedback runs for less than 2 minutes. Number and duration of the orbit-feedback outages are reported in the yearly operation statistics.
Table \ref{tab:distorted-orbit-limits} shows a comparison how “Distorted-orbit” failures are handled at the different facilities.
| Facility | Distortion | Remark |
|---|---|---|
| ALBA | Max +20% or feedback off | recorded |
| BESSY II | RMS \( >3\,\mu m\) or off | recorded |
| Elettra | RMS \( >1.7_x, >1.2_y\,\mu m\) or off | recorded |
| LNLS-UVX | Max \( >30_x, >8_y\,\mu m\) | recorded |
| PETRA III | Max \( >500_x, >250_y\,\mu m\) | beam dumped |
| PETRA III | Max \( >15_x, >5_y\,\mu m\) or off | warning issued |
| SPring-8 | Max \( >10_x, >5_y\,\mu m\) | fast change recorded |
| SLS | feedback off | recorded & published |
Facilities in top-up can keep the beam current constant even with a low beam lifetime. But this will cause an increased frequency of injections and therefore more distortions and background radiation for the experiments. The limit for this failure mode does depend on the facility and the specific operation mode. We propose to define a minimal lifetime \(\tau_{\hbox{low}}\) for each user operation mode of a facility and record whenever the lifetime is below that limit for more than a minute.
ALBA has a nominal beam lifetime that is given by the combination of both the filling pattern and the radio frequency (RF) voltage. The typical lifetime at 100\(\,\)mA is 22 hours. ALBA operates with six RF cavities each fed with two Inductive Output Tubes (IOTs), and a typical “low-beam-lifetime” failure is caused by the trip of one IOT. Since it is the trip of a sub-system, the operator will record it. Normally the operator will recover the IOT and thus recover the nominal beam lifetime. A beam lifetime below 18 hours is considered “low”. Top-up is stopped if the lifetime drops below 10 hours.
SLS has a typical beam lifetime of about 8 hours. Depending on the vacuum and the selected coupling this lifetime may varies in practice between 6 and 10 hours during normal operation. A low beam lifetime leads to more frequent injections from the top-up. A “low-beam-lifetime” failure is automatically recorded when the beam lifetime stays below 4.5 hours for longer than five minutes. The failure mode stops as soon as the lifetime is above 4.5 hours again for longer than one minute.
The failure mode “low-beam-lifetime” is not independently recorded at neither BESSY II, Elettra, LNLS-UVX, PETRA III nor SPring-8 .
Table \ref{tab:lifetime-limits} shows a comparison of what would be considered a “low-beam-lifetime” at the different facilities.
| Facility | Mode | \(\tau_{\hbox{normal}}\) | \(\tau_{\hbox{low}}\) | Remark |
| (h) | (h) | |||
| ALBA | decay | 22 | 18 | |
| ALBA | top-up | 22 | 18 | |
| BESSY II | MB | 7 | 5 | stops top-up |
| BESSY II | SB | 2 | 1 | stops top-up |
| Elettra | 2.0\(\,\)GeV | 23 | 10 | |
| Elettra | 2.4\(\,\)GeV | 27 | 10 | |
| LNLS-UVX | decay | >14 | not defined | |
| PETRA III | continuous | 12 | not defined | |
| PETRA III | timing | 1.5 | not defined | |
| SPring-8 | all modes | 15 …50 | 10 | |
| SLS | top-up | 8 | 4.5 | yearly evaluation |
The beam size should stay constant for a light source, since the emittance is an important parameter. We propose to define vertical and horizontal beam size limits for each operation mode and record whenever the beam dimensions are larger than these limits for more than a minute.
While this failure mode is easy to define, it is very hard to detect for many facilities. A beam height of 10 µm requires costly diagnostics to measure it continuously to 10% precision.
Table \ref{tab:blow-up-limits} shows a comparison of the “Beam-blow-up” failure mode detection.
| Facility | typical size | blowup | Remark |
|---|---|---|---|
| (µm x µm) | |||
| ALBA | 70 x 30 | +20% | recorded |
| BESSY II | 250 x 14 | +30% | recorded |
| Elettra | 260 x 10 | +10% | no on-line measurement yet |
| LNLS-UVX | 1000 x 120 | +10% | recorded |
| PETRA III | 140 x 7 | - | no on-line measurement |
| SPring-8 | 100 x 12 | - | not an independent failure mode |
| SLS | 50 x 10 | +50% | recorded and evaluated |
Some experiments have very strict requirements on the ratio between a filled single bunch and the residual charge in the neighbouring bucket. This again depends on the specific requirements from the experiments. Any deviation from the desired bunch filling may cause problems to some experiments. This failure mode is mainly relevant to time resolved measurements and the usefulness of any definition depends on the requirements of the specific users. For each operation mode an allowed maximum charge deviation \(dQ_{\hbox{max}}\) should be defined.
A bunch purity of \(10^{-8}\) requires a lengthy procedure to be measured to a useful precision. It is therefore not feasible to measure the bunch purity continuously. The filling pattern can be measured down to an accuracy of several percent, but the diagnostics to do that is only available at some light sources.
Table \ref{tab:filling-limits} shows a comparison of the Distorted-filling limits.
| Facility | Remark | |
|---|---|---|
| ALBA | - | planned for 2015 |
| BESSY II | 10% | recorded |
| Elettra | - | no on-line measurement |
| LNLS-UVX | - | no on-line measurement |
| PETRA III | - | no on-line measurement |
| SPring-8 | 10% | recorded, deviation after accumulation neglected |
| SLS | 100% | feedback outages recorded |
Some failures do not affect the beam, but they do affect the user experiments. If the beam is stored and all beam parameters are within the desired limits, there still can be problems that prevent most users from running any experiments. Infrastructure outages like massive control system and IT-infrastructure failures or photon shutter interlocks can lead to those situations. There cannot be a simple rule to calculate the start and stop for all failures of this type; but they should be recorded if they have an influence on a significant number of the experiments.
Currently beam unrelated incidences are considered to be “downtime” at some facilities, if they prevent all beamline to continue their measurements. This is the case at ALBA, Elettra and the SLS. Other facilities do neglect those types of failures for their downtime calculation, as long as the electron beam was not affected, for example PETRA III. At most facilities these failures are evaluated on a case by case basis: for example an interlock of all photon shutter would clearly be considered “downtime”, at least at ALBA, BESSY II and LNLS-UVX; but a problem with the IT infrastructure might not, even if the majority of the users where affected.
Many facilities have a cut off for a minimal time to store the beam. For example if less than one hour is between two beam trips then the time in-between is counted as downtime. This can be defined as an extra failure mode: “short-user-time”. The limit of what time is too short for user experiments depends on the time the facility needs to get into thermal equilibrium and on the typical length of a measurement at an experiment. Each facility should define this cutoff time limit \( T_{\hbox{short-user-time}}\); it may depend on the operation mode.
BESSY II, Elettra, LNLS-UVX and the SLS consider a beam delivery of a total length of less that one hour to be “downtime”; at ALBA the cut-off is at 30 minutes. PETRA III does not record short-user-time as separate fault criteria, but covers this by the rule which adds up to one hour or the length of the downtime to each beam outage. SPring-8 does not have a cutoff for a beam delivery time.
None of these facilities record currently “short-user-uptime” as a separate failure mode.
Table \ref{tab:sf-limits} shows a comparison of secondary failure modes: if they are on-line recorded at the different facilities and which failure modes are reported in yearly statistics to the public.
| Facility | Distorted-orbit | Low-beam-lifetime | Beam-blow-up | Distorted-fill | Short-up-time |
| (h) | |||||
| ALBA | on-line | on-line | on-line | - | 0.5 |
| BESSY II | on-line | on-line | on-line | on-line | 1.0 |
| Elettra | on-line | on-line | on-line | - | 1.0 |
| LNLS-UVX | on-line | on-line | on-line | - | 1.0 |
| PETRA III | on-line | - | - | - | \( \le\)1 |
| SPring-8 | on-line | on-line | on-line | on-line | 0 |
| SLS | on-line | on-line | report | on-line | 1.0 |
“Distorted-orbit” failures are recorded at most facilities but are rarely taken into account in the yearly failure statistics. The limits when an orbit is considered out of specification are varying by orders of magnitudes between different facilities. Publication of these limits and the associated failure rates would be very useful to compare facilities.
“Low-beam-lifetime” failures are apparently rare events at most facilities. The example of ALBA shows that it still makes sense to record and evaluate these faults. The normal beam lifetime varies considerably between facilities and operation modes. The nominal beam lifetime at the SLS would be considered a very low lifetime at ALBA or SPring-8. But a significant decrease in the beam lifetime can cause problems at most facilities and should therefore be recorded to evaluate the reliability of the facility in this respect.
“Beam-blow-up” failures are again infrequent at most facilities. Betatron coupling can affect the occurrence of this failure type, since at very low coupling even small errors can lead to relatively large changes of the vertical beam size. For some facilities the vertical beam size is difficult to measure. Nevertheless one needs to define limits for the tolerated variation of the beam size. The number of reported failures outside these limits would be an essential measure for the reliability of the facility.
“Distorted-filling and Bunch-purity” faults are not relevant at all facilities. Time resolved measurements depend on bunch purity. At present only few facilities have the means to measure the bunch purity on-line. Sophisticated procedures are required to measure a bunch charge ratio of \(10^{-8}\). Many facilities do have the means to detect deviations from the nominal bunch charge distribution. Where these means exist we encourage to publish failure limits and associated data.
“Beam-unrelated” failure should be recorded whenever they have an impact on a significant number of beamlines. These faults are often facility specific.
“Short-user-time” are for most facilities just subtracted in the beam availability calculations; they are not recorded as a failure mode. An independent recording would enable to calculate beam availability with and without accounting for the “short-user-time’. This would improve the comparability between facilities that handle “short-user-time” differently.