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...
We rather suggest a simpler definition: to require the RMS orbit distortion to stay below a nominal, facility and operation mode dependent value $dx_{\hbox{nom}}$, $dy_{\hbox{nom}}$.
{\em 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
on the SOFB 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
SOFB. feedback.
Operators generate a beam incidence entry on the logbook for each
SOFB feedback interruption.
{\em BESSY II} covers all ``orbit-out-of-control'' situations as a ``Distorted-orbit'' failure:
if none of the
corrections FOFB/SOFB orbit feedbacks is usable/active (orbit-feedback-outage)
of 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)
...
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 0.4$\,\mu m$ in horizontal plane and 0.3$\,\mu m$ in vertical plane.
Typical value are RMS x
0.33$\,mm$ 0.33$\,\mu m$ and RMS y 0.25$\,\mu m$.
Orbit feedback outages are recorded if they are longer than 10 sec.
...
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$\,\mu m$ in the vertical plane and 20$\,\mu m$ in the horizontal plane.
All orbit related beam dumps
have to be 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).
{\em SPring-8} stabilises closed-orbit-deviations
in to a sub-micron
level
by the orbit feedback once per second.
There are abrupt Abrupt changes of
orbit by e.g. the
orbit, for example by gap
change changes of an insertion device,
which
are
immediately corrected by the feedback.
The most sensitive user to the photon axis change desire variations
under below 1 micro radian.
At the SPring-8 storage ring this 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.
{\em SLS} does
currently not record
closed and evaluate all outages of the fast orbit
deviations. 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$\,\mu s$.
Instead orbit-feedback outages are automatically recorded if they are longer than 10 seconds.
Large transient or persistent closed orbit deviations will switch off the orbit feedback
to avoid beam losses due to malfunctioning
BPMs. 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.
Table~\ref{tab:distorted-orbit-limits} shows a comparison how ``Distorted-orbit'' failures are handled at the different facilities.
\begin{table}
\centering
...
PETRA III & Max $ >500_x, >250_y\,\mu m$ & beam dumped \\
PETRA III & Max $ >20_x, >5_y\,\mu m$ or off & warning issued \\
SPring-8 & Max $ >10_x, >5_y\,\mu m$ & fast change recorded\\
SLS &
outage feedback off & recorded \& published\\
\end{tabular}
\end{ruledtabular}
\end{table}
...
record whenever the lifetime is below that limit for more than a minute.
{\em ALBA} has a nominal beam lifetime that is given by the combination of both the filling pattern and the
RF radio frequency (RF) voltage.
The typical lifetime at 100$\,$mA is 22 hours.
ALBA operates with
6 six RF cavities each fed with
2 IOTs 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''.
...
{\em 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.
...
\begin{tabular}{lrl}
\textbf{Facility}&\parbox[c]{2.25cm}{\textbf{Bunch charge deviation}}&\textbf{Remark}\\\hline
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 \\
...
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
these types of failure; 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,
...
\subsection{Short-user-time}
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
them this by the rule which adds
up to one hour
(or one downtime) to 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.
...
%\scriptsize
\begin{ruledtabular}
\begin{tabular}{lccccr}
\textbf{Facility}&\textbf{Distorted-orbit}&\textbf{Low-beam-lifetime}&\textbf{Beam-blow-up}&\textbf{Distorted-fill}&\textbf{Short-up-time}\\ \textbf{Facility}&\textbf{Distorted-orbit}
&\textbf{Low-beam-lifetime}
&\textbf{Beam-blow-up}
&\textbf{Distorted-fill}
&\textbf{Short-up-time}\\
& & & & & (h) \\\hline
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 \\
\end{tabular}
\end{ruledtabular}
\end{table*}
...
{\em ``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.
{\em ``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
many most facilities and
should therefore be recorded to evaluate the reliability of the facility in this respect.
{\em ``Beam-blow-up''} failures are again infrequent at most facilities.
...
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
is would be
an essential measure for the reliability of the facility.
{\em ``Distorted-filling and Bunch-purity''} faults are not relevant at all facilities.
Time resolved measurements depend on bunch purity.
...
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.
{\em ``Beam-unrelated''} failure should be recorded whenever they have
an impact on a significant number of beamlines.
These faults
turned out to be rather rare and are often facility specific.
{\em ``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 comparison between facilities that handle ``short-user-time'' differently.
