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\section{DEFINING PRIMARY FAILURE MODES}
Reliability is the ability of a system to provide a certain function over time.
The function of a light source is to provide synchrotron radiation for user experiments.
From the perspective of a user it does not matter much, what type of failure prevented
him to perform his experiment.
But in order to progress to highest reliability light sources one needs to differentiate
the failure modes;
for each mode solutions should be found to increase the mean-time-between-failure,
and to minimize the mean-time-to-recover.
The beam current in a storage ring is the primary parameter for beam delivery.
We can Based on the beam current we define two simple failure modes:
``no-beam'' and ``low-beam-current'' (see Fig.~\ref{fig:primary-failure-modes}).
These two modes are discussed in the next sections with examples.
Beam limits $I_{\hbox{nom}} \ge I_{\hbox{tol}} \ge I_{\hbox{min}}$ should be defined for each operation mode of a facility.
Further failure modes will be discussed in the next section.
\subsection{No-beam}
When the beam current is below $I_{\hbox{min}}$ the ``no-beam'' mode starts.
It stops when the nominal beam current $I_{\hbox{nom}}$ is reached again.
\subsection{Low-beam-current}
This
failure mode starts when the beam current drops below $I_{\hbox{tol}}$, but only if the machine
is not in the ``no-beam'' failures mode (otherwise all ``no-beam'' events would be ``low-beam-current'' events as well).
The ``low-beam-current'' mode stops when the beam current reaches the nominal beam current $I_{\hbox{nom}}$ again.
For facilities in top-up mode the current limit $I_{\hbox{tol}}$ should be slightly
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\subsection{Primary Failure Modes in Practice}
Table~\ref{tab:pf-limits} shows the current limits for the primary
failure
modes modes, partly depending on the operation modes of the seven facilities.
The column $I_{\hbox{nom}}$ shows the maximum current in the given operation mode.
The current $I_{\hbox{inj}}$ is the typical value when injection would start in this operation mode,
below $I_{\hbox{tol}}$ it would be considered a ``low-beam-current'' event.
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For some facilities a ``no-beam'' event stops not when the beam is back to
the nominal current $I_{\hbox{nom}}$ but when the interlock is cleared that prevents
the beamline shutters from opening (ALBA, BESSY II, LNLS-UVX) or
when the insertion devices move back to
the their closed positions (SLS).
PETRA III adds an amount of time to allow for the warm-up of the optical components at the beamlines.
These practices are oriented to use a beam availability understood as photon beam availability.
Facilities are counting the time the photon beam is readily to be used at the beamlines.
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``Low-beam-current'' events do vary significantly between facilities.
At Spring-8, for example, a beam decay of about 0.1\% starts a ``low-beam-current'' event,
while at BESSY-II it starts only at 9\% beam decay.
The reason
of for this difference comes from the required current stability for the experiments
that can
differ vary significantly between light sources.
LNLS-UVX and PETRA III do not account for ``low-beam-current'' events at all (
$I_{\hbox{tol}}$ = $I_{\hbox{min}}$).
...
minutes until the insertion-devices are unblocked, then it should be visible from the failure
data that these were not three but was only one interruption in user operation.
The authors decided to allow arbitrary limits for ``no-beam''
events,
but events;
in practice this does not make a large difference:
situations are rare at the evaluated facilities where the current drops from
the nominal beam current to less than 50\% but not to zero.
