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\section{DEFINING PRIMARY FAILURE MODES}
The beam current in a storage ring is the primary parameter for beam delivery.
We can 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
two next sections with
examples
In general each facility can define beam examples.
Beam limits $I_{\hbox{nom}} \ge I_{\hbox{tol}} \ge I_{\hbox{min}}$
should be defined for each operation
mode. mode of a facility.
\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 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).
...
\subsection{Primary Failure Modes in Practice}
Table~\ref{tab:pf-limits} shows the current limits for the primary
failure modes
for 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.
The column $I_{\hbox{min}}$ shows the condition under which a ``no-beam'' event would start.
...
\scriptsize
\begin{ruledtabular}
\begin{tabular}{lcrrrl}
\textbf{Facility}&\textbf{Mode}&\textbf{$I_{\hbox{nom}}$}&\textbf{$I_{\hbox{inj}}$}&\textbf{$I_{\hbox{tol}}$}&\textbf{$I_{\hbox{min}}$}\\ \textbf{Facility}&\textbf{Operation Mode}&\textbf{$I_{\hbox{nom}}$}&\textbf{$I_{\hbox{inj}}$}&\textbf{$I_{\hbox{tol}}$}&\textbf{$I_{\hbox{min}}$}\\
& & (mA) & (mA) & (mA) & \\\hline
ALBA & decay & 120.0 & 72.0 & 72.0 & photon shutter (PhS)\\
ALBA & top-up & 100.0 & 98.5 & 95.0 & PhS \\
BESSY II & MB & 299.0 & 299.0 & 272.0 & PhS or 200$\,$mA \\
BESSY II & SB & 14.0 & 13.5 & 12.0 & PhS or 8$\,$mA \\
Elettra & 2.0$\,$GeV & 310.0 & 309.0 & 307.5 & 0$\,$mA \\
Elettra & 2.4$\,$GeV &160.0 & 159.0 & 158.0 & 0$\,$mA \\
LNLS-UVX & decay & 250.0 & 130.0 &
- 60.0 & PhS or 60$\,$mA \\
PETRA III & top-up & 101.0 & 100.0 & 75.0 & 75$\,$mA \\
SPring-8 & top-up & 99.5 & 99.5 & 99.4 & 0$\,$mA \\
SLS & top-up & 402.0 & 400.0 & 399.0 & 20$\,$mA \\
\end{tabular}
\end{ruledtabular}
\end{table}
``No-beam'' events are defined to start when the beam current drops below a given limit.
For several facilities this is currently not case. ALBA, BESSY-II and LSLS-UVX consider a
closure of the photon shutters by an interlock equivalent to a loss of the electron beam.
For other 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
...
while at BESSY-II it starts only at 9\% beam decay.
The reason of this difference comes from the required current stability for the experiments
that can differ significantly between light sources.
LNLS-UVX and PETRA III do not account for ``low-beam-current'' events at all (
$I_{\hbox{tol}}$ = $I_{\hbox{min}}$).
\subsection{Discussion of Primary Failure Modes}
A ``downtime'' often only ends after the photon shutter or insertion device control is
...
situations are rare at the evaluated facilities where the current drops from
the nominal beam current to less than 50\% but not to zero.
{\em ``Low-beam-current''} ``Low-beam-current'' events vary significantly with the definition of the $I_{\mbox{tol}}$ limit.
Nevertheless comparing the tolerated beam decay is still
useful. useful to judge the promised current stability of a facility.
For those facilities where the limits are similar, the failure rates will allow a
meaningful comparison of the reliability of the injection process.
