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\section{FACILITY OPERATION MODES}
Many failure modes of a
given facility depend on the
specific operation mode.
A The nominal beam current in single bunch operation
for instance would be considered insufficient
for
any useful multi-bunch
filling operation at the same facility.
Therefore we'll We therefore discuss the different operation modes
of our facilities before we
get to the address actual failure modes.
\subsection{ALBA Operation Modes}
The ALBA light source~\cite{Garc_a_2014}
presently runs
basically with one operation mode: 100$\,$mA multibunch.
The
filling fill pattern consists of 10
bunch trains each with 32
buckets bunches and a 24$\,$ns gap in between.
Since September 2014 ALBA
is running operates in top-up mode, before that it was decay mode.
In top-up mode the beam decays down to 98.5$\,$mA and is then
accumulated re-injected up to
100$\,$mA again. 100$\,$mA.
In the decay mode
we were accumulating to ALBA injected 120$\,$mA
with 2 injections per twice a day and let the beam
decay. decay in between.
\subsection{BESSY II Operation Modes}
The annual beamtime calendar at BESSY II~\cite{Bakker_1999} is organized in weeks of user
operation, machine development and beamline commissioning, and shutdown.
A standard week of scheduled user beam time starts Tuesday
7:00 a.m. and ends Sunday 23:00 providing three basic user operation modes.
Every other Sunday two
eight hour eight-hour shifts are given to the PTB (4$^{th}$ mode).
\begin{description}
\item[Multi bunch hybrid mode] comprises 299$\,$mA total current, kept constant by
permanent top-up injections.
General bunch pattern is an even filling of 300--350 bunches and a gap of about 100--50 bunches (550$\,$MHz, harmonic number 400).
In addition to this multi bunch current, up to five specific bunches are serving dedicated experiments.
The size of the gap is negotiated: the long gap (200$\,$ns) supports pulse picking with a mechanical chopper, the short gap (50$\,$ns) aims at overall stability.
\begin{itemize}
\item A higher current (4$\,$mA) camshaft bunch can be inserted in the middle of the (purity controlled) gap for pump-probe experiments
\item Three higher current (4$\,$mA) bunches can be located opposite of the gap, sequentially sliced with
6$\,$kHz repetition, 20$\,$fs laser pulses, generating 100$\,$fs x-ray pulses.
\item One horizontally excited bunch, typically three buckets away from the end of the gap, can be resonantly excited for pseudo-single bunch
experiments by a specific set-up.
\end{itemize}
In this standard mode two emergency modes of degraded beam conditions
are at disposal:
\begin{itemize}
\item At problems with the LINAC-booster injector chain BESSY still
can easily switch from LINAC to microtron injector: then multi bunch
top-up is feasible, occasional refilling of the custom bunches require a
preparing decay phase and a closure of the beam shutters.
\item If the top-up lifetime constraint $>$5h cannot be met
(e.g. vacuum problems) lowering the nominal current from 299mA to
some 250$\,$mA allows to stay operational. experiments.
\end{itemize}
In this standard mode two emergency modes of degraded beam conditions are possible.
BESSY can easily switch to {\em microtron injector} in case of LINAC problems.
Then multi bunch top-up is feasible, occasional refilling of the custom bunches require a
decay phase and a closure of the beam shutters.
If the top-up lifetime constraint $>$5h cannot be met (e.g. vacuum problems)
BESSY can set a {\em lower nominal current} of \~250$\,$mA to stay operational.
\item[Single bunch mode] consists of 14$\,$mA in a (purity controlled) single bunch, refilled with top-up, used for time resolved experiments (2--3 weeks/y).
This mode specifically depends on the LINAC. The microtron can inject single bunches only with
insufficient low efficiency, thus a lasting LINAC failure results in a decaying beam degraded mode.
\item[Low Alpha mode] even filling of 100$\,$mA (short pulse mode) or 15$\,$mA (THz mode, non- bursting coherent synchrotron radiation) in an alternating, 12 hour period decaying beam sequence (2--3 weeks/y).
\item[National Bureaux of Standards (PTB)] main user mode, beam conditions according to their experimental requirements.
\end{description}
{\em %{\em Top-up constraints} are imposed by the radiation protection prescriptions and are implemented in the top-up interlock at BESSY II, generating a number of enforced combinations of decay mode, closure of the beam shutters, and several refilling modes.
{\em %{\em Decay only}: Enforced by insufficient lifetime ($t<$5$\,$h) or insufficient injection efficiency (four hour average of injection efficiency Eff$_{\mathrm{avg}}<$90\% enforce
$T=4\,\mathrm{h}*(90\%-\mathrm{Eff}_{\mathrm{avg}})/(100\%-90\%)$ %$T=4\,\mathrm{h}*(90\%-\mathrm{Eff}_{\mathrm{avg}})/(100\%-90\%)$ injection free time). W.r.t. accounting
begin %begin and end of this failure condition is handled according to the 'low-beam-current' criterion.
{\em %{\em Beam shutter closure enforced}: triggered by a single shot with efficiency $ <60\%$, beam current $ <200\,$mA in MB mode or $ <10\,$mA in SB, or an unclear/inconsistent status of the top- up interlock, an enforced decay starts.
Any %Any re-injection shot requires to close the beam shutters first. Then a single shot fulfilling the top up requirements
reenables re-enables the opening of the beam shutters.
Beam %Beam shutter closure can be shifted within negotiated time limits, but the failure begins at the moment, the interrupting condition has been met.
The %The event stops when top up conditions and the nominal beam current is reached again
\subsection{Elettra Operation Modes}
...
For radiation protection reason, the booster current must be lower than 0.5$\,$mA.
To be able to maintain the machine in top-up mode some radiation protection constraints must be met.
These are the following:
{\em %{\em Top-up constraints}
The %The integral of the electron charge losses during top-up injection, measured on a relatively long interval time (one hour), must not exceed a prefixed threshold.
This %This value is obtained computing the so-called top-up-lost-charge which is the difference between the injected charge, measured through a toroid installed at the end of the BTS TL and the relative current increment, measured through the ring DCCT.
If %If the integral of the top-up-lost-charge per hour exceeds the prefixed threshold, the {\em top-up process is inhibited for the following hour}.
The %The integral of the electron charge losses during top-up injection, measured on a relatively brief interval time (about 3 to 5$\,$s) is not within a prefixed threshold.
This %This interlock will permit the {\em safety system to interrupt quickly the operations}
(through %(through the inhibition of the Linac gun) if large beam losses occur along the ring.
Another %Another safety interlock will be based on the beam lines gamma monitors.
If %If one of them exceeds a prefixed alarm threshold, the safety system will not interrupt the top-up injection but will {\em close the beam stopper of the affected beam line}.
\subsection{LNLS-UVX Operation Modes}
