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\section{FACILITY OPERATION MODES}
Storage ring based light sources can be operated in a variety of different modes.
Some modes are optimized to
provide the highest for high photon flux
to the experiments (e.g. multi-bunch fillings)
while other modes
are optimized to create short
photon pulses
(e.g. single-bunch and low-$\alpha$ operation).
Some
light sources
do only operate in a single
mode, mode and
others switch from mode to mode for specific
sets of users on a weekly basis.
The total amount of
user annual beam time
provided each year is of course
as well facility dependent.
Many failure modes
of defined at a given facility depend on the specific
operation operational mode.
The
nominal total beam current in single-bunch operation for instance would be considered insufficient
for useful multi-bunch operation at the same facility.
In the following we will illustrate the differences presenting data for seven facilities:
ALBA, BESSY II, Elettra, LNLS-UVX, PETRA III, SPring-8 and SLS.
A reference publication is provided for each light source.
Table~\ref{tab:opmodes} shows an overview of the scheduled operation user time and user modes.
\subsection{ALBA}
The ALBA light source~\cite{Garc_a_2014}
presently runs with one operation mode: 100$\,$mA multi-bunch.
The fill pattern consists of 10 bunch-trains each with 32 bunches and a 24$\,$ns gap in between.
Since September 2014 ALBA 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 re-injected
up to 100$\,$mA.
In the decay mode ALBA injected 120$\,$mA twice a day and let the beam decay
to 50$\,$mA in between.
ALBA scheduled 3888 h for users in 2014.
Extra user time has been provided for compensation in some
of the rare cases of long machine outages.
\subsection{BESSY II}
A standard week of scheduled user beam time at BESSY II~\cite{Bakker_1999} starts Tuesday
7:00 a.m. and ends Sunday 23:00 providing the three basic user operation modes: Multibunch hybrid, single-bunch or low-$\alpha$.
Every other Sunday two eight-hour shifts are given to the National Bureaux of Standards (PTB). The user modes are:
\begin{itemize}
\item {\em Multi-bunch hybrid} mode comprises 299$\,$mA total current, kept constant by top-up injections.
General bunch pattern is an even filling of 300
bunches and a gap of 100 bunches
(200ns, 550MHz, harmonic number
400). (200ns). In addition to this multi-bunch current, five specific
bunches
are serving serve dedicated experiments (see below). The size of the
gap around
the each camshaft bunch is variable to support pulse picking with a
mechanical chopper.
The five specific bunches
are are: one
{\em camshaft-bunch}
of a at higher current (4$\,$mA) inserted in the middle of the (purity controlled) gap for pump-probe experiments;
three {\em slicing bunches} of 4$\,$mA each can be located opposite
of to the gap, sequentially sliced with
6$\,$kHz repetition, 20$\,$fs laser pulses, generating 100$\,$fs x-ray pulses; and
one {\em pseudo-single-bunch} typically three buckets away from the end of the gap, which is resonantly excited for pseudo-single-bunch experiments.
In this standard mode two
emergency backup modes
of for degraded beam conditions are possible.
BESSY can easily switch to microtron
injector injection in case of LINAC problems.
Then multi-bunch top-up is feasible, occasional refilling of the custom bunches
require requires a
decay phase and a closure of the
beam beamline shutters.
If the top-up lifetime constraint $>$5h cannot be met (e.g. due to vacuum problems)
BESSY can set a lower nominal current of $\approx 250\,$mA to stay operational.
\item {\em 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 per year).
...
In a typical run the initial current is 250$\,$mA, injected at 500$\,$MeV and ramped up to 1.37$\,$GeV,
the nominal operation energy.
Injection takes place twice a day and is scheduled to last 30 minutes at most.
By the end of the run, after about 11 hours, the current
is decays down to 130$\,$mA.
Single-bunch shifts are available since 2003, but the provision of
these shifts depends on the demand by the users. In the single-bunch mode the initial current is typically in the
order of 9$\,$mA and is
not less interesting for high-flux, multi-bunch users.
Operation in hybrid mode is impractical due to the characteristics of the UVX injection system.
In 2014 LNLS-UVX was operated 3816 hours for users.
Since UVX is operated from
Mondays Monday to
Saturdays, Saturday, it is common practice to provide
bonus user time on the weekend in case of operational problems during the week;
a total of 87 hours bonus time was provided in 2014.
...
In the continuous filling modes either every fourth bucket (960 bunches), or every eighth (480 bunches) is filled;
in timing mode either 40 bunches (192$\,$ns bunch spacing) or 60 bunches (128$\,$ns bunch spacing) are filled.
PETRA III
was running in 2013 for operated 5812 hours in
user operation 2013
and went into a long shutdown in February 2014 for an expansion project.
Users have not been compensated for long beam outages so far.
\subsection{SPring-8}
SPring-8~\cite{Tanaka_2006} runs in 9 operation modes, one of which is multi-bunch
mode of with twelve 160 bunch-trains,
also several-bunch modes and hybrid modes.
The several-bunch modes consist of equally spaced bunches (or bunch-trains),
i.e. 203 bunches, 84 trains of 4 bunches, and 29 trains of 11 bunches.
The hybrid modes are composed of partially filled
multi-bunch-train multi-bunch-trains and isolated single-bunches,
i.e. 11/29-filling + 1 bunch, 1/7-filling + 5 bunches, 1/14-filling + 12 bunches, 2/29-filling + 26 bunches, and 4/58-filling + 53 bunches.
The impurity of an isolated bunch is maintained under $10^{-8}$ in top-up operation.
...
\subsection{SLS}
The Swiss Light Source (SLS)~\cite{Aiba_2013} runs only in 80\% filling mode (390 out of 480 buckets), 400$\,$mA top-up.
A single-bunch in the gap with a higher charge can be added on
demand - that is demand, the so called "camshaft mode".
Typically the single-bunch is filled to 4 times the charge of the other buckets.
Switching between the modes does not affect users of the 80\% filling, since the beam current is kept constant at 400$\,$mA in both.
A total of 4984 hours of user operation
had been was scheduled in 2014.
Only 4824 hours were pre-scheduled for user experiments.
Two blocks of 80 hours were scheduled in addition to allow to compensate users for longer outages.
Those backup user shifts
are were assigned to user experiments roughly a month in advance,
and
are were given to those users who were not able to complete their experiments.
Two beam availability numbers are published: with and without taking
the backup shift compensation into account.
The compensated beam availability is mainly used for in-house purposes.
Infrequently part of the weekly machine development time is provided
for user experiments, to compensate for recent machine problems.
These shifts are not accounted for in the operation statistics.
\subsection{Operation mode comparison}
Table~\ref{tab:opmodes} shows an overview of the operation user time and user modes.
