HOW TO CONTROL THE NORTH AREA BEAM LINES
The North Area beams H2, H4, H6, H8, P42, K12 and M2 are produced by a
high-intensity primary proton beam, impinging on each of the three
primary targets T2, T4 or T6. The ‘useful’ particles are picked up by
the beam lines and are transported to the user areas. These beam lines
are long (from about 500 to almost 1200 meters) and complex and contain
a large number of elements of different natures, such as targets,
magnets, collimators, dumps, absorbers, converters, detectors, vacuum
pumps and access doors.
Each equipment has its own settings and
readings, which depend on the required operational mode, on the beam
momentum, on the wanted intensity and particle type, on the state of the
access to the areas on the beam line and sometimes even on the settings
in neighboring beam lines.
Most of these settings can be managed by the users themselves via the
‘Cesar’ application for beam line control.
A very succinct description of this software is given in section 2.
Detailed instructions concerning the general features of the Cesar
application are available in a
document. In the present document we rather address in a general way
the most common tasks and how they are best performed.
Every beam line has been designed with specific main purposes and strong
points and therefore the detailed use of the beam lines is different for
each of them. The specificities of each beam line are normally described
in a specific User Manual for the beam line. Information can of course
also be obtained from the AB/ATB/SBA beam line expert for the beam in
This document is organized in 11 sections
Short overview of the Cesar application software
What is a wobbling station
Reference settings and beam files
Beam steering, focusing and control of magnets
Beam intensity, momentum spread and control of collimators
The type of particles in your beam line: targets, absorbers, converters, etcetera
Using the detectors in the beam line to optimize your beam
- Access to the beam area
Tools for quick checks of the beam performance
- Final remarks
Before starting to modify settings in a beam line, it is of obvious
importance to know what one wants to achieve and what equipment one
needs to act on. This information is usually obtained from the
information available on the SBA
web pages, from the beam line experts or SPS operators or from the
more experienced beam line users in your experiment. Particularly useful
Useful information is also available on the
board pages for the North Area. A synoptic of the North Area beam
lines is given in Figure 1.
- The beatch files that give the geometrical layout of the beam lines, in particular the
sequence of equipments along the beam.
- The Transport listings and optics drawings that
show the beam optics for your beam. From those you can read the
approximate beam size at a given location and you may be able to judge
the effect of a particular magnet on the beam steering or of a
particular collimator on your beam intensity.
Figure 1: Synoptical diagram of the North Area beam lines
Your beam line is a complex system that picks up particles emerging from
the primary target and makes a selection in terms of momentum and angle.
The so-called “wobbling station”, described in section 3, ensures that
sufficient particles of the requested type and energy are sent towards
your beam line in a way consistent with the requirements of the other
beam lines derived from the same primary target.
Dipole magnets, called Bends, are used to guide the particles
through the tunnels towards the relevant experimental areas and also to
introduce dispersion, necessary to achieve momentum selection. Trims
are small dipoles that have no nominal deflection angles but allow
corrections to the beam steering. Quadrupoles help to control the
beam size and dispersion along the beam line. Together with (adjustable)
slits, called Collimators, they define the acceptance of the beam
line in momentum bite and angle. The settings of these elements are
collected in so-called beam files, described in section 4. The settings
and control of magnets and collimators are described in sections 5 and
The amount of hadrons as well as the relative abundance of pions, kaons
and protons can be calculated with the
This program calculates the intensity and composition of the secondary
beam. Various absorbers, converters and secondary targets allow to
control the beam composition and intensities taking advantage of
interactions or Bremsstrahlung in their material. In that case the
energy of the beam leaving the absorber or secondary target is typically
significantly lower than the secondary beam momentum. This must be taken
correctly into account in the magnet settings and hence in the beam
files. The control of absorbers, converters and secondary targets is
described in section 7.
Several types of detectors allow checks of beam intensities and the beam
spot along the beam line and consequently also to correct and optimize
the beam steering and focusing. These detector types comprise
scintillators, various types of wire chambers and filament scanners.
Cerenkov counters provide particle identification. Details of their use
are given in section 8.
Dumps are essential parts of the safety system of the beam lines. In
case of access to your experimental area the beam must imperatively be
stopped by moving dumps into the beam and or switching off a number of
well-defined dipole magnets. In special cases other elements such as
primary targets are part of the safety chains, too. Your personal safety
is guaranteed by the access system, which is implemented mostly in
hardware. However, the beam line must be configured especially for
either access or beam. The software tools to achieve this are described
in section 9.
The beam from the SPS is delivered in so-called super-cycles. The
super-cycle configuration changes rather frequently. The presently
active super-cycle and its properties are shown on the so-called
page-1 screens that are
available in most control rooms as well as
on the web. The super cycle contains one or more ‘flat tops’,
typically 4.8 or 9.6 seconds long, during which beam is extracted
towards the North Area targets with a time distribution as uniform as
possible, in principle without RF structure. In between flat tops the
SPS is used to inject and accelerate the beam or to provide beams to
other users (CNGS, LHC, machine studies, …). A detailed planning of
super-cycles can be found on the
SPS Coordinator’s schedule
pages. Many programs are synchronized with particular timing
‘events’ in the super-cycle, such as the start or end of the flat top.
Magnet currents are typically refreshed only at the start of the flat
top, whereas detector readings are typically only updated after the end
of each flat top. The page-1 screen also allows you to check easily
whether or not the SPS is providing beam. Even when there is beam
extracted from the SPS onto your primary target, it may happen that
there is no beam arriving in your experiment. Section 10 describes some
tools and techniques to find out whether there is beam or not in your
area and why.
Finally we conclude in section 11, which gives some hints on what to do
if everything else fails.
2 - A short overview of the Cesar application software
As explained in the
documentation, the Cesar application starts automatically on the
beam terminals in the experiment control rooms after booting. The Cesar
GUI is shown in Figure 2.
Settings can be verified or changed via several mechanisms:
Figure 2:The Cesar application window
- Via the menu bar on the top of the GUI. For the
users who have experience with the (now obsolete) Nodal control system,
the menu structure reflects more or less precisely the Nodal tree
structure. In the text menu items are highlighted in green.
- Via the icons, called ‘task buttons’, located
just below the menu bar. They provide quick and easy access to the most
frequently used tasks. A description is shown when you move the mouse
over the icon. Task buttons are indicated in red throughout the text.
- Via the “physicist tree’ equipment window on the
left hand side in the main working space. This window allows direct
access to individual equipment or groups of equipment. This is often
more efficient than the mechanisms described above, but requires more
Just below the task button bar, you find a list of ‘work spaces’,
corresponding to individual beam lines. In the user barracks, normally
only one work space is accessible. In the central control rooms, please
make sure you have selected correctly the work space tab corresponding
to your beam line.
Most settings and readings can be made via the status panels, activated
by the Status menu, the appropriate task buttons or via the physicist
tree. Often they have an automatic refresh for every flat top. However,
after some time, typically 10 minutes, the automatic refresh stops (to
save on system resources). Please check from time to time!
3 - What is a wobbling station?
As seen in Figure 1, two beam lines (H2 and H4) are derived
simultaneously from the T2 primary target. Similarly up to three beams
are running simultaneously via the T4 target. Each beam line has its own
requirements in terms of beam momentum, charge (positive or negative)
and intensities. On top of that, these requirements change frequently
with time. The required
flexibility is provided by the so-called ‘wobbling stations’. As an
example we show in Figure 3 the T2 wobbling station for one specific
setting, namely +150 GeV/c secondary hadrons in H2 and -150 GeV/c
hadrons in H4.
In order to achieve this, two sets of dipoles just upstream of the
target, B1T and B2T, direct the beam towards the center of a third set
of dipoles B3T, located a few meters downstream of the target. The H2
and H4 beam lines start from the center of B3T at different angles. In
this particular case the protons follow the bisector between the H2 and
H4 axes. The B3T dipoles are set to sweep +150 and -150 GeV/c particles
from that bisector into the H2 and H4 beam lines, where they pass
through relatively small holes in special thick dump-collimators called
TAXes. Different momentum particles, including the primary protons
traversing the target, hit the TAX at different positions, where there
is no hole, so that they are cleanly dumped.
The monitors TBIU and TBID (Upstream and Downstream of the target) that
allow the steering of the protons onto the primary target are displaced
onto the calculated nominal trajectory of the primary proton beam.
By changing the orientation of the proton beam or selecting particles
with non-zero production angle, a large number of requirements can be
met. An alternative option is to use a strong field in B3T to sweep away
all charged particles from the TAX holes and to send only neutral
particles through the holes. The beam lines are set to either pick up
pions of protons from Ko or Lambda decays or electrons from
conversion of photons in a lead converter located just behind the TAX.
Figure 3: The T2 wobbling station with positive hadrons in H2 and negatives in H4
The settings of the H2 and H4 beam lines must be matched to the wobbling
station settings. Not only the beam momenta must match, but also
production angles and angular offsets (‘skew’) of the beam through the
TAX hole with respect to the nominal beam direction must be correctly
taken into account.
A similar system exists for the T4 target, but in this case 3 beam lines
share the same target and the
constraints are stronger.
T6 has no wobbling station, the momentum ratio between the P61 and M2
lines is fixed to exactly ‑2.0, e.g. 400 GeV/c primary beam in P61 (i.e.
P0 from T6) implies -200 GeV/c momentum in M2.
The settings of the wobbling stations are agreed at the weekly schedule
meetings and are prepared by the responsible beam physicists and
executed by the SPS operation teams.
4 - Reference settings and Beam Files
The most frequently changing settings of beam elements are magnet
currents and collimator openings. E.g. in the M2 beam line there are 67
magnet currents (11 bends, 7 trims, 36 groups of quadrupoles, 9 magnetic
collimators and 4 MIBs) and 18 collimator motor positions (for 9
collimators). The management of these settings would be virtually
impossible without using so-called ‘beam files’ containing well defined
and valid reference settings. Separate files exist for different beam
momenta and for secondary and tertiary beams. Often different files
exist for electron, hadron and muon beams. Different wobbling
configurations will often require different files for the beam lines
behind. On top of that different users have different beam zones and
different beam requirements. For all those reasons there may be many
files for a single beam line. Not all of them may work in a given
The list of available beam files can be inspected by invoking the
Beam File Browser. This can be done by either clicking on the
Browsertask button or by selecting
in the menu Files →Browse. The Files Menu also
provides options to only show files compatible with the active wobbling
settings (‘Browse beam files filtered by wobbling’) or files for your specific
experiment or zone (‘Browse by Experiment’). An example of a Beam File Browser window is
shown in Figure 4.
4.1 Producing files
Files can be produced – painfully – by hand via the
View or Edit
commands in the
Files menu. Normally this should
be done by the expert beam physicists. For small modifications the user
can also do it. The View command allows modifying settings one by one
(select a current of collimator, select ‘Write
to File Reference’ and enter the new value). It also allows
modification of the file name. The
Edit command is more convenient when there are many changes,
but it invokes separate spreadsheet programs (Do not forget to send the
edited values to the database at the end!). In practice it is more
convenient to save the current reference settings to a beam file using
→ Selected File’ button.
4.2 Loading files
A selected file can be activated by using the
Load Beamfile button. You are then offered the choice to
apply the settings for magnets or collimators or both. The program will
then send the new reference currents and positions to the hardware and
to the BeamReference for each equipment.
If some settings have not been reached successfully, the user is
informed of this. Please confirm
‘Yes’ that you want to continue
loading the file and take corrective action (see sections 5 and 6).
Please do not forget to check in detail the status of magnets and
collimators after loading a file!
Figure 4: The Beam File Browser window
4.3 Saving new settings into a file
Once the beam file values have been successfully loaded, the user may
change individual settings (see sections 5 and 6), in which case the
BeamRef settings become different from the Beam File values (indicated
by ‘F’ in the corresponding status column). If the new settings are
considered better than the file, the beam file may be updated via the ‘BeamRefs→ Selected File’ button.
4.4 Momentum extrapolation
Files with the same characteristics but at different momentum may be
produced automatically from a selected reference file by using the
Extrapolate button. Specify the number of the new file (it
must be new, the program does not allow to overwrite an existing file!),
the new secondary beam momentum. Special algorithms exist for
high-energy electron files (above ~100 to 150 GeV/c) where the energy
loss of electrons due to synchrotron radiation must be taken into
account: choose the appropriate option, e.g.
Hadron => Electron.
A special situation occurs for tertiary beams, where the upstream part
of the beam has a well-defined beam momentum and the part downstream of
an intermediate target has a lower momentum. In that case you must
indicate at which element the momentum change occurs (ask the beam line
expert) and what the tertiary beam momentum shall be.
4.5 Beam File management
Special buttons have been provided to Delete or
Copy files. You may also select two or more files and Compare them.
5 - Beam steering, focusing and control of magnets
Beam steering and focusing of a beam line are controlled by different kinds of magnets:
- BENDs are groups of dipole magnets that introduce a nominal deflection of the beam axis. The term
BEND is also applied to big spectrometer magnets in an experiment. In each beam line one or several BENDs define
the absolute momentum of the secondary or tertiary beam. The currents in those bends should never be different from the nominal (theoretical)
current, see the documentation of the individual beam lines. Other BENDsmay indeed be used to correct the trajectory of the beam.
- TRIMs are small dipole corrector magnets
(typically 0.4 meters long), that normally have zero nominal bend angle.
They serve for correction and beam steering purposes. Even though their
theoretical current is normally zero, but as these magnets are quite
weak, their current can in fact be quite substantial (tens of Amps,
sometimes even more).
- QUADs are groups of quadrupole magnets
that allow focusing of the beam. Normally they should stay on file
values, or only be changed by experts or experienced users.
- SCRAPERs are magnetized collimators that
provide a toroidal field around the beam pipe, but no field in the ‘good
beam’ region. They serve for halo reduction in the M2 and K12 beam
lines. On top of a current, they also have four motor positions, two
defining the aperture upstream and two the aperture downstream. Their
total length is 5 meters!
- >MIBs are fixed toroidal magnets with
20x20 cm2 gap for muon cleaning. There length is typically
1.6 meters or a multiple of that. MIBs are very important in the M2
5.1 Status and setting of magnets
The status of the magnets is obtained via the
menu or via the
task button. The status of individual magnets can be obtained by
double clicking on the name(s) of the magnet(s) in the physicist tree.
The status panel shows the current reading and the current BeamRef
setting for each magnet. If the run button at the bottom is activated,
these readings are refreshed at every start of flat top, either for the
selected (highlighted) magnets in the panel or for all of them. Also
indicated are the maximum allowed current, some special information and
comments, including error condition reports. The error reports are
shortcuts and rather cryptical for the non-initiated; a more verbose
display is obtained with the ‘Display
You may change the current in a selected magnet via the
Current’ button and entering the new current. If you want to
keep the new current for longer, please make sure that you tick the ‘update
Beam Reference’ box (if not, somebody may suspect a
misbehavior of the rectifier and change the current to the now obsolete
BeamRef!). Once you consider the change permanent, you should also
update the beam file value via the File Browser application (see section
4). In case the Beam Ref is different from the current beam file
reference current, this is indicated by a capital F in the column marked
‘F’. This may be a normal
state of affairs during beam tuning sessions, but could also be a hint
for e.g. a typing mistake in setting a current….
If the current is not (but should be) on the BeamRef current, e.g. in
case of a trip of the rectifier, you may try to correct this using the ‘Set
to beamRef’ button. If
this also fails, you have to use the Rectifier Status.
5.2 Correcting problems with magnets and rectifiers
The rectifier (power supply) status is a more expert oriented
application than the magnet status. It has no link to beam file
references, but allows more actions than the magnet status. It can be
invoked from the magnet status or from the ‘Rectifier
Status’ task button or from
the menu via
→ Rectifier Status. Buttons
Reset a rectifier (clear
faults), switch it on or off, to put it in standby mode or simply to set
a current. In general it is recommended
not to switch rectifiers
but put them in
STANDBY mode, except if the
rectifier will be stopped for very long periods (e.g. many days).
Sometimes, if a rectifier does
not start, it may unblock the situation if you try the opposite polarity
first and then switch to the correct polarity once the rectifier works
It is not possible to start a rectifier as long as the faults have not
been reset. If you do not succeed to reset the faults or if nothing else
helps: please call the SPS operator.
There is also an option to switch a rectifier from
DC mode or vice versa. In pulsed
mode the rectifier only powers the magnet during the flat top. At very
low currents this may lead to instabilities. On the other hand at high
currents the DC mode may lead to overheating of the magnets. In general
it is not recommended to change this setting without consulting the
operators or your beam line expert (i.e. only if you know what you are
Sometimes a magnet is stopped with a RB (Red Button) fault that cannot
be reset. In fact the RB is not only activated by a Red Button on the
magnet (e.g. pushed by people working around the magnet), but also in
case of veto by the access system. Please check that nobody has taken
access to one of the beam areas (see section 9)!
5.3 Degaussing of magnets
The degauss program can be useful in case it is very important to
guarantee absolute precision in the absolute momentum scale. This is
e.g. the case for linearity studies of calorimeters. This program cycles
through a number of current settings for all or a number of selected
power supplies. The inconvenience is that it is a somewhat lengthy
process and the risk of equipment failure is not negligible in case you
degauss many magnets at the time. It is invoked by a
right-hand button mouse click on the magnet(s) selected in the Magnet
Status. A contextual menu appears in which you can select the
Degauss option. Two strategies are proposed:
- Cycle stepwise through a sequence of opposite
and decreasing currents, starting from the maximum allowed current and
then -80%, +60%, -40%, +20% and finally 0 Amps,
- Switch the magnet 3 times successively to
maximum current and zero current.
The latter option gives more reproducible results, but sometimes
slightly larger remnant fields.
5.4 Other useful commands
In the contextual menu you will also find two other useful options:
- The ‘Get Additional Info’ item displays details on the location and
- The ‘Get Logbook Info’ command displays all actions on the element(s) in question between user-specified start and end dates.
6 - Beam intensity, momentum spread and control of collimators
Collimators are adjustable slits, that define the acceptance of the beam
line in angle and/or momentum, depending on their location in terms of
beam line optics.
In the North Area optics drawings, red curves indicate the trajectory of
a particle produced at an angle with respect to the nominal beam optics.
This is also called the sine-like wave or R12, resp R34
term. When that line crosses the axis, the beam has a so-called focus in
that point. Normally the beam momentum and momentum bite are defined by
a collimator located in a ‘dispersive focus’, i.e. a focus where the
dispersion (the blue dotted line in the optics drawing) is large. Such a
collimator is called a momentum slit. The beam flux is normally nicely
proportional with the opening of a momentum slit, but the momentum
spread as well.
Acceptance collimators are located in a place where the sine-like wave
is large. It defines the angular range that can be accepted by the beam
line. The beam flux increases with acceptance angle, but in a non-linear
In many cases the optics is arranged such that there is a non-dispersive
focus at your experiment. If that is the case, the beam spot is to first
order independent of the openings of acceptance and momentum slits.
However, the beam divergence does increase with larger openings. In case
the beam is parallel or if there is strong dispersion at your
experiment, the beam spot will rapidly increase with larger collimator
6.1 Collimator control
Collimator gaps can be observed or changed with the Collimator Status
application, that can be invoked using the
Collimator Status task button
or via the Status → Collimator
menu. The collimator has two jaws, which both must have a
position between the min and max positions indicated in the status
panel. Jaw 2 must be at a more positive position than Jaw 1 and the
minimum opening is about 1 mm. Normally a collimator is considered wide
open with an aperture of ±40 mm (or more), beyond which other apertures
usually take over. There are some exceptions, notably collimator 5 in
the M2 beam which can be opened to ±100 mm. Collimators can act in the
horizontal or vertical plane; this is indicated in the status panel.
The jaws of a selected collimator can be set to a new position using the
Jaw Positions button in the Collimator Status panel. You may
(or may not) update the beam reference as well, depending on whether (or
not) you plan to keep the new setting for longer periods. Please
remember that the new Beam Reference setting is only copied to the beam
file if you explicitly
Save the Beam Refs from the
Browser task. The
Set to BeamRef button sets the
jaws back to the beam reference settings.
6.2 Loading of collimator settings
It is of course also possible to go back to the collimator settings from
the original beam file by loading the beam files (via the File Browser
application) for the collimators only.
7 - The type of particles in your beam line: absorbers, converters, etcetera
The type of particles in your beam line depends on many parameters and
on specific properties of your beam line. Nevertheless there are some
First one should distinguish between various basic operational modes:
This is the 400 GeV proton beam (or whatever beam is being
extracted from the SPS), that has traversed the primary target without
interacting. For radiation safety reasons, this beam cannot be sent to
the surface halls without strong attenuation. This mode is set up for
you by the SBA beam line experts. Only the ECN3 underground cavern is
allowed to receive high-intensity primary proton beams (in practice not
above 1010 protons per spill).
A secondary beam consists of particles that leave the primary
target region and are typically produced in or very near the primary
target. These beams have in general moderately high intensities,
normally between 104 and 107 particles per spill,
somewhat more under very special conditions. Its composition depends on
the beam momentum, production angle and target length and the hadron
content can be calculated with the
Normally, except at very low energies (~20 GeV/c), the secondary beam
consists mainly of different types of hadrons and some small electron
component. Certain wobbling station sections allow to provide
essentially pure electron beams with intensities up to ~106,
depending on the beam momentum and production angle.
||Most beam lines provide the possibility to insert a secondary
target on the secondary beam, typically in the transfer tunnel between
the primary target zone and the experimental hall. The secondaries
interact (or suffer Bremsstrahlung in the case of e± beams)
and produce lower energy interaction products that can be
momentum-selected. By appropriate choices of target material, one may
obtain rather pure hadron or electron beams. Tertiary beams are limited
to lower intensities, typically not exceeding ~104 to 105
particles per spill.
The wobbling station settings, agreed at the weekly schedule meeting,
impose certain constraints on the momentum and characteristics of the
secondary beams. The tertiary beam option recuperates part of the
required flexibility and is therefore frequently used.
7.1 Secondary beam mode
Secondary beams are normally mixed beams, i.e. a mixture of different
hadron species plus some amount of electrons (or positrons). In most
beam lines an ‘absorber’ can be moved onto the beam to remove an
electron component. This absorber is located somewhere half along the
beam line in a focus in both planes. This absorber is almost (but not
quite) transparent to hadrons, but most of the electrons loose a
substantial fraction of their energy in it due to Bremsstrahlung. Those
degraded electrons will not be transported by the second part of the
beam line and are therefore effectively removed from the beam. The
absorber is a sheet of lead, between 3 and 10 mm thick. The thicker the
lead sheet, the more important the reduction factor for electrons
(typically ~10 to > 100), but also the larger the losses of the hadrons,
in particular at the lowest beam momenta.
Another possibility is to use B3T of the wobbling station (the one
immediately following the primary target) to sweep away all charged
particles so that only neutrals reach the first Bend of your beam line. However, the KS
and L particles in the neural
beam may decay and provide charged decay products: pions or
(anti-)protons. By correctly setting the first
in your beam line to compensate for any skew (see section 3), these
pions and protons can be picked up and transported to your experiment.
Alternatively on may insert a so-called ‘converter’ (sheet of Lead, few
mm thick) just downstream of the TAX dump-collimators and upstream of
the first Bend. In that case the photons in the neutral
beam convert into pairs of electron and positrons, which may be momentum
selected and transported to the experiment. In that case the beam is
almost pure in electrons (~98%!). Hence:
- Converter OUT: pure hadron beams
- Converter IN: pure electron (or positron) beams
Secondary beams have normally a unique momentum all along the beam line,
except in the case of high-energy electron (positron) beams, where
synchrotron radiation losses must be corrected for. This is handled by
the beam files (see section4).
7.2 Tertiary mode
A tertiary beam is created by
- inserting a secondary target in between two sets of main bends (e.g. downstream of
the big upward bends at the start of the tunnel leaving the primary
target zone and upstream of the big downward bends at the entrance to
the experimental hall).
- Setting the
magnets according to the secondary momentum before the secondary target
and corresponding to the wanted tertiary momentum for those downstream
of the secondary target.
The second point is handled ‘automatically’ by the beam files. In the
Beam File Browser tertiary beam files are distinguished by a double beam
energy, e.g. “+150/+120” indicates that the file corresponds to a +120
GeV/c tertiary beam derived from a +150 GeV/c secondary beam.
For tertiary electron beams, one normally uses a thin (few mm to 1 cm)
Lead target. Electrons in the secondary beam are momentum degraded in
the target and transported to the experiment. Hadrons keep there initial
momentum and are lost after the secondary target.
For tertiary hadron beams, one uses thicker and often heavier targets:
tens of centimeters of Copper, Poly, Beryllium, …
7.3 Muon beams
Often it is useful to have muon beams. Muons are produced by the decay
of pions. Muon beams are therefore produced by setting up a moderately
high-intensity pion beam. The muons from their decay will partly reach
the experiment. The pions must be stopped, either in a beam dump (XTDX
or XTDV) or in a collimator. For this purpose it is recommended to close
the collimator in an off-axis position, e.g. +45/+46 mm. If the
collimator is located downstream of the last big bend, the muons are
unfiltered in momentum and cover the whole range between 57 and 100% of
the pion momentum. If a collimator is closed upstream of the last big
Bend, the muons will
be roughly momentum selected by that big bend.
The M2 beam line has been designed specifically to provide high-energy
high-intensity muon beams. The user is referred to the M2-specific
documentation for details in this case. Also the K12 case is very
7.4 How to control absorbers, converters, targets
Absorbers, converters and targets are controlled via the
menu or via the
task button. The panel provides the list of obstacles.
Some are in/out movements. The ‘Move IN’ and ‘Move OUT’ buttons allow
moving them into or out of the beam. Others allow continuous position
control. There are pre-defined settings, which you may select with the
(discrete)’ button, which calls up the list of options.
Please note that the movement is slow, of the order of a minute. You may
also change the BeamRef settings.
Status General menu or ‘General
Status’ task button shows a
panel with a summary of all settings of obstacles, collimators and some
useful count rates.
8 - Using the detectors in the beam line to optimize your beam
Each beam line is equipped with a number of detectors. The main detector types are the following:
Allow to count the beam intensity and are sometimes used to
strobe the more complicated detectors. The
Scintillator Status task button
or Detectors → Scintillator
Status menu shows the count rates of each of the scintillators.
The Move IN and
Move OUT buttons allow moving scintillators in and out of the
beam. Beware that they present a fair amount of material on the beam!
There are also buttons to Restore and
Zero the high-voltage. Please note that the HV tuning is
reserved for the equipment experts.
Normally it is wise to move unused scintillators OUT, to reduce
multiple Coulomb scattering and Brems-strahlung tails.
Attention: sometimes a MWPC is moved out by the same motor!
| MWPC (Analog Chambers):
|| These are multiwire proportional chambers with analog readout
that integrates charge per wire over the SPS spill. As the total charge
on a wire is proportional to the #particles on that wire, the result is
a beam profile. The proportionality constant is a strong function of the
high voltage. The wire spacing is 1 mm for XWCA and 2 mm for the larger XWCM type chambers, but
depending on the beam characteristics, the cabling has been made such
that either each wire or only every 2nd or 3rd
wire is read out. As the number of channels is limited to 32 per plane,
only the central third of the chamber is read if every wire is read.
There are task buttons for the ‘Analog
Wire Chamber Status’ and the ‘Analog
Wire Chamber Profile’, as well as the menus
→ Analog Wire Chamber Status and
Tune → Measurement
→ Analog Wire Chamber Profile.
The Status panel has buttons to Move IN and
Move OUT selected chambers, to
Zero the HV and also a button to
Set the HV to a new value (max
4000 Volts). For lower-intensity beams a higher HV is required,
otherwise the signal is within the noise. For high intensity beams of
small spot size, a much lower HV is sufficient. The HV is automatically
reduced by the control electronics if the signal is too strong (> 1024
counts). A good starting HV is normally 2000 V.
In the profile windows you will find a ‘Zoom’
button. This will open a window with a more detailed view and many more
readings. Also you can recalculate the mean and RMS using only channels
with reading above a user-defined cut-off (using the slider on the right
hand side of the profile).
|XDWC (Delay Wire Chambers):
||These are drift chambers with a simple TDC readout over a delay
line. Individual tracks are registered with better than 200 micron
resolution. The application software shows only the integrated beam
profile per spill, but the output signals can be used by the DAQ of the
experiment, using private TDC’s.
The profiles are obtained by theDelay Wire Chambers Profile task button
or the menu Detectors→ Delay Wire Chambers Profile.
The slow control has not yet been implemented. Again a zoom button has
been implemented like for the analog chambers.
|EXPT (Experimental Scalers)
||These are not really detectors, but inputs in a patch panel that
receive NIM signals from detectors belonging to the experiment. These
signals are counted by a VME scaler on a burst-by-burst basis. These are
extremely helpful tools for beam tuning and monitoring. The counts can
be monitored by the Scaler Status
task button or via the Detectors → Experimental Scaler Status
||These are only present in the highest intensity beams. Their readout is included in the
Experimental Scaler Status panel (see above).
||The FISC counters (FIlament SCanners) are motorized scintillator
filaments, 200 microns wide and 4 mm thick along the beam that can be
move at constant speed (Fast mode) or step-by-step (slow mode) through
the beam. Their main advantage is that they are integrated in the vacuum
The FISC Status
task button and the Detectors → FISC Status menu allow to
Position of the filament,
to Zero Position or to the Min and Max positions.
The FISC Profile task button and
Tune → Measure → FISC Profile
menu allow to measure the beam
profile. Precise profiles require the
Slow mode (choose by Radio-button), but take a long time (one
spill per point). Select one or more FISCs and bring them to the right
hand white window for selected FISCs by clicking on the “
”sign. If necessary choose
a normalization counter (the default is normally the incident proton
flux on your incident target). Then define minimum, maximum position and
step size. Press the Scan button and lean back…
Fast mode will make a profile in
one burst. The step size is chosen automatically. However, the result is
less precise and sensitive to non-uniform arrival time of the particles
over the spill.
|XCET (Threshold Cerenkovs)
||Threshold Cerenkov counters allow some level of particle
identification, depending on the gas used (normally Helium or Nitrogen)
and the beam momentum. The gas pressure can be varied between 20 mbar
and 3 bars. The user can get an analog signal from the counter via a
dedicated cable into his private DAQ system. Another output signal is
‘digested’ by the Experimental Area control system and available for
checking the beam composition and/or setting up the counter.
→ Threshold status
menu shows the HV setting and count rates of the threshold
counters. The raw Cerenkov counts are indicated. The detector is
‘strobed’ by a coincidence of two scintillator counters surrounding it
(and which MUST be IN for meaningful results). The so-called
Coincidence rate gives the rate of coincidences between the Scintillator
coincidence and the Threshold counter. Buttons
Restore HV and
Zero HV allow to act on the HV,
but the precise HV is under equipment expert control. You may
the Pressure to your preferred value.
As the threshold pressure calculated may not correctly take into
account the gas (im-)purity and the precision of the pressure gauge, it
is recommended to do a Pressure Scan, using the
Threshold Pressure Scan task
button. Select the threshold counter, define start and end
pressure (from high to low pressure) and a step size. You are also
requested to give a minimum count rate per point. Press the
Scan button and observe the result (after some time). The
theoretical threshold pressures are indicated on the left hand side of
These are very specialized Cerenkov counters with excellent
performance. This is specialized equipment and the user should consult
the beam line expert for more information.
8.1 How to optimize your beam
Normally beam files exist with theoretical or pre-tuned reference
currents and collimator settings. As there may be drifts, due to ground
movements, magnetic history, misalignment and so forth, further
optimization or re-tuning may be required. The most frequently used tool
for this is the Scan program. It can be activated using the
task button or the
Tune → Scan menu.
A scan consists of a series of measurements (usually rates, sometimes
profiles) according to a sequence of settings of another equipment
(magnet current, collimator position) in well-defined steps. E.g. a scan
of a TRIM current from -50 to +50 A in 10 A steps, counting on an EXPT.
Figure 5: The Scan panel
From the scan panel, shown in Figure 5, in the left hand top windows you
select one or more monitors to
be displayed. The Steering Element
Selection pop-up list allows selecting the element that will be
stepped through a list of equidistant settings. The
normalization can usually be left on
the default setting. In the right hand side of the window, you may
define the steps. Once you start the
Scan, a new window sill pop up with the results.
At the end of the scan, the steering element is set back to its BeamRef
position. It is up to the user to judge, based on the results, whether a
different setting should be made. If a setting cannot be reached within
tolerance you are offered the choice to accept it nevertheless. You may
also increase the tolerance somewhat before you start the scan. Typical
tolerances for magnet currents are 0.3 A. The tolerance is set back to
the standard value at the end of the scan.
9 - Access to the beam area
From time to time you may need to take an access to the experimental
area. To allow access in safe conditions, the access system guarantees
that there cannot be beam in your area during the access. The exact
requirements depend on the beam line and beam zone in question and may
be changed (under strict control of the AB-ATB-SBA beam line experts)
depending on beam conditions during your particular run period.
Important:the access procedures are described
Each beam zone is connected to one or more access chains,
containing a number of safety elements, which may be magnets, beam dumps
(XTAX, XTDX, XTDV), targets or TAX ranges. These access chains are
defined in hardware and connected to the access doors in hardware. To
prepare the conditions for access or, at the end of an access, for beam
all safety elements must be put in the required condition. The access
program knows the access chain configuration from a database.
The access program is invoked via the
Command task button or via
→ Access Command
menu. The access panel is shown in Figure 6.
In this panel you must select the door corresponding to your area. You
may then select the required state:
Figure 6: The Access Command panel
To have access with key.
Each person entering the zone MUST take a key, which is put back
whenever the person leaves the area the area is
required at the end of the access.
To have free access, i.e. without key. A search is
required at the end of the access. This search must be done by
authorized persons that have been adequately trained for this very
important and responsible task. This mode is recommended for long
accesses or when many people have to enter (to reduce the risk of loosing
||To switch on the beam once the access is finished and the zone
has been secured. This requires a search in case of leaving free access
and in any case the release of the veto on the beam line by pushing the
“End of Access” button on the access door. After some time (“Sirene”)
the veto is released and you will be allowed to switch on the beam line
elements, after the program has asked you to enter your name.
The progress window will show you the state of progress with messages.
On the right hand side you will be shown the status of the door, the
safety chain and of the various safety elements. The program will inform
you when it has successfully finished.
9.1 What to do if the access command fails?
Sometimes things do not work as expected. There may be many reasons for
- The door is not properly closed, an emergency
passage has been pushed or the End of Access button has not been pushed.
In these cases there is still a veto on the door. Please correct the
state of the door.
- Another door connected to the same safety chain
is not closed. Please check with the
Status task button or with
→ Access Status
menu. Close that door and try again
- There may be a veto from other systems such as
TAX cooling or P0-survey. Please check with the SPS operators.
- A safety element fails. See section 5 for magnet
problems. Problems with TAX, XTDX and XTDV dumps can usually be solved
via the Status
Tdx and Tdv menu or
directly from the Physicist tree.
If all fails, do not hesitate to call the SPS operator who can normally
solve the problem or, if necessary, call the relevant expert.
10 - Tools for quick checks of the beam performance
If you want to find quickly why your beam does not work or just want to
have a quick check of the beam line, the following programs can be of
- Magnet Status allows to check the proper functioning and setting of
all magnets in your beam line and to take corrective action
General Status(activated from the
Status → General Status menu
of via the
task button) shows a beam-specific panel
with all static settings, such as collimators, targets, absorbers and
dumps. It also shows currents in some key magnets (main spectrometers)
and relevant counting rates.
Quick can be launched by
right-clicking on selected scalers in the Scaler, Fisc and Scintillator
status panels. It displays for every burst the counts in the selected
scalers, normalized to the incident proton beam intensity.
Analog Chambers and Fast
Fisc profiles will give you very quickly a good feeling for the beam
If you cannot find the problem with these tools, you may call the SPS
control room for help.
The previous chapters have described in some detail the general aspects
of beam line control and the tools that Cesar provides for this.
However, as pointed out already in the introduction, all beam lines are
different and sometimes they even have their specific equipment and/or
software tools. Some examples are listed below:
Those items are beyond the scope of this document and will be described
in beam specific User Guides and other documentation, but some of them
are self evident to use (they are in many cases accessed from the
||XCLD collimators, Big Fiscs, P0-Survey, Dump Control, special control for Bends 6&7
||Hadron absorbers, SM1&2 interlock, primary target control by user
There are also general programs that may be useful but have not been
mentioned before. Please feel free to have a look at them. Some
particularly useful ones are:
|Status → Vacuum
||to check the vacuum pressure and the status of vacuum pumps and
valves. There is no way to act on the pumps from Cesar
|EA → Check timing
||to check whether the machine timing events arrive correctly.
Normally there are 3 events: WWE, WE arrive 1000, resp 1 msec before the
start of the flat top and EE arrives at the end of the flat top
Sometimes a beam problem is the result of a wrong operation or e.g. a
mis-type. In that case it may be helpful to look at the settings
logfiles. This information can be obtained from most status panels by a
right-hand button mouse click and selecting ‘Get
Logbook Info’ from the pop-up menu. There is also the
TIMBER interface that allows to
interrogate the measurement database. TIMBER is part of (and documented
by) the LHC
If there are still questions or problems that you cannot solve, please
call the operators (77500) or your beam line expert (during working
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