SBA

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[1]. 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 separate 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 question.

This document is organized in 11 sections

  1. Introduction

  2. Short overview of the Cesar application software

  3. What is a wobbling station

  4. Reference settings and beam files

  5. Beam steering, focusing and control of magnets

  6. Beam intensity, momentum spread and control of collimators

  7. The type of particles in your beam line: targets, absorbers, converters, etcetera

  8. Using the detectors in the beam line to optimize your beam

  9. Access to the beam area

  10. Tools for quick checks of the beam performance

  11. Final remarks

Introduction

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 documents are
  • 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.
Useful information is also available on the White board pages for the North Area. A synoptic of the North Area beam lines is given in Figure 1.


    Figure 1: Synoptical diagram of the North Area beam lines

    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 6.

The amount of hadrons as well as the relative abundance of pions, kaons and protons can be calculated with the partprod program. 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 web 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 Cesar 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.

    Figure 2: The Cesar application window

    Figure 2:The Cesar application window
Settings can be verified or changed via several mechanisms:
  • 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 detailed knowledge.

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[2]. 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 situation!

 

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 the ‘BeamRefs 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 beam.

5.1 Status and setting of magnets

The status of the magnets is obtained via the Status Magnets menu or via the Magnet Status 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 Faults’ option.
You may change the current in a selected magnet via the ‘Set 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 Status Rectifier Status. Buttons allow to 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 OFF, 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 again.

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 PULSED to 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 doing).
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 associated rectifier.

  • 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 way.
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 openings.

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 Set 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 File 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 general features.
First one should distinguish between various basic operational modes:

Primary beam:

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).

Secondary beam: 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 partprod program. 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.
Tertiary beam: 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 Bend 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 specific.

7.4 How to control absorbers, converters, targets

Absorbers, converters and targets are controlled via the Status Obstacles menu or via the Obstacle Status 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 ‘Move (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.
The 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:

Scintillators 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 Detectors → 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 Restore or 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’ (Å) 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 menu.
   
Ionization Chambers These are only present in the highest intensity beams. Their readout is included in the Experimental Scaler Status panel (see above).
   
FISC counters 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 system.
The FISC Status task button and the Detectors → FISC Status menu allow to Set Position of the filament, Set 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…
The 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.
The Detectors → 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 Set 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 the panel.
   
CEDAR counters 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 Scan task button or the Tune Scan menu.

  • Figure 5: The Scan panel
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.
From the scan panel, shown in Figure 5, in the left hand top windows you may 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 here.
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 Access Command task button or via the Access Access Command menu. The access panel is shown in Figure 6.

  • Figure 6: The Access Command panel
In this panel you must select the door corresponding to your area. You may then select the required state:

Open: 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.
   
Free:   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 keys!).
   
Beam on: 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 this:
  • 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 Access Status task button or with the Access 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 Tax, 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 use:

  • Magnet Status allows to check the proper functioning and setting of all magnets in your beam line and to take corrective action

  • The General Status(activated from the Status → General Status menu of via the General Status 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.

  • The 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.

  • The Analog Chambers and Fast Fisc profiles will give you very quickly a good feeling for the beam steering.
    If you cannot find the problem with these tools, you may call the SPS control room for help.

Final remarks

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:

H2 Ion splitters
   
H8 Micro-collimators
   
K12 XCLD collimators, Big Fiscs, P0-Survey, Dump Control, special control for Bends 6&7
   
M2 Hadron absorbers, SM1&2 interlock, primary target control by user
   
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 EA menu).
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 Logging project.
If there are still questions or problems that you cannot solve, please call the operators (77500) or your beam line expert (during working hours).


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[1]  Some settings, in particularly those involving risks for personnel or equipment and those affecting several beam lines, are restricted to authorised personnel, such as operators and beam line experts.

[2]       Sometimes the primary beam is the wanted condition in either H2 or H4. In that case special measures must be taken to ensure sufficient attenuation of the beam before it reaches the experimental hall.

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