X5 User Guide

Introduction to the X5 beam

L. Gatignon, SL/EA

The X5 beam is a secondary or tertiary particle beam that provides hadrons, electrons or muons of energies between 5 and 250 GeV. This beam is a part of the West Area test beam complex, that is derived from the H3 secondary beam. This note gives a short introduction on the basic elements of the X5 beam. For more detailed information the users are referred to the X5 handbook or to one of the West Area liaison physicists.

1. The Layout of the West Area test beams

A 400 GeV/c primary proton beam is extracted from the SPS and directed on the T1 primary target. Typical intensities of this primary beam are 2^.10¹² protons per burst. These numbers can be read from the so-called PAGE-1 TV screens in the control rooms. For proper operation of the test beams the symmetry on the T1 target should be at least 80%.
From the T1 target a secondary beam is derived, called the H3 beam. Normally the H3 beam transports 120 GeV/c negative particles, mainly pions and electrons, but both the energy and polarity may change depending on the requirements of the West Area users. The H3 top momentum is 250 GeV/c.
The H3 beam is split into two branches, which each transport up to about 1.5 10⁷ particles to the X5 and X7 secondary targets. The spot sizes are of the order of a few millimetres RMS in each projection.

The steering onto the X5 and X7 targets is the responsibility of the EA liaison physicists. The control over the test beams themselves, i.e. the parts downstream of the secondary targets, is normally left to the test beam users.

Please find below a drawing of the optics of the X5 beam, indicating the positions of all magnets, collimators and detectors. Note that a detailed Postscript file (to scale) is available as well.

Click in the picture for more information

Note : The red curve indicates the beam envelope along the beam. The dotted blue curve indicates the dispersion (horizontal plane only).

2. The Control Tree

The user wants to select the energy and polarity of the particles in his beam, to steer the particles into a selected part of the detector and to adjust the spot size (focussing). He needs to choose the type of beam particles, control the beam intensity and eventually to stop the beam and get access to his experimental zone. He will use the beam instrumentation to check certain properties of the beam. Finally he needs to monitor that all the equipment in the beam is functioning correctly.

All these tasks can be performed from the beam X-terminal, connected to a cluster of HP/UX computers running the NODAL system. From this X-terminal the user controls the beam and related equipment through the so-called TREE program, invoked by the command 'RUN TREE', or if necessary 'RUN<index> TREE' , where the 'index' is 202 for the X5A test facility (normally in the PPE105 zone) and 204 for X5B (normally PPE115). From then onward the user just follows the menus offered by the control tree. A detailed description of the control tree can be obtained from your liaison physicist. Note that for (parasitic) users of the Gamma Irradiation Facility (X5C) the beam status can be observed, but not changed, from index 206.

Note that the NODAL system only accepts upper case! In case you get lost or stuck, you can leave the control tree by typing CTRL-C. You can then enter the tree again by 'RUN TREE'. In case your X-terminal hangs, try to switch it off and on again. Normally it should re-boot correctly and present you with the appropriate tree program

A new control system for the beam lines to the SPS experimental areas is in preparation. The hope is that this system can be made available to the X5 beam users in the course of 2002 or at the latest at the 2003 start-up.

3. Beam Files

Normally each user wants to run his tests or experiment at a number of different beam energies and corresponding beam intensities. The conditions are provided by adequeate settings of magnets and collimators, as well as some auxiliary equipments (targets, absorbers, etcetera). For the magnets and collimators these sets of conditions are described by Beam Files. In each user index (202, 204 or 206 for the X5 beam) there are 10 read-only beam files, reserved for the EA physicists and typically 20 or 30 beam files for use by the experiments. The EA-only files are called X5.A to X5.J and the user files are numbered X5.1 to X5.30. These files contain all magnet and collimator settings. A list of available files is obtained by

          FILES / LIST

The actual conditions in use at any given time are described by a special beam file, called BIM.0.

The user can select new conditions by loading a file from the tree:

          FILES / LOAD / X5.nn

and then answering the questions. In particular one has the option to change only magnets, only collimators or both. Magnet current changes are fast, collimator changes may be much slower. The LOAD command copies the values of file X5.nn into BIM.0 and sets the proper magnet currents and/or collimator positions.
Usually beam files are prepared by or after discussion with the responsible EA liaison physicist, although experienced users will sometimes change files themselves.

It is wise to check that the equipment has responded correctly to the requested changes by typing

          STATUS / MAGNETS   
or   
          STATUS / COLL

and verifying that the currents (positions) read correspond within tolerances to the currents (positions) in BIM.0. Tolerable deviations are 0.2-0.3 Amps for BENDS and QUADS, 0.5 Amps for TRIMS, 0.2 mm for collimators. In case of problems, try once more to load the file. If the problem still persists, call the CRN operator by phone 75566, over the intercom - CRN - or by Natel 16-0137. The liaison physicist can do nothing for you in this case!

4. Fine Steering and Focussing of the Beam

BENDs Steering of a beam is done by BENDing magnets (dipoles) Bend-1 to Bend-4. Normally the currents in the dipole magnets are defined correctly in the beam files and the user should not modify them without discussing with the EA physicists. In particular the Bend-3 and -4 currents should not be changed, as they define the beam momentum. Note that all Bends are horizontal bends.
Note that at very low momentum the beam can be operated with Bends 2 and 4 off and Bends 1 and 3 at correspondingly higher current. This so-called low-energy mode allows to run at 30% lower beam momenta.
BEND-5 is located in the X5B area (PPE115). Its only purpose is to deflect the electron beam tiwards Gex, sideways of the source container, in case the electron beam is sent into the GIF. Otherwise, in normal operation it should always be OFF
QUADs Quadrupoles are like lenses in conventional optics, they are used to (de-)focus the beam and thus change the spot size of the beam. The spot size of the beam at the test zone is controlled by QUADs 6 and 7. Which quad controls what projection depends on the beam file used. In the beam files these quads are defined to minimise the spot size at the main experiment locations, unless mentioned otherwise in the title of thge beam file.
TRIMs Trim magnets are correction dipoles, used for fine steering of the beam. Normally TRIM-1 and TRIM-2 should be set (close) to zero. TRIM-3 and TRIM-4 allow fine steering of the beam onto particular parts of the detector. TRIM 3 moves the beam vertically, TRIM-4 horizontally. Typically one needs 18 Amps/cm for the X5A zone and 12 Amps/cm for the X5B zone at 50 GeV/c beam momentum (proportional to p).

The currents in these magnets can be set using e.g.

          TUNE / SET/ TRIM / 3 / current

These changes are not automatically saved in the beam files!

5. Beam Intensity and Momentum Spread

The beam intensity is normally measured by the count rate in TRIG-7, located at the entrance to the X5A area:

         TUNE / MEAS/ TRIG / 7 / 3 / 3

The intensity is normally controlled by two collimators, namely:

C1 Momentum defining collimator (horizontal),
C2 Acceptance collimator.
C3 Horizontal collimator upstream of X5 target

C4 Vertical collimator upstream of X5 target

The momentum defining collimator C1 defines the momentum byte of the particles transported to your detector. The momentum byte Dp/p is proportional to the opening of the collimator. A gap of ±5 mm gives a Dp/p of approximately 1%. The acceptance collimator C2 should normally not be opened further than ±12 mm. Decreasing its slit results in a (non-linear) reduction of rate. It is not related to the momentum band of the beam. The collimators are controlled by

          TUNE / SET / COLL / 1 / JAWS/ -5 / 5
or
          TUNE / SET / COLL / 2 / SLIT / 24

Note that depending on momentum byte requirements it may be more advantageous to close C1 than C2, or conversely, to open C2 rather than C1.

Sometimes it turns out not to be possible to reduce the rate sufficiently by closing C1 and C2. In those cases the H35 rate can be decreased by collimators in the H3 beam. Normally the X5 beam is operated as a tertiary beam. In that case the above procedures apply. It is also possible to run it as a secondary beam, i.e. the same momentum in the X5 beam as in the H3 parent beam. In that case the intensities have to be reduced first by collimators in the H3 beam. Before loading the beam file for secondary beam operation, you must make sure that the intensity on TRIG7 (similar to the one on TRIG-1) will be below 10⁶ particles per burst (if not, radiation alarms will cut your beam!). The rate is best monitored by running (in a separate window) the programme

          TUNE / SPECIAL / MAXFLUX

which compares the actual rate with the maximum authorised one.
The rate reduction should be done with collimators in the H3 beam (COLLS 1 and 5 in the common part for X5 and X7 and COLLs 6 and 7 in the X5 branch). For this purpose a special programme is provided:

          TUNE / SPECIAL / INTENSITY

which allows to obtain this intensity reduction in successive stages. First the bulk of the reduction is done with the lower jaw of COLL-1 of H3 and the final iterations are done with COLLs 6 and 7 and, if necessary, the lower jaw of COLL-5, all in H3.
The same programme will also be of help when you go back to tertiary beam operation (to increase the intensity again), after having loaded the lower energy beam file. 6. The type of particles in your beam

Normally 5 options exist for the type of particles in your beam:

Secondaries The type of particles is the same as for the H3 secondary beam, which has been defined after consultation with all users at the SPS schedule meetings. The energy is strictly the same as for the H3 beam. You may calculate the hadron composition using the partprod formula. The intensities should be limited by collimation to < 10⁶ particles per SPS cycle (run TUNE / SPECIAL / MAXFLUX in a separate window to monitor this flux). The flux reduction can easily be obtained with a dedicated programme, activated by TUNE / SPECIAL / INTENSITY. See chapter 5 for more details about this procedure.
The target position should normally be OUT (i.e. no target material in the beam) or LEAD (to eliminate any electrons from the beam).
Pure electrons These are produced by using the Pb (lead) target. No absorber or converter should be present in the beam.
In principle electrons can also be produced from photon conversion. This is obtained by putting Trim-1 to 250 Amps and putting the converter in the beam. This electron beam is in many cases somewhat dirtier than the beam mention above. Therefore most experiments prefer the former solution. However, in case of very high (or positive) H3 momenta, the latter option may be of interest.
'Pure' hadrons The most pure tertiary hadron beam available in the X5 beam is obtained from the (40 cm long) Cu target. At average energies very pure hadron beams are obtained even without an absorber, but in particular at the lowest and highest parts of the X5 energy range it may be necessary to improve the purity by using the absorber (5 mm should do). This has however the disadvantage that it reduces the hadron rate and blows up the beam.
Mixed beam A reasonable mixture of electrons and hadrons is obtained by using the (40 cm long) Beryllium target in combination with a 5 mm absorber in the beam.
Muons A useful muon flux is obtained by choosing a beam energy which is slightly above 57% of the H3 (H37) energy. The target is then irrelevant. The hadrons and electrons are stopped by closing the collimators C1 and C2 asymmetrically, e.g. setting the jaws to +45 and +46 mm, respectively. The muon flux is a strongly varying function of the beam energy selected. In X5B it is also possible to take access to stop the electrons and pions in the mobile dump (XTDX) instead of by closing the collimator.

The present situation of your beam can be seen quickly by typing

          TUNE / SPECIAL / MODE / STATUS

which also offers a help facility summarising the above explanations. The target, converter and absorber can be changed by

          TUNE / SPECIAL / MODE / TARGET /Pb

          TUNE / SPECIAL / MODE / ABS / 5
 
          TUNE / SPECIAL / MODE / CONV / 4

The rate should then be re-adjusted with collimators 1 and 2.

7. Access to your zone

Frequently you will need access to your zone in order to modify, adjust, move or repair your apparatus. This is done through the command

          ACCESS / DOOR / 105 / OPEN          (for the X5A zone)
or
          ACCESS / DOOR / 115 / OPEN          (for the X5B zone).

Type in your name when the program asks for it. Then go to the door marked PPE105 or PPE115, wait till the lights 'ACCESS WITH KEY' start flashing, push the button with a key on it, take the key for which the red diode lights up and use it to open the door and enter the zone.

Every person entering the zone should take a key and keep it with him.

When you come out of the zone you should put back the key and turn it into its normal position. When the last person has finished, check that nobody is left in the zone, put back the last key, push the red button marked 'END OF ACCESS' (do not forget - otherwise you will not get beam!) and go back to your barrack. Then go to your beam terminal and type

          ACCESS / DOOR / 105 (or 115) / BEAM ON

type in your name (you are responsible for persons left in the zone!) and wait till beam comes back. It is wise to check that all magnet currents are OK by typing

          STATUS / MAGNETS

If the magnets do not switch on properly, then try "ACCESS/BEAM ON" again or try to set them to their BIM.0 value by TUNE / SET. If the problem persists, call the CRW operators.

Important : In the door itself, next to the door knob, there is a round 'pastille' with a dim red light in it, which should be pushed in emergency cases only! Whenever this button is pushed, it requires an operator to come over and reset the emergency stop manually. This may cause significant loss of beam time, in particular when the operators are working on another problem elsewhere!

The access to the Gamma Irradiation Facility (door PPE125) is a more complex procedure which is described in a separate document.

8. Using the detectors in your beam

The X5 beam is equipped with various detectors:

XWCA (MWPC) Wire chambers that allow to make beam profiles. They only perform reasonably for beam rates well above 1000 particles per burst. These profiles are made by typing
TUNE/ MEAS / MWPC / PROFILE
In the case of the X5 beam, MWPC 1 and 2 show the horizontal and vertical profile of the beam incident on the secondary target. The steering on these chambers is controlled by the EA physicist.
Chambers 3 and 4 show the profiles in the X5A area, chambers 5 and 6 in X5B. These are only useful at intensities exceeding the rate capabilities of the XDWC profile chambers:

XDWC The profile of the beam in your experimental area is obtained by typing
DETECTORS / DELAY / PROFILE / nr bursts

WC_SPECTRO Four spectrometer wire chambers, that give X-position readout for every event. Profiles can be made with the beam computer:
DETECTOR/ WC_SPECTR / PROFILE
You may also read the output of these chambers with your own TDC's and use them for momentum measurement of individual particles in the beam.
The beam can be centred in these chambers with BEND-1 and BEND-2.
Note that in 1997 these chambers will not be available before the middle of May.

TRIG Scintillation counters. Trig-1 counts the rate incident on the X5 target. Trig-7 measures the rate at the end of the beam line. Trig-8 counts the muon flux in the GIF zone within a diameter of 10 cm. Trig-4 and Trig-5 count the rate just upstream of your detectors. Trig-2 and Trig-3 are needed for the spectrometer. Trig-6 is a small counter (2 cm diameter) that selects the particles read out by the new (2002) Leadglass calorimeter (XEMC). Their rates can be measured by e.g.
TUNE/ MEAS/ TRIG/ 5/ USE/ 1/nr bursts
and coincidences between TR2-TR3 and TR4-TR5 are obtained by
TUNE/ SPECIAL/ COINCIDENCES/ nr bursts
Note that the scintillator can be moved in and out of the beam under TUNE/ MEAS.

EXPT Experimental scalers do not count any of our detectors but rather yours. In your barrack there is a panel with four plugs marked 'Multi-User'. In each of the four you can provide a standard NIM-signal that is counted over each burst and read into the SPS computer system. These counts are displayed by
TUNE/ MEAS/ EXPT/ scaler / 1 / nr bursts
where the scaler number depends on the barrack (ask your EA physicist).

The TRIG and EXPT counters are very useful in beam tuning. Steering can be somewhat automatised by the SCAN procedure:

          TUNE / SCAN / EXPT / 1 / 3 / TRIM / 4 / -50 / 50/ 20

will vary the Trim-4 current from -50 to +50 Amps in steps of 20 Amps (one step per burst), measure the count in EXPT-1 (You know what that count means!), normalise it to NORM-3 (see below) and display the ratio of EXPT-1 and NORM-3 versus the TRIM-4 current at the end. This technique of using a normalisation counter helps to be less sensitive to fluctuations in the SPS itself. The SCAN procedure allows to choose the optimum current for e.g. TRIM-4. You can set this current by

          TUNE / SET / TRIM / 4 / current

and eventually save it into the beam file by

          FILES / WRITE / X5.n / comment / TRIM / 4 / current

where the comment is a text of up to 30 characters ('*' leaves the old comment). Alternatively the whole BIM.0 (present setting) can be saved with the command

          FILES / SAVE / X5.n

Three normalisation counters are available:

	Norm-1 :	Interaction rate in primary target T1,
	Norm-2 :	Incident rate on primary target T1,
	Norm-3 :	Number of particles incident on X5 target.

Normally it is recommended to use Norm-3, but it is not possible for scans with TRIG counters.

9. Further problems

1. The HELP command gives you a list of questions and suggestions on what to do.

2. Under INFO / LOGBOOK you find the last changes to magnet currents and collimator settings. You may find hints to what went wrong.

3. Run the STATUS /CHECK and TUNE /SPECIAL /MODE programs to verify the precise status of the beam.

4. Call the CRN operators if all this fails.

Last updated : 29 January 2002 by Lau Gatignon

BENDs	Steering of a beam is done by BENDing magnets (dipoles) Bend-1 to Bend-4. Normally the currents in the dipole magnets are defined correctly in the beam files and the user should not modify them without discussing with the EA physicists. In particular the Bend-3 and -4 currents should not be changed, as they define the beam momentum. Note that all Bends are horizontal bends. Note that at very low momentum the beam can be operated with Bends 2 and 4 off and Bends 1 and 3 at correspondingly higher current. This so-called low-energy mode allows to run at 30% lower beam momenta. BEND-5 is located in the X5B area (PPE115). Its only purpose is to deflect the electron beam tiwards Gex, sideways of the source container, in case the electron beam is sent into the GIF. Otherwise, in normal operation it should always be OFF
QUADs	Quadrupoles are like lenses in conventional optics, they are used to (de-)focus the beam and thus change the spot size of the beam. The spot size of the beam at the test zone is controlled by QUADs 6 and 7. Which quad controls what projection depends on the beam file used. In the beam files these quads are defined to minimise the spot size at the main experiment locations, unless mentioned otherwise in the title of thge beam file.
TRIMs	Trim magnets are correction dipoles, used for fine steering of the beam. Normally TRIM-1 and TRIM-2 should be set (close) to zero. TRIM-3 and TRIM-4 allow fine steering of the beam onto particular parts of the detector. TRIM 3 moves the beam vertically, TRIM-4 horizontally. Typically one needs 18 Amps/cm for the X5A zone and 12 Amps/cm for the X5B zone at 50 GeV/c beam momentum (proportional to p).

C1		Momentum defining collimator (horizontal),
C2		Acceptance collimator.
C3		Horizontal collimator upstream of X5 target
C4		Vertical collimator upstream of X5 target

Secondaries	The type of particles is the same as for the H3 secondary beam, which has been defined after consultation with all users at the SPS schedule meetings. The energy is strictly the same as for the H3 beam. You may calculate the hadron composition using the partprod formula. The intensities should be limited by collimation to < 10⁶ particles per SPS cycle (run `TUNE / SPECIAL / MAXFLUX` in a separate window to monitor this flux). The flux reduction can easily be obtained with a dedicated programme, activated by `TUNE / SPECIAL / INTENSITY`. See chapter 5 for more details about this procedure. The target position should normally be OUT (i.e. no target material in the beam) or LEAD (to eliminate any electrons from the beam).
Pure electrons	These are produced by using the Pb (lead) target. No absorber or converter should be present in the beam. In principle electrons can also be produced from photon conversion. This is obtained by putting Trim-1 to 250 Amps and putting the converter in the beam. This electron beam is in many cases somewhat dirtier than the beam mention above. Therefore most experiments prefer the former solution. However, in case of very high (or positive) H3 momenta, the latter option may be of interest.
'Pure' hadrons	The most pure tertiary hadron beam available in the X5 beam is obtained from the (40 cm long) Cu target. At average energies very pure hadron beams are obtained even without an absorber, but in particular at the lowest and highest parts of the X5 energy range it may be necessary to improve the purity by using the absorber (5 mm should do). This has however the disadvantage that it reduces the hadron rate and blows up the beam.
Mixed beam	A reasonable mixture of electrons and hadrons is obtained by using the (40 cm long) Beryllium target in combination with a 5 mm absorber in the beam.
Muons	A useful muon flux is obtained by choosing a beam energy which is slightly above 57% of the H3 (H37) energy. The target is then irrelevant. The hadrons and electrons are stopped by closing the collimators C1 and C2 asymmetrically, e.g. setting the jaws to +45 and +46 mm, respectively. The muon flux is a strongly varying function of the beam energy selected. In X5B it is also possible to take access to stop the electrons and pions in the mobile dump (XTDX) instead of by closing the collimator.

XWCA (MWPC)	Wire chambers that allow to make beam profiles. They only perform reasonably for beam rates well above 1000 particles per burst. These profiles are made by typing TUNE/ MEAS / MWPC / PROFILE In the case of the X5 beam, MWPC 1 and 2 show the horizontal and vertical profile of the beam incident on the secondary target. The steering on these chambers is controlled by the EA physicist. Chambers 3 and 4 show the profiles in the X5A area, chambers 5 and 6 in X5B. These are only useful at intensities exceeding the rate capabilities of the XDWC profile chambers:
XDWC	The profile of the beam in your experimental area is obtained by typing DETECTORS / DELAY / PROFILE / nr bursts
WC_SPECTRO	Four spectrometer wire chambers, that give X-position readout for every event. Profiles can be made with the beam computer: DETECTOR/ WC_SPECTR / PROFILE You may also read the output of these chambers with your own TDC's and use them for momentum measurement of individual particles in the beam. The beam can be centred in these chambers with BEND-1 and BEND-2. Note that in 1997 these chambers will not be available before the middle of May.
TRIG	Scintillation counters. Trig-1 counts the rate incident on the X5 target. Trig-7 measures the rate at the end of the beam line. Trig-8 counts the muon flux in the GIF zone within a diameter of 10 cm. Trig-4 and Trig-5 count the rate just upstream of your detectors. Trig-2 and Trig-3 are needed for the spectrometer. Trig-6 is a small counter (2 cm diameter) that selects the particles read out by the new (2002) Leadglass calorimeter (XEMC). Their rates can be measured by e.g. TUNE/ MEAS/ TRIG/ 5/ USE/ 1/nr bursts and coincidences between TR2-TR3 and TR4-TR5 are obtained by TUNE/ SPECIAL/ COINCIDENCES/ nr bursts Note that the scintillator can be moved in and out of the beam under TUNE/ MEAS.
EXPT	Experimental scalers do not count any of our detectors but rather yours. In your barrack there is a panel with four plugs marked 'Multi-User'. In each of the four you can provide a standard NIM-signal that is counted over each burst and read into the SPS computer system. These counts are displayed by TUNE/ MEAS/ EXPT/ scaler / 1 / nr bursts where the scaler number depends on the barrack (ask your EA physicist).