Introduction to the H2 beam

(A.Bonifas and N.Doble)


This page is under development


The H2 beam is a secondary particle beam that provides hadrons, electrons or muons of energies between 10 and 400 GeV/c. This beam is installed in the SPS North Area (EHN1). This note gives a short introduction to the basic elements of the H2 beam. For more detailed information the users are referred to the H2 handbook or to one of the SL-EA liaison physicists.

1. The Layout of the H2 beam

(see also the drawings in your H2 handbook)

A 450 GeV/c primary proton beam is extracted from the SPS and directed on the T2 primary target. Typical intensities of this primary beam are several 1012 protons per burst. For proper operation of the beams the symmetry on the T2 target should be at least 80%. These numbers can be read from the so-called PAGE-1 TV screens in the electronics huts and control rooms. From the T2 target, two secondary beams are derived: the H2 and the H4 beams. The momenta, production angles and polarities of the two beams are somewhat correlated. For example, if the H4 beam runs at 450 GeV/c, the H2 beam can run at any negative polarity (-10 to -400 GeV/c) or at positive polarities up to +180 GeV/c at 0 or at +270 GeV/c at +5.0 mrad production angle. Alternatively, both beams can run with tertiary particles derived from neutral secondaries of any low to medium momentum (eg. e+ or e- from photon conversion or pi- from Lambda and Kso decay at a difference of production angles = 9.04 mrad eg. H2 at +4.52 mrad, H4 at -4.52  mrad. Any of these "front-end" changes of the beams should be done by an SL-EA liaison physicist.

a) Different modes of H2 operation

There are four distinct modes of operating the H2 beam:

	1) the high resolution mode
	2) the high transmission mode
	3) the "filter" mode (used when a radiator is introduced into the
beam)
	4) as a heavy ion beam (not discussed in this note)

b) Experimental areas in H2

Like in all other beams in EHN1, we distinguish experimental areas (at present NA43 and NA49 in H2) and test zones (H2A).

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 terminal, connected to a cluster of NORD (soon HP UNIX) computers running the NODAL system. From this 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 233 FOR NA43, 215 for NA49 and 221 for the test zone H2A. 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 the NODAL system only accepts upper case! In case you get lost or stuck, you can leave the control tree by typing an ESC character (also press several times, if necessary...). You can then enter the tree again by 'RUN TREE'. In case of mis-typing, you can backspace by 'Control-A'.

3. Beam Files

Normally each user has a number of different beam energies and corresponding beam intensities at which he wants to run his experiment. These sets of conditions are described by Beam Files. In each index (233,215 or 221) there are 10 read-only beam files, and up to 40 beam files, for use by the experiments. The EA files are called H2.A to H2.J and the user files are numbered H2.1 to H2.40. 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 another beam file, called BIM.0.

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


	FILES / LOAD / H2.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. This command copies the values of file H2.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 / CHECK
or 
	STATUS / MAGNETS  and  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 (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). 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.

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 is controlled by the last QUADs in front of each experiment. Which quad controls what projection depends on the beam file used. In the beam files these quads are usually defined to minimise the spot size at the main experiment locations.

TRIMs Trim magnet are correction dipoles, used for fine steering of the beam. Normally the last TRIMs upstream of each experiment should only be used for steering. Typical values for TRIM7 (V) and TRIM8 (H) steering to the EHS superconducting magnet are 22 mm for a 100 A change at 100 GeV/c beam momentum (proportional to p and inversely proportional to the distance from the TRIM).

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


	TUNE / SET/ TRIM / 8 / current
These changes are not saved in the files (except the BIM.0 file for the present status)!

5. Beam Intensity and Momentum Spread

The beam intensity is normally controlled by three collimators, namely:

	C1  (filter mode)       Horizantal Acceptance collimator,
or	C2  (HR and HT mode)               - " -	
	C3  Momentum defining collimator (vertical),
	C6  Vertical Acceptance collimator.
The momentum defining collimator C3 defines the momentum bite of the particles transported to your detector. The momentum bite p/p is proportional to the opening of the collimator. A gap of 3 mm gives a p/p of approximately 0.1%. Decreasing the opening of C1 or C2 and C6 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 / 3 / JAWS/ -5 / 5
Note that depending on momentum bite requirements it may be more advantageous to close C3 than C1/2, 6, or conversely, to open C1/2, 6 rather than C3.

Sometimes it turns out not to be possible to reduce the rate sufficiently by closing C1/2 and C6. Other collimators can be closed in those cases, e.g. C8 and C10. C9 is used to redefine the vertical image following momentum dispersion by bends 2 and 3 and recombination by bends 4 and 5.

6. The type of particles in your beam

Several options exist for the type of particles in your beam (ions neglected):

'Pure' electrons These are obtained by photon conversion in a lead CONVERTOR and loading the appropriate file and by assuring that subsequent material (lead sheets and TRIG counters etc.) are removed from the beam after the convertor. To this end, use


	TUNE / SPECIAL / CONVERTER / LEAD
and	
	TUNE/MEAS/TRIG/

'Pure' hadrons
  -> (1) secondary beams
direct from the target T2 depend on the setting of the target station 'wobbling' to give the required momentum. Choose the appropriate beam file and put in TRIG1 or TRIG2 to remove electrons.
  ->(2) tertiary beams: These can be obtained with the same target station 'wobbling' as for electrons from photon conversion and are mainly pi- from Lambda and Kso decay. Choose the appropriate file with the CONVERTER=AIR. Momentum selection as above, use TRIG2 (with 6mm Pb) to "kill" the electrons in the tertiary beam.

Mixed beam Do you really want it? You get it by not applying the prescriptions above, e.g. choose a tertiary hadron beam and do not remove electrons by lead sheets.

Muons Choose a secondary hadron beam file and close collimator C9 asymmetrically (i.e. setting the jaws to -45 and -40 mm respectively). The muon momentum is selected by BENDs 4 and 5. For higher muon flux, open collimators upstream of C9.

The CONVERTER can be changed by


	TUNE / SPECIAL / CONVERTER 
and the situation of the TRIG counters can be observed or changed by

	TUNE / MEAS / TRIG /

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 / 142 / OPEN  (for the NA43 zone) 
	
	ACCESS / DOOR / 152 / OPEN  (for the NA49 zone)
	
	ACCESS / DOOR / 172 / OPEN  (for the H2A test zone)

Type in your name when the program asks for it. Then go to the door (e.g. marked PPE152), 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 (PRESS THE DOOR HANDLE SLOWLY!!!). Every person entering the zone has to take a key and keep it with him - your safety depends on this action!!! 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 have to walk back to the door later...!) and go back to your barrack. At your beam terminal, type e.g.


	ACCESS / DOOR / 152 / 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 CRN operators.

Important : In the door itself, next to the handle, 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 because the operators are often working on another problem elsewhere, e.g. at LEP!

8. Using the detectors in your beam

The H2 beam is equipped with various detectors:

XWCA (MWPC) Wire chambers that allow to make beam profiles. They only perform reasonably for beam rates above 1000 particles per burst. These profiles are made by typing


	TUNE / MEAS / MWPC / PROFILE / ...
Often, the first profiles look strange - repeat profiles, because HV adjustment in the chambers is done automatically with every profile.

FISC Scintillation counter filaments of e.g 0.2 mm widths, moving through the beam at 1 step per burst, which provide horizontal and vertical profiles. To use a FISC, type e.g.


	TUNE / MEAS / FISC / PROFILE / fisc # /
		 2 / start mm / end mm / stepsize mm/

TRIG Scintillation counters. Trig-1 counts the rate between the two main BENDs. Trig-1 and Trig-2 are also used to move in and out Pb foils, Trig-3 and Trig-4 are used as trigger counters for the CEDAR Cerenkov counter. If the CEDAR counter is removed fron the beam then Trig-4 is removed with it. Their rates can be measured by


	TUNE/ MEAS/ TRIG/ 3/ USE/ 2/nr bursts
Note that the scintillator can be moved into and out of the beam with 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 / 2 / 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 / 13 / 2 / TRIM / 8 / -50 / 50/ 20 
will vary the Trim-8 current from -50 to +50 Amps in steps of 20 Amps (one step per burst), measure the count in EXPT-13 (you know what that count means!), normalise it to NORM-2 and display the ratio of EXPT-13 and NORM-2 versus the TRIM-8 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-8. You can set this current by

	TUNE / SET / TRIM / 8 / current 
and eventually save it into the beam file by

	FILES / WRITE / H2.nn / comment  / TRIM / 8 / current
where the comment is a text of up to 30 characters ('return' leaves the old comment). Alternatively the whole BIM.0 (present setting) can be saved with the

	FILES / SAVE / H2.nn
command.

Two (or more) normalisation counters are available:

	Norm-1 :	Interaction rate in primary target T2,
	Norm-2 :	Incident rate on primary target T2.
Normally it is recommended to use Norm-2.

9. Further problems

1. The HELP command gives you a general list of questions and suggestions on what to do - even though it might not be specific to your beamline only, it could still be of use... .

A general check of magnets and collimators is obtained by STATUS / CHECK which indicates wrong values in these beamline elements.

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 to assure that no magnet nor collimator has failed, and TUNE / SPECIAL / CONVERTER as well as TUNE / MEAS /TRIG/ to verify the precise status of the beam. In particular, this could help to see why you don't get electrons, etc... .

4.Call the CRN operators (Tel. 75566, beep 13-4190) if all this fails.

Figure 1:

The test beam optics for the H2 beam. Also indicated are the magnets, collimators and detectors along the beamline. The drawing is to scale.


Last updated : 3 July 1996 by Niels Doble