User Guide for the M2 beam
The M2 beam is a secondary or tertiary beam that can provide to the experiment
in experimental hall
EHN2
one of the following types of beam:
- A high-intensity muon beam in the momentum range ±60 to ±190 GeV/c
with fluxes up to 2 108 muons per SPS cycle,
depending on the momentum chosen and limited by radio-protection guidelines,
- A secondary hadron beam of momenta between ±40 and ±280 GeV/c at a
maximum allowed flux of 108 hadrons per SPS cycle,
- A low-intensity electron calibration beam (typically 103
to 104 electrons per SPS cycle), of momenta up to -60 GeV/c.
Note that the quality of the electron beams is limited by
the large amount of air and material on the beam line.
This User Guide gives an introduction to the use and operation
of the M2 beam. Part of the operational procedures are
restricted to the CRN operators or SL/EA liaison physicists.
These parts are indicated in
brown italic.
The M2 has recently been modified significantly to better match the needs of the
COMPASS experiment (NA58). A description
of these modifications is available
on the Web.
The M2 beam for the COMPASS experiment can be operated as a
muon, hadron or electron beam.
During 2008 the standard beam
operation will be in hadron mode with
beam momentum -190 GeV/c.
A
user guide is available, as well as
instructions for operators.
For +190 or -190 GeV/c
and in general up to up to 225 GeV/c,
we use initially the
225 GeV optics, for higher momentum the
280 GeV optics. For -190 GeV a
new optics is under development.
We are now in the
process of validating this new optics (file
M2A.006).
For the hadron mode of operation the 9 hadron
absorbers are moved OUT of the beam line. Two CEDAR counters
allow particle identification. The beam is made parallal in the
CEDAR section.
The beam parallellism at the CEDAR can
be tuned with Q32 and Q33. With D(Q32)/D(Q33)=6.9 one
affects only the horizontal parallellism (A 15 A
increase of Q32 will focus the beam 85 m downstream of
Fisc3+4). With D(Q32)/D(Q33)=0.28) one affects only the
vertical parallellism (DQ32=18
A will focus again at that same position).
The hadrons are produced in the primary target
T6. Their rate (and the muon rate proportionally) is defined by
collimation and by changing the length of the target (5 heads
available). They are momentum selected by the horizontal Bends 1
and 3. The momentum slits are COLL1=COLL3
and COLL5.
A preliminary
tuning procedure is available both for the
2007 and
2008 versions of the hadron beam optics. For the 2007 optics
the steering coefficients for the beam through COMPASS are given
here.
Regularly COMPASS will switch to
muon beam, typically -208/-190
GeV/c. From the T6 target
a high hadron flux is produced and after a wide momentum
range selected transported through a 600 metres long
FODO channel before the pions are dumped on nine 1.1 m
long Beryllium absorber modules. The optics of this
hadron section of the beam is available
here. The muons produced upstream of the absorber by
decay of pions and kaons are momentum selected and
transported to the experimental hall by the
muon section of the beam. At -190
GeV/c the flux is of the
order of 1.5e7 muons per spill for 3e12 protons on the
T6 target.
The changes of mode from hadrons to
muon beam and vice versa are described
here.
The -40 GeV/c
electron beam is a tertiary beam,
derived from a -100 GeV/c
secondary beam produced in the T6 target. Just upstream
of Bend 4 a 5 mm thick Lead target (essentially
transparent for the hadrons) allows to downgrade the
electron momentum by Bremsstrahlung. The part of the
beam downstream of this target is set for -40
GeV/c and picks up the
electrons of that momentum. Like for the muon beam the
optics comes in two parts:
secondary beam part and the
tertiary beam optics. Due to the large amount of
material along the beam (many metres of air,
scintillators, COMPASS detectors) the flux is modest (of
the order of 10k per spill) and the Bremsstrahlung tails
are large. During some periods in 2008 we will run this
electron beam and attempts will be made to operate this
beam at even lower electron momenta, possibly down to
-15 GeV/c (?).
1. The Layout of the M2 beam
A 400 GeV/c primary proton beam is extracted from the SPS towards the North
Experimental Area. A fraction of this beam, selected by two stages of septum
magnets, is directed towards the primary target T6. The proton intensity
incident on this target is decided by the SPS coordinator and may be in the
range between 2 1012 and 1.2 1013 protons per SPS cycle.
For a proper operation of the M2 beam the symmetry of the beam on target should
be about 80%, as can be checked from the so-called
Page1
screens in the control room (explanations are available
here). From the T6
target a secondary beam (positive or negative) at zero production angle is
derived. This beam is either transported directly to the experiment (in the case
of the hadron beam), or tertiary muons or electrons are selected.
A schematic layout of the M2 muon beam is shown below:
 |
<-- |
Please click in the picture
for a larger size image |
A more detailed description of the M2 beam layout is available in the form of a
so-called
Beatch listing,
indicating the exact positions of all magnets, collimators and detectors. A more
graphical view is provided by the optics drawings, which in addition to the
positions of the numerous beam elements, show the different optical terms for
the available beam modes:
Optics version | Postscript |
PDF |
Hadron section of muon beam
Muon section of muon beam |
X
X |
X
X |
| Hadron optics, standard version |
X |
X |
| Hadron optics, compatible with P61 operation |
X |
X |
Secondary beam section of electron beam
Tertiary beam section of electron beam |
X
X |
X
X |
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 239 in case of the M2 beam. From then onward the user just follows
the menus offered by the control tree. A detailed explanation 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 CTRL-C.
You can then enter the tree again by 'RUN TREE' (you may also try the commands
RUN and BACK). 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
3. Beam files
In the M2 beam, most of the beam characteristics are defined by
magnet and collimator settings. In the M2 beam there are 9
independent groups of main bends, 36 quads and 7 correction
dipoles (Trims), as well as 7 magnetic collimator currents. The
currents requested from the corresponding power supplies, as
well as the the collimator apertures are collected in lists,
called beam files. There are 10 read-only beam files, called
M2.A to M2.J, which are reserved for the SL-EA physicists and
contain mostly theoretical currents and collimator openings. In
addition there are up to 40 user files, which may be modified by
the user and which contain tuned settings for the different
types of beams and for different momenta. These are numbered
M2.1 through M2.40. A list of available files is obtained by
FILES / LIST
The actual conditions in use at any given time (i.e. the last
values asked for via the tree program) are available in a
special beam file, called
BIM.0 (
under beam index <123>).
The user can select new conditions by loading a file via the tree program:
FILES / LOAD / M2.nn
where
nn indicates the file number. The program
proposes the following options:
| MAGS: | Only magnet currents, |
| COLLS: | Only (non-magnetic)
collimator settings, |
| SCR: | Magnetic collimator
settings, |
| MIBS: | MIB currents (MIB =
Magnetised Iron
Blocks, |
| ALL: | Magnet currents and
(non-magnetic) collimator settings. |
Note that magnet current changes happen in general much faster than collimator
changes!
Two options refer to equipments not included in the beam files:
- The SCRapers (magnetic collimators) serve to define the momentum band
of the muon beam and to reduce the muon halo around the beam. They are essential
for proper operation of the muon mode of the beam. For the hadron beam they may
have some small, though positive, effect if properly used,
- The MIBs are static magnetic toroids. Their function is similar to
the one of the scrapers, but they cover a larger surface. Usually scrapers are
followed by MIBs.
The MIB currents are handled by a special subprogram (also
accessible via
TUNE / SPECIAL / MIBS
which proposes settings for Muons (Positive or Negative),
Hadrons or Electrons. The latter two correspond to switching the
MIBs off. In practice it may turn out to be better for the beam
quality if they are switched on with the appropriate polarity,
even for the hadron or electron beams.
A scraper has not only a current, but also four positions: two
upstream and two downstream. The motors can be controlled
individually or in pairs, namely via the so-called "
Displacement"
(average position) and "
Angle" of each jaw. The 28 motor
positions are in the normal beam files, but are not handled by
the standard programs in the
FILES menu. The scraper
control is in a special branch:
TUNE / SET / SCRAPER
The
SCR option in the
FILES / LOAD program
will indeed activate the required scraper positions, but this is
only meaningful if these settings have been defined explicitly
beforehand!
Therefore changes to scraper settings should be left to
experienced users!
Note that the FILES / LOAD command (as well as the scraper and MIb
programs) not only sends commands to the hardware, but also updates the BIM.0
file. It is thus possible and strongly recommended 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 Focusing of the Beam
| BENDs |
Steering of a beam is done by BENDing magnets (dipoles)
Bend-1 to Bend-9. 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-6 current should not be changed, as it
defines the beam momentum. |
| 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 35 and 36. 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 magnets are correction dipoles, used for fine steering of the beam.
Their nominal (theoretical) currents are zero, but as the M2 Trims are short,
large-aperture and therefore weak magnets, their tuned currents may be quite
substantial!
|
The currents in these magnets can be set using e.g.
TUNE / SET/ TRIM / 3 / current
These changes are updated in BIM.0, but
not saved in the
files M2.
nn! If required, they can be saved by the
command
FILES / SAVE / M2.nn
where the reply '*' as new title preserves the old title.
In all optical modes the relevant (almost)orthogonal steering elements are:
| Position |
Angle |
Horizontal: | Bend-7 or Bend-9 |
Trim-5 and/or Trim-6
or a linear combination,
depending on the optical mode |
Vertical: | Trim-7 or Bend-8 |
Bend-4 or Bend-5 |
The focusing can be adapted by small adjustments of Q35 and Q36. Q35 is (in most
of the modes ) a horizontally focusing quadrupole, Q36 a vertically focusing
one. Normally single scans of Q35 and Q36 are made to minimise the rate in some
small aperture halo counter (read via a EXPT scaler) close to the nominal target
position. Note that Bend-6 defines the reference momentum and it should always
be kept on nominal current.
The steering and focusing can be monitored by the analog wire chambers in the M2
beam via
TUNE / MEAS / MWPC / PROFILE / ALL (or chamber #) / DICO
The first four chamber frames (2 planes each) are motorised.
They can be moved IN/OUT via a special program
TUNE / SPECIAL / CHAMBERS / In (or Out)
5. Beam Intensity and Momentum Spread
The beam intensity is easily controlled via the reading of XION-2 or several of
the EXPT readings (experiment-dependent). For fluxes below 106
particles per SPS cycle, a more reliable measurement is provided by the
coincidence of TRIGGERs 1 and 2. Convenient programs are the following:
STATUS / GENERAL : Detailed overview of M2 status &
performance
TUNE / SPECIAL / QUICK : Some important counting rates
repeated every SPS cycle
The beam intensity is normally controlled by
- The primary target (T6) head: the longer the target head, the larger
the flux.
The target head can only be changed by the CRN operators
(via
EA / BEAM / TARGET / HEAD).
- Depending on the scheduled activities in the beam, it is possible that
restrictions have been imposed on the maximum target length (via the
EA / BEAM / M2-PROTECT branch). This protection should never be undone
without prior agreement or instruction from an SL-EA physicist!
- During certain periods the M2 beam shares the T6 target with the P61 beam to
the underground area ECN3. During those (usually short) periods, the T6 target
head should
not be changed.
- The collimators and scrapers in the beam line:
- COLL-1 and COLL-3 (horizontal) define the pion momentum band, in
particular for the muon mode of operation,
- COLL-2 and COLL-2 define the vertical acceptance,
- COLL-5 (vertical) defines the momentum band of the hadron and electron
beams,
- COLL-6 and COLL-8 (horizontal), as well as COLL-7 and COLL-9 (vertical) are
cleaning collimators that eliminate backgrounds created in the long air sections
upstream and in the Beam Momentum Station scintillators,
- SCRapers 4 and 5 (vertical) define the muon momentum slit.
The other scrapers, namely SCR 3 and 6 (horizontal) and SCR1, 2, 7 (vertical)
serve as cleaning collimators, mainly for the muon beam. Their tuning is quite
delicate. In general their positions should not be changed lightly!
COLL-1 and COLL3 are used as momentum slits for the hadron decay section
of the muon beam. The momentum band Dp/p is
proportional to the collimator gap with a dispersion of about 5 mm per percent,
with an intrinsic resolution of a few percent. The flux is strictly proportional
to the opening of this collimator. Its gap can be controlled by the command
(Example for COLL-1 to an opening of ±30 mm):
TUNE / SET / COLL / 1 / JAWS / -30 / 30
or
TUNE / SET / COLL / 1 / SLIT / 60
The (usually more relevant) momentum band of the muon beam itself is
controlled by SCRAPERS 4 and 5. These are controlled (for scraper 4) via
TUNE / SET / SCRAPER / MOVE / 4 / UP / UPSTReam / value
DOWN DWNSTReam
ANGLE
DISPLacement
Please note that a significant change to the SCR4 and SCR5 settings may imply a
need to change the settings of SCR7.
COLL-5 (vertical) is the momentum slit of the hadron and electron beams.
The dispersion at the collimator is about 6 mm per % for the hadron beam and 9
mm per % for the electron beam. In both cases the intrinsic resolution is from
optics alone about a percent RMS, but in the electron case, the scattering in
material degrades this resolution significantly.
COLLs 6-9 have as main function the cleaning of the beam. Their settings should
in principle be tuned to minimise the Halo/Beam ratio in the hadron, resp.
electron beams. Flux adjustments should rather be made with the upstream
collimators. In muon mode, COLLs 6-9 should be wide open.
In general adequate collimator and scraper settings are written in the beam
files.
6. The Type of Particles in the M2 Beam
Three basic modes of operation have been implemented for the M2 beam:
Muon beam |
The M2 has historically been designed and operated as a muon beam. A large
acceptance, relatively wide-band (±10% Dp/p) pion
beam, as well as the muons originating from pion decay, are transported through
a 600 metres long decay channel (FODO lattice). At the end of this channel the
muons are focused on a Beryllium absorber (up to 9 units of 1.1 metres of
Beryllium each), which stops all the hadrons in the beam. The muons are picked
up and transported through a second FODO channel (regular lattice of focusing
and defocusing quadrupoles). The muon momentum definition and cleaning is done
in this section, too. At the end of this FODO array the beam is shaped in terms
of spot size and divergence for the experiment. The maximum allowed flux is 2 108
muons per SPS cycle. Typical spot sizes at the target are 8 mm RMS in each
plane, with a divergence of 0.5 mrad RMS in the horizontal plane and less than 1
mrad RMS in the vertical plane. |
Hadron beam |
In this mode of operation the hadron absorbers are moved out of the beam and
the secondary hadrons are transported directly from the primary target to the
experiment. If necessary (in particular for negative beams at momenta below 150
GeV/c) any electron contamination in the beam can be reduced by introducing a 5
mm lead converter in the beam. This converter is placed some 20 metres upstream
of the hadron stopper. The maximum allowed fluxes are 108hadrons per
SPS cycle, limited by radio-protection guidelines. Typical spotsizes are of the
order of 3 to 5 mm RMS. The beam composition (ignoring lepton contaminations) as
a function of beam momentum can be found
here or calculated with the
Atherton formula. |
Electron beam |
Electron beams can only be provided as tertiary beams. A low momentum
negative secondary beam (typically -100 or -120 GeV/c) is derived from the T6
target, transported down the decay FODO channel and focussed on the 5 mm lead
converter. In this mode the converter serves as a secondary target, i.e.
secondary electrons are momentum degraded (by Bremsstahlung). The tertiary
electrons, in a momentum range between some -30 and -60 GeV/c, are momentum
selected and transported to the experiment. Typical fluxes, depending on beam
momentum and collimator settings, are in the range 103 to 104
electrons per SPS cycle. Spot sizes are of the order of 8 mm RMS at the
experiment. |
From the operations point of view the different modes are characterised by a
limited number of settings:
Beam
Mode | Typical
momentum |
Safety
guarantee | T6 target
head length |
Hadron
absorbers | Colls
1 to 5 |
Secondary
Target |
Muons | +177/160 | Absorbers | any OK |
- I I I I I I I - | Open | Out |
Hadrons | +200
-100 | T6 head
Colls 1-5 |
max. 100 up to 500 | - - - - - - - - - | Very
closed |
Out |
Electrons |
-100/40 | Colls 1-5
pEHN2/pT6 |
Must be 500 | - - - - - - - - - | Rather
closed |
In |
In general there is no need to change the scraper positions.
The scraper currents are taken care of by the beam files.
A simple way to get a quick overview of the settings and the performance of the
M2 beam are obtained via the general status program:
STATUS / GENERAL
This programs shows the rates in the available standard beam counters and a
number of EXPT scalers. Also a numbr of passive settings are presented.
At high positive momenta, fluxes well above the agreed maxima, are in principle
possible. Some of the equipments or software protections involved can only be
controlled by SL-EA experts.
Therefore the changes of mode have to be performed by the
CRN operators or EA physicists. Please note that the order of the various
operations is important. Therefore a special program has been written that
guides you through the various steps and checks, wherever it is important, that
the previous steps have been executed correctly before you are allowed to
continue. This program,
EA / BEAM / MODE
is run in a separate window. The various operations are executed via a second
window. The program proposes 6 options:
| 1 | Muons --> Hadrons |
|
2 | Hadrons --> Muons |
| 3 | Muons --> Electrons |
|
4 | Electrons --> Muons |
| 5 | Hadrons --> Electrons |
|
6 | Electrons --> Hadrons |
In general it is of particular importance that
- Never a muon file is loaded while the scrapers are OUT,
- Never the scrapers are moved out while a too long T6 target head is
selected and/or the Collimators 1-5 are too wide open.
In the (rare) periods that the T6 target is shared with the P61 beam, the M2
beam is forced to run at -1/2 times the P61 momentum in the front end (secondary
beam). Usually this corresponds to -200 or -225 GeV/c negative beams with lower
fluxes. During and only
during such periods a simplified procedure may be followed. This procedure will
on those occasions be communicated directly to the CRN operators.
Some of the manipulations involved in the execution of the EA / BEAM / MODE
program involve special instructions:
| EA / BEAM / TARGET / HEAD / nr |
| Change T6 target head |
| EA / BEAM / M2-PROTECT / |
| Protect absorbers 4,5,6
and T6 target head length |
| TUNE / SPECIAL / ABSORBERS / |
| Move absorbers 1-9 in our out |
| TUNE / SPECIAL / CONVERTER / I (O) |
| Move electron target In or OUT |
| TUNE / SPECIAL / CHAMBERS / |
| Move wire chambers MWPC 1-8 in our out |
In the (rare) periods that the T6 target is shared with the P61 beam, the M2
beam is forced to run at -1/2 times the P61 momentum in the front end (secondary
beam). Usually this corresponds to -200 or -225 GeV/c negative beams with lower
fluxes. During and only
during such periods a simplified procedure may be followed. This procedure will
on those occasions be communicated directly to the CRN operators.
7. Access to your Area
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 / 221 / OPEN
Type in your name when the program asks for it. Then go to the door marked
PPE221, 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. The
'End of Access' button serves to clear the veto,
but only after a 1 minute delay. Then go to your beam terminal and type
ACCESS / DOOR / 221 / BEAM ON
type in your name and wait till beam comes back. I t 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.
Often more persons have to enter than the number of available keys (16 for door
PPE221). In that case, or also when a very long access is required, one should
go into FREE ACCESS. In that condition one can enter the zone without
taking a key. The zone will switch to FREE access automatically when the door is
kept open for more than a minute. To come back from FREE ACCESS to KEY ACCESS, a
formal search
has to be performed by a team consisting of the CRN operator and a physicist
from the experiment.
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!
Please note that it is forbidden to operate the crane in EHN2 while the beam is
switched on. Therefore the access to the crane has been blocked off by special
locks. The keys for those locks can be taken from a special key distributor in
door PPE211. The detailed procedure is available
on the web.
|
The formal procedures related to access to SPS experimental areas are described
in a
web page. A
very detailed descriprion (with pictures) of the way to take and end an access
is
available as
well. The specialities related to access in EHN2 are described
here. After
a FREE access, a formal search is required. The guidelines and rules for such a
search are described in
Annex J of the SL Safety manual.
|
8. Using the Detectors in your Beam
The M2 beam is equipped with different types of detectors:
| XWCA (MWPC) |
10 Analog wire chambers with two planes are installed along the beam. They
only provide reasonable profiles for intensities above 105 particles
per SPS cycle. The profiles may be obtained by typing
TUNE / MEAS / MWPC / PROFILE
It is then possible to show all profiles together in a graphics window (called
DICO) or one by one. MWPC 19 and 20 show the horizontal and vertical profile of
the beam at the entrance to the experimental area PPE221, at the exit of the
last quadrupole. |
| TRIGGER |
The name TRIGGER stands for a scintillation counter with a sensitive area of
100 mm diameter. Two of such scintillators are installed in the last section of
the beam line. Their coincidence gives a good measurement of the beam rate for
fluxes below 107 particles per SPS cycle. The counting rates of the
single counters, normalised the incident flux on the T6 target, can be read via
TUNE / MEAS / TRIG / 1 (or 2) / USE / 2 / nr bursts
and the coincidence of the two counters via
TUNE / MEAS / COINC / 1 / 2 / # bursts
Note that the TRIGGERS can only be moved IN or OUT of the beam via the TUNE
/ MEAS
command, options USE or OUT, respectively.
The trigger coincidence is also used for strobing the reading of the CEDAR
counters (see below) through the beam control computer. |
| XION |
Two ionisation chambers have been installed in the beam line. XION-1 is
located at the end of the hadron decay section, a few tens of metres upstream of
the hadron stopper. It allows in particular to count the hadron flux in the muon
mode. XION-2 is installed almost at the end of the beam line and counts the flux
at the entrance to the last doublet of quadrupoles, which for a well-tuned beam
should be close to the beam flux at the experiment.
The counting rate from the ionisation chambers is only meaningful for fluxes
above several 106 particles per SPS cycle. They can be read via
TUNE / MEAS / ION CHAMB / 2 / # bursts
or, together with some other rates, via
TUNE / SPECIAL / QUICK
|
| CEDAR |
CEDAR stands for CErenkov Differential counter with Achromatic
Ring focus. One or two counters can be installed on request and allow
particle identification at high momenta, provided the beam has been made
sufficiently parallel. They work for fluxes up to 10 MHz. Whenever they are in
place, the muon or electron modes of operation are compromised, resp. excluded.
A detailed description of these complex devices is available in a CERN yellow
report 82-13 by C.Bovet et al. Their control is via the branch
DETECTOR / CEDAR
|
| EXPT |
Experimental scalers do not count rates in the standard beam equipment, but
they allow to count rates in equipment belonging to the experiment. A patch
panel with 8 inputs is available in one of the control rooms in EHN2. A NIM
signal connected to one of those ports may be read via a scler connected to the
beam control system via
TUNE / MEAS / EXPT / channel nr / 2 / nr bursts
A nice way to read all scalers plus several other counters for the same burst,
accompanied by explanations and the most relevant static properties of the beam,
is available via the general status program:
STATUS / GENERAL
|
9. A Quick Checkout of the Beam
For a quick checkout of the M2 beam, the following programs are available:
1. STATUS/GENERAL
Presents the (commented) count rates in the experimental scalers and the
standard beam counters in the M2 beam.
It shows also the collimator and scraper settings, as well as the target head,
absorber positions. Below 140 GeV/c only 7 absorber modules are used, at higher
energies all 9 modules should be in.
Of special importance are the MIB current polarities (positive for negative beam
and vice versa).
The first four wire chambers should normally be out of the beam to reduce
radiation damage. They are moved out by typing the Nodal command TUNE /
SPECIAL / CHAMBERS
2. STATUS/MAGNET
Look for magnet currents not on BIM.0 as well as settings marked with an 'F',
indicating that the BIM.0 does not correspond to the beam file loaded.
3. TUNE/MEAS/MWPC/PROFILE
All wire chamber profiles should be more or less centred. Adjust according to
the tuning procedure, chapter 4 in the M2 handbook.
Note that MWPC 19 and 20 show the horizontal and vertical profile of the beam
just a few metres upstream of the experimental target.
4. TUNE/SET/COLL
Note that Collimators 2 and 4 are closed rather tightly around the beam axis at
that position. This beam position may vary with the vertical angle of the
primary beam on T6. If necessary, scan EXPT-1 (i.e. the beam rate) vs COLL-2 and
center the C2 and C4 slits around the maximum found. COLL-5 is the momentum slit
for the hadron beam (slit about 10 mm), wide open for the muon beam.
The rate is modified by COLL-1 and 3. Equal slits for both of them.
10. Further Questions
- The HELP command gives you some suggestions on what to do in case of bad
beam quality.
- Under INFO / LOGBOOK you may find the recent changes to beam
settings, applied via the TREE programs. Note that these logfiles are also
available on the web, via the SL-EA web pages. Sometimes this may give a hint!
- Call the CRN operator, phone 75566, mobile phone 160137, if all this fails.
A number of documents are available, either on the web or in paper form:
- A detailed description of the M2 muon beam was published in Nuclear
Instruments and Methods A343 (1994) 351-362 by N.Doble et al.
- The M2 tuning procedure
- The
modifications for COMPASS
- A summary of the M2 commissioning for
COMPASS
Last updated : 6 February 2001 by
Lau Gatignon
