the m2 beam for compass

Lau Gatignon, 07-03-2006 The M2 beam line for COMPASS 1

L.Gatignon / AB-ATB-EA

• General layout and basic principles of the muon beam

• Optics and collimation of the muon beam

• Operational and performance issues – rate, halo

• Access related issues

• Other beam modes – hadron and electron beams

• Documentation

The M2 beam for COMPASS


SM1SM1

SM2

6LiD Target

160 GeV μ

RICH

ECal & HCal

μ Filter

Trigger-hodoscopes

Silicon

Micromegas

SciFi

Gems

Drift chambers

Straws

MWPC

Two stage spectrometer

Polarized beam and target ~75% ≥50%

SAT, LAT, PID

Merge of two proposals:HMC – muon physicsCHEOPS – hadron spectroscopy

≈ 250 physicists from 28 institutes

COMPASS (M2-BEAM)


CHEOPS:

HMC:

Each has:

a target 2 magnets lots of tracking 2 RICH 2 HCAL 2 ECAL 25 MCHF cost

One detectorSwap beams

COMPASS


COMPASS Spectrometer Layout


COMPASS BEAM REQUIREMENTSThe beam serves sometimes as a muon beam, sometimes as a hadron beam. An electron calibration beam should be available from time to time as well. • The muon beam spot size at the COMPASS target should be smaller than 8 mm RMS in each plane, with a RMS divergence not exceeding 1 mrad, • The muon beam intensity at 160 GeV/c should be at least 2 108 muons per SPS cycle. • The horizontal angle of incidence on the COMPASS polarised target is variable to allow compensation of the 1.05 Tm dipole field in that target,

• The hadron beam will transport secondary hadron beams up to 280 GeV/c, • Particle identification should be done with 2 CEDAR counters, for which a 15 metres long parallel section is required, • The hadron beam should have a spot size of not more than 3 mm RMS and a small divergence, • The hadron intensity can be as high as 108 particles per SPS cycle.

• The electron beam intensity should be several 104 electrons per burst at 40 GeV/c

2006: Concentrate on muon beam running. Only tests with e- and hadron beams


Muons from pion decay

•Pion decay in center of mass:

p* = = 30 MeV/c

m2 – m

2

2 m

E* = = 110 MeV/c2 m

m2 + m

2

(p*, E*)

• Boost to laboratory frame:

E = (E* + p* cos *) with

• Limiting cases:cos = +1 → Emax = 1.0 E

cos = -1 → Emin = 0.57 E 0.57 < E/ E < 1


MUON POLARISATION

SPINMuons from pion decay are naturallypolarised through Parity Violation:

For the typical COMPASSconditions, p / p = 0.92 andthe measured muon polarisation is about -80%

Note:Average pion decay lengthis 55 metres per GeV/c


The muon beam consists of several sections:

The production target T6

Pion acceptance and momentum definition section

A pion decay volume (600 metres long)

A hadron absorber

A muon transport and cleaning section

The final focus section

Choice of several target heads: the longer, the more flux

Large acceptance, p/p up to ±10%

Average pion decay length at 172 GeV/c is ≈ 9.5 km

To stop all hadrons and scatter muons as little as possible

Momentum selection (±3%) and cleaning by magn. colls

Steer and focus propoerly on COMPASS target


• The muon momentum is based on physics requirements.

• The ratio p/p is based on a compromise beween: - Total muon flux - Polarisation (higher for extreme p/p )

• In muon mode there are 3 sections with different momenta:1) The hadron section of the beam, up to the end of the FODO

The momentum is usually p = p / 0.92.Sometimes the M2 is coupled to P61: pM2 = - 1/2 pP61

2) The section from the end of the FODO to the absorberHere pint = p + Nabs * 0.315 GeV to correct for dE/dx in the absorberNormally Nabs = 7 (p<140 GeV) or 9 (p>140 GeV)

3) The muon section of the beam, from the absorber till the end Here the momentum is the required muon momentum.

MUON BEAM – MOMENTUM

2006: p/p = 172 / 160 GeV/c

Momentum defining bend:

B1

B6


Transport pions as well as muons from their decay together

Pions are ‘matched’ into a long decay channel (> 5-10% of ).

The M2 decay channel is about 700 metres long!

The pions themselves have a large momentum spread ( 10%).

Muons from their decay have a lower momentum and an even larger

momentum spread.

They do not come from a single point.

The optics of the decay channel must transport both and .

CONSIDERATIONS FOR M2 OPTICS


Transport muon beam over a sufficient distance

After the hadron stopper (at the end of the decay channel)

the muon momentum selection must be done by magnetic collimation.

The unwanted muons must be separated from the beam axis

and ‘ranged out’ in the earth: the energy loss is about 0.5 GeV/m.

For 200 GeV muons this requires a length of 400 metres!

The origin of the muons is not a point source but a long ‘tube’.


Both problems have a common answer:

FODO channels: Regular array of alternately focusing and defocusing quadrupoles.They have a very large momentum acceptance


0 20 40 60 80 100 120 140 160 180

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

f / L

Phase advance per cell (degrees)

Bo/cell f / L

30 1.9336 1.61745 1.30560 172 0.8590 0.707120 0.578144 0.525180 0.5


Acc

epta

nce

Phase advance per cell (degrees)

60 90 18072

Pions

Muons

Choose 60o/cellfor the pions,

still very good acceptance for

the decay muons


SPECIAL PROBLEMS FOR M2 OPTICS Large momentum spread (up to ± 10%)

Scattering in absorber introduces p/p y, y’correlations

First order optics breaks down:

Rij(p) = Rij(po) + dRij/dp . po . (p/p) + d2Rij/dp2 . po2 . (p/p)2 + …

The (p/p)2 term is no longer negligible for larger p/p

Better use 2nd order optics for p/p >> 1%

Faster particles (p > po) are bent less in B4+5If they are scattered downward in the absorber, they are

lost, if they are scattered upward, they may be kept :

This introduces a y’ p correlation (like dispersion).

Need full Monte Carlosimulation:

HALO


HADRON ABSORBERS: 9 Be modules (1.1 m each) in 3 bends

In/out motorisation


SCRAPERS(Magnetic Collimators)

B

Magnetic force

Have to get polarity right….


4 Motors: 2 upstream, 2 downstreamallows to follow beam divergence

Have to stop current before moving jaws !!!


MIB: Magnetised Iron Block

Same function as scrapersbut bigger and no motors

B

Beam


Steering at the end of beam is very critical


Novelties for 2006:

One new scraper + MIB in front end of beam line (posn 200 m)Should solve a radiation dose problem at the ramp from bldg 892 to EHN1

Move one scraper from posn 685 to 845and add one scraper + MIB at position 1000Allows to reduce muon halo in COMPASS by about 25-30%

500 Tons of passive Iron shield have been installed above thebeam lines in TCC2 to protect EHN1 from ’s from T6 & T4


OPERATIONAL ASPECTS

Initial tuning for 2006 done by EA physicist

Muon operation inherently stable (if equipment in correct state)

Have to keep proton flux on T6 stable and highNominal T6 flux 1.2 1013 ppp

Potentially an increase to 1.45 1013 ppp can be granted,

depending on RP measurements early in the run

Important that at least 7 hadron absorbers are in the beam

during muon operation

COMPASS changes T6 target head themselves (low intensity runs)

Fine steering at the end is done by COMPASS,

based on their own SciFi detectors


MONITORING OF M2 BEHAVIOUR:

General Status

– look at beam intensity on ionisation chamber

– look at halo rates (check with logbooks)

– check static settings (T6, colls, TAX) in case of problems

Wire chamber profiles

Magnet status


GENERAL STATUS (1):

Muon flux

Halo rates

Scr

aper

s


GENERAL STATUS (2):

The Cesar version will be very similar

Static settings

Col

limat

or s

etti

ngs


SM1&SM2 interlock

Pol. Target SM1 SM2

The full beam would plough into theCOMPASS detectors and trip or evendamage them in case of failure of SM1 or SM2

Similarly in case the dipole inside the COMPASS polarised target is switched on

In those cases the SM1&SM2 interlock puts Bends 4&5 on “Delestage” to stop the beam line. No operator intervention is required.The situation re-establishes itself automatically as soon as the situation is healthy.

Please note that COMPASS can disable this interlock from their control room


SM1

SM2

Max. field: 1.8 TCurrent: 5 kAmpsPower: 3 MWWeight: 400 tons


Access Specialities for EHN2

Crane access is forbidden with beam on. Special keys are needed.

Search boxes!


M2


pM2 = -0.5 pP61 if P0 derived from T6 target

Coupling between M2 and P61 beam lines:


HADRON OPTICSThere are 2 versions for the M2 hadron optics:

• An optics compatible with P61 operationRuns out at 225 GeV/cQ1-Q6 and B1 are now same as in P61 at –2 pM2

Momentum definition by COLL 1 and 3 only, giving rather poor resolution.Allows M2 to operate e.g. in the last week of a long proton run

• A high-energy optics Runs up to 280 GeV/c

Q1-Q6 and B1 are now different from P61 Momentum definition by COLL 5, with better resolution.This is the ‘reference’ optics, however still to be commissioned (in 2006?).

In 2006 only used under EA physicist control

1) Short target head2) Small TAX holes3) Single beam momentum all along the beam4) Hadron absorbers OUT


This beam is produced as in the West Area, i.e.:Secondary beam at -100 GeV/c up to 5 mm Pb ‘target’, Tertiary beam of –40 GeV/c from Pb target to experiment.

The Pb target is about 50 m upstream of B4+B5.For this mode there is a special optics with focus at the Pb target.

ELECTRON BEAM MODE

Will only be used for one or a few days,set up by EA physicist


On the SL-EA Web pages you may find (from the BEAMS page):

• A User Guide• A Quick Reference• More detailed optics drawings of all modes• Detailed tuning procedure for the muon beam• Detailed tuning procedure for the hadron beam• Modications to the beam line for COMPASS• Summary of the commissioning progress• Description of access specialities

Examples of tuning sessions can be found in the M2 logbook

INFORMATION ON THE WEB

Cesar related information for M2 will be provided soon


Questions ???

Thanks for your attention!

the m2 beam for compass

Documents

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muon beam momentum2006

muon beam optics

gevcthe muon beam

m2 beam

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beam modes hadron

muon beam spot size