FIGURE OF MERIT FOR MUON IONIZATION COOLING
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FIGURE OF MERIT FOR MUON
IONIZATION COOLING
Ulisse Bravar
University of Oxford
28 July 2004
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100 m cooling channel
• Channel structure from Study II
• Cooling:
d / dx = + equil./
• Goal: 4-D cooling. Reduce transverse emittance from initial value to equil.
• Accurate definition and precise measurement of emittance not that important
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MICE
• Goal: measure small effect with high precision, i.e. ~ 10% to 10-3
• Full MICE (LH + RF)• Empty MICE (no LH, RF)
• Software: ecalc9f
• does not stay constant in empty channel
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The MICE experiment
• Measure a change in e4 with an accuracy of 10-3.
• Measurement must be precise !!!
Incoming muon beam
Diffusers 1&2
Beam PIDTOF 0
CherenkovTOF 1
Trackers 1 & 2 measurement of emittance in and out
Liquid Hydrogen absorbers 1,2,3
Downstreamparticle ID:
TOF 2 Cherenkov
Calorimeter
RF cavities 1 RF cavities 2
Spectrometer solenoid 1
Matching coils 1&2
Focus coils 1 Spectrometer solenoid 2
Coupling Coils 1&2
Focus coils 2 Focus coils 3Matching coils 1&2
The MICE experiment
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Quantities to be measured in MICE
equilibrium emittance = 2.5 mm rad
cooling effect at nominal inputemittance ~10%
Acceptance: beam of 5 cm and 120 mrad rms
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Emittance measurementEach spectrometer measures 6 parameters per particle x y t
x’ = dx/dz = Px/Pz y’ = dy/dz = Py/Pz t’ = dt/dz =E/Pz
Determines, for an ensemble (sample) of N particles, the moments:Averages <x> <y> etc… Second moments: variance(x) x
2 = < x2 - <x>2 > etc… covariance(x) xy = < x.y - <x><y> >
Covariance matrix
M =M =
2't
't'y2
'y
't'x2
'x
'tt2t
'yt2y
'xt'xy'xxxtxy2x
...............
............
............
............
............
2'y'xyx
D4
't'y'xytxD6
)Mdet(
)Mdet(
Evaluate emittance with: CompareCompare in in withwith outout
Getting to e.g.Getting to e.g. x’t’x’t’ is essentially impossibleis essentially impossible with multiparticle bunch with multiparticle bunch measurementsmeasurements
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Emittance in MICE (1)
• Trace space emittance:
tr ~ sqrt (<x2> <x’2>)
(actually, tr comes from the determinant of the 4x4 covariance matrix)
• Cooling in RF
• Heating in LH
• Not good !!!
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Emittance in MICE (2)• Normalised emittance
(the quantity from ecalc9f):
~ sqrt (<x2> <px2>)
(again, from the determinant of the 4x4 covariance matrix)
• Normalised trace space emittance
tr,norm ~ (<pz>/mc) sqrt (<x2> <x’2>)
• The two definitions are equivalent only when pz = 0 (Gruber 2003) !!!
• Expect large spread in pz in cooling channel
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Muon counting in MICE• Alternative technique to
measure cooling: a) fix 4-D phase space volumeb) count number of muons
inside that volume
• Solid lines number of muons in x-px space increases in MICE
• Dashed lines number of muons in x-x’ space decreases
Use x-px space !!!
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Emittance in drift (1)
• Problem: Normalised emittance increases in drift
(e.g. Gallardo 2004)
• Trace space emittance stays constant in drift
(Floettmann 2003)
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Emittance in drift (2)
• x-px correlation builds up: initial final
Emittance increase can be contained by introducing appropriate x-px correlation in initial beam
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Emittance in drift (3)
• Normalised emittance in drift stays constant if we measure at fixed time, not fixed z
• For constant , we need linear eqn. of motion:
a) normalised emittance:
x2 = x1 + t dx/dt = x1 + t px/mb) trace space emittance:
x2 = x1 + z dx/dz = x1 + z x’
• Fixed t not very useful or practical !!!
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Solenoidal field
• Quasi-solenoidal magnetic field:
Bz = 4 T within 1%
• Initial within 1% of nominal value
fluctuates by less than 1 %
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Emittance in a solenoid (1)
• Normalised 4x4 emittance – ecalc9f
• Normalised 2x2 emittance
• Normalised 4x4 trace space emittance
• Normalised 2x2 emittance with canonical angular momentum
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Muon counting in a solenoid
• In a solenoid, things stay more or less constant
• This is 100% true in 4-D x-px phase space
solid lines
• Approximately true in 4-D x-x’ trace space
dashed lines
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Emittance in a solenoid (2)• Use of canonical angular momentum:
px px + eAx/c, Ax = vector potential
to calculate
• Advantages:a) Correlation x,y’ = 1,4 << 1b) 2-D emittance xx’ ~ constant • Note: Numerically, this is the same as subtracting the canonical
angular momentum L introduced by the solenoidal fringe field • Usually x,y’ = 1,4 in 4x4 covariance matrix takes care of this 2nd order
correlation • We may want to study 2-D x and y separately… see next page !!!
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MICE beam from ISIS
• Beam in upstream spectrometer
• Beam after Pb scatterer
x
y y
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How to measure (1)
• Standard MICE• MICE with LH but no RF
• Mismatch in downstream spectrometer
• We are measuring something different from the beam that we are cooling !!!
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How to measure (2)
• Spectrometers close to MICE cooling channel
• Spectrometers far from MICE cooling channel with pseudo-drift space in between
• If spectrometers are too far apart, we are again measuring something different from the beam that we are cooling !!!
increase in “drift”
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Quick fix: x – px correlation
Close spectrometersFar spectrometers
Far spectrometerswith
x-px correlation
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Gaussian beam profiles• Real beams are non-gaussian• Gaussian beams may become
non-gaussian along the cooling channel
• When calculating from 4x4 covariance matrix, non-gaussian beams result in increase
• Can improve emittance measurement by determining the 4-D phase space volume
• In the case of MICE, may not be possible to achieve 10-3
• Cooling that results in twisted phase space distributions is not very useful
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Conclusions
• Use normalised emittance x-px as figure of merit
• Accept increase in in drift space• Consider using 2-D emittance with
canonical angular momentum• Make sure that the measured beam and
the cooled beam are the same thing• Do measure 4-D phase space volume of
beam, but do not use as figure of merit