7.4 Beamline Design
7.4.1 General Description
The proposed MICE muon beam line is based on a conventional pion-muon decay channel, with
provision for emittance tuning and matching into the experiment. The generic layout falls naturally into
four main sections:
Particles from the target (mainly pions and protons) are captured by a first quadrupole triplet, (Q1-Q3)
tuned for as large an acceptance as possible.
The captured beam is then momentum analysed by a rectangular dipole magnet (D1) and injected into a
solenoid decay channel. It is the muons from pion decay in this section which are useful for MICE.
A second dipole (D2) is then used to select muons of a desired momentum, and separate them from the
remaining pions and protons. The muons are subsequently transported down a large acceptance
qudrupole transport channel (Q4-Q9).
Beam is finally focussed onto a Pb. Beam Diffuser for emittance growth and matching into the
experiment.
To provide a tuneable and matched emittance into MICE, the scheme outlined in Figure 7.4-1 is
envisaged.
Figure 7.4-1: Diffuser and beam line focus conditions for emittance tuning & matching
The beam is first focussed on to a thick target, the thickness of which is chosen to fill the phase space
with the desired matched muon distribution: The beam size is matched to the final beam size but the
divergence of the incoming beam is (because of limitations of the beam line) smaller than the matched
beam. The effect of the diffuser is to increase the divergence of the beam and to fill the matched
emittance of the cooling channel.
7.4.2 Specific Design Issues
In addition to the functions described above, the beam line design has evolved with the aims to satisfy
additional features for MICE. The inputs which have been used to guide the design of the muon beam
line are listed in Table7.4-1.
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Table7.4-1: Driving consideration for the beam line design.
Consideration
Muon purity
Q9 saturation
TOF/PID discrimination
Design
Pion/Muon momentum separation should be as large
as possible.
Pion focus before Q4.
Polyethylene absorber in beam line to remove
protons.
Q9 End to End Coil minimum separation 1.2169m.
Upstream detector shielding assumed.
TOF0 to TOF1 minimum separation of 6.94m.
Intra-quadrupole spacing minimum 0.15 m for TOF
stations.
7.4.3 Physical Layout and Beam Line Geometry
The proposed beam line is shown in its location in the synchrotron vault and Hall 5.2, in Figure 7.4-2.
Details of the layout and parameters of the current design are given in the Table 7.4-2. This design is
for a configuration supplying a matched muon beam of normalised emittance εn,y=5.9 π mm rad, at 206
MeV/c.
Figure 7.4-2: Layout of the Beamline on ISIS
Details of the layout and parameters of the above setting are given in Table 7.4-2.
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Table 7.4-2: Beam line parameters for current setup. Final design momentum=206 MeV/c, ε n,y=5.9π
mm rad. The TURTLE source code will be published in a MICE note.
Element
Drift Space
Type 4 NIM Qd -Q1
Drift Space
Type 4 NIM Qd -Q2
Drift Space
Type 4 NIM Qd -Q3
Drift Space
Type 1 NIM Dipole - B1
(bend angle 60°)
Drift Space
Decay Solenoid
Drift Space
C2H4 Proton Absorber
Drift Space
Type 1 NIM Dipole - B2
(bend angle 30°)
Drift Space
Q35 Qd - Q4
Drift Space
Q35 Qd -Q5
Drift Space
Q35 Qd - Q6
Drift Space
Q35 Qd - Q7
Drift Space
Q35 Qd - Q8
Drift Space
Q35 Qd - Q9
Final Drift Space.
0.76cm Pb Diffuser
Position1
Vertical
Aperture
Horizontal
Aperture
10
10.15
10
10.15
10
10.15
10
10
10.15
10
10.15
10
10.15
10
7.4342
8.5212
9.6647
14.6647
15.0459
15.0959
10
10
6
33
10
6
10
33
15.2817
16.3316
17.7348
18.3948
18.9508
19.6108
20.1668
20.8268
24.9765
25.6365
26.1925
26.8525
27.4085
28.0685
29.4784
10
17.82
m
0.0
2.5733
3.4267
3.9733
4.8267
5.3733
6.2267
Quad
Pole
Tip Radius2
Cm
Field
Strengths
Units
1.02463
T/m
-1.28079
T/m
0.89163
T/m
1.25326
T
3.7
T
33
0.36595
T
23.6
23.6
1.61263
T/m
17.82
23.6
23.6
-1.48935
T/m
17.82
23.6
23.6
1.53438
T/m
17.82
23.6
23.6
1.31141
T/m
17.82
23.6
23.6
-1.39495
T/m
17.82
23.6
23.6
1.60845
T/m
25
25
10.15
10.15
10.15
7.4.4 TRANSPORT Design Code Beam Profiles
The following two figures show the second order TRANSPORT beam profiles, corresponding to the
above beam line optics. The initial pion source from the target occupied half widths of 0.255 cm and
0.1 cm in x and y (respectively) and 33.0 mrad and14 mrad in x’ and ,y’, with a mean momentum of
390 MeV/c and uniform Δp/p =± 2.5%. Both figures use a horizontal scale of 0 – 16 metres. The full
width pion beam profile is shown in Figure 7.4-3.
1
2
This refers to the position of the start of the effective length of the magnet along the central trajectory.
The pole tip radius is the radial distance between the central axis of the quadrupole and its pole tip.
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25
B
2
B2
B1
Q
4
Q
5
Q4
Q
6
Q5
Q
7
Q6
Q7
Q
8
Q8
Q
9
Q9
P
b
Pb.Disk
P
T
Vertical
Half-width
(cm)
0
z 16m
Horizontal
Half-width
(cm)
25
Figure 7.4-3. TRANSPORT beam profile for the pion injection and decay section forthe beam optics
parameters given in Table 7.4-2. The Pion source (incomming beam) is described in the text.
Following pion to muon decay, and the passage of the muon beam through the proton absorber, the
second dipole and the following quadrupole channel are set to transport muons of momentum 220
MeV/c to the Pb. Diffuser. The muon beam profile for this section is shown in Figure 7.4-4. This
shows the rms vertical profile for the muons which reach the diffuser, together with the horizontal
profile of the same emittance. The final beam focus is appropriate for generating the matched emittance
of εn,y=5.9π mm rad.
25
Q
1
'
Q1
Q
2
Q2'
Q
3
'
Q3
B1
Solenoid
Vertical
Half-width
(cm)
0
z
16m
Horizontal
Half-width
(cm)
25
Figure 7.4-4. TRANSPORT beam profile for muon extraction for beam opticdescribed in Table 7.4-2.
The RMS muon beam profile at 220 MeV/c is shown.
7.4.5 TURTLE Code Final Beam Distributions (±1% Δp/p Beam Cut)
A Pb. disk of thickness 7.6mm gives sufficient scattering and emittance growth to produce the 6
normalised emittance. Muons of momentum 220MeV/c entering the Pb disk emerge with a momentum
of 206 MeV/c. The final yy’ muon distribution, for muons within a momentum byte of Δp/p=±1% and
mean p of 206 MeV/c, has y-rms = 3.23 cm, y’-rms= 93.1 mrad and ryy’=-0.024 which corresponds to a
matched, normalised, rms emitance of εn,y =5.9π mm rad.
The x-x’ and y-y’ phase space plots for this beam cut are shown in Figure 7.4-5. Due to the present Q4Q9 quadrupole optics, the horizontal x-x’ distribution is slightly larger, corresponding to an emittance,
εn,x of 7.7 π mm rad.
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Figure 7.4-5. TURTLE ±1% Δp/p muon beam profiles after Pb. Diffuser for beam optics described in Table
7.4-2 (xx’ left, yy’ right).
Full Muon Distribution
The x-x’ and y-y’ phase space distributions for the full muon distribution after the diffuser are shown in
Figure 7.4-6.
The plots of the full muon distribution have a number of features additional to the restricted cut. These
can be attributed to the beam dispersion at this point, and the presence of a large population of higher
momentum particles. The overall momentum distribution (which should ideally be centred on 206
MeV/c) is shown in Figure 7.4-6.
Figure 7.4-6: TURTLE full muon beam profiles after Pb. Diffuser for beam optics described in Table 1-2 (xx’ left,
yy’ right)
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Figure 7.4-7: TURTLE muon momentum distribution after the Pb. diffuser for the beam optics described in
Table 7.2
Work is in progress to further understand and address this. All of the above design code results should
be compared with the evaluations described in the following section
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