Overview and Issues of the MEIC
Interaction Region
M. Sullivan
MEIC Accelerator Design Review
September 15-16, 2010
Page 1
Outline
• MEIC IR and detector
• Backgrounds
• SR
• Radiative Bhabhas
• Summary
• Conclusion
Page 2
MEIC Interaction Region Design
• The MEIC Interaction Region Features
• 50 mrad crossing angle
• Detector is aligned along the electron beam
line
• Electron FF magnets start/stop 3.5 m from the IP
• Proton/ion FF magnets start/stop 7 m from the IP
Page 3
Table of Parameters (electrons)
• Electron beam
• Energy range
• Beam-stay-clear
• Emittance (x/y) (5 GeV)
• Betas
• x* = 10 cm
• y* = 2 cm
• Final focus magnets
• Name Z of face L (m)
• QFF1
3.5
0.5
• QFF2
4.2
0.5
• QFFL
6.7
0.5
3-11 GeV
12 beam sigmas
(5.5/1.1) nm-rad
x max = 435 m
y max = 640 m
k
-1.7106
1.7930
-0.6981
G (11 GeV)
-62.765
65.789
-25.615
Page 4
Table of Parameters (proton/ion)
• Proton/ion beam
• Energy range
• Beam-stay-clear
• Emittance (x/y) (60 GeV)
• Betas
• x* = 10 cm
• y* = 2 cm
• Final focus magnets
• Name Z of face L (m)
• QFF1
7.0
1.0
• QFF2
9.0
1.0
• QFFL 11.0
1.0
20-60 GeV
12 beam sigmas
(5.5/1.1) nm-rad
x max = 2195 m
y max = 2580 m
k
-0.3576
0.3192
-0.2000
G (60 GeV)
-71.570
63.884
-40.028
Page 5
Magnet apertures
• The magnet apertures are usually set by the required
BSC
• Making the BSC generous forces larger aperture
final focus magnets – usually these are more difficult
magnets to build
• However, a large BSC improves machine flexibility
by allowing for smaller beta* values (and hence
larger beta max values) if the accelerator can
operate with fewer beam sigmas than that defined by
the BSC and backgrounds are acceptable
• The MEIC 12 sigma BSC definition is a reasonable
compromise
Page 6
Interaction Region and Detector
Small angle
hadron detection
Central detector with endcaps
Low-Q2
Ultra forward
hadron detection
electron detection
ion quads
dipole
Large aperture
electron quads
dipole
IP
dipole
Small diameter
electron quads
~50 mrad crossing
Solenoid yoke + Muon Detector
EM Calorimeter
Hadron Calorimeter
Muon Detector
Tracking
RICH
RICH
HTCC
EM Calorimeter
Solenoid yoke + Hadronic Calorimeter
Courtesy
Pawel Nadel-Tournski
and Alex Bogacz
5 m solenoid
Page 7
Estimate of the detector magnetic field (Bz)
4
The detector magnetic field
will have a significant impact
on the beams. Some of the
final focusing elements will
have to work in this field.
3
Tesla
2
1
QFFP
QFFL
QFFP
QFF2 QFF1
QFF1 QFF2
QFFL
~2 kG
Page 8
Energy range
• Both beam energies have a fairly large energy range
requirement
• The final focus elements must be able to
accommodate these energy ranges
• An attractive alternative for some of the final
focusing elements (especially the electron elements)
is to use permanent magnets – they have a very
small size and do not need power leads
• However, any PM design has to be able to span the
energy range (for instance, only 30% of the final
focus strength can be PM)
Page 9
First look at SR backgrounds
50 mrad
Synchrotron radiation
photons incident on various
surfaces from the last 4
electron quads
38
P+ +
P
8.5x105
2.5W
4.6x104
240
2
FF2
FF1
40 mm
30 mm
50 mm
e1
-1
2
4
3
5
3080
Beam current = 2.32 A
2.9x1010 particles/bunch
X
M. Sullivan
July 20, 2010
F$JLAB_E_3_5M_1A
Z
Electron energy = 11 GeV
x/y = 1.0/0.2 nm-rad
Rate per bunch incident on the
surface > 10 keV
Rate per bunch incident on the
detector beam pipe assuming 1%
reflection coefficient and solid
angle acceptance of 4.4 %
Page 10
e-
5 times larger beam emittances
and lower beam energy
50 mrad
Synchrotron radiation
photons incident on various
surfaces from the last 4
electron quads
7
P+ +
P
1.6x105
0.5W
9.0x104
1.8x105
4
FF2
FF1
40 mm
30 mm
50 mm
e1
-1
2
4
3
5
6.4x105
Beam current = 2.32 A
2.9x1010 particles/bunch
X
M. Sullivan
July 20, 2010
F$JLAB_E_3_5M_1A
Z
Electron energy = 5 GeV
x/y = 5.5/1.1 nm-rad
Rate per bunch incident on the
surface > 10 keV
Rate per bunch incident on the
detector beam pipe assuming 1%
reflection coefficient and solid
angle acceptance of 4.4 %
Page 11
e-
Beam tails
100
 x 2
 x 2
d N
y 2 
y 2 
 exp  2  2  A exp  2 2  2 2 
dxdy
 2  x 2  y 
 2Sx  x 2Sy  y 
2
A = 7.2 10 5
S x = 3.3
S y = 10
Normalized intensity
10-3
y
The tail distribution is
primarily driven by
the beam lifetime
x
10-6
Gaussian
beam profile
(no tail)
The SR background
is dominated by the
beam tail particles
Beam
center
10-9
0
10
Assumed beam tail
distribution is the
same as was used in
PEP-II background
calculations
20
30
x/x or y/y
Page 12
Backgrounds
• Initial look at synchrotron radiation indicates that
this background should not be a problem but a more
thorough study is needed
• Need to look at lost particle backgrounds for both
beams
• Proton beam has been studied extensively
• Generally one can restrict the study to the region
upstream of the IP before the last bend magnet
• A high quality vacuum in this region is sometimes
enough
Page 13
Radiative Bhabha Background
• There is a luminosity background from the electron
beam
• During the collision the electron can radiate a photon
• This was a major source of neutrons in the B-Factory
detectors
Page 14
B-Factory Radiative bhabhas
PEP-II Interaction Region
HER Radiative bhabhas
30
Off energy
beam particles
were swept out
of the beam
and then hit the
local vacuum
pipe
3
2.5
20
1
1
3.
1.5 2
G
3.5
eV
4
0.5
LER gammas
10
cm
0
6
6.5
7
7.5
8
4.5
5 5.5
9 GeV
9 GeV
-10
HER gammas
-20
1
3.
-30
-7.5
3
G
2 1.5
eV
1
0.5
2.5
LER Radiative bhabhas
-5
-2.5
0
m
2.5
5
7.5
M. Sullivan
Feb. 8, 2004
API88k3_R5_RADBHA_TOT_7_5M
Page 15
B-Factory Radiative Bhabhas
KEKB Interaction Region
HER radiative bhabhas
30
Q1ER
Both B-factories
experienced
this luminosity
background
LER radiative bhabhas
20
Q2PL
3
2.5
1.5
1
Detector
5
4
6
7
2
1
2
0.5
0.5
QCSL
QCSR
CSL
3
HER radiative
gammas
CSR
10
LER radiative
gammas
cm
0
3.5 GeV
LER
-10
HER
eV
8G
CSL
CSR
Q2PR
QCSL
QCSR
Q1EL
-20
Detector
-30
-7.5
-5
-2.5
0
m
2.5
5
7.5
M. Sullivan Nov. 9, 2004
B3$KEK2_IR_RADBHA
Page 16
Super B-factories
• The super B-factories (SuperB and superKEKB) have
designed the IR so that this background source is
minimized
• This background has become one of the primary
design drivers for the interaction region
• The other background that is close to being a design
driver is the two-photon process where the produced
background comes from low-energy e+/e- pairs
produced from the interaction
• This background should not be a factor in the MEIC
design (down by another factor of )
Page 17
MEIC
Central detector with endcaps
Low-Q2
Small angle
hadron detection
Ultra forward
hadron detection
electron detection
ion quads
Large aperture
electron quads
dipole
dipole
IP
dipole
Small diameter
electron quads
~50 mrad crossing
• The radiative bhabha background may not be as
important for the MEIC as it was for the B-factories
but there will be beam induced backgrounds in the
low angle detectors for the low-Q2 detectors and this
reaction will be very similar to the physics signal
looked for here
Page 18
Summary
• The IR is one of the more challenging regions to
design
• There are multiple constraints, however, balancing
the various requirements to maximize the physics is
always the primary goal of any design
• The beam induced backgrounds many times control
a large part of the design of the interaction region in
order to allow the detector to operate in what can be
a very hostile environment
Page 19
Conclusion
• A careful study of the beam induced backgrounds
is always an important aspect of any interaction
region design
• As the accelerator design evolves one must
constantly recheck backgrounds to make sure
the changes do not have an adverse effect on
rates
• There is always room for unexpected
backgrounds so diligence in controlling and
understanding backgrounds always tends to pay
off
Page 20