Transcript On the Feasibility of NLC Final Focus with b*=2mm/0.11mm

st
1
and
nd
2
NLC
IR performance
and balancing
the accelerator and detector issues
American Linear Collider Workshop
July 13-16, 2003
Andrei Seryi
SLAC
for the NLC Accelerator Physics Group
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A.Seryi, 07/14/03, ALCW
A. Drozhdin, L. Keller,
T. Markiewicz, T. Maruyama,
N. Mokhov, Yu. Nosochkov,
T. Raubenheimer, A. Seryi,
P. Tenenbaum, M. Woodley
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A.Seryi, 07/14/03, ALCW
1st and 2nd IR
design approach
• Would like to make 1st & 2nd IRs more equivalent, at least up to
1.3TeV CM
– Require that Lumi of two IRs can be equal within 30%
• The two IRs will never be equal
– 1st IR has higher potential (straight tunnel, => multi TeV)
– 2nd IR needs big bend => BDS is shorter => lower energy reach
•
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Luminosity loss in FF scales as dL/L~ g7/4 / L5/2. That means that though
the required FF length scales only as L ~ g 7/10 , the luminosity loss can
be significant when the FF length is decreased
– => Want to make 1st and 2nd IR BDS as close in length as possible
A.Seryi, 07/14/03, ALCW
1st and 2nd IR
layout evolution
• Evolution of IR layout is driven by attempts to solve
this contradiction:
– Need 2nd IR BDS to be as close to the “full length”
as possible
– Need to limit the emittance growth in 2nd IR big
bend
• This drives the big bend length up, and appear to
contradict the previous …
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A.Seryi, 07/14/03, ALCW
NLC layout
evolution
IP2
e+
e-
May 03:
1st IR : full length (1430m) BDS
2nd IR : 2/3 length (970m) BDS
IP1
Big Bend has to be long (600m)
so that de/e<30% @ 650 GeV/beam
June 03:
2nd IR : 2/3 length one way bending BDS
Big Bend shortened twice
e-
e+
Saved 125m in e- and 450m in
e+ beamlines of 2nd IR
July 03:
Lengthen the e+ 2nd IR BDS to full length
The e- 2nd IR BDS is still 2/3 length
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A.Seryi, 07/14/03, ALCW
Picture is still June layout
e-
IP2
e+
IP1
Full length BDS (1430m) in 1st IR
and in e+ side of 2nd IR
Only the e- side of 2nd IR has
shorter BDS (presently 970m
optics, but have space to lengthen
it to 1100m)
Optics for July 03 layout
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A.Seryi, 07/14/03, ALCW
1st and 2nd IR
LO~E
Geometric luminosity (normalized) of NLC BDS. Include effect of aberration
and synchrotron radiation. Beam-beam enhancement is not included.
Same normalized emittances assumed for the entire range.
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The e- 2nd IR BDS can still be lengthened to improve performance
BDS performance (July layout)
A.Seryi, 07/14/03, ALCW
in absolute units
Geometric luminosity for NLC BDS (optics only: include aberrations and
synch.radiation; beam-beam luminosity enhancement is not included).
Same normalized emittances assumed for the entire range.
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lengthening of 2nd IR e- BDS to 1100m will pull it up somewhat
BDS performance (July)
A.Seryi, 07/14/03, ALCW
FF upgrade means (1):
reduce bending angle in FF
E-Collimation
bends:
Increase angle
by 15%
To reduce synch.radiation in FF magnets:
FF bends:
reduce
angle twice
Reduce bending angle in FF twice, and
increase bending angle in E-Collimation
by ~15%.
IP
Location of IP is fixed. BDS magnets need
to be moved by ~20cm. Outgoing angle
change by ~1.6 mrad
One way bending BDS for 2nd IR
“Standard” (two way bending) BDS
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A.Seryi, 07/14/03, ALCW
FF upgrade means (2):
use longer Final Doublet
Longer FD allow to reduce luminosity
degradation due to synch.radiation in FD
(Oide effect).
Short FD
Long FD
2nd IR FD optimized for 90-650 GeV CM range
2nd IR FD optimized for the energy upgrade
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A.Seryi, 07/14/03, ALCW
BDS performance (July)
When luminosity loss is so high at low E,
it can be partly regained by increasing b*
Benefits of the FF upgrade
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Geometric luminosity (normalized). Thin curves show performance if upgrade
(=layout & FD change) was not made, or if one goes back from 1TeV to Z
A.Seryi, 07/14/03, ALCW
Collimation in
NLC BDS
Octupole Doublets
Betatron
Collimation
Energy
Collimation
Collimation system removes the beam halo.
Losses of halo occur in dedicated places.
No losses after last FF collimator.
SR emitted by halo is lost only on dedicated
masks, and do not touch vertex detector.
K=1
K>1
FD aperture
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K=1 corresponds to nominal collimation depth when
SR from the halo do not touch the vertex detector
Entrance
Assumed halo is 0.1% of the beam
IP
A.Seryi, 07/14/03, ALCW
Collimation gaps
collimator
Collimation gaps are defined by requirement to protect
Vertex Detector from synchrotron radiation emitted by beam halo
For a given optics design, gaps are proportional to the Vertex Radius
(and are independent on beam energy)
Smallest spoiler gaps in NLC BDS are +-0.2mm
(or +-0.6mm with tail folding octupoles)
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A.Seryi, 07/14/03, ALCW
Collimation gaps
and wakes
Small gaps is an issue (TRC R3)
because collimator wakes cause
the IP beam jitter to increase
For NLC Ab~1.3 (or 0.7 with Octupoles)
that means that Y-jitter increase by
64% (or 22% with Octupoles) at 500GeV CM
The 22% number would be OK at 500 GeV CM,
however, the effect scales as 1/Energy
NLC spoiler is tapered to
reduce wake-fields
Jitter amplification in y-plane
(due to y’) is (1+ Ab2)0.5 times
At 90 GeV CM, will have Ab ~ 7 (or Ab ~ 3.9 with octupoles). Unacceptable.
Since Ab almost does not depend on optics, have only three choices:
–Solution 0: Ongoing studies will prove that the wake formula
give an overestimate
–Solution 1: Degrade b* and Luminosity expectations at low E
–Solution 2: Increase the vertex detector radius
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A.Seryi, 07/14/03, ALCW
Possible reduction of luminosity
due to collimation wakes
Assume that tolerable Ab
is 0.7 (or 1.4), which
gives 22% (or 72%)
Y-jitter increase
Luminosity reduction at Z
is 2.5 times (or 1.7 times)
For Rvx X 2, the reduction
is 1.4 times (or none)
This assumes the tail
folding Octupoles are ON
(more optimistic case)
Assumed that for typical spoilers, Ab scales
as Ab ~ b N / (
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sz1/2
g
gap3/2
) or, equivalently, as
N L2*
Aβ  * 1/2 3/2
γ β σ z R VX
A.Seryi, 07/14/03, ALCW
Vertex radius
discussion*
Agreed that VX radius is, in certain
extents, a free parameter that accelerator
physicists can optimize
*) This discussion took place in a context
of 500GeV CM. The low energy
requirements need to be discussed again
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From studies by Aaron Chou
A.Seryi, 07/14/03, ALCW
Conclusion
• Performance of NLC BDS optics will allow
almost equal luminosities in 1st and 2nd IR up
to 1.3 TeV CM
• There are potential limitations at Z energy
due to collimation wake fields, which need
to be taken into account when detector
parameters are to be chosen
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A.Seryi, 07/14/03, ALCW