Transcript LHCb

Accelerator Physics
Aspects LHCb
[email protected]
CERN SL/AP
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Layout
Crossing Scheme
Luminosity
Collision Scheme
Electron Cloud
Impedances
Official Schedule
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Layout of the LHC
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Layout of IR8
Dispersion Suppressor
Matching
Triplet
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A few definitions
Longitudinal emittance:
 l  4 E  t
Transverse emittance:
 T ,n   ( T 2 /  T )
Luminosity:
Beam-Beam Parameter:
r N
 p
4  T , n
N 2 kfrev
 1  N  N 
L
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4 T2
4  T   T ,n   t 
Proportional to the beam current
Beam transverse density,
proportional to the beam-beam parameter
Inversely proportional to *
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A few definitions
Beam-Beam Tune Shift Parameter:
rp N

4  T , n
Head-on + Long range
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Spread of the transverse oscillation frequencies
High order transverse resonances and a tune shift
It is limited by the space between dangerous
resonances
Difficult to compensate for: all particles do not
have the same tune shift
Independent of *
Its nominal value is 0.0035
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Crossing Angle
Crossing angle:
n
 n ,T
 *
Beam envelope defined at n
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Avoid unwanted bunch collisions
Must be larger than the divergence of the beam
envelope
Limited by the excursions of the beam trajectories
(aperture limitations in the triplet)
In the expression for the luminosity there is a
reduction factor for the crossing angle (0.1)
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Beam Separation and crossing scheme
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End of triplet
Spectrometer
Spectrometer magnet
compensation: 3 correction
magnets to make local bump
IP
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Correctors
Horizontal crossing
Vertical separation when not in
collision
Correctors
D1
D2
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Beam Separation and crossing scheme
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Limitations by Aperture
Accomodate spectrometer -> 11.22m shift towards
IP7
Beam Separation 2 mm
atot= aspec + aext
atot=345 mrad / 75 mrad depending on spectrometer
polarity
aspec=135 mrad positive or negative
aext =210 mrad constant
Crossing scheme only one direction
Ramping of spectrometer magnet important to
permit both polarities of spectrometer (limitations
at injection)
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Beam Separation and crossing scheme
10mm
0.5mm
10mm
1mm
 =1m
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Optics IR8
 =400mm
 =70mm, =10m
 =160mm, =50
Beam Size  =Sqrt (*/gamma)
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Luminosity vs *
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Limited to 35m
Wanted luminosity range for
LHCb 1-5 1032 cm-2 s-1
Tunability 1m < * < 35m
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5 1032
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75
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Luminosity requirements fulfilled
dynamically by varying *
Nominal
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1
1032
50% of Nominal
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10
20
30
40
50
10 % of Nominal
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Luminosity Lifetime
Initial Beam Intensity Lifetime
from the collisions
N
 0
Lk x
Total cross section (10-25 cm2)
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Number of Interaction points
Scattering from residual gas ignored (10-12 torr)
The beam-beam effect and the intrabeam scattering produce
emittance increase but this is compensated by synchrotron
radiation damping. The net result is a decrease of emittance.
We are left with the formula above giving a lifetime of 26 hours
Beam-gas induced lost rate into the pipe at the triplet under
study
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Luminosity Life Time
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No Beam-Beam Blow up
No synchrotron radiation damping
 decreases
L = 11hours
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Synchrotron radiation (theory)
 constant
L = 25hours
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Synchrotron radiation (theory)
 decreases because of beam
blow up (SppS Collider)
L = 10hours
Run on Beam-beam limit
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Collision scheme
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Distance between IPs = 891 half buckets: collision scheme has to
repeat from one IP to the other
“Holes” (empty buckets) due to injection kickers SPS and LHC,
dump Kicker LHC
There are 2808 filled buckets out of 3564 according to following
scheme:
{[(72b+8e)*3+30e]*2+[(72b+8e)*4+31e]}*3{[(72b+8e)*3+30e]*3+81e}
“Pacman” bunches:
do not encounter bunches of the other beam in one or several
parasitic collision points
“Superpacman” bunches:
as “pacman” but not even at the collision point
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Filling scheme
(72b+8e)*3+30e
(72b+8e)*3+30e+81e
(72b+8e)*4+31e
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Horizontal orbit offsets
Zooming up
Horizontal offset at IP1, in IP8 the situation is similar,
need to scale so that the spread 1/10 of the beam size
Effects coming from
the very start of train
where there is a “big
hole”
Effects coming from
the “small holes”
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Collision scheme
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IP8 shifted by 3 half buckets which means 124 extra
superpacman bunches in IP8
Double bunch spacing no encounters in IP8
IP8
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Triple spacing means less luminosity (bunch current has to be
increased by 31/2 to keep luminosity constant)
Bunch offsets within +-0.1 at collision point, small effects
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Longitudinal Impedance
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Longitudinal impedance can cause longitudinal
instabilities of the beam
The geometry of an element is crucial
All elements in the machine are optimized to give a
minimum contribution to the impedance budget.
Longitudinal impedance budget is very tight
No feedback system in the LHC for longitudinal
instabilities
A longitudinal feedback system is technically very
difficult and expensive
The evaluation of the LHCb experimental beam pipe
longitudinal impedance is done by Nikhef. Has to fit
into total budget of the LHC!
Examples of critical geometries
Sharp edges not good
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Transverse Impedance
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A transverse feedback system is required in the
LHC to cure the effect of transverse impedance
(resistive wall instability).
Aluminum, copper and beryllium are good
materials (stainless steel not so good).
Transverse impedance should not exceed budget
because of emittance conservation (feedback
capabilities are limited)
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Higher Order Modes
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Depends on the geometry of the object
Frequencey spectrum of loss factor should not
overlap, bunch spectrum
Different positioning of the vertex detector gives
different resonance conditions
All positions of the detector have to be evaluated
Heating up change resonance conditions, cooling
down etc. Pumping effect.
Different situations should be carefully evaluated
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Electron Cloud
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Photons, protons, electrons
from gas ionization
Critical dimensions of chamber
Heat Load
Vacuum
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Electron Cloud
Scale different
SEY=1.2
Boxes open,
xb=12cm, yb=3cm
SEY=2.8
Boxes closed,
xb=6mm, yb=6mm
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Official Schedule
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First Beam 01/02/2006
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First Collisions 01/04/2006
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Shut Down 01/05-31/07/2006
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L *=0.5=5 1032cm -2 s-1
Physics Run 01/08/2006-28/02/2007
L *=0.5>= 2 1033cm -2 s-1
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People who Contributed
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Optics: Oliver Brüning
Crossing Scheme: Werner Herr, Oliver Brüning
Electron Cloud: Frank Zimmermann, Oliver Brüning
Impedance: Daniel Brandt, Oliver Brüning
Lattice files: Elena Wildner
Aperture: Bernard Jeanneret
Beam-Beam: H.Grote
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LHC general parameters
Energy at collision
Energy at injection
Dist. aperture axes (1.9 K)
Luminosity
Beam beam parameter
DC beam current
Bunch spacing
Bunch separation
Number of particles per bunch
Norm. transv. emittance (r.m.s.)
Total crossing angle
Luminosity lifetime
Energy loss per turn
Critical photon energy
Total radiated power per beam
Stored energy per beam
Filling time per ring
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450
194
1
3.6
0.56
7.48
24.95
1.1
3.75
300
10
7
44.1
3.8
350
4.3
TeV
GeV
mm
E34 cm-²s-¹
E-3
A
m
ns
E11
µm
µrad
h
keV
eV
kW
MJ
min
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Transverse Parameters
Parameter
Injection
Collision
Unit
Energy
Relativistic factor (gamma)
Magnetic rigidity
Dipole field
Transverse physical emittance
Norm. transv. emittance (r.m.s.)
Maximum beta value in arc (H/V)
Max. beam size, arc (H/V) (r.m.s.)
Beam size at IP1 and IP5 (r.m.s.)
Maximum beta value in insertions
Transv. intrabeam scattering growth time
0.450
479.6
1501
0.535
7.82
3.75
~180
1.20
45
7
7460.6
23349
8.33
0.503
3.75
~180
0.303
15.9
4705
100
TeV
Tm
T
nm
µm
m
mm
µm
m
h
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Longitudinal Parameters
Parameter
Injection
Collision
Unit
Energy
Revolution frequency
RF frequency
RF harmonic number
RF tuning range
RF voltage
Frequency slip factor (eta)
Energy gain/turn (20 min. ramping)
RF power per beam
Synchrotron frequency
Bucket area
Longitudinal emittance (2 r.m.s.)
Energy spread (r.m.s.)
Bunch duration (r.m.s.)
Bunch separation
Stored energy per beam
Long. intrabeam scattering growth time
Synchrotron radiation energy loss per turn
Longitudinal damping time
RF component of batch current
DC beam current
0.450
11.2455
200.395 (*)
17820
10
3
3.43
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2.38
1
0.285
0.62
24.95
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1.20
0.56
7
11.2455
400.790
35640
10
16
3.47
485 (**)
257 (**)
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7.63
2.5
0.105
0.28
24.95
350
60
7
25.8
1.25
0.56
TeV
kHz
MHz
kHz
MV
E-4
keV
kW
Hz
eVs
eVs
E-3
ns
ns
MJ
h
keV
h
A
A
(*) At injection the 400MHz RF system is used as a second harmonic system in addition with
v=0.75MV
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(**) During acceleration