1 Freddy Poirier, 12/06/06, DESY student seminar. ILC Beam Dynamic Freddy Poirier FLC / EUROTEV group It is a project designed to smash together electrons and positrons at the center of mass energy of 0.5 TeV initially and 1 TeV later. The ILC Global Design Effort team, established in 2005, has been making its accelerator design. Recently, it worked out the baseline configuration for the 30-km-long 500 GeV ILC= International Linear Collider
ILC Beam Dynamic. ILC= International Linear Collider. It is a project designed to smash together electrons and positrons at the center of mass energy of 0.5 TeV initially and 1 TeV later. - PowerPoint PPT Presentation
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1
Freddy Poirier, 12/06/06, DESY student seminar.
ILC Beam Dynamic Freddy PoirierFLC / EUROTEV group
It is a project designed to smash together electrons and positrons at the center of mass energy of 0.5 TeV initially and 1 TeV later.
The ILC Global Design Effort team, established in 2005, has been making its accelerator design. Recently, it worked out the baseline configuration for the 30-km-long 500 GeV collider.
ILC= International Linear Collider
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Freddy Poirier, 12/06/06, DESY student seminar.
Why a straight machine?• Synchrotron RadiationBending a particle = loosing some energy
E ~ (E4 /m4 R)
• From a cost point of view:
Rm,E
cost
Energy
CircularCollider
Linear Collider
At high energy,linear collider ismore cost effective
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Freddy Poirier, 12/06/06, DESY student seminar.
Physics at the ILC (1)• Explore new Physics through high precision at
high energy• Study the properties of new particles (Cross-sections, BR’s,
Quantum numbers) ILC=microscope• Study known SM processes to look for tiny deviations
through virtual effects (needs precision of measurements and theoretical predictions)
– Precision measurements will allow:• Distinction of different physics scenarios• Extrapolation to higher energies
ILC=telescope
ILC will provide a detailed map of new physics
Exemple with e+/e- LEP experiment: Indirect determination ofthe top quark mass.Proves high energy reachthrough virtual processes
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Freddy Poirier, 12/06/06, DESY student seminar.
Physics at ILC (2)• Comprehensive and
high precision coverage of energy range from Mz to ~ 1TeV
cross sections few fb to few pb e.g. O(10,000) HZ/yr
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Freddy Poirier, 12/06/06, DESY student seminar.
Luminosity• Parameters for the ILC from physics point of view:
– Ecms adjustable (90500GeV)– Luminosity int Ldt=500 fb-1 in 4 years– Ability to scan – Energy stability and precision below 0.1%– Polarisation of electrons (at least 80%)
To achieve high luminosity small sizes at the interaction point have to be achieved
What is needed to reach high luminosity?
Before going to the world of beam dynamic, let’s have a look at the ILC
Dyx
repb HfNn
L4
:Luminosity2
factort enhancemen beam-Beam
beam)(gaussian size Transverse
rate Repetition bunchper Particles
nbunch/trai ofNumber where
,
D
yx
rep
b
H
fNn
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Freddy Poirier, 12/06/06, DESY student seminar.
Layout of the ILC
500 GeV
Upgraded energy (~1TeV)
Long straight sections (e-/e+)
Nominal:nm 5 ,500, yx
10 km~31 km
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Freddy Poirier, 12/06/06, DESY student seminar.
Scheme of the ILC
Squeeze the beam as small as possible for High luminosity
5 nano m
Electron source
To produce electrons, light from a titanium-sapphire laser hit a target and knock out electrons. The laser emits 2-ns "flashes," each creating billions of electrons. An electric field "sucks" each bunch of particles into a 250-meter-long linear accelerator that speeds up the particles to 5 GeV.
Damping Ring for electron beam
In the 6-kilometer-long damping ring, the electron bunches traverse a wiggler leading to a more uniform, compact spatial distribution of particles.
Each bunch spends roughly 0.2 sec in the ring, making about 10,000 turns before being kicked out. Exiting the damping ring, the bunches are about 6 mm long and thinner than a human hair.
Main Linac
2 main linear accelerators, one for electrons and one for positrons, accelerate bunches of particles up to 250 GeV with 8000 superconducting cavities nestled within cryomodules. The modules use liquid helium to cool the cavities to - 2°K. Two ~10-km-long tunnel segments, house the two accelerators. An adjacent tunnel provides space for support instrumentation, allowing for the maintenance of equipment while the accelerator is running.
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Freddy Poirier, 12/06/06, DESY student seminar.
Beam Dynamic
• Beam dynamic is the study of the evolution of the beam through the various sections:– Here we’ll look at the beam dynamic in the linear
accelerator section.• i.e. after the Bunch compressor and before the Beam
Delivery System (BDS)
– The accelerator section is part of the LET (Low Emittance Transport):
• The goal of game here is to accelerate the beam from 15 GeV up to 250 GeV (for center of mass energy of 500 GeV)
• Keep the emittance growth as low as possible
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Freddy Poirier, 12/06/06, DESY student seminar.
Lattice• The lattice is a series of components (periodic arrangement) in the
beam line– It is constituted mainly of
• Magnets (quadrupoles, dipoles,…)• Accelerating cavities - SuperConducting Radio Frequency (SCRF)• Diagnostic Systems
– The most basic repetitive sequence of components is called a FODO cell (focusing and defocusing quadrupole interspaced with drift space)
1 FODO cell
Trajectory of an individual electron in the FODO lattice.•The magnetic lattice is periodic (2d)
•The pseudo-sinusoidal motion is referred to as the Betatron oscillation.
•The phase advance per FODO cell period is here µ=π.
x
QF QD
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Freddy Poirier, 12/06/06, DESY student seminar.
Betatron Oscillation• Property of the focusing arrangement
phase advance variation
• The betatron oscillation are e.g. dependent on the strength of the quadrupole, (independently) for x and y:
y
x
Nominal focus. Quad. strength |k0|= 0.0524 m-2
Changed to |k1|= 0.0624 m-2
k0
k1
QD QF
)(/1)(' ss
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Freddy Poirier, 12/06/06, DESY student seminar.
Motion• From the equation of motion (Hill’s equa.):
Where K(S) is the quadrupole strength and is periodic i.e. K(S)=K(S+2d) One can get the solution in the form (Floquet’s theorem):
0)('' xsKx
))(cos()( 0 ssx xx
Emittance: (initial condition)
Beta amplitude: periodic(dependant on focusing strength)
Initial phase
Phase advance (dependant on focusing strength)
And get the differentiate along the beam axis:
))(sin()( 0
ssds
dxxx
x
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Freddy Poirier, 12/06/06, DESY student seminar.
Emittance• To talk to an accelerator physicist, talk in
phase-space diagram (x’ vs x):– 1 particle travelling along the linac will
describe in x’,x plane an ellipse (approx.)
– Now we are not dealing with 1 particle but with a bunch of them.
– At 1 location, x’,x plane:– All particles travelling will form an elliptical
surface on the plane.– The ellipse envelope is a characteristic of
the quality of the beam (it encompasses 95% particles). It is called the emittance .
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Freddy Poirier, 12/06/06, DESY student seminar.
Beam Size• The Beam size is computed with
Luminosity is then defined (gaussian beam) by:
yxyxyx ,,,
D
yyxx
repb HfNn
L****
2
4
Defined by focussing arrangement at IP
Quality of the beam at IP and dependent of emittance prior to IP
The challenge with the (normalised) emittance is that along a transport line it can only get worse.
Dispersion not included
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Freddy Poirier, 12/06/06, DESY student seminar.
Degradation of emittance
• In a linac the emittance will inevitably degrade due to:– Synchrotron Radiation– Collective effects
• Wakefields– Residual gas scattering– Accelerator errors:
Wakefields• Passage of charged particle beams induce
electromagnetic field in RF cavities and other structures in accelerator.
• These wakefields act back on the beams and may cause instabilities– Long range Wakes: acts on following beam– Short range W: head of bunch acts on its tail
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Freddy Poirier, 12/06/06, DESY student seminar.
Wakefields
• Bunch ‘current’ generates wake that decelerates trailing bunches.
• Bunch current generates transverse deflecting modes when bunches are not on cavity axis
• Fields build up resonantly: latter bunches are kicked transversely
tb
bunch
0 km 5 km 10 km
head
head
headtailtail
tail
accelerator axis
cavities
y
tail performsoscillation
Long range:
Short range:When bunch is offset wrt cavity axis, transverse (dipole) wake is excited.
Wt α a-3.5
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Freddy Poirier, 12/06/06, DESY student seminar.
Effect of misalignment
Multibunch emittance growth for cavities with 500m RMS misalignment
The misalignements contribute largerly into the emittance growth along the linac.
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Freddy Poirier, 12/06/06, DESY student seminar.
A challengeRMS random misalignments to produce 5%
vertical emittance growthBPM offsets 11 mRF cavity offsets 300 mRF cavity tilts 240 rad
• Impossible to achieve with conventional mechanical alignment and survey techniques
• Typical ‘installation’ tolerance: 300 m RMS– On BPM this would imply an emittance growth of
3800%• At Beginning of linac y=20 nm.rad• At IP y=~40 nm.rad
Beam Based Alignment is crucial
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Freddy Poirier, 12/06/06, DESY student seminar.
Beam Based Alignment• Alignment performed on the beam using
the beam itself.• It involves steerers and BPM (measure beam centroid position)
• BACK to BASIC:
YjYi
Ki
gij 1. A particle arriving non centered on a quad will get a kick.
2. Betatron oscillation surimposed
3. Emittance grows
1
j
j ij i i ji
y g K Y Y
yj
Standard notation used: i.e. focusing for x, but defocusing of y
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Freddy Poirier, 12/06/06, DESY student seminar.
A BBA solution? 1-to-1 steering
• To limitate the kick one could think of a solution:
simply apply one to one steering to orbit: i.e. at each BPM zeroing the orbit with a steerer such that the bunch centroid is in the central axis of the quad.
steererquad mover
dipole corrector
Assuming:
-A BPM adjacent to each quad,
-A ‘steerer at each quad
BPM
1-2-1 corrected orbit But BPM are offset wrt quad.
Dispersion are increased(Particle with different energy will undergo a different angle in electromagnetic field)
Emittance grows
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Freddy Poirier, 12/06/06, DESY student seminar.
BBA - DFS
• Dispersion Free Steering (DFS)– Measure beam orbit (BPM) for a beam at E0– Measure beam orbit for beam(s) at other
energies– Find a set of steerer settings which minimise
the orbit difference. (for the case of curved linac: minimize wrt to the designed orbit difference)
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Freddy Poirier, 12/06/06, DESY student seminar.
BBA (2)• An exemple of results for the BBA:
Good result for DFS technique.
- Benchmarking of the various DFS algorithm are being done
- Dynamic effect of ground motion not included
DFS lower emittance growth
Nor
mal
ized
em
ittan
ce (m
.rad)
DFS along linacWith 2 beams
No position jitter
Energy (GeV)
Transverse Quadrupole 300 µm Wrt to CM axis
Rotation Quadrupole 300 µrad
Transverse BPM Alignment error 200 µm CM
Transverse RF Structure 300 µm CM
Rotation RF Structure 300 µrad CM
Cryomodule Offset 200 µm Accel. Ref
BPM Resolution 5 µm (10 µm in TDR)
With jitter
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Freddy Poirier, 12/06/06, DESY student seminar.
BBA(3)
• BBA when?– In general following a startup, or at regular
intervals (DFS for SLC: monthly basis)
– this process takes time; during which the machine is not integrating luminosity (TT)
– typically takes ~ 100 pulses per focusing magnet; with ~5 different energies.