1. ANKA Seminar
Ultra-low emittance for the CLIC damping rings using super-conducting wigglers
Yannis PAPAPHILIPPOU
October 8th, 2007

2. ANKA Seminar, Y. Papaphilippou
Outline
Overview of the CLIC Project
CLIC damping rings design
Design goals and challenges
Energy
Lattice choice and optics optimisation
Circumference
Wiggler design and parameter scan
Final emittances including Intra-beam scattering
Chromaticity correction and dynamic aperture
Low emittance tuning in the presence of coupling
Summary and open issues
M. Korostelev
(PhD thesis, EPFL 2006)
08/10/2007
ANKA Seminar, Y. Papaphilippou

3. ANKA Seminar, Y. Papaphilippou
The CLIC Project
Compact Linear Collider : multi-TeV electron-positron collider for high energy physics beyond today's particle accelerators
Center-of-mass energy from 0.5 to 3 TeV
RF gradient and frequencies are very high
100 MV/m in room temperature accelerating structures at 12 GHz
Two-beam-acceleration concept
High current “drive” beam, decelerated in special power extraction structures (PETS) , generates RF power for main beam.
Challenges:
Efficient generation of drive beam
PETS generating the required power
12 GHz RF structures for the required gradient
Generation/preservation of small emittance beam
Focusing to nanometer beam size
Precise alignment of the different components
08/10/2007
ANKA Seminar, Y. Papaphilippou

4. Base line configuration
Injector complex
DC gun
Unpolarized e-
3 TeV
Base line configuration
Laser
Polarized e-
Pre-injector
Linac for e-
200 MeV
e-/e+
Target
Linac for e+
Primary beam
2 GeV
Injector Linac
2.2 GeV
e+ DR
e+ PDR
2.424 GeV
360 m
Booster Linac
6.6 GeV
3 GHz
e+ BC1
e- BC1
e+ BC2
e- BC2
e+ Main Linac
e- Main Linac
12 GHz, 100 MV/m, 21 km
1.5 GHz
e- DR
e- PDR
162 MV
12 GHz
2.3 GV
9 GeV
48 km maximum
 30 m
 10 m
 360 m
 150 m
 15 m
L. Rinolfi

5. Damping ring design goals
Ultra-low emittance and high beam polarisation impossible to be produced by conventional particle source:
Ring to damp the beam size to desired values through synchrotron radiation
Intra-beam scattering due to high bunch current blows-up the beam
Equilibrium “IBS dominated” emittance should be reached fast to match collider high repetition rate
Other collective effects (e.g. e--cloud) may increase beam losses
Starting parameter dictated by design criteria of the collider (e.g. luminosity), injected beam characteristics or compatibility with the downstream system parameters (e.g. bunch compressors)
PARAMETER
NLC
CLIC
bunch population (109)
7.5
4.1
bunch spacing [ns]
1.4
0.5
number of bunches/train
192
316
number of trains 3 1
Repetition rate [Hz]
120 50
Extracted hor. normalized emittance [nm]
2370
<680
Extracted ver. normalized emittance [nm]
<30
< 20
Extracted long. normalized emittance [eV m]
10890
<5000
Injected hor. normalized emittance [μm]
150 63
Injected ver. normalized emittance [μm]
1.5
Injected long. normalized emittance [keV m]
13.18
1240

6. ANKA Seminar, Y. Papaphilippou
Ring energy
Choice dictated by spin tune (half integer) for maintaining high-spin polarisation
Frozen on early design stage
Advantage of lower energies:
For same equilibrium emittance
i.e. smaller circumference and radiated power (cost), high momentum compaction (longitudinal stability).
Advantages of higher energy
For fixed damping fraction due to wigglers and wiggler peak field,
i.e. easier magnetic design (lower main field) and smaller total wiggler length
08/10/2007
ANKA Seminar, Y. Papaphilippou

7. Lattice choice
Usually racetrack configuration with Theoretical Minimum Emittance (TME) arcs and damping wigglers in the straights
NLC DR was based on lattice with 32 TME arc cells and wigglers of 62m total length
(A. Wolski et al. 2003)
ILC has a large ring of more than 6km for accepting large number of bunches with reduced e-cloud effect
TME and FODO lattice considered
(A. Wolski et al. 2007)
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8. CLIC damping ring layout
08/10/2007
ANKA Seminar, Y. Papaphilippou

9. TME arc cell
TME cell chosen for compactness and efficient emittance minimisation over Multiple Bend Structures (or achromats) used in light sources
Large phase advance necessary to achieve optimum equilibrium emittance
Very low dispersion
Strong sextupoles needed to correct chromaticity
Impact in dynamic aperture

10. ANKA Seminar, Y. Papaphilippou
Phase advance choice
Optimum horizontal phase advance of cells for minimising zero current emittance is fixed (284o for TME cells)
Vertical phase advance is almost a free parameter
First iteration based on lattice considerations, i.e. comfortable beta functions and relaxed quadrupole strengths and chromaticity
Low horizontal phase advance gives increased momentum compaction factor (high dispersion) but also chromaticity
08/10/2007
ANKA Seminar, Y. Papaphilippou

11. ANKA Seminar, Y. Papaphilippou
Phase advance with IBS
Horizontal phase advance for minimum horizontal emittance with IBS, is found in an area of small horizontal beta and moderate dispersion functions (between π, for CLIC damping rings)
Optimal vertical phase advance quite low (0.2π)
The lowest longitudinal emittance is achieved for high horizontal and low vertical phase advances
The optimal point has to be compromised due to chromaticity considerations and dynamic aperture optimisation
08/10/2007
ANKA Seminar, Y. Papaphilippou 11

12. ANKA Seminar, Y. Papaphilippou
Circumference
Usually chosen big enough to accommodate number of bunches
Drift space increase essential for establishing realistic lattice, reserving enough space for instrumentation and other equipment
For constant number of dipoles (TME cells), zero equilibrium emittance is independent of circumference
Normalised emittance with IBS increases with circumference (no wigglers)
When dipole lengths increase with drifts, emittance grows due to increase of damping time (inversely proportional to radiation integral I2 which decreases with length)
When only drifts increase, smaller emittance growth due to increase of optics functions
Impact on chromaticity + dynamic aperture
Compensation may be achieved due to increase of bunch length with circumference (momentum compaction)
Drifts + dipoles
Only Drifts
Drifts + dipoles
Only Drifts
08/10/2007
ANKA Seminar, Y. Papaphilippou

13. ANKA Seminar, Y. Papaphilippou
Damping wigglers
Damping wigglers are used to increase radiation damping and reduce the effect of IBS in order to reach target emittances
The total length of wigglers is chosen by its dependence with the peak wiggler field and relative damping factor
Damping factor increases for higher fields and longer wiggler occupied straight section
Relative momentum spread is independent of total length but increases with wiggler field
08/10/2007
ANKA Seminar, Y. Papaphilippou

14. Wigglers effect in emittance
For fixed value of wiggler period, equilibrium emittance minimum for particular value of wiggler field
By reducing total length, optimal values necessitate higher fields and lower wiggler periods
Optimum values change when IBS included, necessitating higher fields
Damping rings cannot reach 450nm with normal conducting wigglers
08/10/2007
ANKA Seminar, Y. Papaphilippou

15. Wigglers’ effect with IBS
For higher wiggler field and smaller period the transverse emittance computed with IBS gets smaller
The longitudinal emittance has a different optimum but it can be controlled with the RF voltage The choice of the wiggler parameters is finally dictated by their technological feasibility
ANKA SC
wiggler
BINP SC
BINP PM
The choice of the wiggler parameters is finally dictated by their technological feasibility.
Normal conducting wiggler of 1.7T can be extrapolated by existing designs
Super-conducting options have to designed, built and tested

16. ANKA Seminar, Y. Papaphilippou
Wiggler prototypes
Parameters
BINP
ANKA
Bpeak [T]
2.5
2.7
λW [mm] 50 21
Beam aperture full height [mm] 12 5
Conductor type
NbTi
NbSn3
Operating temperature [K]
4.2
Two wiggler prototypes
2.5Τ, 5cm period, built by BINP
2.7Τ, 2.1cm period, built by ANKA
Aperture reduced for the more challenging design
Current density can be increased by using different conductor type
Short version to be installed and tested at ANKA (energy of 2.5GeV)
Lifetime of 8-10h for lower gap, enough for the beam tests
08/10/2007
ANKA Seminar, Y. Papaphilippou

17. RF voltage and frequency
The smallest transverse emittance is achieved for the lowest RF frequency and higher voltage, while keeping the longitudinal emittance below 5000 eV.m
Reversely the longitudinal emittance is increased for small RF frequency
08/10/2007
ANKA Seminar, Y. Papaphilippou

18. Wiggler FODO cell
Average horizontal β function should be small enough for the wiggler period not to exceed the value producing efficient damping
FODO cell structure chosen with phase advances close to 90o giving average β’s of around 4m and reasonable chromaticity
Quad strength adjusted to cancel wiggler induced tune-shift 18

19. Non-linear dynamics Two sextupole schemes considered
Two families / 9 families of sextupoles
Dynamic aperture is 9σx in the horizontal and 14σy in the vertical plane (comfortable for injection)
Wiggler effect should be included and optimised during the design phase

20. Coupling correction
Coupling effect of wigglers should be included in simulations
Correction with dispersion free steering (orbit and dispersion correction)
Skew quadrupole correctors for correcting dispersion in the arc and emittance minimisation
Iteration of dynamic aperture evaluation and optimisation after correction
In CLIC damping rings, the effect of vertical dispersion is dominant (0.1% of coupling and 0.25μm of dispersion invariant)

21. ANKA Seminar, Y. Papaphilippou
Bunch charge
Approximate scaling laws can be derived for a given damping ring design
For example, for the CLIC damping rings, the horizontal normalized emittance scales approximately as
The above relationship is even more exact when the longitudinal emittance is kept constant (around 5000 eV.m, in the case of the CLIC damping rings)
Vertical and longitudinal emittance are weakly dependent on bunch charge, and almost linear with each other
08/10/2007
ANKA Seminar, Y. Papaphilippou 21 21

22. Damping rings’ parameters
2005: original ring
2006a: super-conducting wiggler considered
2006b: vertical dispersion included
2007a: 12GHz structure
2007b: reduced bunch population
2007c: CLIC_G structure
08/10/2007
CLIC PWG Y. Papaphilippou
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23. ANKA Seminar, Y. Papaphilippou
Concluding remarks
Robust design of the CLIC damping rings, delivering target emittance with the help of super-conducting wigglers
Prototype to be built and tested in ANKA
Areas needing further optimisation
Pre-damping ring optics design
Collective effects including electron cloud
Realistic cell length and magnet design
Sextupole optimisation and non-linear dynamics including wiggler field errors
08/10/2007
ANKA Seminar, Y. Papaphilippou