Ultra-low emittance for the CLIC damping rings using super-conducting wigglers

C L I CC L I C

Ultra-low emittance for the CLIC

damping rings using super-conducting

wigglersOctober 8th, 2007

Yannis PAPAPHILIPPOU

ANKA Seminar

C L I CC L I C

ANKA Seminar, Y. Papaphilippou

208/10/2007

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)

C L I CC L I C


308/10/2007

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

C L I CC L I C

4

Injector complex

DC gunUnpolarized e-

3 TeV

Base line configuration

LaserDC gunPolarized e-

Pre-injector Linac for e-

200 MeV

e-/e+ Target

Pre-injector Linac for e+

200 MeV

Primary beam Linac for e-

2 GeV

Inje

ctor

Lin

ac

2.2

GeV

e+ DR

e+ PDR

2.424 GeV360 m

Boo

ster

Lin

ac

6.6

GeV 3 GHz

e+ BC1 e- BC1

e+ BC2 e- BC2e+ Main Linac e- Main Linac

12 GHz, 100 MV/m, 21 km 12 GHz, 100 MV/m, 21 km

1.5 GHz

e- DR

e- PDR

1.5 GHz 1.5 GHz 1.5 GHz

3 GHz162 MV

3 GHz162 MV

12 GHz2.3 GV

12 GHz2.3 GV

9 GeV48 km maximum

30 m 30 m

10 m 10 m

360 m

150 m

15 m 15 m 150 m

2.424 GeV360 m

2.424 GeV 2.424 GeV

L. Rinolfi

C L I CC L I C

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] 150 1.5

Injected long. normalized emittance [keV m] 13.18 1240

C L I CC L I C


608/10/2007

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

0 5 10 150

1

2

3

4

5

6

7

n0 5 10 15

0

1

2

3

4

5

6

7

n0 5 10 15

0

1

2

3

4

5

6

7

Ene

rgy

[GeV

]

n0 5 10 15

0

1

2

3

4

5

6

7

n

E = E0 (n + 1/2)/

NLC DR

CLIC DR

ILC DR

C L I CC L I C

708/10/2007

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

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. 2003)

(A. Wolski et al. 2007)

C L I CC L I C


808/10/2007

CLIC damping ring layout

C L I CC L I C

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

C L I CC L I C


1008/10/2007

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

C L I CC L I C


1108/10/2007 11

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 1.2-1.3π, 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

C L I CC L I C


1208/10/2007

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

Only Drifts

Drifts + dipoles

C L I CC L I C


1308/10/2007

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

C L I CC L I C


1408/10/2007

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

C L I CC L I C

ANKA SCwiggler

BINP SCwiggler

BINP PMwiggler

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

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

C L I CC L I C


1608/10/2007

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

Parameters BINP

ANKA

Bpeak [T] 2.5 2.7λW [mm] 50 21Beam aperture full height [mm] 12 5

Conductor type NbTi NbSn3

Operating temperature [K] 4.2 4.2

Wiggler prototypes

C L I CC L I C


1708/10/2007

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

C L I CC L I C

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

Wiggler FODO cell

C L I CC L I C

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

C L I CC L I C

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)

C L I CC L I C


2108/10/2007 21

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

Bunch charge

C L I CC L I C


2208/10/2007 CLIC PWG Y. Papaphilippou 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

C L I CC L I C


2308/10/2007

Concluding remarks

Robust design of the CLIC damping rings, delivering target emittance with the help of super-conducting wigglersPrototype to be built and tested in ANKA

Areas needing further optimisationPre-damping ring optics designCollective effects including electron cloudRealistic cell length and magnet designSextupole optimisation and non-linear

dynamics including wiggler field errors

Documents

Ultra-low emittance for the CLIC damping rings using super-conducting wigglers