lecture 5 beam dynamics issues and ilc design

Lecture 5

Beam Dynamics Issues and ILC Design

Carlo PaganiINFN Milano and DESY

On leave from University of Milano

2005 International Workshop-Summer Schoolon physics, detector and accelerator at the linear collider

July 15-20, 2005

Center for High Energy PhysicsTsinghua University, Beijing 100084, China

2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 2

TESLA 500 GeV ParametersFrom the TESLA TDR


The Luminosity Issue

Collider Luminosity [cm-2 s-1] isapproximately given by

where:

Nb = bunches / trainN = particles per bunchfrep = repetition frequencyA = beam cross-section at IPHD = beam-beam enhancement factor

Drepb H

AfNn

L2

=

Dyx

repb HfNn

Lσπσ4

2

=

For Gaussian beam distribution:

Introducing the center of mass energy, Ecmand the RF to beam power efficiency, ηRF RFbeamRFbeamcmrepb PPENfn →== η

Dcmyx

RFRF HENPL

σπση

4=We get

i.e. for a given Ecm theLuminosity is proportional to the RF power


Luminosity Issue: intense beams at IP

choice of linac technology:• efficiency• available power

Beam-Beam effects:• beamstrahlung• disruptionStrong focusing• optical aberrations• stability issues and

tolerances

Dyx

RFRFcm

HN)P(E

L

=

σση

π41

LEP: σxσy ≈ 130×6 µm2

ILC: σxσy ≈ (200-500)×(3-5) nm2

Beam size comparison at the Interaction Point


Luminosity Issue: Beam-Beam - 1

strong mutual focusing of beams (pinch) gives rise to luminosity enhancement HD

As e ± pass through intense field of opposing beam, they radiate hard photons [beamstrahlung] and loose energy Flat Beam

Interaction of beamstrahlung photons with intense field causes copious e +e − pair production [background]

- 6 - 4 - 2 0 2 4 6- 3000

- 2000

- 1000

0

1000

2000

3000

E y(M

V/cm

)

y/σy

σx » σy


Luminosity Issue: Beam-Beam - 2 see lecture 2 on beam-beam

beam-beam characterised by Disruption Parameter:

Enhancement factor (typically HD ~ 2) is given by:

In a LC, hence

for storage rings, andzbeamf σ>> 1<<y,xD

beam

z

yxy,x

zey,x f)(

NrD σσσγσ

σ≈

+=

2 σz = bunch length,

fbeam = focal length of beam-lens

2010 ÷≈y,xD zbeamf σ<

( )

++

++=

z

yxyx

yx

yxyxyDx D

DD

DHσβ .

,3,

3,4/1

,,

8.0ln21ln

11


Luminosity Issue: Beamstrahlung see lecture 2 on beam-beam

rms relative energy loss induced by Beamstrahlung

we would like to make (σxσy) small to maximise luminosity

and keep (σx+σy) large to reduce δSB

Rule:

make σx large to limit δSB to few % for background

make σy as small as possible to achieve high luminosity.

( )2

2

20

3

286.0

yxz

cmeBS

NEcm

erσσσ

δ+

=

Trick: use “flat beams” with σx >> σy 2

2

xz

cmBS

NEσσ

δ

=∝


Luminosity Issue: Beamstrahlung

Returning to our L scaling law, and ignoring HD

From flat-beam beamstrahlung

hence

yxcm

RFRF NEPL

σση 1

∝

y

zBS

cm

RFRF

EPL

σσδη

2/3∝

cm

zBS

x EN σδσ

∝


Luminosity Issue: story so far

high RF-beam conversion efficiency ηRF

high RF power PRF

small vertical beam size σy

large bunch length σz (will come back to this one)

could also allow higher beamstrahlung δBS if willing to live with the consequences

For high Luminosity we need:

Next question: how to make a small σy

y

zBS

cm

RFRF

EPL

σσδη

2/3∝


Luminosity Issue: A final scaling law?

where εn,y is the normalised vertical emittance, and βy is the verticalβ-function at the IP. Substituting:

hour glass constraint

βy is the same ‘depth of focus’ β for hour-glass effect. Hence zyβ σ≥

y

zBS

cm

RFRF

EPL

σσδη

2/3∝γεβ

σ ynyy

,=

y

z

yn

BS

cm

RFRF

y

z

yn

BS

cm

RFRF

EP

EPL

βσ

εδη

βσ

εγδη

,,2/3 ∝∝


Luminosity Issue: A final scaling law?

high RF-beam conversion efficiency ηRFhigh RF power PRFsmall normalised vertical emittance εn,ystrong focusing at IP (small βy and hence small σz)could also allow higher beamstrahlung δBS if willing to live with the consequences

Above result is for the low beamstrahlung regime where δBS ~ few %Slightly different result for high beamstrahlung regime

Dyn

BS

cm

RFRF HEPL

,εδη

∝ zy σβ ≈


Luminosity as a function of βy

200 400 600 800 1000

1´ 1034

2´ 1034

3´ 1034

4´ 1034

5´ 1034

300z mσ µ=

100z mσ µ=

500 mµ

700 mµ

900 mµ

( )y mβ µ

2 1( )L cm s− −

2

4bx y

n N fL πσ σ=

1BS z

δ σ∝


Transverse Wakes: The Emittance Killer!

Bunch current also generates transverse deflecting modes when bunches are not on cavity axis

Fields build up resonantly: latter bunches are kicked transversely

⇒ multi- and single-bunch beam breakup (MBBU, SBBU)

bunch

0 km 5 km 10 km

head

head

headtailtail

tail

accelerator axis

cavities

∆y

tail performsoscillation

∆tb

Wake Fields in a TW structure

Alignment tolerance δYRMSdetermines the emittance growLow frequency is preferred: For a given ∆ε , δYRMS scales as

βδ acc

RMSE

NfY

3−

∝


Machine Overview - 1

Electron Source Positron Source


Electron Sources

TDR design has two polarised RF guns

120 kV

electrons

laser photons

GaAscathode

λ = 840 nm

20 mm

• laser-driven photo injector• circ. polarised photons on

GaAs cathode → long. polarised e-

• laser pulse modulated to give required time structure

• very high vacuum requirements for GaAs (<10-11 mbar)

• beam quality is dominated by space charge(note v ~ 0.2c)


Positron Source

• Photons (γ) produced in undulator by the high energy electron beam upstream of BDS and IR

• Option for polarised e+ with s.c. helical undulator

• Thin target converts γ to positrons

• High energy electrons ( > 150 GeV) required for positron beam


Positron Source

Advantages

significantly reduced power deposition in thin target (~5 kW)smaller emittance beam produced

– less multiple coulomb scattering– reduced acceptance requirements

for DR• no pre-DR foreseen

much cheaper / less complex than equivalent ‘conventional source’ for TESLANaturally allows upgrade topolarised e+ source

Disadvantages

Requires e-linac with ≥150 GeV– TDR solution to use main e-

linac– coupling e- to e+ production

raises questions of• operability• reliability• commissioning strategy

Never been done before– although physics is well

understood!– E166 experiment at SLAC


Machine Overview - 2

Damping Rings Beam Delivery System (BDS)


Damping Rings

(storage) ring in which the bunch train is stored for Tstore ~20-200 msemittances are reduced via the interplay of synchrotron radiation and RF acceleration

final emittanceequilibriumemittance

initial emittance(~0.01m for e+)

damping time

see lecture 5

DTeqieqf e τεεεε /2)( −−+=


Damping Rings

Need to compress 300 km (~1ms) bunch train into ring

Compression ratio (i.e. ring circumference) depends on speed of injection/extraction kicker.


TESLA TDR Damping Rings

TESLA bunch train 2820 × 337 ns = 950 ms⇒ 285 km long

Extract every bunch separately, bunch spacing given by shortest kicker rise/fall time

⇒20 ns × 2820 ≈ 56 ms ⇒ 17 km longSave tunnel cost: DR in main linac tunnel and short return arcs

⇒ dogbone

337 ns

40 ns

0.6 mrad ±0.05%0.01 Tm

Ripple:0.05%

• 2820 pulses with 3 MHz repetition rate• 5 Hz repetition rate of macro-pulse

rise 20nsτ ≤

Kicker Specs


Dogbone DR Concept

Need ~ 450m of wiggler for the required 28 ms damping time

– ∫B2dl= 605 T2m – Permanent Magnet Wiggler with

Bmax = 1.6 T, λ=0.4 m– Radiated Power (160 mA) over 450 m

: 3 MWTime varying stray fields at linac beam pulse could be an issue ( > 1 mT measured)


DR Design Approaches: Example # 1

The TESLA TDR lattice



The FNAL 6 km Lattice



The KEK 3 km Lattice


Bunch Compression

bunch length from ring ~ few mmrequired at IP 100-300 µm

RF

z

∆E/E

z

∆E/E

z

∆E/E

z

∆E/E

z

∆E/Elong.phasespace

dispersive section

see lecture 6


Beam Delivery System Functionality

Focus and collide nanobeams at the interaction point (IP)

Remove (collimate) the beam halo to reduce detector background

Provide beam diagnostics for the upstream machine (linac)

Each one of these is a challenge!


• 1st IP has no crossing angle

• 2nd (optional) IP has crossing angle of 34mrad for γ−γ option

• FFS not based on FFTB/SLC design (later reviewed)

Beam Delivery System


Focusing and Colliding Nanobeams

Correction of chromatic and geometric aberrations becomes principle design challengeA consequence: systems have extremely tight alignment (vibration) tolerances: stabilisation techniques a must!

xQS DKK /=

xS

QQS D

KK

ββ

=21

horizontaldispersion

final lens IP

geometric cancellation

geometric cancellation

xδ2 cancellation xQS DKK /

xS

QQS D

KK

ββ

=21

chromatic correction

Local correctionwith D’ at IP[Raimondi, 2000]

Non-local correction(CCS)[Brown, 1985]


Beam-beam kick

Long bunch train:

~ 3000 bunches

tb = 337 ns

Multiple feedbacksystems will be mandatory to maintain the nanobeams in collision

IP Fast (Orbit) Feedback


ILC Possibilities

33km

47 k

m

US Options Study (2003)500 GeV (1.3 TeV)

TESLA TDR (2001)500 GeV (800 GeV)


BDS Strawman Model

Discussion on angles between the Linacs was again hot:• Multi-TeV upgradeability argument is favoured by many• Small crossing angle is disfavoured by some


Luminosity Stability

Ground motion– vibration; slow drifts

Fast Intra-Train Feedback– beam-beam collision feedback

Effect of slow drifts– Importance of orbit control (BDS: critical)

High-Disruption Regime– beam-beam kink instability makes TESLA like ILC ‘sensitive’

Brinkmann, Napoly, Schulte, TESLA-01-16


Ground motion spectra


Reliability / Operability

A major issue for ILC – needs much more workCurrent state-of-the-art is Tom Himel study for USCWO


520

cm

190

cm440 cm

80 cm

90 c

m

30 cm

125

cm

210 cm

275

cm

Single Tunnel layout

Tunnel Layout as in the TDR

Reviewed version


LINAC tunnel housing

Two-tunnel (possible) optionklystrons/modulators(?)/LLRF/PS is Service Tunnel to allow access during operation (availability arguments).

450

cm

600 cm

950 cm

350 cm 315 cm

75 c

m

410

cm


Conclusions

The two major advantages of the COLD technology are: – The frequency– The high conversion efficiency

At the level of design, construction and qualification of a few complete accelerating modules, TESLA Collaboration did great.

Working prototypes of most of the subsystems have been developed and successfully tested.

Final ILC design must reconsider some of the “Historical” parameters, eventually finding a new optimization

Re-invent or just improve hot water is quite dangerous

Reliability and availability analysis set up by Tom Himel must be extended and used as a basis for design choices.


Start of the Global Design Initiative

~ 220 participants from 3 regionsmost of them accelerator experts

Next Meeting at SnowmassAugust 14th, 2005


The Global Design Effort GDE

3 Regional Design TeamsCentral Group with Director:

Barry Barish

Goal:Produce an internal full costed ILC Technical Design Report by 2008

EuropeanDesignGroup

USDesignGroup

Int.DesignGroup Asian

DesignGroup


Project Timelines

2006 2007 2008 2015

CDRTDR

GDE process

constructioncommissioning

physics

EUROTeV

preparation

2010 2012

constructionoperation

2005

CARE

EURO XFEL

ILC

UK LC-ABD


Final Message

ILC is a great opportunity for HEP

Physics expectations are great

The interest for the cold technology is enormous

As in the past, HEP can have a leading role in technology development for scientific and human applications

lecture 5 beam dynamics issues and ilc design

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