Lecture 5 Beam Dynamics Issues and ILC Design
Transcript of 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
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 3
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
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 4
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
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 5
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
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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
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 7
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
=∝
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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
∝
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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∝
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 10
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 ∝∝
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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 ≈
2005 ILC School - Lecture 5Beijing, 19 July 2005Carlo Pagani 12
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∝
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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−
∝
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Machine Overview - 1
Electron Source Positron Source
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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)
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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
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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
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Machine Overview - 2
Damping Rings Beam Delivery System (BDS)
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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)( −−+=
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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.
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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
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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)
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DR Design Approaches: Example # 1
The TESLA TDR lattice
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DR Design Approaches: Example # 2
The FNAL 6 km Lattice
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DR Design Approaches: Example # 3
The KEK 3 km Lattice
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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
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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!
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• 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
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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]
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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
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ILC Possibilities
33km
47 k
m
US Options Study (2003)500 GeV (1.3 TeV)
TESLA TDR (2001)500 GeV (800 GeV)
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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
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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
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Ground motion spectra
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Reliability / Operability
A major issue for ILC – needs much more workCurrent state-of-the-art is Tom Himel study for USCWO
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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
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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
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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.
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Start of the Global Design Initiative
~ 220 participants from 3 regionsmost of them accelerator experts
Next Meeting at SnowmassAugust 14th, 2005
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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
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Project Timelines
2006 2007 2008 2015
CDRTDR
GDE process
constructioncommissioning
physics
EUROTeV
preparation
2010 2012
constructionoperation
2005
CARE
EURO XFEL
ILC
UK LC-ABD
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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