Challenges of modern
e+e- colliders
Michael Koratzinos, University of Geneva
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
Contents
•
•
•
•
Figures of merit
The physics landscape
Challenges of circular colliders
conclusions
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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The butterfly plot
• Figure of merit for all accelerators: energy vs
luminosity
Luminosity
FCCee
CEPC
CLIC
ILC
CM Energy
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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The physics landscape
• …is not a challenge. It is merely an input to
the discussion of if a machine should get the
go-ahead or not. And it is completely beyond
our control.
• For instance, if an exciting new object is found
or not with a mass of around 750GeV will
have a profound effect on which machine is
favoured for construction
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Challenge no. 1: the lack of challenge
• Arguably the most difficult challenge to my
mind.
• Synchrotrons have been around for a very
long time: Edwin McMillan designed the first
electron synchrotron in 1945 (university of
California). Energy: 350MeV
• …surely in the 21st century we should move on
to a different concept?
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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The case for e+e- synchrotrons as
flagship machines
• In history, many times progress is made by
incrementally improving a known concept,
simply making it bigger (and better)
• Best known example:
1967
1926
Robert Goddard
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Challenge number 2: luminosity
• e+e- colliders cannot compete with linear
colliders in terms of energy. But they can
compensate in terms of luminosity
• This necessitates operating the machine at a
new regime that was never reached before:
the beamstrahlung dominated regime
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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The circular e+e- collider approach
For the high luminosities aimed at, the beam lifetimes due to
natural physics processes (mainly radiative Bhabha scattering) are
of the order of a few minutes – the accelerator is ‘burning’ the
beams up very efficiently
A “top-up” scheme (a la B factories) is a must
Booster ring
Main ring
•
•
injector
A. Blondel
Booster ring the same size as main ring, tops up the main ring every ~O(10s)
Main ring does not ramp up or down
• What kind of luminosities can be achieved?
• How big a ring needs to be?
• How much power will it consume?
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Luminosity of a circular lepton
collider
𝑅ℎ𝑔
3 𝑚𝑒 𝑐 2 2
𝜌
ℒℒ== 𝑐𝑜𝑛𝑠𝑡 2× 𝑃𝑡𝑜𝑡 3 𝜉𝑦 ∗
8𝜋 𝑟𝑒
𝛽𝑦
𝐸0
The maximum luminosity is bound by the total power dissipated,
the maximum achievable beam-beam parameter, the bending
radius, the beam energy, the amount of vertical squeezing 𝛽𝑦∗ ,
and the hourglass effect, a geometrical factor (which is a
function of σz and 𝛽𝑦∗ )
ℒ = 6.0 × 1034
𝑃𝑡𝑜𝑡
50𝑀𝑊
𝜌
10𝑘𝑚
120𝐺𝑒𝑉
𝐸0
3
𝜉𝑦
0.1
𝑅ℎ𝑔
0.83
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
1𝑚𝑚
𝑐𝑚−2 𝑠 −1
∗
𝛽𝑦
9
The beam-beam parameter 𝜉
𝑁𝑏 𝑟𝑒 𝛽𝑦∗
𝜉𝑦 =
2𝜋𝛾𝜎𝑥 𝜎𝑦
• The beam-beam parameter (closely related to tune shift) is a
measure of the blow-up of one beam as it goes through the
other and has a maximum value on every implementation
• Increasing the beam current or squeezing more when the
beam-beam limit has been reached will not increase
luminosity
• The more damping in the machine (higher energy, smaller
radius) the higher the maximum beam-beam parameter
• The maximum beam-beam parameter has been a limit in the
performance of circular e+e- accelerators for the last 50
years…
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Beamstrahlung
• Beamstrahlung is a phenomenon that affects future, veryhigh-squeeze machines.
• A single hard photon exchange between an electron and the
collective electromagnetic field of the opposing bunch
changes the momentum of the electron. This can have two
adverse effects in a circular accelerator:
– The bunch length is increased (main effect at low beam energies)
– The electron can fall out of the momentum acceptance of the
machine and beam lifetime is affected
• (In a linear accelerator, beamstrahlung modifies the ECM
profile which is no longer monochromatic)
𝜎𝑥 𝜎𝑧
• Beam lifetime increases with 𝜂
i.e it depends on the
𝑁𝑏
momentum acceptance 𝜂, the beam sizes in x and z (but not
in y!) and the electron bunch population 𝑁𝑏
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Two limits for the beam-beam
parameter
• Putting the two limits together defines the performance of a circular accelerator
• At low energies the beam-beam parameter 𝜉 saturates at the beam-beam limit
(normal operation, for ways to circumvent this limit, see next slides)
• At high energies, the beamstrahlung limit arrives first
vertical beam_beam parameter
0.300
0.250
0.200
0.150
Beam-beam
0.100
Beamstrahlung
(lifetime=300s)
allowed
0.050
0.000
100
120
140
160
beam energy (GeV)
180
Parameters of
FCC-ee-175
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Can we do better?
• Using the crab waist scheme we can gain
substantially wrt the beam-beam limit
P. Raimondi, 2006
x
βy
e-
y
e+
Beamstrahlung

z

 z  
tg  – Piwinski angle, should be >> 1
x  2
Beam-beam
Beam energy
allowed
CW
Colliding at an angle, with long beams suppresses instabilities
(in other words the machine can operate at larger beambeam values)
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Vertical emittance
• Beamstrahlung does not depend on the
vertical beam size, but luminosity does.
• Achieving a very small vertical beam size is
beneficial for luminosity without aggravating
the beamstrahlung limit
• Minimizing vertical emittance minimizes
vertical beam size (also use a small a beta* as
possible!)
𝜀𝑦,𝑡𝑜𝑡𝑎𝑙 = 𝜀𝑦,𝑎𝑟𝑐𝑠 + 𝜀𝑦,𝐼𝑃𝑠
Mainly from coupling
Mainly from dispersion
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Vertical emittance II
• The goal for (vertical)
emittances is not lower
than future (or even
current) electron rings
• However, such low
emittances were never
achieved for a ring of the
size envisaged for FCC-ee
or CEPC
• This might prove to be a
formidable challenge, but
should ultimately be
achievable
Emittances of past and future machines
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
LEP2
FCC-ee
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Recap luminosity challenge
• Modern circular machines can be designed to
operate at the limit of physical bounds – maximizing
luminosity
• We can ‘circumvent’ the beam-beam limit that was
the limit of the previous generation e+e- colliders
(LEP), but the beamstrahlung limit remains a
challenge
• High momentum acceptance and low vertical
emittance is key to increasing the beamstrahlung
limit and a lot of effort should be put in this
direction.
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Challenge number 3: power consumption
• These machines are very power hungry (300-500MW for
100MW beam power)
• Luminosity and RF (beam) power are directly proportional
• The energy consumption is high (~1TWh per year, costing
~50MCHF at current CERN contract prices), but still
corresponds to less than 1% of the construction cost of the
facility per year
• But “energy costs might not be a true reflection of its value to
society”, so every effort should be made to reduce this number
• Largest consumer: RF system, where our efforts must be
concentrated
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Power consumption table
For 100MW beam power:
CEPC(1)
TLEP(2)
RF
250
180
Cryogenics
20
30
Power converters
90
20
Rest (cooling, ventilation,
general services)
130
90
total
500MW
310MW
FCC-ee: no official
value released yet
(1) W. Chou, Future Circular Colliders and R&D, EPS-HEP Conference
July 22-29, 2015, Vienna, Austria
(2) TLEP power consumption in arXiv:1308.2629 [physics.acc-ph] and
arXiv:1305.6498 [physics.acc-ph]
A big chunk is RF power consumption
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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RF power consumption
Modulator η≈ 93%
Klystron saturation η ≈ 64%
IOT η ≈ 65%
overhead for LLRF, Qo,
Qext, HOM power, power
distribution,…
loss
wall plug
AC/DC
power
converter
loss
RF power
source
loss
useable RF
beam
Φ & loss
~50% of
wall plug
power
One single efficiency that, if improved, would have the largest impact: RF
power source efficiency
• Klystron efficiency currently ~65%, R&D to take this to ~90%
• Other technologies: IOTs (inductive Output Tube), Solid state amplifiers
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Energy per Higgs particle produced
complex
integrated
Energy/
power on/ Energy per luminosity
# higgs per
Luminosity/y
higgs
off (MW) year (TWh) (cm-2s-1)
year
(2IPs)
fb-1 (2 IPs)
(MWh)
TLEP full power
310/90
1.4
1.1E+35
2.20E+03
4.40E+05
3.1
TLEP 50% power
200/90
1.1
5.5E+34
1.10E+03
2.20E+05
4.8
CEPC full power
500/120
2.1
2.0E+34
6.00E+02
1.21E+05
25.9
CEPC 50% power 310/120
1.5
1.0E+34
4.00E+02
8.08E+04
38.6
It is always more efficient to run at full power (and for shorter period)
CERN electricity price: ~50CHF per MWh
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Recap power consumption challenge
• Every effort should be made to increase
klystron efficiency from ~65% currently to
~90%
• CEPC seems much more conservative than
TLEP/FCC-ee when it comes to estimates of
power consumption
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Challenge no. 4: The interaction region
Is very complicated for the following reasons:
• Crab waist needs an opening angle of around
30mrad and two beam pipes
• The magnetic field of the experimental solenoid is
large (about 2T) and it is not in the direction of the
electrons – electrons experience a vertical kick that
gives rise to vertical emittance blow up
• L* is 2m, meaning that the final focus quads are
close together and inside the detector
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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The interaction region
e+
Detector
eFinal quads
Main
detector
solenoid
Quad
screening
solenoid
Compensati
ng solenoid
An artist’s impression of the forward region around the IP
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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A promising solution
• Various layouts tried, the following gives best performance: emittance
blow up of 0.11pm for two IPs
Solution comprises:
• Compensation solenoid (-2T)
• Anti-solenoid (-5T)
Incoming e+
Anti-solenoid
(-5T)
Compensation
solenoid (-2T)
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Final focus quadrupoles
•
We need:
– Excellent field quality (O(10-4))
– Very compact design
– Ability to compensate unwanted interference from nearby quadrupole
•
The solution: CCT (canted cosine theta) quadrupoles. Advantages:
–
–
–
–
Very good field quality
‘bespoke’ design – can be designed to compensate neighbouring quad
Fast prototyping: can be 3D printed
No iron
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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First piece of hardware of FCC-ee at
CERN
• Prototype FCC-ee final focus magnet
– 20cm length
• Will be wound with available NbTi
cable (cross section 4mm2)
• Fast prototyping: 3D printed in
‘bluestone’
• Real magnet will be ~3m long
CAD drawing
Magnet ready to be wound
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Final challenge: political
• Luminosity is (almost) proportional to machine
circumference
• The political and financial challenge is enormous:
• HEP is entering an era where discovery is not
guaranteed – this affects all types of machines, not
only circular ones
• To be entrusted with the funds, we need to either
instigate pride to a nation(s) or make them dream
• For the politicians we need to point out the collateral
benefits, highlight technology added value and make
sure industry that will benefit lobbies strongly for us
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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Summary
• Modern e+e- machines are based on proven
principles but push the envelope of design to the
limit. “Lack of challenge” is a fallacy.
• Highest luminosities can be achieved by running with
very low emittances, very high momentum
acceptance, high power (and large machine
circumference). All these represent formidable
challenges.
• It is up to our community to answer those challenges
and create the circular e+e- collider for the 21st
century
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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End
Thank you
M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016
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