M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016 Challenges of...

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016

Challenges of modern e+e- colliders

Michael Koratzinos, University of Geneva


Contents

• Figures of merit• The physics landscape• Challenges of circular colliders• conclusions

2


The butterfly plot• Figure of merit for all accelerators: energy vs

luminosity

3

CM Energy

Lum

inos

ity

FCC-ee

CEPC

ILC

CLICcircular

linear


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

4


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?

5


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:

6

19261967

Robert Goddard


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

7


The circular e+e- collider approach

• What kind of luminosities can be achieved?• How big a ring needs to be?• How much power will it consume?

8

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

injectorBooster ring

Main ringA. 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


Luminosity of a circular lepton collider

9

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 , and the hourglass effect, a geometrical factor (which is a function of σz and )

ℒ=6.0×1034( 𝑃 𝑡𝑜𝑡

50𝑀𝑊 )( 𝜌10𝑘𝑚 )(120𝐺𝑒𝑉

𝐸0 )3( 𝜉 𝑦

0.1 )( 𝑅h𝑔

0.83 )( 1𝑚𝑚𝛽 𝑦∗ )𝑐𝑚− 2𝑠− 1


The beam-beam parameter

• 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…

10

𝜉 𝑦=𝑁𝑏𝑟𝑒 𝛽𝑦

∗

2𝜋𝛾𝜎 𝑥𝜎 𝑦


Beamstrahlung• Beamstrahlung is a phenomenon that affects future, very-

high-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)• Beam lifetime increases with i.e it depends on the

momentum acceptance , the beam sizes in x and z (but not in y!) and the electron bunch population

11


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

12

100 110 120 130 140 150 160 170 1800.000

0.050

0.100

0.150

0.200

0.250

0.300

Beam-beamBeamstrahlung (lifetime=300s)Power (Beamstrahlung (lifetime=300s))

beam energy (GeV)

verti

cal b

eam

_bea

m p

aram

eter

Parameters of FCC-ee-175

allowed


Can we do better?

• Using the crab waist scheme we can gain substantially wrt the beam-beam limit

13

z

e+ e-

xβy

P. Raimondi, 2006

2

tgx

z – Piwinski angle, should be >> 1

Colliding at an angle, with long beams suppresses instabilities (in other words the machine can operate at larger beam-beam values)

Beam-beam

Beamstrahlung

Beam energy

y

allowed

CW


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!)

14

Mainly from coupling Mainly from dispersion


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

15

FCC-ee

LEP2Emittances of past and future machines


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.

16


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

17


Power consumption table

CEPC(1) TLEP(2)

RF 250 180

Cryogenics 20 30

Power converters 90 20

Rest (cooling, ventilation, general services)

130 90

total 500MW 310MW

18

(1) W. Chou, Future Circular Colliders and R&D, EPS-HEP ConferenceJuly 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

For 100MW beam power:

FCC-ee: no official value released yet

http://arxiv.org/abs/arXiv:1308.2629

http://arxiv.org/abs/arXiv:1305.6498


RF power consumption

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

19

wall plugAC/DC power

converter RF power source useable RF

beam

loss

lossloss

Φ & loss

Modulator η≈ 93%

Klystron saturation η ≈ 64%IOT η ≈ 65%

overhead for LLRF, Qo, Qext, HOM power, power distribution,…

~50% of wall plug power


Energy per Higgs particle produced

complex power on/ off (MW)

Energy per year (TWh)

luminosity (cm-2s-1)

integrated Luminosity/y fb-1 (2 IPs)

# higgs per year (2IPs)

Energy/higgs (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

20

CERN electricity price: ~50CHF per MWh

It is always more efficient to run at full power (and for shorter period)


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

21


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

22


Detector

The interaction region

23

Main detector solenoid

Quad screening solenoid

Compensating solenoid

Final quads

An artist’s impression of the forward region around the IP

e+ e-


A promising solution

24

• 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)


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

25


First piece of hardware of FCC-ee at CERN

26

• 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 longCAD drawing

Magnet ready to be wound


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

27


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

28

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, 18-21 January 2016 29

End Thank you

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