M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Challenges of modern e+e- colliders Michael Koratzinos, University of Geneva

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Contents Figures of merit The physics landscape Challenges of circular colliders conclusions 2

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 The butterfly plot Figure of merit for all accelerators: energy vs luminosity 3 CM Energy Luminosity FCC- ee CEPC ILC CLIC circular linear

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 21 st century we should move on to a different concept? 5

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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: Robert Goddard

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 injector Booster ring Main ring 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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Luminosity of a circular lepton collider 9

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Beamstrahlung 11

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 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 Parameters of FCC-ee-175 allowed

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Can we do better? Using the crab waist scheme we can gain substantially wrt the beam-beam limit 13 z e+e+ e-e- x βyβy  P. Raimondi, 2006 – 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y allowed CW

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Vertical emittance 14 Mainly from coupling Mainly from dispersion

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 LEP2 Emittances of past and future machines

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Challenge number 3: power consumption These machines are very power hungry ( MW 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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Power consumption table CEPC (1) TLEP (2) RF Cryogenics2030 Power converters9020 Rest (cooling, ventilation, general services) total500MW310MW 18 (1) W. Chou, Future Circular Colliders and R&D, EPS-HEP Conference July 22-29, 2015, Vienna, Austria (2) TLEP power consumption in arXiv: [physics.acc-ph] and arXiv: [physics.acc-ph]arXiv: arXiv: A big chunk is RF power consumption For 100MW beam power: FCC-ee: no official value released yet

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 plug AC/DC power converter RF power source useable RF beam loss Φ & loss Modulator η≈ 93% Klystron saturation η ≈ 64% IOT η ≈ 65% overhead for LLRF, Q o, Q ext, HOM power, power distribution,… ~50% of wall plug power

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Energy per Higgs particle produced complex power on/ off (MW) Energy per year (TWh) luminosity (cm -2 s -1 ) integrated Luminosity/y fb -1 (2 IPs) # higgs per year (2IPs) Energy/ higgs (MWh) TLEP full power310/ E E E TLEP 50% power200/ E E E CEPC full power500/ E E E CEPC 50% power310/ E E E CERN electricity price: ~50CHF per MWh It is always more efficient to run at full power (and for shorter period)

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

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 Detector The interaction region 23 Main detector solenoid Quad screening solenoid Compensati ng solenoid Final Final quads An artist’s impression of the forward region around the IP e+e+ e-e-

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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)

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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 4mm 2 ) 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, January 2016 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

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January 2016 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 21 st century 28

M. Koratzinos, HKUST Jockey Club Institute of Advanced Study, January End Thank you