Undulator based polarized positron source for Circular electron-positron colliders Wei Gai Tsinghua University/ANL a seminar for IHEP, 4/8/2015

Currently, two viable options CEPC FCC of CERN Positron production: conventional approach What we propose: undulator based approach, use the experience we gained from ILC and CLIC Compton ring.

CEPC lattice layout IP1 IP2 RF Critical parameters for CEPC: Circumference: 50 km SR power: 50 MW/beam 16*arcs 2*IPs 8 RF cavity sections (distributed) 6 straights (for injection and dump) Filling factor of the ring: ~80% Revolution time ~ 0.18 ms

Main Parameters Main Collider Booster Energy (GeV)12010~120 Circumference (Km)50 Bunch Number50 Emittance x/y (nm)6.8/0.0224/ Life time (min)30 Beam Current (mA)

Injection linac Challenge 1.N bunch e+ =2  nC/bunch e + 2.Polarization Main parameters ParameterSymbolUnitValue E - beam energyE e- GeV6 E + beam energyE e+ GeV6 Pulse widthΔtns0.7 Repetition ratef rep Hz100 E - bunch populationN e- 2×10 10 E + bunch populationN e+ 2×10 10 Energy spread (E + /E - )σEσE <1×10 -3

In general For 0.09 m-rad normalized emittance, （ 9mm-mrad at 5 GeV) the yield would be 1 for 4 GeV drive beam. For m-rad as required by, CEPC, the estimated yield would be ~ 0.1 (much less than 1). Drive beam needs to be

Is there an alternative solution? Based the ILC and CLIC studies, a combination of the undulator based and Compton ring based scheme could produce polarized positron while simplifying the CEPC overall configuration.

Injection Options Geometrical Arrangement Booster Main Collider 2 m

4 meter Helical undulators Either in booster or Main ring Target, capturing for positron and acceleration 6 GeV for both electron and positron Stacking as in CLIC design and with some damping damping Injection into booster ring at 6 GeV, (Tang made direction right) Overall layout of the Concept- Simpler than ILC and CLIC, much less demand ~ 50 km of main ring and booster

13 ILC TDR positron source location Photon collimator for pol. upgrade Optical Matching Device for e+ capture Main e- beam from electron main linac Target for e+ production PTAPA (~125MeV) PPA ( MeV) PBSTR 400MeV-5GeV  dump e- dump Damping ring 147 m helical undulator for photon production  PCAP PLTR: Energy compression and spin rotation Main e- beam to IP 150 GeV beam to dump

OK, what to do next? Insert a small section of undulator as ILC, but much smaller. 120 GeV electron would produce 3 photons/meter 1 good e+/100 photons 4 meters undulator (loss 50 – 100 MeV/turn) Use Compton stacking (50 turns) Production 50 (turns) x 50 (bunches) = 6 ms Or can be re-arranged in many different configurations

Comparing with ILC and CLIC Timing made easy with the ring scheme. The booster only operate a part time (a few sec for every 30 mins) Target stress level is much less than ILC, no major issues here. Polarization can be as high as ~ 80% (even higher?). Overall optimization paradigm can be drastically different, probably CEPC is much easier

lu lu i -i RDR undulator based positron source Undulator parameter: K=0.9, l u=1.15cm Length of undulator: 231m long Target: 0.4 X0 Ti Drift between undulator and target: 400m Photon collimator: None Optical matching device: ¼ wave transformer

18 Photon number spectrum and distribution functions The spectrum of photon generated by helical undulator is known as: Photon number spectrum in terms of harmonics (1) Harmonics distribution function Energy distribution function (2) (3) (4)

ILC RDR undulator photon number spectrum

20 Yield contribution from different harmonics 231m RDR undulator, 150GeV drive beam, 400m drift from the end of undulator to target In CEPC case, it would be more of 1 st harmonic dominated.

21 Yield and polarization of RDR configuration for different drive beam energy Drive beam energy YieldPolarizatio n 50GeV GeV GeV GeV GeV Drive beam energy Energy lost per 100m Energy lost for 1.5 yield 50GeV~225MeVN/A 100GeV~900MeV~9.9GeV 150GeV~2GeV~4.6GeV 200GeV~3.6GeV~3.7GeV 250GeV~5.6GeV~3.96GeV

22 Drive beam energy Energy lost per 100m Energy lost for 1.5 yield and 60% polarization 150GeV~2GeV~8.8GeV 231m RDR undulator, 150GeV drive beam, ¼ wave transformer Polarization upgrade With QWT, with a photon collimator to upgrade the polarization to 60%, the positron yield will drop to ~0.8

CEPC alternative e+ source -- Yield and Polarization The results showing that in order to achieve yield of 1 by stacking 50 e+ bunches together, the K needs to be 0.9. The associated polarization will be about 33%

What possible differences are at CEPC: Lower K: less photons, better polarizations. Target is much less likely be damaged and conventional facilities are much easier. Possible no remote handling is needed. No need for a drive beam generation. Much lower cost than ILC and CLIC. A 6 GeV linac for both electron and positron sources. Techs developed for ILC and CLIC are readily adapted to CEPC. 24

What if I were….. All the works below are not challenging, but tedious. Good understanding of the timing, timing and timing Study the effect of low K on polarization and yield, capturing. Perform end to end simulations. Re-examine the CLIC stacking/pre damping ring. Injection into the booster and spin de-polarizations. Total power consumption estimations. 25