Super Tau-Charm @ Hefei: Concept, Status and Prospects Qing Luo National Synchrotron Radiation Laboratory University of Science and Technology of China 中国科学技术大学，国家同步辐射实验室 罗箐

Outline Goal and Preliminary Concept of the Accelerators Design Goal How to achieve high polarization beams Key Technologies General Description Examples: Impedance & Vertical Emittance Measurement, etc. Prospects Hefei Integrated National Science Center A Test Facility for Future Super Tau-Charm What’s next?

What’s HIEPA? A High Luminosity e-e+ Collider @ 2-7GeV Luminosity: ×1035cm-2s-1 @ 4GeV (center of mass) Beam Energy Range: 1-3.5GeV Polarization: >80% longitudinal e- @ IP With possibility to upgrade to polarized e+ e- collision May provide 3rd generation synchrotron Symmetric Dual-ring, with Linac injection Full energy linac, no booster, easy to alter beam energy Also provide possible X-FEL and other test facility

Parameters and Plans Accelerator Parameters Steps: Pilot: 0.5×1035 2 Circumference/m ~600 Beam Energy/GeV Current/A 1.5 E𝐦𝐢𝐭𝐭𝐚𝐧𝐜𝐞 𝜀 𝑥 / 𝜀 𝑦 /nm·rad 5/0.05 β Function @ IP 𝛽 𝑥 ∗ /𝛽 𝑦 ∗ /mm 100/0.9 67/0.6 Collision Angle(full θ)/mrad 60 Tune Shift 𝜉 𝑦 0.06 0.08 Hour-glass Factor 0.8 Luminosity/×1035cm-2s-1 ~0.5 ~1.0 Steps: Pilot: 0.5×1035 Nominal: 1.0×1035 Final: Polarized e- Upgrade: Polarized e+

Interaction Region: Large Piwinski Angle Collision Layout Interaction Region: Large Piwinski Angle Collision +Crabbed Waist Detector Snakes Crab Sextupole Wigglers Injector Linac 0.5-3.5 GeV e+0.5GeV e- 0.5GeV About injector: For e+ and e-, no booster, 0.5GeV->1~3.5GeV e+, a convertor, a linac and a damping ring, 0.5GeV e-, a polarized e- source, accelerated to 0.5GeV

Accelerator Design Design Method Further Discussion Design the ring & IR respectively Match the lattice function at both sides 30 DBA cells are enough for a 5nm·rad emittance (SSRF, 20 cells, 4 nm, 432 m) Iterative Method Further Discussion thousandth What if we increase the energy? What about the higher order components and tolerance of the magnets? ×0.1‰？

Accelerator Design Is MBA Lattice Useful? 1-3.5GeV While designing a diffraction limit synchrotron ring ‘HALS’ Good: smaller emittance and beam size Bad: more severe collective effects and smaller dynamic aperture Balances all requirements Could definitely use some help when design the IR 6BA，~600m Natural ε 50pm·rad Full Current 400pm·rad ε∝E2C-3

To achieve high polarization Siberia Snakes Mini-rotator Very large trace bending angle, increases emittance Spin resonance How many snakes? 𝜏 𝑑𝑎𝑚𝑝 ∝ 𝐸 −7 𝑁 𝑠𝑛𝑎𝑘𝑒 2 For Energy of 3.5 GeV in future, 7 snakes Courtesy of Prof. Dong Wang, SINAP A.W. Chao, Problems in obtaining polarized e+ and e- beams and perspectives for PEP, High-Energy Physics with Polarized Beams and Polarized Targets Volume 38 of the series EXS 38: Experientia Supplementum, p. 15-26, 1981 V. V. Anashin, V.M. Aulchenko, et al., A project of super c-τ factory in Novosibirsk, 2011 https://ctd.inp.nsk.su/c-tau/Project/CDR_en_ScTau.pdf Courtesy of Dr. Anashin, Dr. Aulchenko, et al., BINP Tau Charm Report

Polarized e- e+ Source Polarized Beam Injection Polarized e- beam 𝑃 𝑎𝑣𝑔 = 𝑃 0 𝜏 𝑝,𝑒𝑞 𝜏 𝑝,𝑒𝑞 + 𝜏 𝑏 + 𝑃 𝑒𝑞 𝜏 𝑏 𝜏 𝑝,𝑒𝑞 + 𝜏 𝑏 P0 injection beam polarization 𝜏 𝑏 beam life, 𝜏 𝑝,𝑒𝑞 equilibrium time 𝑃 𝑒𝑞 equilibrium beam polarization Polarized e- beam NEA Photocathode Electron Gun Quantum Efficiency vs Polarization High Current vs High Quantity of Electric Charge QE>1%, P>90% Negative electron affinity

Polarized e+ Source Methods of Polarized e+ Source: from γ ray 1 Linac & Helical Undulator to Circular Polarized γ 2008, SLAC, test, 46.6GeV e- 》80%+ polarized 6MeV e+ 2016, DESY, design, 150GeV e- 》 60%+ polarized e+, e->1.5e+ High energy, long linac, long undulator Best yield, but not appropriate for super τ-C, only use for ILC 2 Compton Backscattering to Circular Polarized γ 2014, LAL(France), 2GeV e- 》20% γ 》5.1‰ e+/e-，18.9MeV, 35.6% Polarization 1979年，前苏联科学家Balakin和Mikhailichenko提出，可以由非极化的电子束通过螺旋型波荡器辐射来获得圆极化γ光，并打靶产生极化束流[25]。这种办法可以充分利用光源领域不断发展的前沿技术，能够获得高亮度的极化束流，因此很快为大家接受。2008年，国际科学家合作在SLAC直线加速器和FFTB验证装置上利用46.6±0.1GeV的电子束获得了纵向极化度达到80%以上的6MeV正电子（以及电子）[26]，证明这种方法完全适用于ILC计划，2016年，DESY为ILC考虑的150GeV驱动极化电子束配置就采用了这一方案，将ILC baseline设计中的147m超导螺旋波荡器再延长73.5m，可以将极化度提高到60%，且满足平均每个驱动电子可以产生1.5个正电子的需求；另外，其正电子产额随能量下降而降低，且能量降低到120GeV时极化度也下降到30-40%[27]。 上述方法要求电子束的能量达到几十乃至上百GeV，建设这样的巨型直线加速器成本很高。对TeV级的巨型对撞机，这个做法有其合理性；但对于几个GeV、周长几百米级别的对撞机就不适宜了。1996年，日本的东京都立大学的Okugi和KEK的Kurihara等提出可以利用非极化的电子束与圆极化激光进行逆康普顿散射（inverse Compton scattering，又称康普顿背散射或反散射，Compton backscattering）来获得圆极化的γ光，从而在KEK的ATF验证装置上打靶获得极化正电子[28]，这对电子束能量的要求大为降低，为紧凑型的正电子源指出了一个重要方向。KEK之后又提出，为了进一步提高产额，可以将逆康普顿散射腔安装在储存环中，并进行多次碰撞[29]。2008年，法国LPNL和俄罗斯BINP合作，为ILC和SuperB按照这一原理设计了（极化）正电子源，其超导直线加速器可使用ERL，用于产生γ光的Radiator既可以是晶体，也可以是康普顿腔；当利用ERL为逆康普顿散射提供1.3GeV电子束，可以获得平均纵向极化度47%、平均能量12MeV的正电子[30]。2014年，法国LAL利用2GeV的电子束获得了20%的γ光子->正电子产额和总计5.1‰的e+/e-产额，正电子平均能量18.9MeV，平均极化度35.6%[31]。 G. Alexander, J. Barley, Observation of polarized positrons from an undulator-based source, Physical Review Letters, Vol 100, 210801, May 2008 A. Vauth, J. List, Beam polarization at the ILC: physics case and realization, Spin Physics (SPIN2014) International Journal of Modern Physics: Conference Series Vol. 40 (2016) 1660003, DOI: 10.1142/S201019451660003X T. Okugi, Y. Kurihara, et al. Proposed method to produce a highly polarized e+ beam for future linear colliders. Jpn. J. Appl. Phys. Vol 35 Part 1, No.6A, Jun 1996, p.3677-3680康普顿背散射产生γ，再产生纵向极化正电子 S. Araki, Y. Higashi, et al. Conceptual design of a polarised positron source based on laser Compton scattering— a proposal submitted to Snowmass 2005 https://arxiv.org/pdf/physics/0509016.pdf X. Artrua, R. Chehab, et al., Polarized and unpolarized positron sources for electron–positron colliders, Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, Vol 266, Issue7, Sept 2008, p. 3868–3875

Polarized e+ Source Methods of Polarized e+ Source 3 Polarized e- 》Bremsstrahlung Comparison Method 1, highest yield and cost, for huge colliders Method 3, lowest yield and cost, for small facilities 2016, Jlab, PEPPo 8.19 MeV, 85.2% Polarized e- 6.25 MeV, 82% Polarized e+ 100 MeV-> 75% Polarization Transfer, 10-4 e+/e- 第三种产生圆极化γ光子的途径是极化电子束打高Z材料靶产生的轫致辐射（bremsstrahlung）。1997年，俄国的Potylitsin结合1959年Olsen等人的研究结果，指出纵向极化的电子束打薄非晶体靶的轫致辐射中包含圆极化的γ光子，而圆极化的γ光子在同一靶上可以产生正负电子对，这些正负电子对中的高能成分拥有与光子圆极化度接近的纵向极化度，因此可以直接利用极化电子束[32]。2011年，JLab提出根据这种办法在CEBAF注入器上进行验证工作[33]，随后发起了称为PEPPo（The Polarized Electrons for Polarized Positrons）的合作计划，该合作计划涉及美、法多个研究单位，到2016年，报道了由8.19MeV、85.2%极化度的电子束打1mm钨靶，产生6.25MeV、82%极化度的正电子的实验结果，并推测可以用100MeV的电子束获得75%的极化传递效果和10-4e+/e-的产额[34]。显然，这个方法需要的电子束能量更低，装置更紧凑，是一个非常有潜力的选项。 D. Abbott, P. Adderley, et al., Production of highly polarized positrons using polarized electrons at MeV energies, Physical Review Letter, Vol116, 214801, 27 May 2016

Concept Design for a Plasma Injector W. Lu et al Concept Design for a Plasma Injector W. Lu et al., CEPC Workshop, April 17-20, 2017 RF gun Linac1 ~200MeV Linac2 2-4GeV BC1 FC PWFA DB LPA/photo-gun linac >100MeV 20nC e+ Target Damping ring BC2 TR=6-12 Linac3 ~300MeV

Polarized e+ Source Plasma Wakefield Accelerator (PWFA) as a Booster 2~3.5GeV -> 40~50GeV, Helical Undulator Polarized γ -> Polarized e+ A PWFA-based Polarized e+ Injector 40~50GeV 3.5GeV Linac PWFA TR=10-20 Good: High yield for high energy beam Relatively low cost Weakness Low repetition beam PRL 100, 210801 (2008)

Collective Effects For Super Tau-Charm Factory High Current; Many Small Bunches Low Energy (compared to B factory, FCC-ee/CEPC…) Collective Effects for Low Emittance Rings For Storage Ring: Like 3rd Generation Light Source For Collider: Beam-beam Effects Single Bunch: Microwave instability, Bunch Lengthening, TMCI, IBS, etc. Multi-bunch: Coupled-Bunch Instability, HOM, Electron Cloud, etc.

Impedance Study Purpose Methods Estimate and Control the Collective Effects Methods Simulation (CST Studio, etc.) Bench Measurement of Coupling Impedance Coaxial-wire method measure S-parameters For devices with complex structure and NEG-coated chamber Junctions and transition sections excite noise; wire thickness; spectrum of higher frequency is hard to measure On-line Measurement? Tune measurement, use tune vs current to estimate the impedance of the whole ring

Vertical Size Measurement Very Small Vertical Bunch Size ~1% Coupling, @IP: σx~20μm; σy~0.2μm Other point, βx≈1m @BM，σy~1μm Traditional Method as X-ray Pinhole or FZP SR X-ray Image: Diffraction Limit(Fundamental Theory) Interferometer: Short SR Wavelength, higher resolution Method Detail Resolution Price Image X-ray Image ~μm High X-ray Pinhole ~10μm Low Interferometer Visible to X-ray 0.01-1μm Medium

Vertical Size Measurement SR X-ray Interferometer Resolution: ∆𝜎~ 𝜆𝐿 𝜋 𝑑 𝑢 1 𝛾 8𝑙𝑛 1 𝛾 ∆𝛾 𝜎 𝑦𝑚𝑖𝑛 = 𝜆 2𝜋𝜓 1 2 𝑙𝑛 1 𝛾 𝑦 ; 𝜓= 1 𝛾 𝜆 𝜆 𝑐 1 3 = 3𝜆 4𝜋𝜌 1 3 ∝ 𝜆 𝜌 1 3 ; 𝜎 𝑦𝑚𝑖𝑛 ∝ 𝜆 2 3 𝜌 1 3 For a X-ray Interferometer of Vertical Beam Size σ 𝑦 = 𝜆𝐿 𝜋 𝑑 𝑦 1 2 ln 1 𝛾 ， 𝛾 𝑢 =𝑒𝑥𝑝 − 1 2 2𝜋 𝑑 𝑢 σ 𝑢 𝜆𝐿 2 𝑑 𝑦 =20μm,𝜆=0.1 nm

Vertical Size Measurement SR X-ray Interferometer Picture below shows a UV system for horizontal size system of 1μm resolution X-ray needs more optimization and better optical devices X-ray multi-layer reflector ; X-ray monochrometer (instead of bandpass filter); X-ray CCD; X-ray polarizing prism; etc.

Other Key Technologies New injection method Top-up injection Minimizing injection-induced detector background Superconducting Ultrahigh vacuum Other high precision beam diagnostics Beam control and feedback system Power supply, Stability Technology such as Bracing, etc.

Prospects Hefei Integrated National Science Center A national science center which will play an important part in ‘Megascience’ of China in near future Pay a lot of attention to accelerator facilities Hefei Advanced Light Source is already under design HIEPA is listed in future plan 10M RMB start fund proved by USTC

Prospects National Synchrotron Radiation Laboratory: HLS II 注入器 储存环 800MeV, 38nm·rad, 4 DBA Cells 注入器 由直线加速器和输运线组成 加速结构：等梯度 能量为800MeV 储存环 发射度38nm·rad 八个直线段，现安装5台插入件

Prospects Hefei Advanced Light Source Parameters A diffraction limit ring based SR light source Parameters Beam Energy ~2.0 GeV Circumference ~600 m Lattice 6BA Natural Emittance < 50 pm.rad Current 500±0.5 mA

Test Facility for HIEPA? What kind of facility it should be? What to do: Bench Test or Online Test? Technique develop and test with beam What kind of accelerator: Ring or Linac? Mainly linac; tests based on small ring is not that useful for large ring collider; built a ring is more expensive Can also provide FEL or other utilities What kind of technology should be tested? Online: Superconducting, Photocathode Guns, Magnets, etc. Bench test: Magnets, Impedance Measurements, Interferometers, etc.

Test Facility for HIEPA? How can we achieve that? Linac with NEA photoinjector Polarized electron directly for polarized positron Also can be used in Positron Annihilation Technique Compton Backscattering in test facility (or HLS II) e- Test NEA Photocathode Gun 100MeV~2 GeV Linac Radiator or Target e+ Test LEPD low energy positron diffraction RHEPD Reflection High Energy Positron Diffraction ERL Loop Future addition

Collaboration Needed Accelerator Physics Accelerator Technologies IR Design Polarization: Spin Rotation and Maintenance Collective Effects: Simulation and Bench Measurements Advanced Computational Accelerator Physics Accelerator Technologies Superconducting Cavities and Magnets Polarized Beam Sources Ultrahigh Vacuum Chamber with Small Aperture, Optimized Impedance and Low SEE

Conclusion Conditions for a super tau-charm factory to be approaching Time Window: 2020-2025 Budget: RMB ~4 Billion Apply for support from Central Government of China and local authority Short of Faculties and Staffs Technologies Readiness: need to be invested Still a lot of work to do

Thanks for Attention! 感谢各位！