Proposal for the DA F NE Variable Energy Design Study

1. Proposal for the DAFNE Variable Energy Design Study C. Milardi, D. Alesini, A. Gallo, M. Preger, A. Drago, P. Raimondi, B. Spataro, S. Tommasini, C. Vaccarezza, M. Zobov Accelerator Division. C. Sanelli, A Clozza, Technical Division. D. Babusci, Scientific Division. D. Moricciani, Roma2. 41st LNF Scientific Committee 22 – 23 November 2010

2. Outline • Preamble • Proposal • Main issues • Collaboration • Conclusions

3. Preamble Proposal for the DAFNE energy upgrade, named DAFNE_VE, in the framework of the FP7 call for infrastructures 2011 (deadline 25 November 2010) The idea is not new a letter of intent has been already presented in 2006 for a collider called DANAE DAFNE has been upgraded including a new conceptual approach to the beam-beam interaction, the Crab-Waist collision scheme which was promising to give in principle a luminosity of the order of 1033 cm-2s-1. The Crab-Waist concept has been successfully tested (L = 4.5 1032 cm-2s-1 ) Since then it is reasonable to consider the Crab-Waist as a basic concept for a variable energy collider. The submission of this project to the European Community is still sub iudice: the INFN management should take a decision in few days (23 November 2010).

4. Design study strategy • The proposed design study is aimed at verifying the feasibility of an electron/positron collider working at variable center of mass energy: • 0.6 GeV ≤ ECM ≤ 3.0 GeV • providing aluminosity • 1032 ≤ L ≤ 1033 cm-2s-1 • reusing as much as possible the infrastructure of the DAΦNE accelerator complex in Frascati. • Search in the field of high energy physics requires large and expensive infrastructures requiring long time for design and construction. For this reason there is a generalized tendency around the world in reusing as much as possible existing facilities; it is the case for several colliders: BEPC2 in China, VEPP2000 in Russia and Super KEKB in Japan.

5. Why changing the energy @ DAFNE is not possible? • DAΦNE with Crab-Waist achieved quite high luminosity, very close to the experiment requirement for this project, and still there is room for further improvements • However due to: • The IR based on permanent magnets elements (quadrupoles and dipoles) • The injection system • RF system specification • only minimal variation around the nominal energy are possible • -2.% ≤ ECM ≤ +2.% • with a progressive luminosity reduction up to a factor two -1.3% ≤ DEcm ≤ 0.7% MeV Lpeak -13. MeV ≤ Decm ≤ 7. MeV

6. About a variable energy collider • The ring lattice needs to be flexible from the optics point of view in order to allow a wide range of variability for the betatron functions, emittance, momentum compaction and to guarantee suitable dynamic aperture and lifetime • Lowering En (nominal energy): • tTouschek decreases ~ E-3 • tdamping increases ~ E-3 • the beam becomes in general more unstable and storing high currents is more difficoult • Luminosity decreases • Increasing En requires a proper injection system • Injection septa e TL magnets cannot deal with higher beam energy • Varying En requires a tunable compensation mechanism to keep under control the beam trajectory in the transverse space

7. Colliding Rings Lattice • Double ring collider • One interaction region based on the CRAB-Waist collision scheme • Ring arcs will be redesigned relaxing the long straight sections • Wigglers necessary to provide damping, essential for low energy operation, could be moved in the straight section opposite to the interaction region • Bending magnets in the arc must be compatible with operation at different energies; a first evaluation suggests that a normal conducting option is feasible DAFNE layout after the 2007 upgrade

8. Rings can fit in the DAFNE hall DAFNE-VE DAFNE B (.5 GeV) ~ 1.2 T (DAFNE) B (1.5 GeV) ~ 1.7 T B (0.3 GeV) ~ 0.33 T (DAFNE-VE) Main Rings Toy-Layout

9. Interaction Region The interaction region has to meet the different requirements coming from operation at different energies and with different values of the detector magnetic field. It will be based on the Crab-Waist collision scheme The low–b section will be based on superconducting quadrupoles including skew and correction coils to provide betatron coupling and beam trajectory correction respectively The interaction region design must include purpose developed screens to shield the background hitting the experimental apparatus. Their design must be compatible with the vacuum chamber layout and the impedance budget of the rings.

10. Beam-beam effects • Simulations of beam-beam interaction should indicate which ultimate luminosity can be achieved in the upgraded DAΦNE at different energies and help in optimization of the collider performance in terms of lifetime and luminosity in realistic operational conditions. • For this purpose we plan: • to perform a series of numerical simulations of beam-beam collisions taking into account all kinds of nonlinear lattice elements such as wigglers, sextupoles, octupoles etc. • to develop a fast numerical code capable to simulate a self-consistent beam size evolution of both colliding beams (“quasi strong-strong approach”)

11. Dynamic Aperture Dynamic aperture (area of particle’s stable motion) and energy acceptance are of crucial importance for beam quality and beam lifetime in an accelerator. These are defined by nonlinear machine optics, working point choice and crosstalk between beam-beam effects and lattice nonlinearities. A careful numerical modeling of particle’s motion in the 6D phase space is necessary in order to optimize the collider performance in all the energy range. This item is particularly important for the DAΦNE_VE due to the presence of strong nonlinear CW sextupoles installed in the interaction region.

12. Radio Frequency • The RF system of the rings will provide turn-by-turn restore of the energy lost by the beam due to synchrotron radiation emission and parasitic interaction with the vacuum chamber elements, as well as beam longitudinal focusing. • The RF system design study is intended to define the RF frequency value, number and type of RF cavities (including special parts such as input couplers and tuners), number, type and rating of the RF power sources, specifications and structure design of the low-level RF control, how much hardware of the existing infrastructure can be reused and what performance are expected. • the beam parameters at the maximum nominal energy define: • RF power request (> 100 kW) • maximum accelerating voltage • low energy will set the design criteria for system stability and low-level control

13. Impedance and Instability • Parasitic beam interaction with the surrounding vacuum chamber can result in several harmful effects: • destructive beam instabilities, both single- and multi-bunch, excessive overheating of vacuum chamber components • damage of collider diagnostics • In order to avoid these phenomena all the new vacuum chamber components will be designed taking care of beam coupling impedance minimization and suppression of parasitic higher order modes.

14. Bunch by bunch Feedback systems Lepton collider performances in terms of luminosity strongly depend on the maximum achievable beam currents and, in turn, on the bunch-by-bunch feedback systems used to damp both synchrotron and betatron instabilities. The very fast technological advance of the electronic components has also given the opportunity to make more compact and powerful digital feedback systems. Feedback systems have proved to be a powerful diagnostic tool to investigate several aspects related to the instability formation and build up. In the specific case is necessary to evaluate carefully the bunch-by-bunch feedback design in the framework of a complex scenario including different optics layouts and radio frequency setups as well as flexible energy range and injection system schemes.

15. Injection System • Low Energy: • Operation below 0.51 GeV poses no problems with the present injection system • High Energy operation (above 0.51 GeV) • Injection at .51 GeV and ramping in the Main Rings • Beam must be dumped at every injection • Top-up injection unfeasible • t must be long • Insert a 1 GeV Linac between the Accumulator and the Main Rings • The necessity to have a large energy gain per unit length suggests the use of the C-band technology. • A bunch compressor is required to confine the bunches in the C-band accelerating crest region to limit the beam energy spread growth during acceleration. • Build a slowly cycling 1.5 GeV Synchrotron (≈ 1 Hz) where the particles are stored with multiple pulses at 0.51 GeV, or lower and then ramped up to the desired energy and injected into the Main Rings

16. Radio Frequency for the Injection System ring RF system for the energy ramping synchrotron option has to be studied and designed, at a frequency lower than the collider rings one to optimize the synchrotron acceptance. Study about the accumulator RF system are also necessary to investigate the feasibility of a different injection option based on a post acceleration in a C-band linac of the beam extracted from the accumulator ring

17. Vacuum system • The design study of DAΦNE_VE vacuum system will address issues concerning the following items: • Vacuum chamber design for the interaction region compatible with the detector and the collider requirements. • Design of new vacuum chamber for the ring arcs suitable to cope with the higher synchrotron radiation power emission. • - Thin film deposition techniques to reduce the vacuum chamber wall secondary electron yield to reduce the positron beam instabilities arising from e-cloud build up.

18. High precision measurements of the beam energy • The measurement of R (ratio of electron-positron annihilation cross section into hadrons to muon pair) requires the knowledge of the energy of the beams circulating in the collider with an accuracy of 50 - 100 keV. • A technique to perform a precise measurement of the energy of an electron beam is based on the Compton backscattering of laser photons against the electron beam. • A typical experimental apparatus includes: • CO2 laser (w0 = 0.117 eV, ∆w0/w0 = 10-7) • high-purity germanium detector for energy spectrum measurements

19. Partners • LNF(Laboratori Nazionali di Frascati I) • UNILIV(University of Liverpool UK) • UU(Uppsala University, Department of Physics and Astronomy SE) • BINP (Novosibirsk RU) Partner expertise fields: Lattice Optics Magnet design High field magnet Beam-beam Beam Dynamics Vacuum System RF Systems Accelerating structure Detector interface Contact with industry

20. Conclusions • The presented R&D activities will lead to:an accurate study for the collider • the definition of the costs for its realization • the evaluation of its the impact on the existing LNF infrastructures

21. (from DANAE LOI)