Thomas Jefferson National Accelerator Facility Newport News, Virginia, USA

1. Solenoid spin ELIC: A HIGH LUMINOSITY AND EFFICIENT SPIN MANIPULATION ELECTRON-LIGHT ION COLLIDER BASED AT CEBAF* L. Merminga, Ya. Derbenev, A. Hutton, M. Poelker, and Y. Zhang, Jefferson Lab, 12000 Jefferson Ave., Newport News, VA 23606 Thomas Jefferson National Accelerator Facility Newport News, Virginia, USA Abstract Electron-light ion colliders with center of mass energy between 20 and 100 GeV, luminosity between 1033 and 1035 cm-2 sec-1, and polarization of both beams at or above 80% has been proposed for the study of hadronic structure. The proposed scheme would accelerate the electron beam using the CEBAF recirculating linac with energy recovery. If 20-40 MV/m accelerating structures are installed in the CEBAF tunnel, then a single recirculation can result in electron beam energy of about 5-10 GeV. After colliding with protons/light ions circulating in a storage ring under electron cooling at an energy of 50-100 GeV or above, the electrons are re-injected into the CEBAF accelerator for deceleration and energy recovery. In this report several innovative features of electron and ion beam designs and their advantages in delivering the luminosity and spin are described. These features include: electron circulator ring to reduce electron polarized source and energy recovering linac requirements, twisted spin booster and collider ring; low energy electron cooling accumulator as option for stacking beams from positive polarized ion sources; interaction points with low beta-star and crab-crossing using the short, cooled ion bunches. Accelerator physics and technology issues for both protons/ions and electrons are discussed. The feasibility of an integrated fixed target program at 25 GeV and collider program with center of mass energy between 20 and 45 GeV is also explored. INTEGRATION WITH 25 GeV FIXED TARGET PROGRAM [1] NUCLEAR PHYSICS MOTIVATION • A high luminosity polarized electron – light ion collider has been proposed as a powerful new microscope to probe the partonic structure of matter • Over the past two decades we have learned a great amount about the hadronic structure • Some crucial questions remain open: • What is the structure of hadrons in terms of their quark and gluon constituents? • How do quarks and gluons evolve into hadrons? The same electron accelerator can also provide 25 GeV electrons for fixed target experiments for physics. THE ELIC PROPOSAL [1] INTRABEAM SCATTERING AND FLAT BEAMS [2] ELECTRON COOLING IN ELIC [4] NUCLEAR PHYSICS REQUIREMENTS It is important to distinguish between multiple IBS and single scattering or Touschek effect. Multiple scattering contributs to ion Focker-Plank equilibrium i.e. beam core, while single scattering kicks the particles out of the core. To overwhelm multiple IBS, current of cooling beam must well exceed a critical value, proportional to ion current. After the cooling starts, the ion beam will shrink to the Focker-Plank equilibrium. Following this stage, an interplay between Touschek scattering and particle damping due to electron cooling beyond the core will determine the core i.e. luminosity lifetime. At ion energies far above transition value, the area of the cooling beam should frequently exceed that of the ion beam, in order to extend the ion core lifetime. Using this phenomenology one can estimate an optimum set of parameters for maximum average luminosity of a collider. See Table below. At energies above the transition value, energy exchange at intra-beam collisions leads to horizontal emittance blow up due to energy-orbit coupling, and vertical emittance – due to x-y coupling. Since luminosity is determined by the product of two emittances, reduction of transverse coupling to a minimum - while conserving the beam area - would result in decrease of energy scattering, hence, decrease of the impact of IBS on luminosity. Electron cooling then leads to a flat equilibrium with a large aspect ratio [5]. A flat ion beam should collide with a correspondently flat electron beam. Electron cooling time grows with beam energy in the first or second power and with normalized beam emittances - in the third power. Therefore, it seems critically important to organize the cooling process in two stages: cool the ion beam initially at injection energy after stacking it in collider ring (in parallel or after re-bunching), and continue the cooling during and/or after acceleration to a high energy. • Implement 5-pass recirculator, at 5 GeV/pass, as in present CEBAF (One accelerating & one decelerating pass through CEBAF  20-45 GeV CM Collider Program) • Center-of-mass energy between 20 GeV and 100 GeV with energy asymmetry of ~10, which yields • Ee ~ 3 GeV on Ei ~ 30 GeV up to Ee ~ 5 GeV on Ei ~ 100 GeV • CW Luminosity from 1033 to 1035 cm-2 sec-1 • Ion species of interest: protons, deuterons, 3He • Longitudinal polarization of both beams in the interaction region  80% required for the study of generalized parton distributions and transversity • Transverse polarizationof ions extremely desirable • Spin-flip of both beams extremely desirable R&D STRATEGY Electron Cooler for ELIC • Conceptual development • “Circulator Ring” concept promises to ease high current polarized photoinjector and ERL requirements significantly • Additional concepts for luminosity improvements are being explored • Analysis/Simulations • Electron cooling and short bunches • Beam-beam physics • Circulator ring dynamics • ERL physics • Experiments • JLab FEL (10mA), Cornell/JLab ERL Prototype (100mA), BNL Cooling Prototype(100mA)to address high current ERL issues • CEBAF-ER: The Energy Recovery experiment at CEBAF to address ERL issues in large scale systems [6] • ELIC DESIGN INNOVATIONS [2,3] • Electron circulator ring • Twisted spin booster and collider ring • Short ion bunches resulting from electron cooling • Low beta-star (~ 5 mm) • Crab-crossing • Traveling ion focus BASIS OF ELIC PROPOSAL CEBAF with upgraded cryomodules and energy recovery “Figure 8” storage ring and boosters CONCLUSIONS • The hadron physics community is asking for a high luminosity, polarized electron-light ion collider • Our design studies have led to an approach that promises luminosities up to 1035 cm-2 sec-1 • This design can be realized cost-effectively using energy recovery on the JLab site and can be integrated with a 25 GeV fixed target program for physics • Planned R&D will address open issues Circulator Ring • CEBAF with energy recovery is used for rf power savings and beam dump requirements • “Figure-8” storage ring is used for the ions for flexible spin manipulations of all light-ion species of interest • Circulator ring for the electrons may be used to ease high current polarized photoinjector requirements [1] L. Merminga, et al., “ELIC: An Electron-Light Ion Collider Based at CEBAF,” Proceedings of EPAC 2002 [2] Ya. Derbenev, “Luminosity Potentials in Colliders with Electron Cooling,” These proceedings TPPB081 [3] Ya. Derbenev, “Advanced Concepts for Electron-Ion Collider,” Proceedings of EPAC 2002 [4] Ya. Derbenev, Proceedings of ECOOL’03 Workshop [5] Ya. Derbenev, Proceedings of EPAC 2000 [6] A. Bogacz et al., and C. Tennant et al., These proceedings TOAC006 and RPPG032 REFERENCES *Work supported by the U.S. Department of Energy, contract DE-AC05-84ER401050