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The nuclear physics community worldwide has suggested that a high-luminosity, at or above 1033 cm-2sec-1, polarized Electron-Ion Collider with variable center-of-mass range ?s in the range of 20 to 100 GeV would allow us to probe the hadronic structure of matter and provide answers to these questions. The 2001 Long Range Plan for the next decade, outlining opportunities in nuclear science, put an Electron-Ion Collider forward as the next major facility to consider for the field. They emphasized the need to refine the scientific case, and to pursue the accelerator R&D necessary to ensure that the optimum technical design could be chosen. The 2002 Ad-hoc Facilities NSAC Subcommittee identified the research program of such a facility as "absolutely central to Nuclear Physics".

Machine Design Goal

The nuclear physics programs outlined in the previous chapter provide a set of high-level requirements for MEIC at Jefferson Lab as follows: 1. Energy The center-of-mass (CM) energy of this collider should be between 15 and 65 GeV. (The value of s=(4EeEu)1⁄2 is from a few hundred to a few thousand GeV2, where Ee and Eu are kinetic energies of electron and nucleon) Thus energies of the colliding beams should range - from 3 to 11 GeV for electrons, - from 20 to 100 GeV for protons, and - up to 40 GeV per nucleon for ions. Protons or ions with energies below 20 GeV per nucleon are also interesting to investigate certain potentially important physics processes. 2. Ion species Ion species of interest include polarized protons, deuterons, and helium-3. Other polarized light ions are also desirable. Heavy ions up to lead do not have to be polarized. All ions are fully stripped at collision. 3. Multiple detectors The facility should be able to accommodate up to three detectors with at least two of them available for collisions of electrons with medium energy ions. A third detector is desirable for collisions of electrons with ions whose energies are lower than 20 GeV/u. 4. Luminosity 33 34 -2 -1 The luminosity should be in the range of mid 10 to above 10 cm s per interaction point over a broad energy range. Further, optimization of luminosity should be centered around 45 to 50 GeV CM energy (the value of s is around 2000 to 2500 GeV2). 22 5. Polarization Longitudinal polarization for both electron and light-ion beams at the collision points should be achieved with greater than 70% polarization. Transverse polarization of the ions at the collision points and spin-flip of both beams are extremely desirable. High- precision (1–2%) ion polarimetry is required. 6. Positrons Polarized positron beams colliding with ions are desirable, with a high luminosity similar to that of the electron-ion collisions. In addition, an MEIC accelerator design should be flexible to allow an option of a future energy upgrade for reaching electron energy up to 20 GeV, proton energy up to 250 GeV, and ion energy up to 100 GeV per nucleon.


ELIC is an electron-ion collider with center of mass energy of 20 to 90 GeV and luminosity up to 8x1034 cm-2s-1. This high luminosity collider is envisioned as a future upgrade of CEBAF, beyond the 12 GeV Upgrade, and compatible with simultaneous operation of the 12 GeV CEBAF (or a potential extension to 24 GeV) for fixed-target experiments.

The CEBAF accelerator with polarized injector can be used as a full energy injector into a 3-9 GeV electron storage ring. A positron source is envisioned as an addition to the CEBAF injector, for generating positrons that can be accelerated in CEBAF, accumulated and polarized in the electron storage ring, and collide with ions with luminosity similar to the electron/ion collisions.

The ELIC facility is designed to produce a variety of polarized light ion species: p, d, 3He and Li, and unpolarized ion species. To attain the required ion beams, an ion facility must be constructed, a major component of which is a 225 GeV collider ring located in the same tunnel and below the electron storage ring. A critical component of the ion complex is an ERL-based continuous electron cooling facility, anticipated to provide low emittance and simultaneously very short ion bunches.

ELIC is designed to accommodate up to four interaction regions (IR's), consistent with realistic detector designs. Longitudinal polarization is guaranteed for protons, electrons, and positrons in all four IR's simultaneously and for deuterons in up to two IR's simultaneously.

An alternate design approach for ELIC is based on the linac-ring concept, in which CEBAF operates as a single-pass Energy Recovery Linac (ERL) providing full energy electrons for collisions with the ions. Although this approach promises potentially higher luminosity than the ring-ring option, it requires significant technological advances and associated R&D. A linac-ring ELIC design is an ultimate Upgrade of ELIC, fully compatible with and a natural extension of the ring-ring scheme.







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