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.
The present MEIC baseline is a traditional ring-ring collider [1,2,3] in which the colliding electron and ion beams are stored in two collider rings. This choice, which has evolved from an ERL-ring collider of an earlier design stage , was adopted in 2007  after it was realized that an electron-ERL/ion-ring design could not significantly improve luminosities beyond a ring-ring collider that utilized a high bunch repetition rate. Moreover, an ERL-ring collider scenario would add a substantial burden on the polarized electron source, and many-pass energy recovery is not well established. A brief comparison of two collider scenarios—ring-ring and ERL-ring—for the MEIC design is presented in section 3.7. The central part of the proposed facility is a set of figure-8 shape electron and ion storage rings as shown in Figure 3.1. The electron ring is made of normal conducting magnets and will store an electron beam of 3 to 11 GeV. The CEBAF SRF linac  serves as a full-energy injector into the electron collider ring, requiring no further upgrade for energy, beam current, or polarization beyond the 12 GeV upgrade. The ion collider ring is made of high-field superconducting magnets and will store a beam with energy of 20 to 100 GeV for protons or up to 40 GeV per nucleon for light to heavy ions. The ion beams are generated and accelerated in a new ion injector complex that will be described below. The two collider rings are stacked vertically and housed in the same underground tunnel as shown in Figure 3.2. They have nearly identical circumferences of approximately 1.4 km, occupying a compact footprint of 500 m by 170 m, which is actually smaller than that of CEBAF, as shown in a Jefferson Lab site map in Figure 3.3. In addition, there is depicted in the figures a large figure-8 ring (in light grey color in Figure 3.1 and dashed red line in Figure 3.3) which represents two high energy collider rings (2.5 km or larger) for a future energy upgrade for reaching up to 20 GeV electrons, and up to 250 GeV protons or 100 GeV/u ions. The upgraded high-energy collider can use the same experimental halls and, possibly, the detectors of MEIC, and the medium-energy ion collider ring would then serve as the final booster in staged acceleration of ion beams.
- MEIC physics meeting on Friday, 21 March 2013, 9:30am-11:00am in F326/327 (https://eic.jlab.org/internal/index.php/General_Meeting_Agenda_6/21/13). For remote participation please check the link below.