An Assessment of U.S.-Based Electron-Ion Collider Science (2018) / Chapter Skim
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4 Accelerator Science, Technology, and Detectors Needed for a U.S.-Based Electron-Ion Collider
4 Accelerator Science, Technology, and Detectors Needed for a U.S.-Based Electron-Ion Collider
Pages 53-81
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Polarized colliding beams have been achieved before only at HERA (with electrons and positrons only) and Relativistic Heavy Ion Collider (RHIC; with protons only)
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The extensive energy vari ability and elaborate interaction region of an EIC require advanced supercon ducting magnet designs beyond state of the art. To attain the highest luminosities demanded by the science, cooling of the hadron beam is essential.
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tunnel con taining two superconducting hadron rings. The addition of a polarized electron source, full-energy injector linac and high-energy electron ring in the same RHIC tunnel opens up the possibility of polar ized electron-hadron collisions.
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In addition, in order to reach the required luminosity, the ERL must be capable of accelerating an unprecedented average beam current of about 500 mA, which presents a number of other challenges, including the superconducting radio frequency systems and interaction region design.2 FIGURE 4.1.1 Schematic of the linac-ring concept of eRHIC at Brookhaven National Labora tory, which would require construction of an electron beam facility (red) to collide with the Relativistic Heavy Ion Collider blue beam at up to three interaction points.
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SOURCE: V Ptitsyn, 2017, "Progress in eRHIC Design," EIC Collaboration Meeting, Brookhaven National Laboratory, October.
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The electron ring is designed to store beam currents up to 2.5A in 1,320 bunches per ring, a performance similar to B-factories. With the hadron emittance and density provided by the injectors, the eRHIC design should achieve a peak e-p luminosity of 4.4 × 1033 cm−2s−1.
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This is avoided by rotating both beams to achieve full overlap using "crab cavities" in the interaction region. These are a special design of SRF cavities, which deflect, rather than accelerate, the beam.
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Radio Frequency Systems At a maximum energy of 18 GeV, the eRHIC electron ring would require 41 MV of peak circumferential voltage to compensate synchrotron radiation losses and ensure adequate beam lifetime. Considerations of cost, space, and mitigation of the unwanted higher order mode power have led to the selection of a 563 MHz 2-cell cavity for the electron ring radio frequency (RF)
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From page 61... ...
This radiative self-polarization mechanism3 deteriorates the polarization of those electron bunches that are initially oriented along the bending field on a time scale ranging from a few tens of minutes to a few hours, depending on the beam energy and magnetic field. Additionally, spin diffusion due to synchrotron radiation may enhance the polarization decay, especially near energies corresponding to specific resonance conditions between the spin preces sion and orbital oscillations.
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and an ion collider ring. The stored ion beam current is up to 0.75 A
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An essential element of the JLEIC luminosity concept is electron cooling for reducing the ion beam emittance. To achieve the required high efficiency, JLEIC adopts a multi-phased cooling scheme, which utilizes two electron coolers, a mag netized DC cooler in the booster synchrotron, and a magnetized bunched-beam cooler based on an ERL for the collider ring.
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Magnets and RF System The bunch repetition rate of the JLEIC stored beams is 476 MHz, driven by the plan to reuse PEP-II warm RF cavities and RF stations for the electron collider ring. A conceptual scheme has been developed for injecting the electron bunches from the CEBAF SRF linac (which has a frequency of 1.497 GHz)
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RMS, root mean square. SOURCE: Yuhong Zhang, talk at EIC Collaboration meeting, October 2017, Brookhaven National Lab, Upton, N.Y., https://indico.bnl.gov/getFile.py/access?
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SOURCE: Y Zhang, 2017, "Progress in JLEIC Design," EIC Accelerator Collaboration Meeting, Brookhaven National Laboratory, October.
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Magnet technology R&D is required for the JLEIC ion ring magnets, the interaction region magnets for both designs, and the solenoids for the electron cooler and spin control. In the case of the eRHIC design, the crossing angle of 22 mrad calls for com bination of active and passive shielding to provide a field-free pass of the electron beam inside the IR quadrupole (see Figure 4.6a)
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Ptitsyn, 2017, "Progress in eRHIC Design," EIC Collaboration Meet ing, October, Brookhaven National Laboratory, Upton N.Y.: https://indico.bnl.gov/conferenceDisplay.
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is being subjected to a proof-of-principle test at RHIC. JLEIC Multiphase Electron Cooling As a critical part of the luminosity concept, JLEIC employs conventional electron cooling technology for reducing the ion beam emittance.
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FIGURE 4.8 Conceptual design of the Jefferson Laboratory Electron-Ion Collider bunched-beam energy recovery linac cooler. SOURCE: Stephen Benson, talk at EIC Collaboration Meeting, October 2017, Brookhaven National Laboratory, Upton, N.Y., https://indico.bnl.gov/getFile.py/access?
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Coherent Electron Cooling, as Applied to eRHIC One of the most remarkable innovations at RHIC was the implementation of bunched-beam stochastic cooling of the heavy-ion beams at full energy in collision. Stochastic cooling, in which an RF "pick-up" measures fluctuations in the particle distribution that are later corrected in a subsequent kicker stage, has been effective in reversing the beam size increase due to intra-beam scattering and has resulted in an increase in luminosity and considerable increase in the physics reach of the RHIC collider.
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Energy Recovery Linacs ERLs, a high-performance and high-efficiency type of recirculating linac, pres ently offer the only credible concept for electron cooling of high-energy colliding beams. The idea of energy recovery in a recirculating RF linac is based on the fact that the RF fields, by proper choice of the time of arrival of the electron bunches in the linac, may be used to both accelerate and decelerate the same beam.
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concept for a variety of applications, including higher-power FELs, synchrotron radiation sources, electron cooling devices, and high-luminosity EICs. The major ity of operating and proposed ERLs are based on SRF linacs, due to their greater efficiency of energy recovery.
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Like eRHIC, the Jefferson Laboratory Electron Ion Collider (JLEIC) design of an EIC also employs an ERL as an electron cooler to achieve low-emittance ion beams.
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Crab Cavity Operation in Hadron Ring To reach the ultimate luminosity goals, both EIC design concepts require "crab crossing." In a storage ring collider with beams crossing at an angle, some luminos ity is lost because the colliding bunches do not overlap completely. A crab crossing FIGURE 4.2.1 Simplified schematic layout of the CBETA test accelerator.
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Extensive simulations of hadron beams with crab cavities, long c bunches, and beam-beam collisions are necessary to evaluate the performance of the proposed EICs. In addition, crab cavity tests with hadron or high current elec tron beams will be critical at the project definition stage of an EIC.
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8 Report of the Electron Ion Collider Advisory Committee, November 2-3, 2009. 9 Report of the Community Review of EIC Accelerator R&D for the Office of Nuclear Physics, February 13, 2017.
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The range of EIC beam energies, ion species, collision rates, and collision characteristics of the processes of interest each present particular challenges for the design of such detectors, as do the envisioned measure ment accuracies, including those of luminosity and polarization. The successful development of suitable EIC detectors relies crucially on HERA experience and on technological developments for other large-scale detectors in high-energy and nuclear physics, in particular those at the LHC and those envisioned for the ILC.
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This range, or acceptance, well exceeds that of the existing and planned detectors at JLab and at RHIC. Like the angles, the energies of the scattered electrons span a broad range from just a small fraction of the EIC electron beam energy up to the ion beam energy.
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The broad range of hadron total momenta and the relatively compact central detector designs require consideration of a similarly broad range of technolo gies for particle identification (PID) , a key capability for essentially all but the inclusive measurements at the EIC.
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The instrumentation associated with luminosity and polarization measurements is typically located very close to the beams and can, in the case of the electron beam, be designed to considerably expand the acceptance of the central detector for scattered electrons to very shallow angles, a region of considerable scientific interest dominated by photo-production processes. 12 Measurement of EIC hadron beam polarization is likely to adopt the methods employed at RHIC, which have achieved 3 to 4 percent accuracy and will take place in a dedicated location away from the central detectors.
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