Increasing ESR Polarization
Collider Cove (3rd Floor)
1005s
Meeting Overview
The Electron-Ion Collider (EIC) aims at providing high luminosity collisions between longitudinally spin-polarized electrons and protons as well as heavy ions in the electron-proton equivalent center-of-mass energy range from 29 to 140 GeV. This large range is achieved by colliding electrons in the energy range from 5 to 18 GeV with protons ranging in energy from 41 to 275 GeV.
The present EIC design assumes a rapid cycling, “spin resonance-free” synchrotron (RCS) that accelerates electron bunches injected from a polarized source up to full energy (18 GeV). These polarized bunches are used to continuously replace bunches in the Electron Storage Ring (ESR) that have fallen below acceptable polarization levels due to radiation. However, an alternative scheme is under consideration to reduce the overall cost of the facility: in this scheme, the RCS maximum energy would be reduced to 9 GeV, and acceleration to 18 GeV would then have to be accomplished in the ESR. Because polarization would be lost during this acceleration in the ESR, this scheme relies on Sokolov-Ternov self-polarization at 18 GeV. This concept is only acceptable for the physics program if a time-averaged polarization of at least 60% can be achieved. This meeting is to discuss possible methods of increasing the asymptotic polarization to above 60% in the 18 GeV case of the Electron Storage Ring (ESR) of the Electron-Ion Collider (EIC).
Furthermore, in order to reduce systematic effects, bunches with "spin up" and "spin down" need to be stored either simultaneously in the same fill, or the polarization direction of entire fills has to be alternated in a fast succession. This complication must also be considered in any proposed method.
Technical Details
Below we include a few paragraphs summarizing the technical details of polarization in the ESR. However, we kindly request that you read through PDF "EIC_ESR_Polarization.pdf" provided below, which contains a more in-depth description of the ESR’s spin rotator design and history, prior to the meeting
In summary, the spin rotator of the ESR consists of a symmetric layout of bend modules and solenoid modules, whose strengths are chosen to rotate the spin from vertical in the arc, to longitudinal at the IP, and back to vertical in the arc for a given beam energy. Two methods have already been applied in order to minimize the spin-orbit coupling function at 18 GeV (and thus maximize the asymptotic polarization in this ring):
- A horizontal “strong synchro-beta” spin match from arc-to-arc
- Vertical bumps computed via the “BAGELS” method
In simulations of an ideal 18 GeV lattice, these two methods together have achieved a 50% asymptotic polarization, which is unfortunately still below the >60% needed to realize such a cost reduction. Of course, we should in simulation achieve some safety margin above 60% to ensure real-life performance.