The JLab Electron-Ion Collider (JLEIC) is proposed to collider spin-polarized beams of electrons and ions at high energy and high luminosity- Sustaining high luminosity requires the use of bunched-beam electron cooling to control emittance growth in the circulating ion beam- The electron cooling requires that a bunched electron beam be coalesced with the ion bunches so that they travel with the same vector velocity through a long straight section in the collider ring- The electron beam must be confined by a ~1 T solenoidal magnetic field, so that each is confined to a tiny-radius spiral around a field line and cannot recoil transversely when it scatters from an ion- The cooling process requires extremely tight alignment of the magnetic field lines with the beam axis, with a tolerance that is less than the angle spread in the ion beam- For the desired ion beam emittance inn JLEIC, that alignment must be better than 10 ?rad over a cooling length of ~60 m- Such alignment has never been achieved experimentally in a solenoid- Misalignments can arise from irregularities in the solenoid windings, material variations in the flux return steel, and misalignments of winding modules- ATC has developed a design for a flux-washer method to suppress transverse components of magnetic field in the aperture of a solenoid- A stack of interleaved thin annular steel washers and ceramic spacers is assembled with micro-alignment and bonded to form long cylindrical columns- The columns are supported within the solenoid- When the solenoid is operated at ~1T field, each steel washer is unsaturated, and its flat face presents a strong boundary condition to shunt field components parallel to the face- Any transverse magnetic field that is generated by imperfections in the solenoid is thereby shunted through the washers so that it is shielded from affecting the beams traversing the cooling channel- Simulations indicate that this method should yield a factor ~10 suppression of transverse fields, so the alignmetn for cooling at JLEIC can be met
