Abstract
The future electron-ion collider (eRHIC) at Brookhaven National Laboratory demands a high-current, polarized, bunched electron beam [http://www.bnl.gov/cad/eRhic]. One of the challenges here is to combine the bunched beams generated by multiple cathodes so to address the issue of designing and prototyping a combiner with high-frequency (700 kHz) rotational magnetic fields. This article presents its design, and simulation, and details some of the test results from this unprecedented device.
Highlights
The planned upgrade of Brookhaven’s Relativistic Heavy Ion Collider (RHIC) is the future electron— Relativistic Heavy Ion Collider project
The state-of-the-art polarized electron cathode [1] delivers much less than 50 mA; its current is limited by the space charge near the cathode and the quantum efficiency of the polarized source
We decided to follow this method; it led us to developing rotating magnetic fields
Summary
The planned upgrade of Brookhaven’s Relativistic Heavy Ion Collider (RHIC) is the future electron— Relativistic Heavy Ion Collider (eRHIC) project. Another important issue must be resolved: Because of the geometry of the combining scheme, in the ideal situation, viz., a hard-edged distribution of the magnetic field (the bending field is uniform between the combiner’s end surfaces but is zero elsewhere), the beam will lack focusing in the bending direction (the X direction); focusing in the field direction (Y direction) after bending engenders bunch-tobunch variation of the transverse divergences If we correct this by adding a rotating quadrupole field downstream of the rotating dipole field, the effective emittance still will increase due to overlapping of the ellipses from bunch to bunch. The injection path deviates from the combiner’s axis and is close to the combiner’s nonlinear field region, which may increase the emittance
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