Symmetric achromatic low-beta collider interaction region design concept

We present a new symmetry-based concept for an achromatic low-beta collider interaction region design. A specially-designed symmetric Chromaticity Compensation Block (CCB) induces an angle spread in the passing beam such that it cancels the chromatic kick of the final focusing quadrupoles. Two such CCBs placed symmetrically around an interaction point allow simultaneous compensation of the 1st-order chromaticities and chromatic beam smear at the IP without inducing significant 2nd-order aberrations to the particle trajectory. We first develop an analytic description of this approach and explicitly formulate 2nd-order aberration compensation conditions at the interaction point. The concept is next applied to develop an interaction region design for the ion collider ring of an electron-ion collider. We numerically evaluate performance of the design in terms of momentum acceptance and dynamic aperture. The advantages of the new concept are illustrated by comparing it to the conventional distributed-sextupole chromaticity compensation scheme.

Wide spin resonance with an rf-bunched proton beam

We recently used an rf solenoid to study the widths of rf spin resonances with both unbunched and bunched beams of 2.1 GeV_c polarized protons stored in the COSY synchrotron. A map, with unbunched beam at different fixed rf-solenoid frequencies, showed a very shallow possible depolarization dip at the resonance. Next we made frequency sweeps of 400Hz, centered at similar frequencies, which greatly enhanced the dip. But, with a bunched proton beam, both the fixed-frequency and frequency-sweep techniques produced similar maps, and both bunched maps showed full beam depolarization over a wide region. Moreover, both were more than twice as wide as the unbunched dip. This widening of the proton resonance due to bunching is exactly opposite to the recently observed narrowing of deuteron resonances due to bunching.