The LHC dynamic aperture

In 1996, the expected field errors in the dipoles and quadrupoles yielded a long-term dynamic aperture of some 8σ at injection. The target was set to 12σ to account for the limitations of our model (imperfections and dynamics). From scaling laws and tracking, a specification for the field imperfections yielding the target dynamic aperture was deduced. The gap between specification and expected errors is being bridged by (i) an improvement of the dipole field quality, (ii) a balance between geometric and persistent current errors, (iii) additional correction circuits (a₃,b₄). With the goal in view, the emphasis has now turned to the sensitivity of the dynamic aperture to the optical parameters. The distortion of the dynamics at the lower amplitudes effectively reached by the particles is minimized by optimizing the distribution of the betatron phase advance. At collision energy, the dynamic aperture is limited by the field imperfections of the low-β triplets, enhanced by the crossing angle. With correction of the most important aberrations, the dynamic aperture reaches the target set to 10