Dispersion Free Steering for YASP and dispersion correction for TI8

The LHC injection tests performed in August 2008 revealed a horizontal dispersion mismatch for both the TI2 and the TI8 transfer lines. While the error for TI2 is acceptable and most likely due to an error of the initial conditions at the entrance of the line, the TI8 dispersion error is rather severe. No unambiguous source of the dispersion mismatch could be identified, even tough large strength errors on some quadrupoles could explain part of the problem [1]. Steering and energy matching of both TI2 and TI8 are tricky due to the limited sampling of the trajectories by the beam position monitors. The possibility that the dispersion error may be due to the kicks from misaligned quadrupoles and from orbit correctors was pointed by E. Gianfelice and first tests yielded encouraging results [2]. The SPS and LHC steering program YASP was therefore upgraded to be able to perform combined orbit (or trajectory) and dispersion corrections, so called Dispersion Free Steering (DFS) that was already used at LEP [3]. This note describes the principle of DFS and analytic expressions to build the dispersion response needed to apply the algorithm. Results of DFS applied to the TI8 and LHC dispersion data are presented. The results indicate that the dominant source of the dispersion error is not the steering, even though it is probably contributing to the error. The LHC injection tests performed in August 2008 revealed a horizontal dispersion mismatch for both the TI2 and the TI8 transfer lines. While the error for TI2 is acceptable and most likely due to an error of the initial conditions at the entrance of the line, the TI8 dispersion error is rather severe. No unambiguous source of the dispersion mismatch could be identified, even though large strength errors on some quadrupoles could explain part of the problem [1]. Steering and energy matching of both TI2 and TI8 are tricky due to the limited sampling of the trajectories by the beam position monitors. The possibility that the dispersion error may be due to the kicks from misaligned quadrupoles and from orbit correctors was pointed by E. Gianfelice and first tests yielded encouraging results [2]. The SPS and LHC steering program YASP was therefore upgraded to be able to perform combined orbit (or trajectory) and dispersion corrections, so called Dispersion Free Steering (DFS) that was already used at LEP [3]. This note describes the principle of DFS and analytic expressions to build the dispersion response needed to apply the algorithm. Results of DFS applied to the TI8 and LHC dispersion data are presented. The results indicate that the dominant source of the dispersion error is not the steering, even though it is probably contributing to the error.