Recent results from the dynamic aperture experiment at the SPS

Since 1986 dynamic aperture studies have been performed at the SPS in view of the LHC, the projected superconducting proton accelerator in the LEP tunnel. The aim of these experiments is to understand the aperture limiting effects of non-linearities and to define a model which allows the prediction of these effects in tracking studies. To this end the SPS, which is a very linear machine, is made non-linear in a controlled manner by 8 strong sextupoles powered in such 8 way that high order effects prevail. In this machine the short-term particle losses after seconds are well understood and in agreement with simulations. It has also been shown previously that power supply ripple in conjunction with the non-linearities can cause long-term losses after minutes. The 1992 experiment was aimed at providing enough date to allow a more quantitative comparison with a simulation model. Much care had to be taken to achieve stable and reproducible machine conditions and to perfect the instrumental tools needed for these delicate investigations