ABSTRACT
Since 1986 experiments are performed in the CERN SPS to study the beam dynamics in presence of strong non-linear fields intentionally introduced along the accelerator circumference. They are motivated by the need to refine the aperture and field quality criteria for the design of future large hadron accelerators like LHC, SSC, or RHIC. The experimental procedure consists in exciting already existing sextupoles in order to introduce in a controlled fashion non-linearities in an otherwise linear lattice. To probe large amplitudes, we give a large coherent deflection to a pencil beam of small emittance and momentum spread, or we create a large emittance beam with repetitive small amplitude coherent kicks. In the first case, a few hundred turns are sufficient to create a ‘hollow’ distribution of charges around the central orbit due to non-linear filamentation, in the second case, few seconds are required to spread-out the particles all around the available physical aperture of the accelerator. The beam behaviour is observed with several instruments: current transformers record lifetime, Schottky noise detectors give tune and tune-spread, flying wires provide transverse profile, and orthogonal pairs of position monitors are able to produce a phase space portrait. More recently, sinusoidal and random tune modulations have been added to simulate the effect on the beam stability of the unavoidable ripples in power supplies of the guiding magnets and the speed of the induced diffusion process has been measured using collimators and loss detectors. A review of the experimental results and of their interpretation is presented below.
