  In this paper, we investigate the stability of the particle trajectories in
Fixed Field alternating gradient Accelerators (FFA) in the presence of field
errors: The emphasis is on the scaling radial sector FFA type: a collaboration
work is on-going in view of better understanding the properties of the 150 MeV
scaling FFA at KURRI in Japan, and progress towards high intensity operation.
Analysis of certain types of field imperfections revealed some interesting
features about this machine that explain some of the experimental results and
generalize the concept of a scaling FFA to a non-scaling one for which the tune
variations obey a well defined law. A compensation scheme of tune variations in
imperfect scaling FFAs is presented. This is the cornerstone of a novel concept
of a non-linear non-scaling radial sector fixed tune FFA that we present and
discuss in details in the last part of this paper.
