  Understanding turbulent transport of charged particles in magnetized plasmas
often requires a model for the description of random variations in the
particle's pitch angle. The Fokker-Planck coefficient of pitch-angle
scattering, which is used to describe scattering parallel to the mean magnetic
field, is therefore of central importance. Whereas quasi-linear theory assumes
a homogeneous mean magnetic field, such a condition is often not fulfilled,
especially for high-energy particles. Here, a new derivation of the
quasi-linear approach is given that is based on the unperturbed orbit found for
an adiabatically focused mean magnetic field. The results show that, depending
on the ratio of the focusing length and the particle's Larmor radius, the
Fokker-Planck coefficient is significantly modified but agrees with the
classical expression in the limit of a homogeneous mean magnetic field.
