  Momentum diffusion of the energetic charged particles is an important
mechanism of the transport process in astrophysics, physics of the fusion
devices, and laboratory plasmas. In addition to the uniform field momentum
diffusion, we obtain the modifying term due to the focusing effect of the
large-scale magnetic field. After evaluating the modifying term, we find that
it is determined by the sign of the focusing characteristic length and the
Fokker-Planck coefficients $D_{\mu\mu}$, $D_{\mu p}$, $D_{p\mu}$, and $D_{pp}$.
It is shown that we get a new second order acceleration mechanism in this work.
