Study of momentum diffusion with the effect of adiabatic focusing

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.