[en] Drift-cone instability is an obstacle when creating thermonuclear reactor based on an open magnetic trap. Additional plasma losses due to instability development may be calculated by quasilinear diffusion equation. The paper is aimed at consequent derivation of quasilinear diffusion equation as well as obtaining of a number of simplified expressions for quasilinear diffusion coefficient. Equation of ion quasilinear diffusion in an adiabatic trap at oscillation frequency close to ion cyclotron one is obtained. Accurate diffusion coefficient value is compared with approximate values received at different approximations