Test particle velocity diffusion coefficients for the fiducial simulation at high scattering rates (solid curves), corresponding to the non-resonant regime discussed in Section 2.2. Scatterers are stationary in the fluid frame. For high velocities, the numerical results are reasonably well approximated by Equation (9) (dashed curve), which describes velocity diffusion by non-resonant interactions with turbulent density fluctuations. The rapid increase in the diffusion coefficient with velocity at intermediate velocities ~few  c _s can produce a non-thermal tail in the distribution function (see Figure 9).