Neoclassical radial electric fields are calculated in a tokamak with a flow by using a δf Monte Carlo particle simulation code[1]. The δf method solves the time development of the perturbation part of a particle distribution function δf=f-f_M, where f_M is the local Maxwellian. To include the effect of the plasma flow, the distribution function with a given flow is treated as a perturbation to the background f_M in the simulation code. The time development of the radial electric field E_r is solved from the obtained ion radial flux Γ_i. Using the δf code, the relaxation process of E_r towards the neoclassical steady-state, in which Γ_i vanishes, is simulated. It is known that the neoclassical steady state is described as a relationship between the parallel flow and radial electric field[2,3], relaxed states of E_r differ according to the given initial flow distribution. Some examples of calculation results are shown in the poster.
A simple analytic model of the time development of the radial electric field is also shown in a case with no temperature gradient. In the analysis, linearized drift kinetic equation is solved up to the order of O(r/R₀) and O(u²_||/v²_th), where u_|| is the background parallel flow. The solution of the steady state value of E_r with a given u_|| is shown to be in a good agreement with the simulation results.

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