Probe method for measuring the diffusion coefficient in edge plasma

A probe method to measure the diffusion coefficient and the ion temperature in the edge plasma is proposed. The probe is constructed as an asymmetric double probe, where one electrode is sensitive to the direction of magnetic field while the other is insensitive to it. The ratio of ion saturation currents to two electrodes is used to determine the diffusion coefficient from which the ion temperature is estimated. The saturation currents are calculated by taking the anisotropic diffusion to the probe into account. The present probe method is applicable in the edge plasma even with such a complicated magnetic field as that in helical devices.