This paper provides a systematic investigation on how plasma conditions (magnetic configuration, collisionality and beta) affect the edge radial electric field (Er) structure derived from charge exchange spectroscopic measurements in the Large Helical Device. We found that the location at which the spatial derivative in the Er structure (∇Er) had a local maximum value shifts outwards from the vacuum last closed flux surface location as the volume-averaged plasma beta (the ratio of the plasma pressure to the magnetic pressure, β ≡ 〈βdia〉 measured by a diamagnetic loop) becomes large. The loss of electrons on the open field lines, leaving the ions behind, might be the physics that determines the positive Er structure just outside the plasma boundary. The collisionality dependence of the experimentally observed Er values inside the plasma boundary is found to be consistent with a neoclassical ambipolar condition, while the Er formation outside the plasma boundary is different from the trend that is predicted by the neoclassical theory inside the plasma boundary. Furthermore, experimental data on saturation in the shift of the location even in a higher β regime (3% ⩽ β ⩽ 4.5%) are presented, inferring and predicting shrinkage of the plasma (i.e. a reduction in the averaged minor radius) in the higher β regime in which the Shafranov shift becomes larger.
Acceleration of plasma electrons