A new diagnostic of the plasma density profile near the edge transport barrier from measurements of the radial profile of the Hα line intensity

An edge transport barrier is now one of the most important subjects of controlled fusion research. The edge transport barrier is located in the plasma region where hydrogen atoms readily penetrate, so the intensity of the Hα (Dα) line is high enough. A new diagnostic method uses the well-known property of hydrogen atoms that the ratio of the ionization rate Si to the excitation rate Sv for the Hα line is nearly constant over a wide range of plasma temperatures and densities. An expression has been derived that relates the radial profiles of the plasma density and Hα intensity. The use of charge coupled device detectors makes it possible to measure the radial profile of Hα line intensity with a resolution ∼0.1 cm; a high intensity of the Hα line ensures a high time resolution ∼1 ms. A high resolution is thus achieved for the density profile calculated from the Hα intensity profile. The method was tested when studying the plasma density profile in the region of edge transport barrier in the L-2M stellarator. It has been shown that the density gradient varies during the barrier formation and that a fine structure of the density profile correlates with a character of the plasma transport near resonance magnetic flux surfaces