Fusion Science and Technology / Volume 51 / Number 1 / January 2007 / Pages 46-53
Technical Paper / Stellarators
Various types of transport barriers have been studied in the Compact Helical System. In addition to the neoclassical transport barrier, the edge transport barrier (H-mode) was studied using the high-power heating of two coinjection neutral beam injections. A density pedestal is formed after the transition that is indicated by the drop of H emission signal. The heating power threshold for the transition was investigated by varying the heating power. Its dependence on the density and the magnetic field is close to the H-mode scaling obtained in tokamaks. The dependence of the power threshold on the magnetic field configuration was also found. Local density fluctuation was measured with beam emission spectroscopy, which observed harmonic oscillations appearing after the density pedestal was formed. For L-mode plasma, long-distance coherence of the potential fluctuations were measured with two sets of heavy ion beam probes (HIBPs). Those coherent modes are supposed to be the geodesic acoustic mode part of zonal flow. Turbulent particle flux was also measured with HIBP, and its change with internal transport barrier formation is demonstrated.