Enhanced confinement observation in the presence of negative currents at the edge of the S.I.N.P. tokamak

Recently, an enhanced confinement regime has been observed in the S.I.N.P. tokamak. This regime has been realized by tuning the Vertical Magnetic Field (Bv) so as to fast ramp-up the current initially, resulting in the formation of hollow current density profiles. The subsequent current decay has been observed to drive negative edge currents within the discharge column, which subsequently help in a transition to an improved confinement properties regime. The observation of the edge negative currents has been made using Internal Langmuir Magnetic Probes (ILMP) and an Internal Rogowskii Coil (IRC) [R. Narayanan and A.N. Sekar Iyengar, RSI, 77,33503 (2006)]. These experimental results in the light of recent observations of central zero current density in JET [N.C. Hawkes et al, PRL, 87, 115001 (2001)] and JT-60U [T. Fujita et al, PRL, 87, 245001 (2001)] has an important impact on the present fusion research. These experimental results provide experimental verifications, that of generating negative currents within the discharge column. These negative currents can, in turn, effectively lead to an L-H transition of the plasma column. This paper will present the experimental results of this enhanced confinement discharge and a plausible model for the same. (author)