Simultaneous evolution of plasma rotation, radial electric field, MHD activity and plasma confinement in the STOR-M tokamak

Abstract

Radial electric field shear and poloidal plasma rotation are important factors affecting transport and confinement in tokamaks. Alteration of the electric field and plasma rotation in the vicinity of magnetic islands is also an important factor in tokamak plasma confinement. In the STOR-M tokamak, fast (∼1 ms) simultaneous alterations of the radial electric field, plasma rotation (M || = 0-0.4 in the plasma current direction), floating potential fluctuations in the periphery and MHD activity generated by rotating islands have been observed experimentally during normal ohmic discharges. The observed time and magnitude of the changes depend on the average electron density and poloidal beta at the beginning of the discharge. In discharges with high initial poloidal beta these changes are accompanied by a reduction in H α emission and an increase in the line averaged density. Drastic decreases in H α and increases in line averaged electron density and estimation of poloidal beta suggest that STOR-M confinement is significantly affected in ohmic discharges without an external additional energy input or biasing. MHD activity in STOR-M is damped when a negative electric field is observed at the limiter region of the plasma edge. MHD frequency is observed to decrease with the negative electric field.

Plasma confinement modification and convective transport suppression in the scrape-off layer using additional gas puffing in the STOR-M tokamak

“…The plasma confinement modification in the plasma bulk is accompanied by a strong variation of plasma rotation in the SOL region, similar to that observed previously in STOR-M [21]. However, the radial profiles of electron temperature, floating potential, ion saturation current and floating potential fluctuations do not seem to suggest the formation of a transport barrier in the SOL region.…”

Diamagnetic measurements in the STOR-M tokamak by a flux loop system exterior to the vacuum vessel

“…Reasonable agreement of about 10% between the two independent methods is shown. The Spitzer resistivity calculations suffer from the assumption that the effective ion charge of the plasma Z eff is constant at a value of 1.5 in STOR-M. A nominal value of Z eff = 1.5 is typically assumed in STOR-M. 18 It is also as- sumed that T i is 1/3 of the electron temperature T e . It is likely that Z eff shows some variation during the breakdown and current ramp up phase of the discharge, 19 and the value of 1.5 needs verification.…”

Interference of robust tori on the resonance overlap

“…We also point out that even though there are non-twist maps which approach the plasma dynamics in Tokamaks [19,20] they do not carry any robust tori. So, the inclusion of robust tori in the non-twist maps can improve the theoretical understanding of plasma confinement and simultaneously to suggest some new experimental setup [21]. This question will be present in a further work.…”