Multiple Gas Puff Induced Improved Confinement Concomitant With Cold Pulse Propagation In ADITYA-U Tokamak.

Multiple gas puff of H$_{\mathrm{2}}$ and D$_{\mathrm{2}}$ of appropriate magnitude are applied during current flat-top in ADITYA-U tokamak to study the cold-pulse propagation and effect of these puffs on plasma confinement. The results indicate the simultaneous occurrence of plasma detachment along with propagation of a cold pulse, i.e., a decrease in the edge temperature ($\rho $ \textasciitilde 0.9-1.0) and an increase in the core temperature on a time-scale less than the energy confinement time, after each gas puff. Initial increase in the radiated power, H$_{\mathrm{\alpha }}$ and CIII signals and subsequent improvement in confinement indicate plasma detachment from the limiter. The increase in energy confinement time by a factor of 2-3 is due to the density peaking along with the suppression of edge density fluctuations due to flattening of density profile in the edge due to gas puff. Both the cold-pulse and the detachment phenomena have a density threshold, i.e., above n$_{\mathrm{e}}$ \textasciitilde 2.7 x 10$^{\mathrm{19}}$ m$^{\mathrm{-3}}$, no detachment and propagation have been observed.