Speaker
Dr
Irakli Nanobashvili
(Andronikashvili Institute of Physics of the Ivane Javakhishvili Tbilisi State University)
Description
As it is well known transport in magnetised plasma is highly bursty, intermittent and has strong convective character. Coherent turbulent structures bring important contribution to such transport. They are formed intermittently on diffusive background and propagate radially
outwards at a speed which is a fraction of ion sound speed.
Different methods are used to study turbulent transport in magnetised plasma. We proposed method – hereafter “method of bursts” – which is based on selection of large amplitude fluctuations – bursts of plasma density (the method can be applied to any other fluctuating signal – floating potential etc.). After the selection of bursts from fluctuating signal we calculate their temporal characteristics - burst rate, inter-burst time and burst duration. Radial dependence of these characteristics together with statistical properties of fluctuations allow better understanding of plasma turbulent transport.
Method of bursts has been applied to the analysis of plasma turbulent fluctuations measured at the edge of Tore Supra tokamak. Physical picture, which is based on dynamics of coherent turbulent structures, for the explanation of experimental observations has been proposed. The method has been also used for the analysis of data generated by one of the two-dimensional fluid codes. Good agreement has been found between experiment and modeling and all this will be presented in detail.
Comparative analysis of plasma turbulent fluctuations measured on Tore Supra and CASTOR tokamaks has been performed. This analysis by means of the method of bursts allowed to conclude that blob-like structures should be responsible for plasma turbulent transport at the edge of Tore Supra tokamak, while at the edge of CASTOR we have elongated streamer-like structures.
Method of bursts has been also used to analyse the experimental data obtained during electrode biasing discharges on the CASTOR tokamak. Method of bursts allowed for the first time to demonstrate experimentally, that biasing splits coherent turbulent structures into larger number of smaller structures thereby reducing plasma transport and improving the confinement. The main reason is that biasing modifies the radial electric field and imposes strongly sheared poloidal rotation on plasma through Er × Bt electric drift.
Primary author
Dr
Irakli Nanobashvili
(Andronikashvili Institute of Physics of the Ivane Javakhishvili Tbilisi State University)
