Role of poloidal flows on the particle confinement in a current-less toroidal device

It is well known that fusion devices such as Tokamaks have complex magnetic geometries and hence performing a detailed study of an individual physics phenomenon is often an involved task. A current-less toroidal device (CTD) provides an alternative scenario to serve as a testbed to perform studies of basic plasma instabilities which are relevant to Edge/SOL region of fusion devices such as a Tokamak. In a CTD, plasma is confined by the application of toroidal and vertical magnetic field only. Therefore, an effective rotational transform ceases to exist. The device BETA at the Institute for Plasma Research (IPR) is one such CTD with a plasma major radius of 45 cm and minor radius of 15 cm and a maximum toroidal field of 0.1 Tesla. An experimental study of particle confinement with variation in parallel connection length for two different sources has been performed in BETA. Two sources used are hot cathode and Microwave source. In the hot cathode source, a tungsten filament of 2 mm diameter and 20 cm length has been mounted at the minor axis at one particular toroidal location. The discharge is struck between the hot cathode and the vessel wall. It creates a potential well and hence produces a radial electric field. This mean radial electric field provides a poloidal rotation to the plasma. In Microwave source, a 2.4 GHz microwave wave is launched from low field side and resonance occurs at one toroidal plane to produce the discharge and has very weak zeroth order radial electric field. In the present work, detailed experiments have been performed with the above mentioned sources and with variation in parallel connection length. As reported earlier, mean parallel connection length plays a significant role in controlling the nature of equilibrium, fluctuation and flow in a current-less toroidal device. It has been observed that the particle confinement improves significantly with the variation in connection length and it also depends on the presence of poloidal flow. The details of experimental findings from above mentioned experiments will be presented. (author)