Origin and dynamics of plasma blob

Abstract

Convective transport of plasma at edge and Scrape-off-layer (SOL) of plasma is an important issue in all the fusion machines needs to be addressed [1-6] for future fusion machines. Moreover, it is observed in all the fusion devices that the edge and SOL region where plasma detachment region is observed is having higher base pressure and the electron temperature of plasma is very low ∼ 1 eV [4-6]. Here the effect of base/ambient pressure on plasma parameters is reported. Electron density profile of plasma is calculated using Langmuir probe. Langmuir probe measurement at ∼ 8cm away from Gun mouth shows high density fluctuations. The FFT of density fluctuations shows a peak at ∼25 kHz. Detail analysis suggests R-T instability at this region. Variation of electron temperature of argon plasma with base pressure is estimated using emission spectroscopy data. It is observed that the electron temperature of plasma is ∼ 0.3–1 eV. However, at higher base pressure ∼ 1 mb or above a number of prominent argon lines are observed unlike that at lower base pressure. This indicates formation of more fast neutrals inside bulk plasma. Fast imaging results (∼9100 fps) shows blob formation and transport from main plasma column at base pressure 1 mb and above. The velocity of blob is ∼ 213 m/s, calculated using time of flight technique. For these experimental findings it can be proposed that the asymmetry force due to fast neutrals inside bulk plasma and slow neutrals outside main plasma column is responsible for blob formation at higher base pressure [7]. The detail mechanism is presented in this communication.