The studies of impurities in tokamak plasma become important due to enhancement of the bremsstrahlung emission in high density operation, the dilution of fuel ions, and the energy loss by radiation. Considering that impurity behaviour has been studied using visible spectroscopic system in the recent campaigns of SST-1 tokamak. It is a steady state superconducting tokamak and having major and minor radiuses of 1.1 m and 0.2 m respectively, equipped with a copper-based central solenoid to provide the required loop voltage 1. At present, it is regularly operated with toroidal magnetic field, BT of 1.5 T and is having circular plasma with graphite limiter. The plasma current is in the range 70 - 95 kA and maximum electron density ~ 1x1019 m-3 and Te of 200 - 275 eV. Helium glow discharge cleaning is routinely carried out before the plasma operation.
Figure 1: Variation of O II and C III intensities plotted with Ip for SST-1 tokamak plasmas
In SST-1 tokamak, visible spectroscopy has been extensively used to study the impurity behaviour. 7 channel fiberscope system, where light is transported using optical fibres of 1 mm core diameter and NA 0.22, wavelength selected by an interference filter with 1 nm band width, and detected by photomultiplier tube, is regularly used to monitor the emissions from fuel gas (hydrogen) and low Z (carbon, oxygen, helium) species from the plasma. Visible Continuum emission around 536.0 nm with a bandwidth of = 3 nm, which is the spectral line free region, is also monitored using similar type of above mentioned PMT based system to infer the Zeff of plasma.
Figure 2 Temporal evolution of estimated Zeff plotted for two SST-1 discharges
It has been found that the plasma current shows increasing trend with the decrease of intensity of carbon and oxygen, which come into the plasma from the limiter due to plasma wall interaction. The increase in plasma current might be related to effective usages of Ohmic input power for the IP increases as the lower input power is gone to the radiation loss from carbon and oxygen as indicated by lower intensities of . The Zeff of the plasma is also estimated using the absolute intensity of bremsstrahlung emission, central line averaged electron density and core plasma electron temperature under the assumption of parabolic profile of ne and Te. Figure 2 shows the temporal evolution of plasma current (Ip), electron density (ne), central electron temperature (Te) and Zeff of two discharges. It is found that Zeff varies between ~ 2.0 to 4.0 in the discharges as shown in figure 2. Not only have those, variations of hydrogen recycling, carbon and oxygen behaviour with BT, and plasma electron density been also studied.
1 Saxena, Y. C., SST1 Team, Nucl. Fusion, 40 (6), 1069, 2000
|Affiliation||Institute for Plasma Research|
|Country or International Organization||India|