Experimental observations of local plasma parameters in the COMPASS divertor in NBI-assisted L-mode plasmas
Abstract
This paper presents an experimental characterization of the local plasma parameters in the COMPASS open divertor in deuterium, L-mode plasmas by means of Langmuir probes during neutral beam injection (NBI) heating. The effect of NBI heating of the core plasma on the local divertor plasma parameters is investigated under different plasma configurations and different directions of the magnetic field and plasma current for different line-averaged electron densities.
At low line-averaged densities below 6 x 10(19) m(-3), a low NBI heating power level does not lead to any significant changes in the plasma parameters in the divertor region. It is also found that for a reversed magnetic field the influence of NBI in the divertor is stronger at higher line-averaged electron density and plasma current.
In the divertor, the impact of the NBI heating is more pronounced at high delivered powers (above P-NBI = 320 kW) and line-averaged densities (above 7 x 10(19) m(-3)). The electron temperature and the floating potential at the outer target increases respectively with 10-50% and 100%, while the plasma potential in the inner divertor increases twofold.
At a higher line-averaged density, the NBI heating affects the edge plasma, namely, the electron temperature in the midplane scrape-off layer and the heating moves toward the divertor. Thus, a twice as high electron temperature is observed in the divertor and a bi-Maxwellian electron energy distribution function arises.