Measurement of plasma-electron temperature and density in front of a neutraliser plate by He I line intensity ratios

Abstract

The plasma-electron temperature T e and density n e in front of a neutraliser plate of a limiter have been obtained from the line intensity ratio of He I line transitions. The emission is due to the interaction of the scrape-off-layer plasma with the helium recycling neutrals. In this region typical plasma parameters are: 5eV < T e < 30eV and 10 18m −3 < n e < 10 19m −3. The same atomic transitions are used as in the helium thermal beam technique used for n e and T e profile measurement in the tokamak plasma boundary. The choice of the correct line of view determines the capacity of this method to obtain valuable information and determines its spatial resolution, since the passive plasma emission is integrated over the whole observing chord. A good correlation has been found between n e and T e estimations by this technique and local Langmuir probe results. A high signal-to-noise ratio are obtained even in deuterium discharges with down to 15% helium. Due to its good time resolution and non-intrusivity this new method would be attractive for basic plasma edge studies in actual devices. In future magnetic fusion reactors, operating with high convective power to the limiters or divertors, the plasma damage would make difficult the use of fixed Langmuir probes. If helium is naturally present as fusion ashes in the plasma the presented non-intrusive method would be an alternative, permitting a continuous monitoring of T e and n e in front of the neutraliser plates over large surfaces.