Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources

U Fantz (1, 2) , C Wimmer (2)

1 IPP - Max-Planck-Institut für Plasmaphysik [Garching]
2 Lst. f. Experimentelle Plasmaphysik

The performance of negative hydrogen ion sources, which rely on the formation of negative hydrogen ions on a surface with low work function, depends strongly on the caesium dynamics in the source. A quantitative measurement of the amount of caesium in the source during plasma-on and plasma-off (vacuum phase) is highly desirable. The laser absorption technique is optimized for the diagnostics of neutral caesium densities close to the extraction surface on which the negative hydrogen ions are generated. The setup has been simplified as much as possible utilizing also an automatic data evaluation for online measurements at high power rf sources. The setup has been tested and calibrated in a small scale laboratory experiment. The system and the analysis of the D 2 caesium line at 852.1 nm is described in detail, including effects of line saturation and density depletion. The system is sensitive in the density range of 10 13 - 10 17 m -3 (path length of about 15 cm), allowing also for a temporal resolution of 40 ms. First very promising results from the negative hydrogen ion source are presented, such as the increase of the caesium density due to the caesium evaporation and time traces before, during, and after the discharge indicating a strong caesium redistribution.

Mots clés: Physical Sciences

U Fantz, C Wimmer. Optimizing the laser absorption technique for quantification of caesium densities in negative hydrogen ion sources. Journal of Physics D: Applied Physics, 2011, 44 (33), pp.335202. ⟨10.1088/0022-3727/44/33/335202⟩. ⟨hal-00644267⟩