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

Abstract

To cite this version: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, IOP Publishing, 2011, 44 (33) Abstract. 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.

Dependence of the source performance on plasma parameters at the BATMAN test facility

“…In order to cancel out the effect of the particular conditioning status of the source, the source has been conditioned for stable Cs conditions and stable performance in all following figures. The stability of the Cs conditions has been monitored with a Cs absorption diagnostics [15] applied at a vertical line of sight in front of the plasma grid, indicating a constant neutral Cs density in the order of 1 × 10 15 m −3 during the plasma pulses. Thus, the work function of the PG is expected to be at a stable value.…”

A collisional radiative model for caesium and its application to an RF source for negative hydrogen ions

“…For the caesium ground state density n(6 2 S) a lower limit between 6 10 14 m -3 and 7 10 14 m -3 has been determined during the present campaign by laser absorption spectroscopy [24] along a vertical LOS (YB1, see Fig. 4).…”

“…Due to low caesium ground state densities (lower as 10 16 m -3 , [24]) resulting in a high mean free path of photons and additionally a low intensity of the photon field, processes like self-absorption due to optical thickness of emission lines [16] or photo ionization can be neglected.…”

NIO1 diagnostics

“…Laser Absorption Diagnostic (LAD) allows measuring Cs density, inside the plasma source, in a more direct way than measuring the photon emissivity of excited neutral or ionized Cs atoms [5]. It is based on the absorption spectrum of a laser tuned to a resonant transition of atomic Cs, and it doesn't need the a-priori knowledge of the electron density and temperature.…”

“…It is based on the absorption spectrum of a laser tuned to a resonant transition of atomic Cs, and it doesn't need the a-priori knowledge of the electron density and temperature. Adopting the same layout used in the BATMAN (BAvarian Test MAchine for Negative ions source) experiment [5], in NIO1 LAD uses a distributed feedback laser diode with a power of about 150 mW, tuneable around the caesium resonant transition at 852 nm. A laser controller allows the coarse and the fine tunings of the laser wavelength through the modulation of the temperature and the current, respectively.…”