Density measurement techniques for negative hydrogen ion source

Negative hydrogen sources have applications in a variety of fields ranging from the neutral beam heating systems for nuclear fusion devices to particle accelerators. To characterize a negative hydrogen ion source it is imperative to measure the fraction of negative hydrogen ions (H^-) produced in the plasma. Typically, the density is small compared to ions and this makes their detection difficult and error prone. In a wave heated magnetized rf plasma device (HELEN-I) with no Caesium injection, the negative hydrogen ion production is quantified with a laser photo detachment (LPD) diagnostic. Laser photo detachment method uses a Langmuir probe to measure the current (I_PD) due to the photo detachment of electrons from negative hydrogen ions. The Langmuir probe is sufficiently positively biased to draw the electron saturation current Iess. When the laser is shot, a sharp increase in the current takes place. This increment in current corresponds to the IPD. A lock-in technique is used to obtain this small IPD from the total current drawn by the probe. This gives more reliable and less error prone results than conventional LPD analysis. The results have been verified with the Optical Emission Spectroscopy, which makes use of, and lines of the hydrogen spectrum to calculate H^- density. (author)