Cavity ring-down spectroscopy on a high power rf driven source for negative hydrogen ions

Cavity ring-down spectroscopy (CRDS) is a very sensitive diagnostic technique for absorption measurements. It is capable of measuring the absolute line-of-sight (LOS) integrated density of negative hydrogen ions (H-, D-) which induce a weak absorption (a similar or equal to 10(-6) cm(-1)) along a LOS in plasmas containing negative hydrogen ions. CRDS has been applied to a high power rf driven negative ion source which is now the reference source for the ITER neutral beam injection system. The rf source operates at low pressure (typically 0.3 Pa). Negative hydrogen ions are produced mainly by the conversion of hydrogen particles at a caesium coated surface achieving negative ion densities comparable to the electron density near the surface. It is shown that CRDS very reliably measures the absolute volume density of negative hydrogen ions in these sources. The densities range from 10(16) m(-3) in volume operation to 10(17) m(-3) in caesium seeded operation. The measured volume density close to the extraction system and the extracted current density change consistently while varying different source parameters, such as the total pressure or the input power applied to the source. Results are shown for measurements in hydrogen and deuterium discharges with caesium seeding. An additional absorption is measured in the afterglow of the discharge and is attributed to the caesium dimer Cs-2.