Atomic Hydrogen Densities in Expanding Thermal Plasmas

A cascaded arc produced hydrogen plasma expansion was studied by means of two photon absorption laser induced fluorescence (TALIF). In a weakly magnetised jet (up to 40 mT) a characteristic colour change from red to blue has been reported earlier. Particularly the intense plasma background radiation at the detected fluorescence transition ($H_\alpha $ line) hampers the application of a straightforward TALIF detection setup. Therefore a gated photomultiplier tube was used to measure and compare the TALIF signals under non- and weakly magnetised conditions. Results of the axial and radial H densities will be presented.