High density plasma devices are important for a range of current and future applications including tokamak divertor research, negative ion sources and recently as plasma sources for wakefield accelerators. One such plasma source is the MAGPIE linear helicon device at the ANU. A major concern is the apparent density limit observed in helicon devices which is directly related to the occurrence of neutral depletion in the plasma.

This research has involved the setting up of a two-photon absorbed laser-induced fluorescence (TALIF) diagnostic on the MAGPIE device to measure the neutral atomic hydrogen species. The TALIF technique will be discussed in detail including the theory and operation of the diagnostic.

The density and temperature of atomic hydrogen in high power (<20kW) plasma discharges will be presented. Observations of neutral depletion of the atomic hydrogen and the mechanisms that are causing the depletion will be discussed. To the authors knowledge these are the first experimental measurements of neutral depletion of atomic hydrogen in a linear plasma device.