Laser Induced Breakdown Spectroscopy from fs to ns laser pulses for the detection of deuterium in fusion plasma walls

The problem of fuel retention in the walls in contact with the plasma in Tokamak’s devices can be addressed by LIBS surface diagnostics or both insitu and in machines dedicated to plasma-wall interaction studies. The interest in the depth profile of fuel adsorption makes it attractive to employ a fs laser which is reported to produce geometrically regular craters with more vertical walls compared to common ns lasers. On the other hand, it is expected that the plasma plume produced by fs shots is less energetic than that of ns shots, giving rise to less rich and intense spectra of atomic lines. The geometry of light collection is also an issue. Collinear detection is preferable for its simplicity, that better adapts to complex machines where only one optical window is available. It nevertheless suffers the overlap of a blackbody continuum radiation whose source is located on the sample surface. This overlap can limit the signal to noise ratio. On the other hand, 90 deg. observation at a distance from the target, while suppressing the continuum radiation, greatly reduces the signal strength and calls for two optical viewports on the machine. Here we report on studies addressing these issues, together with further variable conditions, like the gas pressure in the environment of the plume.