Laser-induced fluorescence (LIF) is a well established technique used to determine the neutral hydrogen density in fusion plasmas.There is a requirement to extend these measurements to the Divertor section of large plasma devices, but this is problematical due to the high levels of stray laser light and background plasma emission. Here we describe a method, based on harmonic saturated spectroscopy which depends on the nonlinear response of a saturated atomic system to discriminate against intrusive noise sources.
