Improved signal-to-noise in fluorescence scattering by means of nonlinear pumping with a frequency modulated dye laser

Abstract

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. For a pump laser modulated such that the intensity varies as I=I0 sin2 wt, the Fourier transform amplitude coefficients of the resulting fluorescence scattered signal take on finite values. Therefore selective frequency detection at, say, 4w makes possible high rejection of the dc component of plasma emission and also stray light at the modulation frequency of the laser. Excitation to the required atomic level is by a flash tube pumped frequency modulated Balmer Beta dye laser at 486 nm, with detection at the same wavelength. The optimization of the nonlinear pumping process, i.e., the degree of saturation for the most efficient production of detectable harmonics, is outlined. Also the anticipated signal-to-noise levels for the pertinent plasma conditions are given.