A comparison between laser induced fluorescence at Balmer-alpha and at Lyman-alpha for the measurement of neutral hydrogen densities in magnetically contained fusion plasmas

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Abstract
The authors examine the relative merits of laser induced fluorescence scattering at the Lyman-alpha and Balmer-alpha wavelengths for the measurement of both the spatial distribution and absolute magnitude of neutral hydrogen densities in fusion plasmas. The enhancements of the fluorescence signals for present and for saturation laser intensities are determined and signal to noise ratios are presented for a range of fusion plasma conditions. Details of the interpretation of the fluorescence signals, and the limitations, at the two wavelengths are discussed. In the hotter regions of the plasma, Balmer-alpha fluorescence scattering allows the measurement of neutral densities as low as 107 cm-3, whereas present intensities from Lyman-alpha sources permit fluorescence measurements only for neutral hydrogen densities approaching 1010 cm-3. Also presented is a technique for the spatial measurement of the electron number density based upon fluorescence scattering on the Balmer transitions, this process not being possible at Lyman-alpha for the range of electron densities in magnetically contained fusion plasmas. This may have applications in regions of the plasma otherwise difficult to diagnose, for example, near divertors and limiters.