Measurement of hydrogenn-α-line Stark profiles in a turbulent plasma

Two hydrogen $n$-$\alpha$ transitions, $n=13\mathrm{}\to 12$ at $\lambda 88.7$ μm and $n=12\mathrm{}\to 11$ at $\lambda 69.0$ μm, were observed in a large-diameter $\theta$ discharge at electron densities between ${10}^{14}$ and 3 × ${10}^{14}$ ${\mathrm{cm}}^{-3\mathrm{}}$. Line profiles were measured with a Czerny-Turner infrared monochromator and a Ge:Ga photodetector at liquid-helium temperature. Both $n$-$\alpha$ lines exhibit strong and symmetric satellite features, with intensities comparable to their parent lines and displacements close to the electron plasma frequency ${\omega }_p$. These satellites can be interpreted in terms of plasmonatom collisions if long-wavelength fluctuations in this turbulent plasma are enhanced by a factor ≳ ${10}^3$ above the thermal level at $T_e\simeq 0.7$ eV. As an independent estimate for the level of turbulence, the continuum emission in the regions corresponding to ${\omega }_p$ and $2{\omega }_p$ was measured, and found to be well above the level expected for a thermal plasma. Comparison with fluid theory, after radiative transfer corrections, shows satisfactory consistency with the $n$-$\alpha$ satellite diagnostics.