New Method for Spectroscopic Diagnostics of Tokamak Edge Plasmas Based on a Peculiar Stark Broadening of Hydrogen or Deuterium Spectral Lines Emitted by the Injected Neutral Beam

We study the situation practically relevant to the magnetic fusion machines, especially to International Thermonuclear Experimental Reactor (ITER), where a highly energetic beam of hydrogen or deuterium atoms is injected into a strongly magnetized plasma. Using an advanced formalism, we provide analytical results for the anisotropic Stark broadening of the hydrogen or deuterium spectral lines emitted by the beam atoms. We find three fundamental new features. First, we show that in the situation of ITER, both the plasma ions and the plasma electrons contribute equally to the Stark broadening, which amounts to doubling the electron density. This is a counterintuitive result that was never found previously in the literature. Second, we demonstrate that in the situation of ITER, this anisotropic Stark broadening does not depend on the reduced mass of the "radiator–perturber" pair. This is a clear distinction from the isotropic Stark broadening that depends on the reduced mass of the "radiator–perturber" pair (the latter being especially significant for the dynamical broadening by plasma ions). Third, we show that in the situation of ITER, the anisotropic Stark broadening is strictly linear with respect to the electron density <tex-math notation="LaTeX">Nₑ</tex-math> —in distinction to the isotropic Stark broadening previously studied in the literature in a variety of versions. The strictly linear <tex-math notation="LaTeX">Nₑ</tex-math> -dependence of the anisotropic Stark broadening, analyzed in this article, is yet another counterintuitive result. On the practical side, we propose a new method for measuring the electron density <tex-math notation="LaTeX">Nₑ</tex-math> of the edge plasmas of ITER based on measuring the width of the Paschen-alpha line, emitted by either the deuterium beam atoms or by the hydrogen beam atoms. We provide analytical results for the Stark width of these lines in the conditions of ITER. We explain the advantage of the proposed new diagnostic method over the traditional Stark-broadening-based method for deducing <tex-math notation="LaTeX">Nₑ</tex-math> .