Fusion-product spectral linewidth in D-D, D-T, and 3He-D plasmas as an ion speed-distribution diagnostic

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Abstract
A Monte Carlo simulation has been used to study the fusion‐product spectral line shape in high‐temperature thermonuclear and beam‐driven plasmas in order to determine whether line‐shape measurements are a viable technique to determine the reacting ion speed distribution and its mean energy. Specific reactions considered include D(d,n), T(d,n), and 3He(d,p). Several speed distributions characteristic of ‘‘mirror‐confined’’ and beam‐driven plasmas were assumed, including Maxwellian, monoenergetic, three‐component Gaussian (characteristic of injected neutrals), and an injected beam reacting with Maxwellian targets. Mean energies in these distributions were varied over the range 1–100 keV. Results indicate that symmetrical Gaussian spectral lines are generated for isotropic plasmas regardless of ion energy or speed‐distribution form. However, the location and width of the Gaussian depends not only on the mean ion energy, but also on the form of the ion speed distribution and its angular distribution. Generally, the linewidth is proportional to the square root of the mean ion energy, but the proportionality constant varies significantly among the speed and angular distributions considered. Thus the angular distribution and functional form of the reacting ion speed distribution must be known before measured Doppler width in the product spectrum may be used to infer the mean ion energy of the reacting plasma.

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