Collisional excitation and emission ofH_αStark multiplet in fusion plasmas

Abstract

We study the excitation of parabolic Stark states in hydrogen atoms by collisions with fast ions. It is shown that excitation cross sections are very sensitive to the angle between the electric field and the projectile velocity. The calculated collisional data are implemented in a newly developed collisional–radiative model involving parabolic quantum states of hydrogen. Our simulations are shown to explain the frequently observed non-statistical behaviour of the Hα component intensities under typical conditions of a motional Stark effect (MSE). A good agreement with the MSE data from the Joint European Torus (JET) for emission of the σ and π components (Mandl et al 1993 Plasma Phys. Control Fusion 35 1373) is obtained for the first time.

Forward Modeling of Motional Stark Effect Spectra

“…More sophisticated collisional radiative models, e.g. to account for alignment effects [18,19], could be used instead.…”

“…However, deviations from statistical distributions are observed the larger the more the density decreases from about n < 10 (e.g. [10,19,30]). In other words, even the wavelength integrated MSE emission is polarized.…”

“…For analyses, this model underestimates the n = 3 population and cannot resolve non-statistical distributions in sublevels. More detailed CRMs for beam particle populations are reported, e.g., in [19,33].…”

“…Partly overlapping, partly newly developed, the forward model presented here involves a collisional-radiative beam-plasma model simulating the beam attenuation. Alignment effects [18,19] and resulting deviations in emission line ratios are not considered in the forward model but the model is structured to include respective model extensions. In a fit version of the code, a parameter involving corresponding deviations (polarization factor) is included.…”

Excitation of the n=2 levels of hydrogen by proton impact for hydrogen beam diagnostics

“…These diagnostics, in turn, depend critically on the existence and accuracy of atomic data for charge transfer and excitation. In particular, collisional-radiative models need not only cross sections for atomic collisions but also the full set of density matrix elements [1,2]. The data needed for such modeling underpinning motional Stark effect (MSE) diagnostics, the subject of the present work, includes the density matrix elements for excitation of atomic hydrogen (from neutral beam injection) by plasma protons, fusion alpha particles, and ions of the most significant impurities, beryllium and carbon.…”