Convergent coupling of helium to the H/D background in magnetically confined plasmas

Abstract

The alpha particles are coupled to the hydrogen/deuterium background plasma via charge exchange processes. Population kinetics calculations show that in the framework of standard models, the charge transfer from hydrogen/deuterium excited states to helium leads to a strong divergence of the atomic populations and to a critical dependence on the number of quantum states retained in the simulations. The divergence is related with the large crosssections for charge transfer from excited states of the background atoms. For typical parameter conditions (e.g., ITER) the charge-exchange-induced population flow may even reach the level of flow typically induced by turbulent diffusion. A self-consistent excited-states coupling approach of atomic kinetics is proposed, which avoids divergences and critical dependence on the number of states. This approach also removes free parameters from the system and provides therefore the ideal basis for a pure diffusion analysis in space which is requested to test global effects of turbulent transport theories.The confinement of the fusion plasma is one of the most important issues in the magnetic fusion research and intensive efforts have therefore been devoted to the understanding of the particle transport. However, the physical processes that underlie plasma transport in torodially confined plasmas are not so well understood. The plasma transport induced by Coulomb collisions (the so-called classical or neo-classical transport) is often much less than what is actually observed [1, 2] and thus the transport is called anomalous.Methods which determine the particle transport independent of theoretical plasma models are therefore of extraordinary and fundamental importance in the magnetic fusion research. Spectroscopic methods have turned out to be very effective. One of the most powerful spectroscopic methods is based on the space-and time-resolved observation of line emission from impurity ions [1][2][3]. This emission is simulated from an atomic physics model with given temperature and density profiles [1,4] and contrasted with the experimental data. Diffusion and convective velocity parameters, D and V , respectively are then determined by matching best the experimental observations [1-3].

Isotope effect in charge-transfer collisions of H with He+

“…4, for details), so that in the interaction region the order of these two states is reversed. We do not consider the three n = 1 states (two 1 are much lower in energy and thus not expected to give a significant contribution to the cross section. Moreover, it was previously shown [2] that it is not necessary to include the n = 3 manifold to get an accurate description of the charge-transfer reaction (1) involving n = 2 states, so we will neglect them in our calculations.…”

“…On the other hand, the plasma of a tokamak is composed of H (or rather of the heavier isotopes D and T) which fuse to form alpha particles. The helium ash can be considered an impurity, and the emission lines of He resulting from the charge transfer of He + ions with hydrogen can be used in the diagnostic of the plasma [1].…”

Ab initiocalculation ofH+He+charge-transfer cross sections for plasma physics

“…When the rotational couplings are taken into account, the only difference is an additional sum over Λ ′ in equation (20). The total cross sections from the H(nl) states, with n = 2, 3 are shown in figure 9 on a log-log scale.…”

Ab initiocalculation of the 66 low-lying electronic states of HeH+: adiabatic and diabatic representations

“…However, to model the intensity of these emission lines, knowledge of the cross sections of the charge transfer processes which populate the emitting levels of helium up to n = 4 is essential. At low or very low collisional energy, the theoretical description of charge transfer requires a molecular approach and the calculation of the excited states of the HeH + quasi-molecule (Rosmej et al 2006). From a theoretical point of view, the hydrohelium cation is the simplest closed-shell heteronuclear molecule and therefore a considerable amount of work has been dedicated to highprecision calculations of its ground state, including accurate description of relativistic and non-adiabatic effects (Stanke et al 2008).…”