State selective charge-exchange cross sections in collisions between C^q+ ions with sodium atoms

Monitoring physical parameters of the plasma is essential to investigate the efficiency of the confinement. The neutral alkali beams, such as lithium and sodium, were shown to be invaluable for measuring turbulence and electron density profiles in the boundary plasma [1]. These beams have also been proven to be useful for measuring local impurity properties using charge-exchange recombination spectroscopy [2]. The cross section calculations are vital to determine which spectral line can be significantly modulated by the charge-exchange (CX) processes between beam atoms and plasma ions.

In this work, the 3-body classical trajectory Monte Carlo (CTMC) method [3] was used to determine the principal quantum number (n) and the orbital angular momentum quantum number (l) dependent CX cross sections for sodium atom and carbon ion collisions. The projectile carbon ion charge state was taken into account from the single charge till the fully stripped ion state. We performed the calculations for 35 keV and 50 keV impact energies.

We found that the CTMC modelling and the experiments on the alkali beam of stellarator Wendelstein 7-X agree that for q=6 carbon ions the electron capture to the n=8 state occurs with the largest probability, while in case of the q=5 carbon ions the same holds for the n=7 states. The CTMC calculations indicate that in these cases the cross sections have a positive correlation with l.