Production of Highly Excited Neutral Atoms for Injection into Plasma Devices

We have considered the production of highly excited states of atomic hydrogen by charge-exchange reactions, discussing first the reaction ${\mathrm{H}}^{+}+\mathrm{H}\mathrm{}\left(1s\right)\mathrm{}\to \mathrm{H} \left(\mathrm{excited}\right)+{\mathrm{H}}^{+}$; we have then used the results for this reaction as a basis with which to compare excited-state production in hydrogen atoms by charge exchange in gases other than atomic hydrogen. We have expressed our results in terms of the equilibrium ratio $\frac{i_n}{i_c}$, that is, the ratio of the flux of excited states with principal quantum number $n$ to the (constant) flux of protons incident on the neutralizer. We find that with atomic hydrogen as neutralizer the ratio $\frac{i_n}{i_c}$ has a maximum of about $\frac{0.75}{n^3}$ at 20 keV. For alkali atoms such a lithium and sodium, the maximum value of $\frac{i_n}{i_c}$ occurs at about 10 keV, being of similar magnitude to, but probably smaller than, the value for atomic hydrogen. In the case of the inert gases helium, neon, and argon, the maximum value of $\frac{i_n}{i_c}$ occurs in the vicinity of 50-70 keV and again is of similar magnitude to, but probably slightly larger than, the maximum value for atomic hydrogen. The inert gases thus offer a number of advantages over alkali gases as neutralizers: they probably yield as large if not larger values of $\frac{i_n}{i_c}$ at maximum; the maximum occurs at higher energies, so that the natural decay lengths of the excited atoms are correspondingly longer; they may be very much more easily handled experimentally.