In recent years, extremely rapid recombination as well as other effects in expanding thermal arc plasmas have been attributed to wall association of radicals (mainly H) and recirculation in the expansion vessel. Coherent anti-Stokes Raman scattering measurements on H 2 and HD when the arc burns on a mixture of H 2 and D 2 show new evidence for this view.
Meulenbroeks, R. F. G., Schram, D. C., Sanden, van de, M. C. M., & Mullen, van der, J. J. A. M. (1996). Wall association and recirculation in expanding thermal arc plasmas. Physical Review Letters, 76(11), 1840-1843. https://doi.org/10.1103/PhysRevLett.76.1840