On a heating mechanism for cold hydrogen and deuterium atoms produced at the edge of a tokamak plasma

Abstract

Spectroscopic measurements of the D and H line profiles emitted within the edge region of a tokamak plasma, have revealed the existence of a cold central component, broadened mainly by the Zeeman effect arising from the confining magnetic field. Evaluation of the Doppler broadening suggests that the cold component is probably produced by electron impact-induced molecular dissociation, dissociative excitation being one of the few mechanisms which can explain the formation of atoms of kinetic energy around 0.2 eV against a background of comparatively hot electrons and ions. Further analysis of these line profiles, observed along different directions in the equatorial plane and under various tokamak discharge conditions, reveals, in addition to this effective `cold temperature', an effective `lukewarm temperature', which we explain in terms of an appreciable collisional heating mechanism. Estimates of the rates for ion-induced dipole and ion-induced quadrupole collisions with excited atoms, yield values of the correct order of magnitude for this observed `lukewarm temperature'. In addition, measurements of Balmer- line profiles, radiated from a gas discharge in a magnetic field of similar magnitude, are analysed and their shapes compared with those from the tokamak plasma.