Measurements of the electron source distribution and implications for particle transport

The electron source profile is critical in the study of particle transport in tokamak plasmas. It arises from a number of atomic processes, namely the ionization of neutrals penetrating from the plasma boundary via multiple charge exchange processes, the ionization of neutrals arising from recombination processes (unless a local balance between ionization and recombination is established), and the ionization of partially ionised impurities. The contribution from neutrals is usually inferred using neutral transport codes, based on Monte-Carlo algorithms, and requires detailed knowledge of plasma parameters and atomic cross-sections. The contribution from impurity ionization additionally requires knowledge of charged-particle transport. Because of the large number of required parameters calculations of the electron source are subject to considerable errors. The experimental verification of these calculations is usually also unsatisfactory as it relies on spectroscopic data, with the concomitant uncertainties arising from the interpretation of line intensities and the Abel inversion of chordal data, or charge exchange measurements of the neutral particle distribution, which are not directly related to the electron source. (author) 3 refs., 4 figs