Improvement of ion-temperature-profile determination from charge exchange measurements by inclusion of total-neutral-flux data

摘要

The reconstruction of ion temperature profiles from energy-resolved neutral flux measurements in the energy range from 20 eV to 20 keV reveals an uncertainty in positioning the steep gradient region at the plasma edge, even if the measured energy range of the neutral fluxes includes ion temperature values of the gradient region. This empirical observation is studied analytically with a simple model for the neutral particles. Using the line integral of the electron density one finds that translations of the whole ion temperature spectrum with respect to this coordinate cannot be resolved below a certain value. It turns out that the lowest energy for which fluxes can be measured as well as the accuracy of the flux measurements determine the amount of uncertainty, which is in our case. To reduce this uncertainty, the total neutral flux which is determined mainly by neutrals with energies of a few eV or less, has been fitted simultaneously with the absolutely calibrated energy-resolved flux spectra. The total neutral flux can be measured with an ion gauge and is closely related to theemissivity. The fit of the total neutral flux or thesignal requires simulation of the ionization zone in the scrape-off layer, i.e. measured electron temperatures must be combined with an accurate model for the dissociation and ionization of molecules and atoms.