Study of the discrete-to-continuum transition in a Balmer spectrum from Alcator C-Mod divertor plasmas

Under detached plasma conditions in the Alcator C-Mod tokamak, the measured spectra show pronounced merging of the Balmer series Lines and a photo-rec ombination continuum edge which is not a sharp step. This phenomenon, known as a smooth discrete-to-continuum (D-C) transition, is typical only for hi gh-density (greater than or equal to 10(21) m(-3)), low-temperature (T-e ap proximate to 1 eV), and recombining plasma. As we will discuss, this type o f transition emphasizes the degree of plasma non-ideality. A theoretical model capable of treating spectra from a detached divertor pl asma, and those with a smooth D-C transition in particular, has been develo ped. It is comprised of three parts: (i) a collisional-radiative model for the population densities of the excited states, (ii) atomic structure and c ollision rates for an atom affected by statistical plasma microfields, and (iii) a model for calculating the line profiles and the extended photo-reco mbination continuum. The effects of statistical plasma microfields on the population densities o f excited states, on the profiles of Balmer series lines, and on the photo- recombination continuum edge are discussed. The changes in Spectrum charact eristics with plasma parameter variation, leading to the smooth D-C transit ion, are analysed. The relevance of volumetric plasma recombination to the spectra observed from a detached divertor plasma is discussed A comparison of the calculated and measured spectra is used to determine the plasma para meters in the recombining plasma region. Along with other properties of mea sured spectrum, the smooth D-C transition provides evidence in support of t he recombining state of the plasma attained under detached divertor conditi ons.