SIGNATURES OF DEUTERIUM RECOMBINATION IN THE DIII-D DIVERTOR

Thomson scattering measurements performed in the divertor of the DIII- D tokamak [Plasma Physics Controlled Nuclear Fusion Research, 1986 (In ternational Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] durin g detached operation show that the electron temperatures are typically between 0.8 and 2.0 eV throughout a region which may extend several c entimeters above the target plate. At such low temperatures the excita tion of recycling deuterium atoms or impurities should be weak. Nevert heless, significant radiation is frequently detected in these location s. It has been suggested that recombination processes, which become im portant only below about 1.5 eV for deuterium, are responsible for the observed emission. This hypothesis has been investigated by comparing ratios of deuterium lines from attached and detached plasmas with the oretical ratios expected for ionizing or recombining conditions. The a nalysis of several discharges indicates that the mechanism for product ion of the emission changes from being collisional excitation of atomi c deuterium to a mixture of collisional-radiative recombination and co llisional excitation as plasmas evolve from attached to detached state s. Localization of D-a emission to low-temperature regions using tange ntially viewing camera data together with Thomson scattering results a nd measurements of deuterium atom temperatures are consistent with the se conclusions. (C) 1997 American Institute of Physics.