Visible spectroscopy in the DIII-D divertor

Spectroscopy measurements in the DIII-D divertor have been carried out with a survey spectrometer that provides simultaneous registration of the visible spectrum over the region 400-900 nm with a resolution of 0.25 nm. Broad spectral coverage is achieved through the use of a fiberoptic transformer assembly to map the curved focal plane of a fast (f/3) Rowland-circle spectrograph [Tugarinov et al., Rev. Sci. Instrum. 66, 603 (1995)] into a rastered format on the rectangular sensor area of a two-dimensional charge-coupled device (CCD) camera. Vertical grouping of pixels during CCD readout integrates the signal intensity over the height of each spectral segment in the rastered image, minimizing readout time. For the full visible spectrum, readout time is 50 ms. Faster response time (<10 ms) may be obtained by selecting for readout just a small number of the 20 spectral segments in the image on the CCD. Simultaneous recording of low charge states of carbon, oxygen, and injected impurities has yielded information about gas recycling and impurity behavior at the divertor strike points. Transport of lithium to the divertor region during lithium pellet injection has been studied, as well as cumulative deposition of lithium on the divertor targets from pellet injection over many successive discharges.