Author(s): , , , ,
Institution(s): 1. Lawrence Livermore National Laboratory, 2. Princeton Plasma Physics Laboratory, 3. University of California
We report on emission spectra in the extreme ultraviolet region 150-200 Å recorded at the National Spherical Torus Experiment (NSTX). This region is typically dominated by M-shell iron emission that is used extensively for solar observations, e.g. Hinode and the Solar Dynamics Observatory, and stellar atmospheres, e.g., the Extreme Ultraviolet Explorer (EUVE). We find that significant emission occurs from several heretofore unmeasured lines, which can persist throughout the plasma duration. We attribute these lines to emission from K-shell carbon ions (C V and C VI). Spectral modeling of collisional excitation fails to account for these lines, but modeling of charge exchange provides a good match with the observation. Our spectral model shows that the lines are formed by charge exchange of bare and hydrogenlike carbon with neutral hydrogen. The high abundance of bare and hydrogenlike carbon in the solar wind suggests that these lines may be formed in the heliosphere and may be part of the soft X-ray background. They may thus be observed by energy dispersive instruments, such as microcalorimeters.
This work was supported by the DOE General Plasma Science program. Work was performed by Lawrence Livermore National Laboratory and Princeton Plasma Physics Laboratory under the auspices of the U. S. Department of Energy under Contracts DEAC52-07NA27344 and DE-AC02-09CH11466.