Charge-exchange-excited line radiation in a tokamak (ASDEX) with neutral-particle-beam injection

Abstract

Time sequences of the spectral emission from the ASDEX tokamak at the Max-Planck-Institute für Plasmaphysik have been recorded using multichannel survey spectrometers operating over the wavelength range 100–7000 Å. During injection of neutral beams of hydrogen or deuterium, charge-exchange- (CX) excited lines, mostly Δn=1,2 transitions from hydrogenic ions of intrinsic impurity elements or injected trace gases, are prominent throughout this entire spectral region. The charge-exchange origin of the lines is established by the temporal and spatial dependence on the injected beam current and also, in the case of visible transitions, by their relatively large spectral widths. With a beam power of 1.7 MW and an extraction potential of 41.5 keV, excitation of intrinsic impurity ions, e.g., ${\mathrm{O}}^{7+}$ (n<11) and ${\mathrm{C}}^{5+}$ (n<9) occurs routinely while, depending on the plasma conditions, other trace impurities, for example, ${\mathrm{F}}^{8+}$ (n=9), ${\mathrm{Cl}}^{16+}$ (n=13), ${\mathrm{S}}^{13+}$ (n≤15), and ${\mathrm{Kr}}^{25+}$ (n≤22) are also observed. From the absolute intensities of the ${\mathrm{C}}^{5+}$ and ${\mathrm{O}}^{7+}$ CX lines, impurity concentrations are deduced. Comparison of the concentration data derived from the vacuum-ultraviolet and separately from the visible intensities allows an assessment of the theoretical cross sections for charge-transfer recombination into different quantum states. Spectral line profiles of the CX transitions in the visible region involving high-n Rydberg states are consistent with bulk plasma and ion thermal motion. Coincidence between Rydberg levels of different ions with the same effective nuclear charge offers a plausible explanation for the appearance of some visible CX lines in the absence of atomic beams.