Measurement of residual carbon in chamber of Shenguang II laser facility

It is reported that in National Ignition Facility's shock timing campaign, there is a 2 μm layer of residual air condensation on the cold laser entrance hole window, and residual gas level of ∼5×10−6 Torr (∼6.6×10−4 Pa) in the chamber [Robey et al., Phys. Plasmas 19, 042706 (2012)]. On Shenguang (SG) II and III laser facilities, a nominal residual gas level in the chamber is ∼10−2 Pa in experiment. In this work, the residual Carbon (C) level in the SG II chamber is investigated experimentally using a transmission grating spectrometer (TGS). The experimental result shows that there is a clear C K edge (280–300 eV), which implies that there is residual C condensation on the charge-coupled device (CCD) silicon oxide layer. The effect of the C condensation on the CCD response is studied with a condensed-C-foil model. With the modified CCD responses, the unfolded spectra by TGS are shown, and the significant C K edge decreases as the C foil thickness increases. Compared to the simulated C K edge depth, the measured spectrum presents that the condensed C foil thickness is 27 nm in the SG II chamber. The correction ratio for the TGS measured flux is 1.165, and there is an insignificant effect on the M band (>1800 eV) fraction measurement in SG II.