A method for density measurements employing an asymmetry of lineshapes in dense plasmas and its implementation in a vacuum spark discharge

Kudrin and Sholin (Sov. Phys. Doklady 1963;7;1015) pointed out that the non uniformity of the ion microfield is the leading cause for the asymmetry of hydrogen lineshapes in dense plasmas. They suggested using a relative diffe rence of intensities of the blue and red maxima of the line profile for den sity measurements. We have developed an enhanced diagnostic method which is more accurate than the one suggested by Kudrin and Sholin, and it is exten ded from neutral hydrogen to multicharged, hydrogen-like ions, and thus to significantly higher densities. We have implemented the method for a vacuum spark discharge in carbon, which has a pinch diameter estimated from an eq ual density profile scan to be 100 mu m. A carbon spectrum is obtained from the micropinches of a vacuum spark discharged at 12 kV using a grazing inc idence hat-held spectrometer. Hydrogen-like and helium-like lines are ident ified in the spectrum. The intensity ratio of the Lyman alpha to the helium -like gamma line [1s4p --> 1s(2)] gives an electron temperature of 90-95 eV . The absence of the helium intercombination line in the carbon plasma indi cates that the electron density is greater than 1 x 10(19) cm(-3). Hydrogen -like lines up to Lyman delta are clearly visible in the spectrum, while Ly man epsilon is located at the borderline with the quasi-continuum and is th erefore faint. The diagnostic method that we developed is applied to the Ly man delta line: its profile shows a significant asymmetry, the blue part of the profile being about 16% more intense than the red one. This method yie lds an electron density of (2.8 +/- 1) x 10(20) cm(-3). We compared this re sult with a comparative analysis of the experimental FWHM of the Lyman beta and Lyman delta lines. In this way we estimated an electron density of (3. 4 +/- 0.4) x 10(20) cm(-3) and a bulk plasma speed of about 1.4 x 10(7) cm/ s. Thus, we demonstrated that our method for density measurements employing an asymmetry of lineshape in dense plasmas yields results consistent with the analysis of the FWHM of the lines in the spectral series, However, it r equires obtaining the profile of just one spectral line, which is an obviou s advantage. (C) 2000 Elsevier Science Ltd. All rights reserved.