Direct and accurate measurements of electron densities of plasmas
Isaac Newton's discovery in the mid-1600s that white light consists of a spectrum of rainbow colors, and then in the early 1800s Joseph von Fraunhofer's observation of lines in the solar spectrum laid the foundations for modern day spectroscopy—the workhorse of astronomers analyzing the chemical compositions of plasmas that form the basis of stars and galaxies.
Recently, astronomers are particularly interested in accurate measurements of the electron density of thermal plasmas to determine the evolution of the universe. The electron density is determined by measuring the relative intensities of two characteristic spectral lines that fluctuate with electron density. However, in practice it is challenging to obtain accurate density dependent ratio measurements with ground based instruments, which is critical for verifying space based observations.
Here, Erina Shimizu and Safdar Ali at the University of Electro-Communications, Tokyo, and colleagues, report on experimental measurements of electron density dependent lines ratios of highly charged Fe X, XI and XII—ions for which there are discrepancies between astrophysical observations and theoretical simulations.
The measurements were made using a flat field grazing incidence spectrometer in the extreme ultraviolet (EUV) spectral wavelength range 16 to 20 nm. The researchers state: "Rather than estimating electron density from the theoretical electron beam width as reported previously. We obtained it experimentally by directly imaging electron beam and observing spatial distribution of the trapped ions."
Notably, the research yielded good agreement between the experimental and theoretical calculations—findings attributed to the determining electron densities experimentally with a combination of a pin hole camera and visible spectrometer.