Extreme ultraviolet: Constructing an experimental database

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  • Published: Aug 1, 2016
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Extreme ultraviolet: Constructing an experimental database

Heavy

This graph shows changes in the terbium light emission spectrum as a parameter of electron temperature. The peaks with the wavelength of 7.023 nanometer and 6.852 nanometer correspond to Tb36+ and Tb18+ ions, respectively. CREDIT Chihiro Suzuki

The identification of new emission lines from heavy ions is allowing researchers in Japan to build a database of extreme ultraviolet spectra. The data could find use in the development of extreme ultraviolet lithography light sources and other plasma applications.

Chihiro Suzuki and Izumi Murakami's research team at the National Institute for Fusion Science, working with Fumihiro Koike of Sophia University, have generated ions of various heavy elements - tin, gold and others - in Large Helical Device (LHD) plasmas. They then measured the emission spectra of these species at extreme ultraviolet wavelengths and discovered a new spectral line. The team describe details at the 43rd European Physical Society Conference on Plasma Physics at KU Leuven in Belgium held during July 2016.

Extreme

For elements of high atomic number from the fifth period and beyond of the periodic table the complete spectra of the plasma state have not been obtained. The high energies needed to generate such fully ionised elements limits research in this area to those with access to suitable equipment, such as the LHD. Some of the spectra have been predicted theoretically but experimental validation of spectral lines as fundamental physical quantities is always desirable to take the science forward.

One element of particular interest is tungsten, which is an important impurity element for the International Thermonuclear Experimental Reactor (ITER) and the development of nuclear fusion energy sources that rely on the creation of stable, confined plasmas. Tin and lanthanides are possible candidates for the EUV lithography light sources and so understanding their fundamental spectroscopic properties is also needed. Experimental verification of any theoretical model becomes essential before intellectual and economic investment in new technology that relies on a clearer understanding.

The LHD can confine a high-temperature plasma in a stable manner for a long period of time and this allows a large quantity of impurities, deliberately added, to be studied in their highly charged ionic condition.

Pellet

The team used the Tracer Encapsulated Solid Pellet (TESPEL) system developed for investigating the behaviour of such impurities as tin, gadolinium, tungsten, gold, bismuth, and other elements including terbium, holmium, and thulium. The elements are first encapsulated in a pellet and the pellet injected into an LHD high-temperature plasma. A Grazing Incidence Vacuum Ultraviolet Spectrometer allows the team to record spectra in the 1 to 15 nanometre range systematically. The team could control temperature and obtain spectra at an energy of more than 2 kiloelectronvolts (keV).

Across a temperature range they could see dramatic changes in the spectra. The spectral lines observed match well the wavelengths predicted by theory, corroborating the calculations and providing experimental evidence for subsequent studies and the further development of theory. The team has now built a database of two-thirds of the elements from atomic number 50 to 83 The experiment database gained from this research will offer fundamental data helpful for improving the accuracy of simulations.

Related Links

KU Leuven 2016, online: "43rd European Physical Society Conference on Plasma Physics"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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