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July 8, 2013
Investigating the Light from Highly-charged Ions: Basic Research in Highly-charged Ions Using CoBIT

In the future, in order to realize fusion energy it will be necessary to convert hydrogen, which is a fuel gas, into a plasma state in which ions and electrons are moving independently and to raise the plasma temperature to more than 120,000,000 degrees. However, when iron or other impurities enter a high-temperature hydrogen plasma, the electrons that these atoms hold are stripped away and become highly-charged ions, and the plasma temperature is lowered because plasma energy is lost through emitting light. Here, using a device developed independently that produces highly-charged ions, we will introduce research that elucidates the behavior of highly-charged ions in high-temperature plasma.

Atoms are made from the nucleus that holds positive-charged electricity in the central part and from electrons that have much negative-charged electricity that envelopes the atoms. If one electron is removed from the atom, that atom becomes an ion. We call that ion a singly-charged ion. It is possible to remove two, three, or numerous electrons, and we call those ions doubly-, triply-, and highly-charged ions.

Hydrogen that is utilized as fuel gas by fusion plasma will not become a highly-charged ion because it has only one electron. But when atoms that hold many electrons, such as carbon, iron, and other atoms, enter a high-temperature plasma, electrons are stripped away one after another through collisions with high-energy electrons that are in the plasma, and these atoms become highly-charged ions. As plasma facing material, the desired tungsten atoms making of high heat resistant metals have 74 electrons. When this tungsten mixes into a plasma as an impurity due to its interaction with the plasma, the electrons are gradually stripped away, and they may become highly-charged tungsten ions, such as 40- charged or 50-charged ions. Such highly-charged ions to the extent that they are of a high charge end up cooling the high-temperature plasma in some cases because they emit much energy as light. For that reason, in order to realize fusion energy for the future, it will become extremely important to know the behavior of tungsten and other multi-charged ions in a plasma and the conditions of light emission.

At the National Institute for Fusion Science we are developing devices for making highly-charged ions, that is, the electron Beam Ion Trap device (CoBIT), generating highly-charged ions from tungsten and other sources by controlling valence conditions, and advancing basic research on the generation process for highly-charged ions such as tungsten which may perhaps exist in high-temperature plasma and on the light emission process. CoBIT injects from outside an electron beam controlled by energy, and is a device that makes possible the highly efficient generation of highly-charged ions. For that reason, we are able to force changes in the charge state of the confined highly-charged ions through the energy of the electron beam. Further, through our own spectrometer developed here we can continuously measure light emission from highly-charged ions. Comparing the light emission lines from highly-charged tungsten ions, whose generation conditions are clear and measured by CoBIT, to the accumulated complicated light emission lines from the highly-charged tungsten ions that have various charge states that are measured by the high-temperature plasma of the LHD, we analyzed which light emission line was being emitted from a highly-charged ion. As a result, we were able to identify the light emission lines of highly-charged tungsten ions of a charge state from 21 to 40, including the light emission process.

In this way, basic research regarding highly-charged ions that uses CoBIT is providing important basic data in research on high-temperature plasma in the LHD and is supporting academic facets of research that aims toward the realization of fusion.