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September 17, 2013
Viewing Atoms in a Plasma’s Core: Extreme Spectroscopy

In order to realize fusion energy in the future it is necessary to raise the performance of plasma confinement, generate high-temperature as well as high-density plasma at temperatures exceeding 120,000,000 degrees, and examine in detail the characteristics of plasma. To generate plasma of even higher temperatures and of even higher density, measuring instruments for accurately assessing the performance of plasma confinement are necessary. The light that plasma emits is used in measuring the performance of various plasma, beginning with the temperature and the density. Here, to assess the performance of the confinement of particles such as ions and electrons that compose plasma, we will introduce the extreme spectroscopy being conducted in the Large Helical Device (LHD).

Plasma is in a state in which the ions and the electrons are separate. The ions and electrons, which are the particles that compose plasma, are confined by the magnetic field container, which is in a nested state from the core area to the plasma edge. Because increasing or decreasing the number of particles in a plasma is determined by the number of particles generated in that plasma and the number of particles that escape from that plasma, if we know the increase or decrease in the number of particles in the plasma and the number of particles generated in the plasma, we can know how many particles are escaping from the plasma. That is, we can evaluate the performance of the particle confinement.

The number of particles in a plasma, that is, the increase or decrease in density, can be known with precision through measurement of the plasma density. However, to date it has been difficult to measure the number of particles generated in a plasma. Even if we were to call this the generation of particles, particles do not suddenly appear from nowhere. Electrons separate from atoms. That is, when a hydrogen atom ionizes, the ion (the atomic nucleus) and the electron separate. In other words, generation of the particles called ions and electrons occurs from the ionization of the atom and the separation into an ion and an electron. For that reason, when an atom collides with particles in the plasma and ionizes, suddenly we can see what seems to be generation of the ion and the electron. The atoms enter from outside the plasma, and because most of the atoms end up being ionized after having entered into the plasma but a few centimeters, only a very small number of atoms are able to approach the core part of the plasma. Because the atoms emit light when they are excited and ionize, we can estimate the number of ionizations, that is, the number of particles generated, by the strength of the light emitted by the atoms. However, because the number of atoms that reach the core of the plasma is small, the intensity of the light is weak and, further, because that light is hidden by the stronger light from the plasma edge area where ionizations are numerous, evaluation of the number of particles generated in the central part of the plasma is normally problematic.

Examination through computer simulations continue, and we know that the strength of the light emitted by the atoms in the core part of the plasma is less than 1/10,000 of the total light intensity. In order to observe such weak light it is necessary to measure at a high level of precision that will overcome the weak light. In the method used to date, separating the light, which is at an intensity of 1/1,000, and detecting it was the best that could be done. Thus, we have improved our measuring equipment and are endeavoring to collect light approximately thirty times more efficiently than in the past. Further, using the characteristics of the LHD that make it possible to generate stable plasma, we have improved measuring precision by devising methods for accumulating and processing data, and have succeeded in separating light at an intensity of 1/1,000,000 of the total light and then detecting that light. Through such advances in technology, we have become able to count the number of particles generated in a plasma’s core. Based upon this, we can look forward to new developments in research on plasma particle confinement performance.