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March 11, 2015
Measuring Negative Hydrogen Ion Density in Plasma: Seeking to Enhance High Performance Negative Ion Sources

In the Large Helical Device (LHD), in order to heat plasma confined by the magnetic field to a high temperature we use a neutral particle beam injection device. This device injects into the plasma hydrogen particles that have been accelerated to a high energy, and it heats the plasma through the beam’s energy. This particle beam is generated by an instrument called the hydrogen ion source. In the LHD, in order to generate a high-energy particle beam we were the first in the world to develop a negative ion source that uses negative ions in a beam. And we are moving forward with research that increases further the performance of this negative ion source. Here, we will introduce research in diagnostics for measuring negative ion density inside a negative ion source plasma, which is one aspect of our research that aims at enhancing the performance of the negative ion source.

In the hydrogen ion source we first generate a plasma that includes hydrogen ions. Because a hydrogen ion bears electricity, we apply voltage to the plasma and extract the hydrogen ions. Further, by applying a still higher voltage and accelerating ions, we generate a high energy hydrogen ion beam. Because a normal plasma is composed from a positive ion that is positively charged and an electron that is negatively charged, to date a positive hydrogen ion had been used as the beam. However, in a large-scale device such as the LHD, a high-energy beam that has been accelerated by voltage exceeding several hundred kilovolts is necessary. Thus, we must make a beam using a negative hydrogen ion.

A positive hydrogen ion is under a condition in which the electron has been stripped from the hydrogen atom, but the negative hydrogen ion is under a condition in which the electron has been affixed to the hydrogen atom. For a normal plasma, because negative hydrogen ions are almost completely absent, we established a method by which electrons are added to hydrogens from a plasma that has come into contact with a metal surface, and succeeded in raising the density of the negative hydrogen ions in the plasma. In order to improve the performance of this negative ion source, it is necessary to generate high-density negative hydrogen ions and to extract the negative ion source effectively. Aiming at achieving that, we are engaged in clarifying the detailed mechanism from having generated negative hydrogen ions to having extracted the negative hydrogen ions as a beam. Thus, we are utilizing various diagnostics methods. Among them, we have made important advances in diagnostics methods for negative hydrogen ion density.

In the negative hydrogen ion diagnostic that we have developed, which is called the cavity-ring-down method (CRD), we inject the laser light into the optical resonator consisting of two facing mirrors that surround the plasma. While the laser light is moving back and forth more than several tens of thousands of times, by measuring the attenuation of the light absorbed by the negative ions we measure the density of the negative ions in the optical resonator. While precisely adjusting the position of the laser light to be consistent with that of the optical resonator, and at the same time making it possible to move in two dimensions vertically and horizontally, we achieved for the first time in the world a measurement of the two-dimensional distribution of negative hydrogen ions through CRD. Through this, we achieved a result in which the density of the negative ions decreased as the distance from the electrode that extracts negative ions from the plasma is increased. We are seeking to become able to generate negative ions on the electrode, which is a metal surface. Results obtained have demonstrated this, and, furthermore, based upon results that have matched our expectations, we have received high evaluations internationally. In the future, we will combine this two-dimensional negative hydrogen ion density distribution diagnostic with other diagnostics and seek to clarify the mechanism from the generation of the negative hydrogen ion to the extraction of a beam. We will continue to move forward with our research.