National Institute for Fusion Science
Many spectrometers are installed in the Large Helical Device (LHD), i.e., a fusion experiment device by the magnetic confinement, and they can measure spectra from the visible to the X-ray region. Some of the spectrometers can also measure the spatial distribution. Visible spectrometers are mainly used to measure the plasma periphery, including the divertor region, and the observation area in the plasma shifts from the periphery to the center as the wavelength decreases in the order of vacuum ultraviolet, extreme ultraviolet, and X-ray. The following is an overview of the main research topics considered by the Plasma Spectroscopy Group.
In ITER, tungsten is used in the divertor, and there is a concern about tungsten ion contamination of the plasma. In the LHD, tungsten ions are introduced into the plasma by impurity pellet injection, which enables us to measure the emission spectrum of tungsten ions over a wide wavelength range. The atomic data related research based on the spectroscopic measurement of various heavy ions, not only tungsten, will continue to be a central issue for the Plasma Spectroscopy Group.
On the other hand, molecular hydrogen and hydrogen atoms originating from hydrogen gas supplied to the plasma as fuel are widely present from the divertor region to the core region, and are considered to play an important role in the plasma confinement characteristics. These neutral particles emitted from the wall surface enter the plasma through various atomic processes. In the plasma, they also reach the core region through repeated charge exchange reactions. Understanding these transport properties of neutral particles is important for understanding the precise particle balance. In addition, it has been recently confirmed that there is a significant amount of fast ions lost by charge exchange with neutrals in LHD, which points out the importance of neutrals. Such studies focusing on hydrogen molecules and atoms are also important issues to be promoted by the Plasma Spectroscopy Group.
In general, the main target of plasma spectroscopy is the emission lines from atoms and ions excited by electron impact, but the facilities for so-called charge exchange spectroscopy, which measures emission lines from excited states produced by charge exchange reactions with neutral beams, are very well equipped. In charge exchange spectroscopy, it is possible to measure the density and temperature of fully ionized ions. From detailed analysis of the line profiles, we have recently succeeded in detecting deviations of the ion velocity distribution function from the Maxwell distribution. On the other hand, in general spectroscopic measurements, the polarization of emission lines from excited states produced by anisotropic electron collisions has been utilized to investigate the anisotropy of the electron velocity distribution function from the polarization of the emission lines. Such deviations of the velocity distribution function of particles from thermal equilibrium are important from the following points of view.
(1) Direct effect on the confinement properties of particles
(2) Impact on collisional-radiative model calculations through rate coefficients for electron impact ionization and excitation
(3) Understanding the non-uniformity or asymmetry of heat exchange between particles and waves.
The Plasma Spectroscopy Group will also be actively involved in research related to the non-thermal or anisotropic velocity distribution functions of such particles. The existence of non-thermal and anisotropic particles is not a problem unique to fusion plasmas, but has been recognized in various plasma studies. For example, in process plasmas, many complex collisional processes play an important role in the radical production for oxygen and nitrogen molecules, and when considering the rate coefficients of these atomic processes, it is now essential to take into account the non-thermal velocity distribution function of the colliding particles. In addition, in the recent solar research, magnetic field measurement using the anisotropy of the radiation field in the atmosphere has been attempted, and this measurement technique has already been applied to LHD experiments. The Plasma Spectroscopy Group considers the promotion of collaborative research based on such common subjects with other fields to be an important theme.
National Institute for Fusion Science, Toki 509-5292, Japan