Analysis of the effect of neutral hydrogen for the particle balance in the GAMMA 10 tandem mirror

Y. Kubota, M. Yoshikawa, Y. Nakashima, K. Sawada 1), T. Kobayashi, M. Saito, Y. Higashizono, A. Itakura and T. Cho

Plasma Research Center, University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan
1) Faculty of Engineering, Shinshu University, Nagano 380-8553, Japan

In magnetically confined plasmas, a neutral hydrogen behavior is an important subject to clarify the particle balance and the particle confinement. In the GAMMA 10 tandem mirror, the ionization rate is estimated experimentally by the measurements of an electron density and Hα line emission. The ionization rate and Hα line emission depend on the hydrogen atom and molecular hydrogen. However it is difficult to separate the effects of the hydrogen atom and the molecular hydrogen on the measurement of Hα line emission. Then we studied the effects of hydrogen atom and molecular hydrogen for an ionization rate and Hα line emission.
From a calculation results of collisional-radiative model (CR-model), the effect of molecular hydrogen for Hα line emission that originates a dissociative-excitation is about one tenth on the effect of hydrogen atom at the plasma parameter of the GAMMA 10 central cell. In the central cell, typical electron density, electron temperature and ion temperature are 1x1012 cm-3, 50 eV and 1 keV, respectively. For Hα line emission, hydrogen atom is dominant compared with molecular hydrogen. Therefore neutral hydrogen atomic density based on CR-model can be estimated by the Hα line emission measurement, however molecular hydrogen density is not clarified from Hα line emission.
The ionization rate is determined by atom-electron, atom-ion, molecule-electron and molecule-ion collisions. These collisions include several reactions those are ionization, ionization of molecule, dissociation, etc. As the results of calculating the practical rate coefficient for ionization rate with typical plasma parameter in the central cell, it is found that the effect of molecule is several times larger than that of atom. Especially, the effect of molecule for ionization rate is dominant in the peripheral region of the plasma.
In this paper, the calculation results of the effect of hydrogen atom and molecule in the GAMMA 10 parameter range are presented and the correlation between neutral hydrogen and the particle balance will be discussed.