A simplified collisional-radiative model has been constructed for the system of the ground state, electronically excited stable states, and the ionic state of molecular hydrogen in plasma. Effective rate coefficients have been calculated for production of electrons, molecular ions, protons, and hydrogen atoms from molecular hydrogen. The ratio of the effective ionization rate of molecular hydrogen to the I3aImer cr photon emission rate and the effective rate coefficients for radiation and energy losses are also presented.

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This paper describes the recent progress in divertor simulation research using the GAMMA 10/PDX tandem mirror towards the development of divertors in fusion reactors. During a plasma flow generation experiment in the end cell of the GAMMA 10/PDX, ICRF heating in the anchor cell successfully extended the particle flux up to 3.3  ×  1023 m2 s−1. Superimposing the short pulse of the ECH also attained a maximum heat flux of ~30 MW m−2. We have succeeded in achieving and characterizing the detachment of the high-temperature plasma, which is equivalent to the SOL plasma of tokamaks, by using the divertor simulation experimental module (D-module) in the GAMMA 10/PDX end cell, in spite of using a linear device with a short magnetic field line connection length. Various gases (Ar, Xe, Ne and N2) are examined to evaluate the effect of radiation cooling against the plasma flow at the MW m−2 level in the divertor simulation region and the following results are obtained: (i) Xe gas was most effective in the reduction of heat and particle fluxes (1%, 3%, respectively) and has a stronger effect on electron cooling (down to ~1.6 eV) in the used gas species. (ii) Ne gas was less effective. On the other hand, (iii) N2 gas showed more favorable effects than Ar in the lower pressure range. These results will contribute to the progress in detached plasma operation and in clarifying the radiation cooling mechanism towards the development of future divertors.

On the basis of the collisional-radiative models for neutral hydrogen, and neutral and ionized helium, the relationship between the ionization flux or the recombination flux and the photon emission rate of a representative visible line of each species is investigated. It is found that both fluxes are proportional to the photon emission rate and that the proportionality factor depends rather weakly on the plasma parameters in the ranges of practical interest. This implies that the observed emission line intensity can be a good measure of the ionization flux or the recombination flux. The relation between the total radiation power rate and the ionization or recombination flux is also considered. For a hydrogen plasma in ionization balance the Balmer-␣ line intensity takes the maximum value near the optimum temperature of 1.3 eV, while for plasmas out of ionization balance it takes the minimum near that temperature. This latter characteristic corresponds to the recently observed ''inverse edge-localized mode'' in divertor plasmas. For neutral hydrogen and ionized helium, it is found that in the recombining plasma of low electron temperature, T e , and density, n e , the radiation energy close to the ionization potential of the ground state is emitted during one recombination event. In the ionizing plasma of high T e and low n e , a similar amount of energy is emitted during one ionization event. Emission line intensities of hydrogen and helium were measured in the Large Helical Device, and the time variation of n e at the initial and final phases of a discharge was estimated. The results agreed well with the interferometer measurement, and this indicated that the variation of n e was dominated by their ionization or recombination processes rather than by diffusion. The total radiation energy of hydrogen and helium in the recombining phase was found to be less than 1% of the stored energy of the plasma.

Our previous calculation of the excited atom population [T. Fujimoto, K. Sawada, and K. Takahata, J. Appl. Phys. 66, 23 15 ( 1989)] is revised on the basis of new assessment of the cross sections for excitation from atomic hydrogen and dissociative excitation from molecular hydrogen. The eifective ionization rate of molecular hydrogen is also calculated by the method of the collisional-radiative model. Its ratio to the Balmer (r photon emission rate is higher than that for the atomic hydrogen case by more than one order of magnitude. 8122