Fig. 2-3 Spectral line profiles emitted from helium atoms near the divertor region (a) Spectral line profiles in front of the divertor plates are narrow, because the emission is mainly coming from low-energy helium atoms desorbed from the divertor plates. (b) Spectral line profiles become broad at a distance of about 12 cm from the divertor plates, because the helium atoms obtain kinetic energy through elastic collisions with protons in the divertor plasma. As shown, the simulation result including the effect of elastic collisions agrees well with the experimentally-obtained, absorbed spectral profile. (c) Cross section of the divertor region. Positions observed by the high resolution spectrometers are indicated.

The effective removal from the reactor core of helium ash, i.e. helium ions which have slowed down after heating the core plasma, is a crucial issue for a steady-state tokamak fusion reactor. While the effectiveness of a divertor for helium ash removal has already been demonstrated by a simulation experiment in JT-60, detailed knowledge about the physics processes of the helium atoms in the divertor region is still needed to establish a high efficiency helium ash exhaust technology. We have made extensive measurements of the spectral lines emitted from the helium atoms in the divertor plasma with a high resolution visible spectrometer, and obtained information on the velocity distribution of the helium atoms and their fundamental transport processes. The behavior of helium atoms in the divertor region has been studied by comparing the experimental data with the result of a simulation analysis that includes the evaluation of various microscopic physical processes occurring in the divertor plasma. Figure 2-3 (a) and (b) show spectral line profiles emitted from helium atoms in the divertor region. As seen in Fig. 2-3 (b) the spectral line profile becomes broad at a distance from the divertor plates (which are dumps for impurity particles and extra-heat) as compared to the spectral profile just in front of the divertor plates (Fig. 2-3 (a)), where the emission is mainly from the low-energy helium atoms desorbed from the divertor plates. The line profile broadening can be attributed to the effect of elastic scattering of desorbed helium atoms from the divertor plates by proton collisions; since the desorbed helium atoms obtain kinetic energy through elastic collisions with protons, the spectral profile emitted by them broadens. This is clearly shown in Fig. 2-3 (b) where the observed spectral line profile is well reproduced by the calculation including the effect of the elastic scattering of helium atoms by proton collisions. Owing to this elastic collision process, it is estimated that the helium atom flux to the gap for helium pumping increases by about 30% compared to the calculation that does not include the effect of elastic collisions, thus a higher pumping efficiency of the divertor is expected.

Reference H. Kubo et al., The Special Profile of the He I Singlet Line (667.82 nm) Emitted from the Divertor Region of JT-60U, Plasma Phys. Contol. Fusion, 41, 747 (1999).

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