The spectral profile of the He I singlet line (667.82 nm) emitted from the divertor region of JT-60U

Abstract

In order to investigate the behaviour of helium atoms in the divertor region of JT-60U, the spectral profile of a He I singlet line (667.82 nm) has been observed with a high-resolution visible spectrometer. In attached divertor plasmas, the spectral profile is narrow in front of the divertor plates, because the line is mainly emitted from low-energy helium atoms desorbed from the plates. In the upper stream of the divertor plasma, as the electron density increases, the profile becomes broader and the temperature of helium atoms corresponding to the broadening increases from 0.2 to 1.5 eV. In detached divertor plasmas, the temperature is 0.9-1.5 eV. The profiles in attached divertor plasmas have been reproduced by simulation with a neutral transport code considering elastic scattering by proton collision. The broadening of the profile is attributed to the elastic scattering. The elastic scattering affects helium concentration in the main plasma and helium pumping efficiency. In the simulation for an attached divertor plasma, the penetration probability of helium atoms from the divertor plates to the main plasma and the helium atom flux to the gap for pumping increase by 30% due to the elastic scattering.