Velocity distribution of helium-atoms in front of the TEXTOR limiter

In tokamak discharges in helium, the velocity distribution of helium atoms released from the limiter has been determined by measuring in radial direction the Doppler broadening of a singlet line (λ=6678 A), the Zeeman effect splitting of which has been eliminated by a polarizer. The spectral line profile shows a central component, which is slightly shifted to the blue and which originates from desorbed helium atoms, and a high-energy blue wing, which results from particles with energies up to 150 eV. The high-energy helium particles are still present when discharges are operated in hydrogen with small amounts of helium added. Since under such conditions the resonant charge exchange of helium is strongly reduced, it clearly appears that the dominant part of the fast particles originates from reflected helium atoms. (orig.)