Sensitivity of neutral helium beam stopping in fusion plasmas to atomic collision cross-section

It is shown that the enhancement of the helium beam stopping cross-section in a fusion plasma due to multistep collision processes is determined mostly by the rate of the population of highly excited singlet states (n≥4). The major atomic processes are the dipole transitions from the ground state by proton and electron impact, including transitions to the levels lying above the Lorentz ionization limit. The contribution of metastable states is about 10% in the equilibrium case. The effect of multistep collision processes on the helium beam attenuation is large at integrated line plasma densities above 1016cm-2 as expected for next generation tokamaks. (author). 21 refs, 6 figs, 1 tab