Investigation of fast-atom fluxes in the high-density plasma in Alcator

A five-channel atomic particle analyser developed at the A.F. Ioffe Physico-Technical Institute of the USSR Academy of Sciences has been used for investigating fast-atom fluxes in Alcator. The analyser employs a pulsed helium stripping cell for increased sensitivity. It is shown that the intensity of the atomic flux diminishes by nearly a factor of 50 when the spatially averaged plasma density e increases from 1.1014 cm−3 to 4.1014 cm−3. The spectra of the charge-exchange atoms, up to an energy of 6 keV, were measured. The ion temperature obtained from the slope of the spectra rises with increasing e and becomes nearly equal to Thomson-scattered electron temperature at e = (4–5) × 1014 cm−3. The spatial distribution of atomic flux with energy of several keV is rather narrow and symmetrical about the toroidal plane at e = 5 × 1014 cm−3. It verifies that an atomic flux of sufficiently high energy is emitted by the central regions of the plasma column. The evaluation of the neutral-atom density in the central region of the plasma column gives n0(0) ∼ 107–108 cm−3 at e = 5 × 1014 cm−3. This exceeds, by several orders of magnitude, the value n0(0) expected from the known atomic flux incoming to the plasma and its penetration to the centre of the plasma column.