The low-energy neutral fluxes and their impurity production at the walls of ASDEX

Abstract

With the low-energy neutral particle analyser LENA at ASDEX the density dependence of the neutral flux, its mean energy and its impurity production at the walls has been investigated for ohmic hydrogen and deuterium discharges. For hydrogen a large increase of the neutral flux with increasing density was observed, while the mean energy of the neutrals decreases. This holds for Hydrogen over the whole range of densities. At constant density the low energy neutral flux increases with the safety factor q. This is in contrast to the high energy flux as measured with the charge exchange analyser. The flux increase of low energy neutrals can be correlated with the increased decay length of the electron density for higher q-values. For deuterium the same behavior up to densities of n ̄ e ∼- 3.5 × 10 13 cm −3 was observed. For higher densities the flux behaves different corresponding to the different confinement regimes. Only in the regime of saturated ohmic confinement, known so far, the flux increases further with increasing n ̄ e ,. With the operational conditions of the new divertor configuration in ASDEX a further increase of τ e with n ̄ e up to the density limit was observed (improved ohmic confinement). In this regime the neutral flux saturates with increasing n ̄ e . In spite of the reduced impurity production at the walls, Z eff increases in this case.