Effect of fast Ti-deposition on gas recycling at the first wall and on fast ion losses in the GDT experiment

Fast Ti-deposition was applied in the gas-dynamic trap (GDT) facility in th e regimes with 14-17.5 keV, 2.5-4 MW neutral beam (NB) injection to control gas recycling at the first wall and thereby reduce charge-exchange losses of energetic ions. The charge-exchange losses of the fast ions turned out t o be much less than in former, non-conditioned discharges. The temporal and spatial variation of the neutral gas density was measured during typical s hots and compared with the results obtained by means of a Monte Carlo trans port code. The comparison of the numerical results with the experimental da ta shows that the recycling coefficient of the chamber wall which has been freshly coated by titanium is close to that of the pure metallic surface. ( C) 1999 Elsevier Science B.V. All rights reserved.