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Absolute densities of the dominant light impurities in the plasma centre are measured during neutral beam heating using charge exchange spectroscopy. The behaviour of carbon, oxygen and helium (in helium discharges) is investigated for limiter and X-point discharges. When the plasma is in the X-point configuration, the central carbon concentration is significantly lower than in the material limiter configuration. After the transition from L to H-mode, a strong impurity build up is observed in the plasma centre which significantly reduces the central dilution factor n(d)/n(e) from initial values near 0.8 to about 0.6 at the collapse of the H-mode. Nonetheless, during the H-mode, the product of deuteron density and ion temperature n(d)T(i) (both derived by the charge exchange spectroscopy diagnostic) shows a clear enhancement with respect to the L-mode at the same input power. Helium discharges, as compared to deuterium discharges, have a higher carbon concentration (consistent with physical sputtering) and a much reduced oxygen concentration n(o)/n(e) < 0.005 ( consistent with chemical release of oxygen). The transient behaviour of oxygen and carbon following deuterium pellet injection is discussed using a comparable non-pellet reference discharge. An increase of the oxygen content in the plasma centre is observed some 600 msec after injection of a deuterium pellet, while the central carbon density appears only transiently diluted by the sudden addition of pure deuterium.

Additional information

Authors: BOILEAU A, Centre Canadien de Fusion Magnetique, 1804 Montée-Ste-Julie, Varennes (CA);VON HELLERMANN M, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB);SUMMERS H P, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB);MORGAN P D, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB);HORTON L D, ORNL, P.O. Box Y, Oak Ridge TN, 37831 (US)
Bibliographic Reference: Report: JET-P(89)18 EN (1989)
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 198911015 / Last updated on: 1994-12-01
Original language: en
Available languages: en