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An analytical simple fluid theory (SFT) is derived to predict when a tokamak divertor should retain impurities. Only the simplest, one-dimensional case of leakage as ions from the point of ionisation, along magnetic field lines to points far upstream is considered. The SFT builds directly on earlier 1-D treatments of divertor retention of impurities. It is found essential to introduce cross-field leakage into the SFT, associated with the two-dimensionality of the actual divertor situation, otherwise the upstream regions suffer catastrophic - and unphysical - impurity accumulation. Without this correction, divertor leakage is predicted to occur even under rather cold (collisional) divertor operation. With this correction, leakage is predicted to occur only for divertor temperatures which are so high as to be unlikely. The most critical factor governing retention is the location at which the impurity neutrals are first ionised. The predictions of SFT are compared with results using the Monte Carlo impurity transport code DIVIMP (divertor impurity). Close agreement is found for plasma conditions which are strongly collisional, but for weaker collisionality the SFT is found to overestimate leakage.

Additional information

Authors: STANGEBY P C, JET Joint Undertaking, Abingdon, Oxon. (GB);ELDER J D, JET Joint Undertaking, Abingdon, Oxon. (GB)
Bibliographic Reference: Report: JET-P(95)03 EN (1995) 50 pp.
Availability: Available from the Publications Officer, JET Joint Undertaking, Abingdon, Oxon. OX14 3EA (GB)
Record Number: 199511209 / Last updated on: 1995-10-10
Original language: en
Available languages: en