Title ofprogram: TOPIC Nature ofphysical problem In high temperature-low density tokamak plasmas, radiation Catalogue number: ACDY cooling by impurity atoms can be an important energy loss mechanism[1,2], since radiation is not reabsorbed. The coupled Program obtainable from: CPC Program Library, Queen's Urn-set of time-dependent diffusion equations for ionized tokainak versity of Belfast, N. Ireland (see application form in this issue) plasma impurities is solved in conjunction with a simple model for neutral impurities, on the assumption of a cylindrical Computer: CDC Cyber 170-720; Installation: Central Institute symmetry. The resultant density distributions are used in the of Physics, Bucharest-Mägurele, Romania subsequent computation of the related energy losses. Operating system: NOS1P4 552/552 Method ofsolution The set of coupled second order differential equations, describ-Programming language used: FORTRAN IV ing the diffusion of impurities in the MHD approach, is discretized using a finite difference scheme. The resultant linear High speed storage required: 0.7 Kwords for the test ru~system, with a three-diagonal block matrix structure, is then solved by a special forward elimination-backward substitution No. of bits in a word: 60 technique. Peripherals used: card reader, line printer, magnetic disk Typical running time About 6 s for the test run output No. of lines in combined program and test deck: 1217