COMPARISON OF NUMERICAL CALCULATIONS OF EXTERNAL KINKS WITH LARGE ASPECT RATIO THEORY
The role of toroidicity for the structure of toroidal modes in tokamaks is investigated by examining circular cross-section equilibria with ERATO. Specifically we have examined a Shafranov step current profile, rounded off for numerical purposes, with beta=0. For q-"rm s" < 2 we find that detailed predictions of the inverse aspect ratio theory for the "it n/" = 1 external kink become invalid as "it R/a/" changes from 10 to 2.5. A Fourier-decomposition of the mode in poloidal mode number "it m/" shows that the "it m/" = 2 diagonal term in delta"it W/" changes sign. With a conducting wall at "it b/a/" = 1.5 the lower aspect ratio equilibrium remains unstable for lower values of "it q/"-"rm o" (> 1) and the unstable mode shows strong coupling of "it m/" = 1, 2 and 3. The radial plots of the delta"it W/"-"rm mm'" show a large cancellation in the diagonal terms. Analytically, delta"it W/"-"rm mm'", derived for low "it m/" modes by partial integration, reduces to Newcomb's expression in the large aspect ratio limit and resolves this large cancellation.
Bibliographic Reference: WORKSHOP ON THEORY OF FUSION PLASMAS, VARENNA/I, AUG. 24-28, 1987, PUBL. 1987, BY: SOCIETA ITALIANA DI FISICA
Record Number: 1989126084900 / Last updated on: 1989-05-01
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