MHD and current profile control studies of reversed shear tokamak configurations
- M.I.T. Plasma Fusion Center, Cambridge, MA (United States)
Given the observed enhancement of energy and particle confinement, and the possibility of achieving high betas and high bootstrap current fractions, reversed magnetic shear equilibria have become the leading concept in advanced tokamak scenarios. The performance limits for these configurations are most likely to be set by MHD stability constraints. The other essential issue for this advanced tokamak concept is the non-inductive, steady state sustainment of the current profile. These two topics are covered in the present work. We concentrate on equilibria with minimum value of q greater than 2 so as to avoid double-tearing mode problems and to have global access to the n={infinity} ballooning second stability region. Then the MHD limit is set by the n=1 (pressure-current driven) external mode. The critical parameters for the onset of the n=1 instability in terms of the plasma beta and current are obtained for several classes of equilibria, allowing for a systematic study of geometrical shaping and profile effects. It is shown that triangular shapes and broad profiles yield optimal results. The realistic sustainment of the reversed shear current profiles is investigated by generating equilibria with the ACCOME code. This simulation model combines a free boundary solution of the Grad-Shafranov equation with the calculation of driven currents due to neutral beam injection, lower hybrid waves, ICRF fast waves and bootstrap effects. Such equilibrium code is coupled to the JSOLVER and PEST-II codes in order to perform the stability analysis. Specific simulations for the ALCATOR C-mod and ITER tokamaks are provided. Optimal parameters for completely stable equilibria without conducting walls thus obtained reach {beta}{sub N}=3.7, {beta}=3.7%, {beta}*=5.0% and bootstrap fraction in excess of 75%.
- OSTI ID:
- 489510
- Report Number(s):
- CONF-960354-; TRN: 97:011657
- Resource Relation:
- Conference: International Sherwood fusion theory conference, Philadelphia, PA (United States), 18-20 Mar 1996; Other Information: PBD: 1996; Related Information: Is Part Of 1996 international Sherwood fusion theory conference; PB: 244 p.
- Country of Publication:
- United States
- Language:
- English
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