Practical beta limit in ITER-shaped discharges in DIII-D and its increase by higher collisionality
Successful steady-state tokamak operation requires operating at the highest possible beta value while avoiding both ideal and resistive magnetohydrodynamics (MHD) instabilities which reduce confinement and induce disruption. The maximum operational beta in single-null divertor (SND), long-pulse discharges in DIII-D with a cross-sectional shape similar to the proposed ITER tokamak is found to be limited not by ideal modes but by the onset of resistive MHD instabilities. Two possible means have been identified as the cause of the onset of these instabilities. Resistive tearing modes that occur at rational surfaces q = m/n cause reconnection into islands of full width w. The island onset and growth can be due to either free energy from an unstable current profile or to a helical bootstrap current which amplifies a seed island. These mechanisms are tested using accurate MHD equilibria reconstructions with the code EFIT using the external magnetics, local measurements of the internal poloidal field with the 16 channel motional Stark effect diagnostic and the measured pressure profile.
Bibliographic Reference: Article: Proceedings of the 16th IAEA Fusion Energy Conference (1996) pp. 148-149
Record Number: 199810144 / Last updated on: 1998-02-12
Original language: en
Available languages: en