Calculation of the Non-Inductive Current Profile in High-Performance NSTX Plasmas
Authors: S.P. Gerhardt, E. Fredrickson, D. Gates, S. Kaye, J. Menard, M.G. Bell, R.E. Bell, B.P. Le Blanc, H. Kugel, S.A. Sabbagh, and H. Yuh
Abstract:
The constituents of the current profile have been computed for a wide range of high-performance plasmas in NSTX [M. Ono, et al., Nuclear Fusion 40, 557 (2000)]; these include cases designed to maximize the non-inductive fraction, pulse length, toroidal-β, or stored energy. In the absence of low-frequency MHD activity, good agreement is found between the reconstructed current profile and that predicted by summing the independently calculated inductive, pressure-driven, and neutral beam currents, without the need to invoke any anomalous beam ion diffusion. Exceptions occur, for instance, when there are toroidal Alfven eigenmode avalanches or coupled m/n=1/1+2/1 kink-tearing modes. In these cases, the addition of a spatially and temporally dependent fast ion diffusivity can reduce the core beam current drive, restoring agreement between the reconstructed profile and the summed constituents, as well as bringing better agreement between the simulated and measured neutron emission rate. An upper bound on the fast ion diffusivity of ~0.5-1 m2/sec is found in “MHD-free” discharges, based on the neutron emission, time rate of change of the neutron signal when a neutral beam is stepped, and reconstructed on-axis current density.
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Submitted to: Nuclear Fusion (September 2010)
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Download PPPL-4600 (pdf 4.3 MB 55 pp)
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