The Dependence of H-mode Energy Cofinement and Transport on Collisionality in NSTX
Authors: S.M.. Kaye, S. Gerhardt, W. Guttenfelder, R. Maingi, R.E. Bell, A. Diallo, B.P. LeBlanc and M. Podesta
Abstract:
Understanding the dependence of confi nement on collisionality in tokamaks is important
for the design of next-step devices, which will operate at collisionalities at least one order
of magnitude lower than in present generation. A wide range of collisionality has been obtained in the National Spherical Torus Experiment (NSTX) by employing two different wall
conditioning techniques, one with boronization and between-shot helium glow discharge conditioning (HeGDC+B), and one using lithium evaporation (Li EVAP). Previous studies of
HeGDC+B plasmas indicated a strong and favorable dependence of normalized con nement
on collisionality. Discharges with lithium conditioning discussed in the present study gen-
erally achieved lower collisionality, extending the accessible range of collisionality by almost
an order of unity. While the confinement dependences on dimensional, engineering variables of the HeGDC+B and Li EVAP datasets differed, collisionality was found to unify the
trends, with the lower collisionality lithium conditioned discharges extending the trend of
increasing normalized confi nement time with decreasing collisionality when other dimension
less variables were held as fi xed as possible. This increase of confi nement with decreasing
collisionality was driven by a large reduction in electron transport in the outer region of
the plasma. This result is consistent with gyrokinetic calculations that show microtearing
and Electron Temperature Gradient modes to be more stable for the lower collisionality
discharges. Ion transport, near neoclassical at high collisionality, became more anomalous
at lower collisionality, possibly due to the growth of hybrid TEM/KBM modes in the outer
regions of the plasma.
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Submitted to: Nuclear Fusion (November 2012)
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Download PPPL-4833 (pdf 1.4 MB 24 pp)
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