Microturbulent drift mode stability before internal transport barrier formation in the Alcator C-Mod radio frequency heated H-mode

Abstract

H-mode experiments on Alcator C-Mod [I. H. Hutchinson, et al., Phys. Plas. 1, 1511(1994 which exhibit an internal transport barrier (ITB), have been examined with gyrokinetic simulations, near the ITB onset time. Linear simulations support the picture of ion and electron temperature gradient (ITG, ETG) microturbulence driving high c i and c e , respectively, and that stable ITG correlates with reduced particle transport and improved ion thermal confinement on C-Mod. In the barrier region ITG is weakly unstable, with a critical temperature gradient higher than expected from standard models. Nonlinear calculations and the role of ExB shear suppression of turbulence outside the plasma core are discussed in light of recent profile measurements for the toroidal velocity. The gyrokinetic model benchmarks successfully against experiment in the plasma core.

Microturbulent drift mode suppression as a trigger mechanism for internal transport barriers on Alcator C-Mod

“…This suggests that the major difference in particle transport occurs due to suppression in ITG growth rates. Stability analysis on Alcator C-Mod [18] and throughout the database on other machines [e.g., Ref. 42] shows that plasma core is generally stable to microturbulent drift modes to relatively large r/a values.…”

“…Redi et al [18] performed microturbulent drift mode stability analyses of the C-Mod H-mode plasmas, supporting a link between turbulence suppression and ITB formation. It has been demonstrated experimentally that small amounts of on-axis ICRF can halt the density rise, allowing control of the ITB [4,6].…”

Testing Gyrokinetics on C-Mod and NSTX

“…Calculations of the ITG mode instability threshold and the dependence of instability on temperature and density scaling length have been used to test the GS2 gyrokinetic model for ITG microturbulence on C-Mod [4][5]. Transport analysis at the ITB onset time in Hmode experiments [6] is in rough agreement with the linear mixing length model γ/<k ⊥ 2 >.…”

Computational Center for Studies of Plasma Microturbulence

“…CCSM supported detailed analyses of data from the National Spherical Torus Experiment (NSTX) at the Princeton Plasma Physics Laboratory, [1,2] from the Joint European Torus (JET) in the United Kingdom, [3,4] from the Doublet III-D (DIII-D) tokamak at General Atomics (San Diego, CA), [5,6,7] from the Frascati Tokamak Upgrade (FTU) in Italy, [8] from the Mega-Ampere Spherical Tokamak (MAST) in the United Kingdom, [9,10] and from the Alcator C-Modified (Alcator C-Mod) tokamak at the Massachusetts Institute of Technology. [11,12] Clear identification of key confinement trends and correlation with fluctuation measurements was made in this work, which focused on confinement in the "core" plasma as well as in "internal transport barriers" (ITB's).…”