We study the effect of the rotational transform profile on the L-H confinement transitions in the neutral beam-heated plasmas in the TJ-II stellarator. The rotational transform profile in the vacuum is determined by the external coil currents but is modified by the plasma current, (Formula presented.). We find that L-H confinement transitions systematically occur when the configuration and plasma current are such that a low-order rational is placed in the plasma edge region, with a distribution centered around (Formula presented.). It is suggested that magnetohydrodynamic turbulence plays an important role in triggering the L¿H transitions at TJ-II.
Classification
subjects
Physics
keywords
magnetic confinement; stellarator; turbulence; confinement transitions; rotational transform; rational surfaces