A review of impurity transport characteristics in the LHD

Abstract

The behavior of impurities in the Large Helical Device (LHD) is reviewed based on the knowledge acquired since LHD experiments started in 1998. As a result, a consistent physical picture of impurity transport is obtained in the vast plasma parameter range. The essential points are: (1) the impurity confinement time increases monotonically as the bulk electron density increases in the plasma core; (2) the balance between the friction force and the thermal force on the impurities plays an important role in determining impurity transport in the stochastic layers; (3) a positive electric field leads to outward convection, and a negative electric field generally leads to inward convection (except for the impurity hole case); and (4) in the case of the impurity hole phenomenon, with a high ion temperature plasma and a steep ion temperature gradient, outward convection of the impurities in the plasma core is apparent in spite of the negative electric field. The mechanism for producing outward convection in the impurity hole plasma has not yet been clarified. The effects of the magnetic axis shift and the magnetic island are summarized, and some possible methods for impurity control are also discussed.

Measurements of Impurity Transport Due to Drift-Wave Turbulence in a Toroidal Plasma

“…In these scenarios, it is thought that non-collisional transport resulting from plasma turbulence helps expel impurities [4]. Similar circumstances have been studied in other tokamak, stellarator, and reverse field pinch plasmas [5][6][7][8][9].…”

Isotopic dependence of impurity transport driven by ion temperature gradient turbulence

“…Apart from the helical systems 4 and reversed field pinch (RFP), 5 impurity transport has also been intensively studied in tokamak plasmas by neo-classical theory [6][7][8] and anomalous transport theory. The anomalous transport is attributed to the presence of drift wave-type turbulence.…”

Stellarator Research Opportunities: A Report of the National Stellarator Coordinating Committee

“…The other noteworthy regime is the "impurity hole" in LHD [91,92,89], which in contrast to HDH mode is found at low rather than high electron density. The regime is associated with neutral beam heating and a peaked T i profile.…”

“…In contrast to the core, high density in the scrape off layer (SOL) is believed to reduce the accumulation of impurities that originate in the edge [88,89]. The reason is that the impurities experience friction with the main ions, which stream along B in the SOL towards the divertor.…”