Abstract
Recent long pulse discharges in the large helical device (LHD) have been interrupted by radiation collapse induced by the emission of dust from the surface of the vacuum vessel and the divertor region. The dust shielding effect by intrinsic ergodic magnetic field line structures (ergodic layer) formed around the main plasma confinement region is investigated using a three‐dimensional peripheral plasma fluid code (EMC3‐EIRENE) coupled with a dust transport simulation code (DUSTT). Simulations performed in three different magnetic configurations with narrow, medium, and wide ergodic layers show that the wide ergodic layer is not always effective in shielding the main plasma from the dust emission. Optimum operational regimes for controlling the influence of the dust emission on the sustainment of plasma discharges are found by investigating the impurity ion content in the peripheral plasma for various plasma heating powers and plasma densities in the three magnetic configurations.
Highlights
The Large Helical Device (LHD) consists of superconducting helical and poloidal coils for forming plasma confinement magnetic field line configurations without the toroidal plasma current, which is advantageous for the sustainment of steady-state plasma discharges [1]
An impurity transport simulation by a fully three-dimensional peripheral plasma fluid code (EMC3-EIRENE) [7] coupled with a dust transport simulation code (DUSTT) [8,9,10] shows that the plasma discharges are interrupted by much lower amounts of iron dust emission from the helical coil can compared to that of carbon dust emission from the divertor region [11]
The iron dust shielding effect by the ergodic layer intrinsically formed in the LHD peripheral plasma was investigated using a three-dimensional peripheral plasma fluid code (EMC3-EIRENE) coupled with a dust transport simulation code (DUSTT) in the three magnetic configurations having narrow, medium, and wide ergodic layers
Summary
The Large Helical Device (LHD) consists of superconducting helical and poloidal coils for forming plasma confinement magnetic field line configurations without the toroidal plasma current, which is advantageous for the sustainment of steady-state plasma discharges [1]. Recent long pulse plasma discharges in LHD have often been interrupted by radiation collapse by impurities (mainly carbon and iron) induced by the emission of dust. An observation with a stereoscopic fast framing camera revealed that iron dust released from the arcing point penetrated into the main plasma confinement region to cause the radiation collapse [6]. It is likely that the iron dust emission from near the main plasma confinement region such as the helical coil can will be a serious obstacle for sustaining long pulse discharges in LHD. The peripheral plasma formed near the Last Closed Flux Surface (LCFS) in the ergodic layer can effectively prevent the iron dust from penetrating into the main plasma by evaporating the dust due to the plasma heat load
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