The divertor plasma characteristics in the Large Helical Device

TLDR

Langmuir probes have been used to study divertor plasma characteristics in the Large Helical Device. Measurements on LHD’s divertor plates reveal a clear three‑dimensional helical structure in particle and power deposition, a linear increase of particle flux with line‑averaged density, and a radial decline of electron density and temperature in the stochastic layer, all of which agree with field‑line tracing and show no high‑recycling or detachment even at high density and low power.

Abstract

Divertor plasma characteristics in the Large Helical Device (LHD) have been investigated mainly by using Langmuir probes. The three-dimensional structure of the helical divertor, which is naturally produced in the heliotron-type magnetic configuration, is clearly seen in the measured particle and power deposition profiles on the divertor plates. These observations are consistent with the numerical results of field line tracing. The particle flux to the divertor plates increases almost linearly with the line averaged density. The high-recycling regime and divertor detachment, which are observed in tokamaks, have not been observed even during high density discharges with low input power. Both electron density and temperature decrease with increasing radius in the stochastic layer with open field lines, and at the divertor plate they become fairly low compared with those at the last closed flux surface. This means the reduction of pressure along the magnetic field lines occurs in the open field line region in LHD.