Space-resolved visible spectroscopy for two-dimensional measurement of hydrogen and impurity emission spectra and of plasma flow in the edge stochastic layer of LHD

Abstract

A space-resolved visible spectrometer system has been developed for two-dimensional (2D) distribution measurements of hydrogen and impurity emission spectra and of plasma flow in the edge stochastic layer of Large Helical Device (LHD). Astigmatism of the spectrometer has been suppressed by introducing additional toroidal and spherical mirrors. A good focal image at the exit slit is realized in a wide wavelength range (75 nm) as well as in a wide slit height direction (26 mm) with a 300 grooves/mm grating. The capability of the spectrometer optical system for the 2D measurement and further possible improvements are discussed in detail. An optical fiber array of 130 channels with a lens unit is used to spatially resolve the edge plasma into different magnetic field structure components: divertor strike points, divertor legs, X-point of the legs, the stochastic layer, and the last closed flux surface. With a 300 grooves/mm grating, the 2D distributions of several hydrogen and impurity line emissions are simultaneously obtained with absolute intensities. A clear correlation is obtained between the magnetic field structure and the emission intensity. With a 2400 grooves/mm grating with a good spectral resolution (0.03 nm/pixel), the 2D distributions of impurity flow velocity are obtained from the Doppler shift measurement. The wavelength position is accurately calibrated by investigating the wavelength dispersion as well as by correcting a mechanical error of the optical setting in the spectrometer. The uncertainty in the velocity is reduced to less than 10% of a typical impurity velocity ∼10 m/s. A temporal change in the flow directions is observed at different spatial locations in divertor detachment plasma.

Direct measurements of counter-streaming flows in a low-shear stellarator magnetic island topology

“…Thus, the ability to measure detailed two-dimensional flow-maps, such as the ones presented, should help improve our understanding of such phenomena. The importance of 2D measurements for better stellarator physics insights have been already demonstrated on the Large Helical Device (LHD) as well [28,29].…”

Three-dimensional simulations of edge impurity flow obtained by the vacuum ultraviolet emission diagnostics in the Large Helical Device with EMC3-EIRENE

“…This is important for understanding the convective losses for energy transport [17] and for assessing the efficiency of impurity screening from the core plasma [18]. On the other hand, the transport properties of edge impurities in the stochastic layer have been investigated by using the spectroscopic measurement of impurity emissivity in the experiments of the Large Helical Device (LHD) [19][20][21][22][23][24]. The complex magnetic field structure of the LHD leads to a more stochastic transport characteristic compared to the axisymmetric magnetic geometry [25], and this provides a good platform to investigate the three-dimensional (3D) effects of a magnetic field structure on impurity transport.…”

Estimations of edge plasma and impurity performance on HL-2M with EMC3-EIRENE modelling

“…The 3D edge transport code EMC3-EIRENE [21,22] has been widely employed on simulations of edge plasma and impurity transports [23][24][25][26][27][28][29]. The EMC3 code solves a set of time-independent Bragnskii's fluid equations [30] for particles, momentum, energy transports of ions and electrons with Monte Carlo (MC) method, and is coupled with the EIRENE code [22], which is employed to treat the transports of neutral atoms and molecules by solving Boltzmann equation for kinetic transport of atoms and molecules.…”