Vertical profiles and two-dimensional distributions of carbon line emissions from C2+−C5+ ions in attached and RMP-assisted detached plasmas of large helical device
Abstract:In Large Helical Device (LHD), the detached plasma is obtained without external impurity gas feed by supplying an m/n = 1/1 resonant magnetic perturbation (RMP) field to a plasma with an outwardly shifted plasma axis position of Rax = 3.90 m where the magnetic resonance exists in the stochastic magnetic field layer outside the last closed flux surface. The plasma detachment is triggered by the appearance of an m/n = 1/1 island when the density, increased using hydrogen gas feed, exceeds a threshold density. Th… Show more
“…A cross-field transport process related to the differences of the impurities gyroradii, as has been observed by Zhang et al. (2017) in LHD, could explain the different responses, although the role of the ion charge remains unclear at present. Figure 5.Impurities intensity amount of mitigation (per cent) increasing trend with ion mass.…”
Section: Experimental Data Acquisitionmentioning
confidence: 68%
“…The mitigation of high-Z impurity emissions seems to be related to I RMP rather than to variations in n e , T e or W MHD which only change modestly. Even the spatial profiles of n e , T e from the TS diagnostic (Zang et al 2010) and T i from XCS (Lyu et al 2014(Lyu et al , 2016, shown in figure 4, do not appear to account for the mitigation levels, with n e , T e slightly increasing with RMP (particularly more so in central plasma regions) and T i staying relatively the same. Figure 5 shows the relative reduction of emissions of the different impurities surveyed in response to the use of RMPs.…”
Spatial profiles of impurity emission measurements in the extreme ultraviolet (EUV) spectroscopic range in radiofrequency (RF)-heated discharges are combined with one-dimensional and three-dimensional transport simulations to study the effects of resonant magnetic perturbations (RMPs) on core impurity accumulation at EAST. The amount of impurity line emission mitigation by RMPs appears to be correlated with the ion Z for lithium, carbon, iron and tungsten monitored, i.e. stronger suppression of accumulation for heavier ions. The targeted effect on the most detrimental high-Z impurities suggests a possible advantage using RMPs for impurity control. Profiles of transport coefficients are calculated with the STRAHL one-dimensional impurity transport code, keeping
$\nu /D$
fixed and using the measured spatial profiles of
$\textrm{F}{\textrm{e}^{20 + }}$
,
$\textrm{F}{\textrm{e}^{21 + }}$
and
$\textrm{F}{\textrm{e}^{22 + }}$
to disentangle the transport coefficients. The iron diffusion coefficient
${D_{\textrm{Fe}}}$
increases from
$1.0- 2.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$
to
$1.5- 3.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$
from the core region to the edge region
$(\rho \gt 0.5)$
after the onset of RMPs. Meanwhile, an inward pinch of iron convective velocity
${\nu _{\textrm{Fe}}}$
decreases in magnitude in the inner core region and increases significantly in the outer confined region, simultaneously contributing to preserving centrally peaked
$\textrm{Fe}$
profiles and exhausting the impurities. The
${D_{\textrm{Fe}}}$
and
${\nu _{\textrm{Fe}}}$
variations lead to reduced impurity contents in the plasma. The three-dimensional edge impurity transport code EMC3-EIRENE was also applied for a case of RMP-mitigated high-Z accumulation at EAST and compared to that of low-Z carbon. The exhaust of
${\textrm{C}^{6 + }}$
toward the scrape-off layer accompanying an overall suppression of heavier
${\textrm{W}^{30 + }}$
is observed when using RMPs.
“…A cross-field transport process related to the differences of the impurities gyroradii, as has been observed by Zhang et al. (2017) in LHD, could explain the different responses, although the role of the ion charge remains unclear at present. Figure 5.Impurities intensity amount of mitigation (per cent) increasing trend with ion mass.…”
Section: Experimental Data Acquisitionmentioning
confidence: 68%
“…The mitigation of high-Z impurity emissions seems to be related to I RMP rather than to variations in n e , T e or W MHD which only change modestly. Even the spatial profiles of n e , T e from the TS diagnostic (Zang et al 2010) and T i from XCS (Lyu et al 2014(Lyu et al , 2016, shown in figure 4, do not appear to account for the mitigation levels, with n e , T e slightly increasing with RMP (particularly more so in central plasma regions) and T i staying relatively the same. Figure 5 shows the relative reduction of emissions of the different impurities surveyed in response to the use of RMPs.…”
Spatial profiles of impurity emission measurements in the extreme ultraviolet (EUV) spectroscopic range in radiofrequency (RF)-heated discharges are combined with one-dimensional and three-dimensional transport simulations to study the effects of resonant magnetic perturbations (RMPs) on core impurity accumulation at EAST. The amount of impurity line emission mitigation by RMPs appears to be correlated with the ion Z for lithium, carbon, iron and tungsten monitored, i.e. stronger suppression of accumulation for heavier ions. The targeted effect on the most detrimental high-Z impurities suggests a possible advantage using RMPs for impurity control. Profiles of transport coefficients are calculated with the STRAHL one-dimensional impurity transport code, keeping
$\nu /D$
fixed and using the measured spatial profiles of
$\textrm{F}{\textrm{e}^{20 + }}$
,
$\textrm{F}{\textrm{e}^{21 + }}$
and
$\textrm{F}{\textrm{e}^{22 + }}$
to disentangle the transport coefficients. The iron diffusion coefficient
${D_{\textrm{Fe}}}$
increases from
$1.0- 2.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$
to
$1.5- 3.0\;{\textrm{m}^2}\;{\textrm{s}^{ - 1}}$
from the core region to the edge region
$(\rho \gt 0.5)$
after the onset of RMPs. Meanwhile, an inward pinch of iron convective velocity
${\nu _{\textrm{Fe}}}$
decreases in magnitude in the inner core region and increases significantly in the outer confined region, simultaneously contributing to preserving centrally peaked
$\textrm{Fe}$
profiles and exhausting the impurities. The
${D_{\textrm{Fe}}}$
and
${\nu _{\textrm{Fe}}}$
variations lead to reduced impurity contents in the plasma. The three-dimensional edge impurity transport code EMC3-EIRENE was also applied for a case of RMP-mitigated high-Z accumulation at EAST and compared to that of low-Z carbon. The exhaust of
${\textrm{C}^{6 + }}$
toward the scrape-off layer accompanying an overall suppression of heavier
${\textrm{W}^{30 + }}$
is observed when using RMPs.
“…In the previous simulations for LHD, it is found that the enhanced impurity perpendicular transport plays an important role in the distributions of carbon impurity density and emission (Dai et al 2016b;Zhang et al 2017). Given that the edge magnetic field structure of LHD is stochastic intrinsically, the stochastic fields induced by RMP application on EAST probably cause the enhanced perpendicular transport FIGURE 13.…”
Section: Change Of the CVI Emission Intensity Ratiomentioning
confidence: 90%
“…The transport properties of edge impurities with and without RMP fields have been studied in LHD by the extreme ultraviolet (EUV) measurements of impurity emission (Zhang et al 2017). However, impacts of the RMP fields on edge impurity transport on tokamaks are not well understood yet.…”
The modelling of edge carbon transport and emission on EAST tokamak under resonant magnetic perturbation (RMP) fields has been conducted with the three-dimensional edge transport code EMC3-EIRENE. The measured vertical distribution of CVI emission by the extreme ultraviolet spectrometer system for the perturbed case shows a reduction in the CVI emission by 20 % compared to the equilibrium case. The chord-integrated CVI emission can be reconstructed by EMC3-EIRENE modelling, which presents an increase in the CVI emission with RMP fields. The discrepancy between experiments and simulations has been investigated by parameter study to examine the sensitivity of the simulation results on the edge plasma conditions and the impurity perpendicular transport. It is found that the variation of edge plasma conditions for the equilibrium case cannot resolve the discrepancy in the CVI emission between simulations and measurements. The simulations with enhanced impurity perpendicular transport coefficient allows a reasonable agreement with the measured reduction of CVI emission.
“…The resulting impurity distribution then impacts on emission distributions, and thereby affects also detachment stability [3]. In LHD, the impurity emission measurements have been conducted, and correlation with the magnetic field structure and impurity transport are discussed [4]. However, the impurity transport property in the edge region is not yet fully understood.…”
In order to understand plasma transport in the edge stochastic layers, a visible spectrometer has been developed. Two-dimensional impurity emission intensity distributions in the edge stochastic layer of LHD, including divertor plate, divertor legs, X-point, and the last closed flux surface, have been measured. Carbon emission intensities and their distributions are found to be clearly different depending on the different magnetic field configuration and on the charge state. Effects of the edge magnetic field structure on these results are discussed.
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