EMC3-EIRENE Modelling Research Articles

Impacts of Lithium Injection Positions on Lithium Transport and Heat Flux on EAST with EMC3-EIRENE Modelling

Impacts of Lithium Injection Positions on Lithium Transport and Heat Flux on EAST with EMC3-EIRENE Modelling

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

The three-dimensional edge transport code EMC3-EIRENE has been employed to investigate the edge plasma behavior and carbon impurity transport in the HuanLiuqi-2M (HL-2M) tokamak. It is found that the outboard divertor target has a narrower flux deposition width and therefore undergoes more intense peak heat flux than the inboard one. Detailed analyses of carbon impurity have been performed to obtain deeper insights into the mechanisms of edge impurity transport and mitigation of heat loads. As the increment in upstream density, a larger region in the intrinsic friction force dominant regime has been achieved in divertor region, while less impurity ionization source locates in the thermal force dominant region, and hence a better impurity screening has been obtained. The total power loss induced by carbon impurity shows no significant differences as the upstream density increases when it is higher than 1.5 × 1019 m−3, even though the total number of carbon ions in space obviously decreases. Further study on impacts of the impurity source locations has been performed. The carbon impurity eroded from the vertical part of the outboard target is likely to leak out into the upstream, which is attributed to that the region where the intrinsic friction force dominates is thin. On the other hand, the carbon impurity eroded from the inboard target and the horizontal part of the outboard target is under good control due to the wider region in the intrinsic friction force dominant regime.

Impacts of resonant magnetic perturbations on edge carbon transport and emission on EAST with EMC3-EIRENE modelling

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.

Edge plasma measurements on the OP 1.2a divertor plasmas at W7-X using the combined probe

During its second operational campaign (OP 1.2a) W7-X operated with an edge island divertor. Much higher densities (1020m−3) than in to the previous, limiter campaign were achieved. In order to asses this improved performance and the nature of transport with regard to the expected higher densities and other parameters such as the electron and ion temperature, the electric field and plasma flow, measurements at the plasma edge were conducted with reciprocating probe, mounted on a Manipulator. The probe contains a set of Langmuir pins, Mach probes, compensation coils and a set of 3D pick up coils for the measurement of the local magnetic field and fluctuations, and an ion sensitive probe. It has been observed, in addition to differences due to the magnetic configurations, that the magnetic topology is sensitive to the plasma conditions, especially to changes due to a evolving toroidal plasma current. Measurements from divertor cameras showed that an increasing toroidal current in the standard configuration broadened the heat flux and shifted the strike line considerably. These down-stream observation can be complemented with the measurements at the mid-plane with the manipulator mounted probes. During the experiments the edge island remnant was identified with the measured edge plasma profiles and compared with the predictions from the field line tracing code. Those electron densities and temperatures have been used as an input for understanding edge transport with the EMC3-EIRENE modeling. The results from EMC3-EIRENE also help improve the caculated density profiles, since the modelling provides a profile of the effective charge Zeff.

Effect of magnetic perturbations for ELM control on divertor power loads, detachment and consequences of field penetration in ASDEX Upgrade

Magnetic perturbation (MP) fields are currently studied in ASDEX Upgrade and many other tokamaks in terms of edge localized mode control and the implication onto steady state divertor power load and access to detachment. Previous studies at ASDEX Upgrade in low density, attached L-mode (Faitsch et al 2017 Plasma Phys. Control. Fusion 59 095006) are combined with high density L- and H-mode studies (Brida et al 2017 Nucl. Fusion 57 116006). A consistent interpretation is presented for the steady state power load variation due to the 3D MP. The deviation from an axisymmetric power load is reducing with increasing density, being an effect of the increasing broadening in the divertor region. No deleterious effect on the access to a detached divertor regime is found. Experimental results from ASDEX Upgrade reveal similar power load profiles for plasmas without and a toroidal averaged profile in presence of an external MP. EMC3-Eirene modeling is showing agreement only if screening currents in the plasma as a response to the external magnetic field are added. In plasma scenarios with field penetration and locking of internal modes a fundamentally different power load pattern is observed. The pattern is in agreement with edge ergodization and the extent can be used to determine the magnitude of current needed to describe the internal mode.

Impact of magnetic islands in the plasma edge on particle fueling and exhaust in the HSX and W7-X stellarators

The edge magnetic structure in the Helically Symmetric eXperiment (HSX) and Wendelstein 7X (W7-X) stellarators has been shown to have a significant impact on the particle fueling and exhaust of the plasma main species (hydrogen) as well as impurity helium. For HSX, the plasma sourcing to exhaust ratio, quantified by the effective and global particle confinement times τp* and τp,H, respectively, increases when a magnetic island chain is located in the plasma edge. The fueling efficiency is reduced by 25% when the plasma boundary is deformed by the magnetic islands. The X-point geometry also yields higher plasma temperatures in front of the main recycling region. When the island is moved radially inward, both τp* and τp decrease by 10%–25% depending on plasma density. The τp,H results rely heavily on EMC3-EIRENE modeling which confirms reduced fueling efficiency due to more rapid ionization in the outward shifted island position. These findings suggest that for a helically optimized system like HSX, the plasma fueling from the recycling source, as well as from active gas injection, can be controlled by the magnetic island chain in the plasma edge—which is a basic requirement for a divertor system. This process is also effective for the control of effective helium exhaust times, as τp,He* measured by perturbative gas puff experiments is reduced by up to 40% when the islands are shifted inwards. For Wendelstein 7-X, a similar reduction of τp,He* was inferred when magnetic islands were moved from the far plasma edge into the confined plasma region. However, the effective confinement features of H as the main plasma species were not affected due to the non-optimal position of the magnetic islands with respect to the highly localized ionization domain during the limiter startup campaign.

Investigation of edge impurity transport derived from the first wall on EAST with EMC3-EIRENE modelling

The long-term operations of the EAST experiments result in a substantial redeposition of impurities on the first wall, which is considered as the potential impurity source during the discharge. The transport properties of lithium impurity in the scrape-off layer (SOL) of EAST have been studied by the three-dimensional (3D) edge transport code, the EMC3-EIRENE. The neutral lithium impurities from four impurity source positions on the first wall have been used in the modelling. The detailed analysis of the relationship between the impurity source positions and the density profiles of the lithium impurity for different charge states has been carried out. It is found that the density profiles of the Li1+ ions are determined by the impurity source positions, whereas the density profiles of the Li2+ and Li3+ ions are not related to the first wall source positions.

Investigation of heat flux deposition on divertor target on the Large Helical Device with EMC3-EIRENE modelling

The measured divertor heat flux profiles are compared to the EMC3-EIRENE simulations for two different times of an LHD discharge, corresponding to higher and lower edge temperatures. The relation between the three-dimensional magnetic field structure and the heat flux distributions on the divertor has been analysed. The modelled heat flux for the lower plasma temperature case has a better agreement with the experimental result obtained by the Langmuir probes, which shows a qualitative reproduction of the experimental profile shape. However, the heat flux distribution for the high plasma temperature case shows a different behaviour between the simulation results and the experimental measurements. The detailed analysis of the heat flux distribution for the higher temperature case which has a larger discrepancy has been performed, both quantitatively and qualitatively. The radiation of the eroded impurity from divertor target plates has a minor effect on the heat flux distribution. Non-uniform cross-field transport coefficients are used in the simulations and its impact on the heat flux distributions is discussed for the case of the high plasma temperature.

Changes in divertor conditions in response to changing core density with RMPs

The effects of changes in core density on divertor electron temperature, density and heat flux when resonant magnetic perturbations (RMPs) are applied are presented, notably a reduction in RMP induced secondary radial peaks in the electron temperature profile at the target plate is observed when the core density is increased, which is consistent with modeling. RMPs is used here to indicate non-axisymmetric magnetic field perturbations, created using in-vessel control coils, which have at least one but typically many resonances with the rotational transform of the plasma (Evans et al 2006 Phys. Plasmas 13 056121). RMPs are found to alter inter-ELM heat flux to the divertor by modifying the core plasma density. It is shown that applying RMPs reduces the core density and increases the inter-ELM heat flux to both the inner and outer targets. Using gas puffing to return the core density to the pre-RMP levels more than eliminates the increase in inter-ELM heat flux, but a broadening of the heat flux to the outer target remains. These measurements were made at a single toroidal location, but the peak in the heat flux profile was found near the outer strike point where simulations indicate little toroidal variation should exist and tangentially viewing diagnostics showed no evidence of strong asymmetries. In experiments where divertor Thomson scattering measurements were available it is shown that local secondary peaks in the divertor electron temperature profile near the target plate are reduced as the core density is increased, while peaks in the divertor electron density profile near the target are increased. These trends observed in the divertor electron temperature and density are qualitatively reproduced by scanning the upstream density in EMC3-Eirene modeling. Measurements are presented showing that higher densities are needed to induce detachment of the outer strike point in a case where an increase in electron temperature, likely due to a change in MHD activity, is seen after RMPs are applied.

Dynamics of three-dimensional radiative structures during RMP assisted detached plasmas on the large helical device and its comparison with EMC3-EIRENE modeling

The resonant magnetic perturbation (RMP) island introduced in the stochastic edge of the large helical device (LHD) plasma plays an important role in the stabilization of the plasma detachment (Kobayashi et al 2013 Nucl. Fusion 53 093032). The plasma enters in the sustained detachment phase in the presence of an RMP once the line averaged density exceeds a critical value with a given input power. During detachment the enhanced radiation from the stochastic edge of the LHD undergoes several spatiotemporal changes which are studied quantitatively by an infrared imaging video bolometer (IRVB) diagnostic. The experimental results are compared qualitatively and quantitatively with the radiation predicted by the 3D transport simulation with fluid model, EMC3-EIRENE. A fair amount of qualitative agreement, before and after the detachment, is reported. The issue of overestimated radiation from the model is addressed by changing the free parameters in the EMC3-EIRENE code till the total radiation and the radiation profiles match closely, within a factor of two with the experimental observations. A better quantitative match between the model and the experiment is achieved at higher cross-field impurity diffusion coefficient and lower sputtering coefficient after the detachment. In this article a comparison, the first of its kind, is established between the quantified radiation from the experiments and the synthetic image obtained from the simulation code. This exercise is aimed towards validating the model assumptions against the experimentally measured radiation.

Characterization of the island divertor plasma of W7-AS stellarator in the deeply detached state with volume recombination

In the high-density H-mode of the Stellarator Wendelstein 7-AS, the plasma detaches from the island divertor targets when the line-averaged density exceeds a critical value. This quasi-stationary detachment is found to be partial and shows edge-localized, poloidally asymmetric radiation. The spectroscopic characteristics of the deeply detached plasma are reported, including evidence for volume recombination. The detached plasma radiates up to 90% of the absorbed power with larger contributions from the locations close to magnetic X-points outside the divertor region. The spectral analysis of the Balmer series indicate very high densities and low temperatures at the detached regions. The results of the spectral analysis underline the importance of three-dimensional modelling. An initial comparison is made with the latest results from EMC3-EIRENE modelling.