Outer Divertor Target Plate Research Articles

A long-pulse small edge-localized-mode high-confinement plasma with detachment feedback control by floating potential in an experimental advanced superconducting tokamak in a metal wall environment

Abstract One of the key challenges facing the magnetic fusion research is to demonstrate the compatibility between high confinement and radiative divertor in long-pulse discharges with a metal wall environment. A small edge localized mode (ELM) high confinement plasma is obtained in the upgraded lower divertor of Experimental Advanced Superconducting Tokamak (EAST) with an energy confinement factor H98~1.1 and a Greenwald density fraction fGW ~ 0.65 maintained for 26 s, and periodical detachment is achieved through active control of neon impurity seeding in this long-pulse discharge. For the divertor region, partial detachment is achieved periodically on the outer divertor target plates with the plasma temperature near the outer strike point decreased to below 5 eV and peak surface temperature on the outer divertor target plates maintained below 350C. The peak heat flux of the lower outer divertor decreases significantly and its profile along the target becomes very flat in the detached state. Two low-frequency (<10 kHz) fluctuations that are related to rippling mode caused by a resistive instability appear in the detached state. For the pedestal region, the electron pressure profile is flatter and ELM amplitude is smaller in the detached state than that in the attached state. Edge coherent mode (ECM) appears in the attached state and disappears in the detached state. To achieve this experimentally, a new impurity seeding feedback control scheme is applied, where the floating potential measured by divertor Langmuir probes is used as the feedback sensor, which is more reliable in the long-pulse discharges with high heat fluxes, thus more suitable for application in future devices. This work provides a new approach for the actively controlled radiative divertor as a solution to the divertor heat loads of the future fusion reactors.

Turbulence simulations with BOUT++ by using SOLPS grids for SOLPS/BOUT++ coupling

AbstractThe coupling of transport code SOLPS with the turbulence code BOUT++ was reported in Reference [D. R. Zhang et al., Phys. Plasmas 26, 012508 (2019)], while the grids of SOLPS and BOUT++ are not completely consistent with each other, especially in the divertor region. In the present work, a method of replacing the grids of BOUT++ with the grids of SOLPS is proposed to make the simulation region fully consistent with each other for the SOLPS/BOUT++ coupling. A SOLPS grid file is generated with an MHD equilibrium and used in BOUT++ code to simulate the profiles of plasma density, ion temperature, and electron temperature with the six‐field two‐fluid model. The profiles of the main plasma parameters simulated with the SOLPS grids are similar with the profiles simulated with the BOUT++ grids at the midplane, while the profiles are deformed compared with the profiles simulated with the BOUT++ grids at the outer divertor target because of the differences of the distributions of SOLPS grids and BOUT++ grids in the divertor region. The radial particle transport coefficient and heat transport coefficients are also calculated by using the BOUT++ code with the two grids, and the comparisons of the radial particle transport coefficient and heat transport coefficients simulated with the two grids at the midplane and outer divertor target plate are discussed.

Electromagnetic turbulence simulation of tokamak edge plasma dynamics and divertor heat load during thermal quench

The edge plasma turbulence and transport dynamics, as well as the divertor power loads during the thermal-quench phase of tokamak disruptions, are numerically investigated with BOUT++’s flux-driven six-field electromagnetic turbulence model. Here, transient yet intense particle and energy sources are applied at the pedestal top to mimic the plasma power drive at the edge induced by a core thermal collapse, which flattens the core temperature profile. Interesting features, such as surging of divertor heat load (up to 50 times) and broadening of heat-flux width (up to four times) on the outer-divertor target plate, are observed in the simulation, in qualitative agreement with experimental observations. The dramatic changes in divertor heat load and width are due to the enhanced plasma turbulence activities inside the separatrix. Two cross-field transport mechanisms, namely, the E × B turbulent convection and the stochastic parallel advection/conduction, are identified to play important roles in this process. First, an elevated edge pressure gradient drives instabilities and subsequent turbulence in the entire pedestal region. The enhanced turbulence not only transports particles and energy radially across the separatrix via the E × B convection, which causes the initial divertor heat-load burst, but it also induces amplified magnetic fluctuation . Once themagnetic fluctuation is large enough to break the magnetic flux surface, magnetic flutter effect provides an additional radial transport channel. In the late stage of our simulation, reaches to 10−4 level that completely breaks magnetic flux surfaces such that stochastic field lines are directly connecting pedestal top plasma to the divertor target plates or first wall, further contributing to the divertor heat-flux width broadening.

Type-I ELM power loads on the closed outer divertor targets in the HL-2A tokamak

The HL-2A tokamak has a very closed divertor geometry, and a new infrared camera has been installed for high resolution studies of edge-localized mode (ELM) heat load onto the outer divertor targets. The characteristics of power deposition patterns on the lower outer divertor target plates during ELMs are systematically analysed with infrared thermography. The ELM energy loss is in the range of 3%–8% of the total plasma stored energy. The peak heat flux on the outer divertor targets during ELMs currently achieved in HL-2A is about 1.5–3.2 MW m−2, the wetted area is about 0.5–0.7 m2, and the corresponding integrated power decay length at the midplane is about 25–40 mm. The rise time of the ELM power deposition is in the range of about 100 μs to 400 μs, and the decay time is typically 1.5 to 4 times longer than the corresponding rise time. Convective transport along open field lines during the ELM rise phase from the midplane towards the divertor targets is implied due to the correlation of parallel transport time in the scrape-off layer (SOL) and ELM power rise time. The peak ELM energy fluence is compared with those predicted by models and with experimental data from JET, ASDEX Upgrade, MAST, and COMPASS. The results, as a whole, show a good agreement.

Study of Temperature and Heat Flux on the EAST Divertor Target Plate in LHW+ NBI/ICRH H-Mode

The heat fluxes on the lower outer divertor plate were calculated using the DFLUX code with the surface temperature measured by the IR camera on the experimental advanced superconducting tokamak. The analyzed discharges were lower single-null divertor configuration discharges with different auxiliary heating: lower hybrid wave current drive (LHCD) combined with neutral beam injection (NBI) and LHCD combined with ion cyclotron resonant heating (ICRH). The surface temperature of the lower outer divertor target plate was about 170 °C in the LHCD + NBI H-mode, with the high temperature mostly distributed in the location near the joint of the dome and the outer target plate. The temperature increased as the NBI was turned on and fluctuated due to edge localized modes (ELMs). The heat flux reached nearly 4 MW/m2 when ELMs appeared and declined to less than 1 MW/m2 when the NBI was turned off. Temperature distribution with extreme toroidal asymmetry was observed in the selected LHCD + ICRH H-mode discharge. The surface temperature on the lower outer divertor target plate was up to 220 °C in one region with broader temperature distribution. During the LHCD + ICRH H-mode, the peak heat flux was not seen during the period of ICRH injection, though a brief spike in heat flux was observed at the beginning of the ICRH injection. The full-width at half-maximum of the heat flux profile as a function of divertor target plate location of the LHCD + ICRH H-mode is three times that of the LHCD + NBI H-mode.

Experiments on transient melting of tungsten by ELMs in ASDEX Upgrade

Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the divertor manipulator II (DIM-II) system (Herrmann et al 2015 Fusion Eng. Des. 98–9 1496–9). Designed as near replicas of the geometries used also in separate experiments on the JET tokamak (Coenen et al 2015 J. Nucl. Mater. 463 78–84; Coenen et al 2015 Nucl. Fusion 55 023010; Matthews et al 2016 Phys. Scr. T167 7), the samples featured a misaligned leading edge and a sloped ridge respectively. Both structures protrude above the default target plate surface thus receiving an increased fraction of the parallel power flux. Transient melting by ELMs was induced by moving the outer strike point to the sample location. The temporal evolution of the measured current flow from the samples to vessel potential confirmed transient melting. Current magnitude and dependency from surface temperature provided strong evidence for thermionic electron emission as main origin of the replacement current driving the melt motion. The different melt patterns observed after exposures at the two sample geometries support the thermionic electron emission model used in the MEMOS melt motion code, which assumes a strong decrease of the thermionic net current at shallow magnetic field to surface angles (Pitts et al 2017 Nucl. Mater. Energy 12 60–74). Post exposure ex situ analysis of the retrieved samples show recrystallization of tungsten at the exposed surface areas to a depth of up to several mm. The melt layer transport to less exposed surface areas leads to ratcheting pile up of re-solidified debris with zonal growth extending from the already enlarged grains at the surface.

Investigation of transient melting of tungsten by ELMs in ASDEX Upgrade

Repetitive melting of tungsten by power transients originating from edge localized modes (ELMs) has been studied in the tokamak experiment ASDEX Upgrade. Tungsten samples were exposed to H-mode discharges at the outer divertor target plate using the Divertor Manipulator II system. The exposed sample was designed with an elevated sloped surface inclined against the incident magnetic field to increase the projected parallel power flux to a level were transient melting by ELMs would occur. Sample exposure was controlled by moving the outer strike point to the sample location. As extension to previous melt studies in the new experiment both the current flow from the sample to vessel potential and the local surface temperature were measured with sufficient time resolution to resolve individual ELMs. The experiment provided for the first time a direct link of current flow and surface temperature during transient ELM events. This allows to further constrain the MEMOS melt motion code predictions and to improve the validation of its underlying model assumptions. Post exposure ex situ analysis of the retrieved samples confirms the decreased melt motion observed at shallower magnetic field line to surface angles compared to that at leading edges exposed to the parallel power flux.

Heat Flux on EAST Divertor Plate in H-mode with LHCD/LHCD+NBI**Supported by the National Natural Science Foundation of China under Grant Nos 11505290, 51576208 and 11575239, and the National Magnetic Confinement Fusion Science Program of China under Grant Nos 2013GB113004 and 2015GB102004.

Based on the surface temperature measured by the infrared camera on the experimental advanced superconducting tokamak (EAST), the heat fluxes on the lower outer divertor target plate during H-mode with the lower-hybrid wave current drive (LHCD) only and with the LHCD combined with the neutral beam injection (NBI) are calculated by the DFLUX code and compared. The analyzed discharges are lower single null divertor configuration discharges. In the case with the LHCD only (, ), ELM-free appears after L-H transition with the peak heat flux on the lower outer target plate less than 1 MW/m2. However, there is no ELM-free appearing after the L-H transition in the case with the LHCD+NBI (, ). The results show that the peak heat fluxes on the lower outer target plate in the LHCD+NBI H-mode cases are larger than those in the LHCD H-mode under the similar auxiliary heating power. This is because the heat flux profiles of the lower outer target plate as a function of plate location in ELMing with the LHCD+NBI are narrower than those with the LHCD only. The results are consistent with the results in terms of the scrape-off layer width observed in the EAST.

Advanced divertor concept design and analysis for HL-2M

HL-2M is a tokamak device that is under construction and will be put into operation in the near future. Based on the magnetic coil design of HL-2M, standard divertor, snowflake divertor and tripod divertor configurations have been designed. The potential properties of snowflake divertor configurations have been analyzed, such as the low poloidal field (Bp) area around the X-point, the connection length, target plate and magnetic field shear. The linear peeling-ballooning (P-B) mode is studied by BOUT++ code for snowflake divertor configurations. According to the divertor configuration properties of HL-2M, asymmetric target plates have been concept designed to be compatible with the intended single null (SN) divertor configurations as well as double null (DN) divertor configurations. The SOLPS5.0 code is used to predict the details of the divertor plasma under the conditions of the divertor configurations noted above without impurities. This result shows that the peak heat load on an outer target plate of the advanced divertor is about 40% of that of the standard divertor. But more power will be transported to the inner target plate of advanced divertor, and this will cause a higher peak heat load on the inner target plate. The advanced divertor will also have to work under low plasma recycling conditions with high particle temperature and low density in an open divertor target geometry. When the SN configuration changes to a DN tripod divertor configuration, most of the power exhaust is handled by the outer divertor target plates and the peak heat load on these is about 4.1MW/m2 (with a power exhaust of 20MW). This range of optimized divertor configurations and target geometry will enable the study of advanced divertor physics and high performance plasmas in HL-2M tokamak.

Resistive reduced MHD modeling of multi-edge-localized-mode cycles in Tokamak X-point plasmas.

The full dynamics of a multi-edge-localized-mode (ELM) cycle is modeled for the first time in realistic tokamak X-point geometry with the nonlinear reduced MHD code jorek. The diamagnetic rotation is found to be instrumental to stabilize the plasma after an ELM crash and to model the cyclic reconstruction and collapse of the plasma pressure profile. ELM relaxations are cyclically initiated each time the pedestal gradient crosses a triggering threshold. Diamagnetic drifts are also found to yield a near-symmetric ELM power deposition on the inner and outer divertor target plates, consistent with experimental measurements.

Non-linear MHD modeling of edge localized mode cycles and mitigation by resonant magnetic perturbations

The dynamics of a multi-edge localized mode (ELM) cycle as well as the ELM mitigation by resonant magnetic perturbations (RMPs) are modeled in realistic tokamak X-point geometry with the non-linear reduced MHD code JOREK. The diamagnetic rotation is found to be a key parameter enabling us to reproduce the cyclical dynamics of the plasma relaxations and to model the near-symmetric ELM power deposition on the inner and outer divertor target plates consistently with experimental measurements. Moreover, the non-linear coupling of the RMPs with unstable modes are found to modify the edge magnetic topology and induce a continuous MHD activity in place of a large ELM crash, resulting in the mitigation of the ELMs. At larger diamagnetic rotation, a bifurcation from unmitigated ELMs—at low RMP current—towards fully suppressed ELMs—at large RMP current—is obtained.

Controlled tungsten melting and droplet ejection studies in ASDEX Upgrade

Tungsten rods of 1×1×3 mm3 were exposed in single H-mode discharges at the outer divertor target plate of ASDEX Upgrade using the divertor manipulator system. Melting of the W rod at a pre-defined time was induced by moving the initially far away outer strike point close to the W-rod position. Visible light emissions of both the W pin and consecutively ejected W droplets were recorded by two fast cameras with crossed viewing cones. The time evolution of the local W source at the pin location was measured by spectroscopic observation of the WI line emission at 400.9 nm and compared to the subsequent increase of tungsten concentration in the confined plasma derived from tungsten vacuum UV line emission. Combining these measurements with the total amount of released tungsten due to the pin melt events and ejected droplets allowed us to derive an estimate of the screening factor for this type of tungsten source. The resulting values of the tungsten divertor retention in the range 10–20 agree with those found in previous studies using a W source of sublimated W(CO)6 vapour at the same exposure location. Ejected droplets were found to be always accelerated in the general direction of the plasma flow, attributed to friction forces and to rocket forces. Furthermore, the vertically inclined target plates cause the droplets, which are repelled by the target plate surface potential due to their electric charge, to move upwards against gravity due to the centrifugal force component parallel to the target plate.

Scaling of the power exhaust channel in Alcator C-Mod

Parametric dependences of the heat flux footprint on the outer divertor target plate are explored in EDA H-mode and ohmic L-mode plasmas over a wide range of parameters with attached plasma conditions. Heat flux profile shapes are found to be independent of toroidal field strength, independent of power flow along magnetic field lines and insensitive to x-point topology (single-null versus double-null). The magnitudes and widths closely follow that of the “upstream” pressure profile, which are correlated to plasma thermal energy content and plasma current. Heat flux decay lengths near the strike-point in H- and L-mode plasmas scale approximately with the inverse of plasma current, with a diminished dependence at high collisionality in L-mode. Consistent with previous studies, pressure gradients in the boundary scale with plasma current squared, holding the magnetohydrodynamic ballooning parameter approximately invariant at fixed collisionality—strong evidence that critical-gradient transport physics plays a key role in setting the power exhaust channel.

Effect of localized gas puffing on divertor plasma behavior in EAST

Localized D2 puffing from various divertor locations has been carried out under double null (DN) and lower single null (LSN) divertor configurations to investigate the effect of gas puff locations on the divertor behavior in ohmic L-mode discharges in EAST. Localized gas puffing from the dome has a higher fueling efficiency than that from the inner and outer targets for both DN and LSN configurations. Under the DN configuration, gas puffing from the inner target exhibits a much better fueling efficiency than that from the outer target. In contrast, the gas fueling efficiency shows little difference between the inner and outer divertor gas puff locations in the LSN configuration. In LSN, localized gas puffing from the outer divertor target tends to promote detachment at the outer target. This will be employed as a means to control heat fluxes to the outer divertor target plates for high power long pulse operations.

Type-I ELM power deposition profile width and temporal shape in JET

A new infra red camera (IR) for high resolution infra red studies for the outer divertor target plate in JET has been installed. Shot integrated energy balance between tile embedded thermocouples and IR based estimation of deposited energy on the outer tile gives fair agreement in the range of 80–120%. The assumptions of the temporal evolution of type-I ELMs power load as made for ITER define a lower, conservative boundary within the observed variation of the data. The broadening of the ELM induced power profiles are, in contrast to earlier results based on a lower resolution IR system at JET, found to be in the range of 1.4–4.3 when compared to the inter-ELM wetted area.

Effect of neutrals on the power decay length at the divertor target

EDGE2D modelling of JET divertor plasmas with wide variation of main plasma parameters was aimed at identification of operational domains of the applicability of the 2/7×λTe scaling for predicting the power deposition profile width at the outer divertor target plate in high recycling conditions. Contrary to expectations, under no conditions, including lowest density plasmas just entering the conduction regime, was this scaling capable of correctly predicting the width and peak value of target power deposition profiles. Poorer ion parallel heat conduction, compared to the electron channel, in combination with reduced energy convection and volumetric power losses in divertor, of which charge exchange was found to play the dominant role, resulted in large deviations from the predictions of the 2/7×λTe scaling.

ERO modelling of local deposition of injected 13C tracer at the outer divertor of JET

The 2004 tracer experiment of JET with the injection of 13CH4 into H-mode plasma at the outer divertor has been modelled with the Monte Carlo impurity transport code ERO. EDGE2D solutions for inter-ELM and ELM-peak phases were used as plasma backgrounds. Local two-dimensional (2D) deposition patterns at the vertical outer divertor target plate were obtained for comparison with post-mortem surface analyses. ERO also provides emission profiles for comparison with radially resolved spectroscopic measurements. Modelling indicates that enhanced re-erosion of deposited carbon layers is essential in explaining the amount of local deposition. Assuming negligible effective sticking of hydrocarbons, the measured local deposition of 20–34% is reproduced if re-erosion of deposits is enhanced by a factor of 2.5–7 compared to graphite erosion. If deposits are treated like the substrate, the modelled deposition is 55%. Deposition measurements at the shadowed area around injectors can be well explained by assuming negligible re-erosion but similar sticking behaviour there as on plasma-wetted surfaces.

A conceptual model of the magnetic topology and nonlinear dynamics of ELMs

A conceptual model is introduced describing the 3D magnetic topology and nonlinear evolution of Type-I edge localized modes (ELMs), which immediately follow the initial linear peeling-ballooning growth phase. The model requires feedback amplification of stable and unstable invariant manifolds that increases the helical perturbation. The amplification process is caused by a rapid growth of field-aligned helical thermoelectric currents that flow through relatively short pedestal plasma flux tubes connecting the inner and outer divertor target plates. It is shown that the model qualitatively agrees with the formation of global field-aligned emission filaments and with fast IR heat flux splitting patterns of the inner and outer strike points observed experimentally. In addition, the model predicts an increase in the size of the ELMs as the pedestal collisionality drops. Experimental data and modeling results are presented supporting the basic conceptual elements of the model.

Divertor Experiments with MBI and Strong Gas Puffing on HL-2A

In the HL-2A 2004 experiment campaign, pulsed molecular beam injection (MBI) and strong hydrogen gas puffing under the divertor configuration were used for gas fueling. The experimental results show that the MBI of hydrogen can reduce the heat flux to the divertor target plate. The electron temperature measured by the Langmuir probe array decreases significantly during the injection of the molecular beam whereas the electron density increases. This indicates that the plasma pressure near the target plates tends to be constant at a new equilibrium level. In the divertor plasmas with strong hydrogen gas puffing a high plasma density up to 4.4 × 1019 m−3 was achieved. In addition, a phenomenon similar to the partially detached divertor regime was observed, which is being studied in open divertor tokamaks such as DIII-D to reduce the peak heat flux on the target plates near the separatrix. After a strong gas puffing the electron temperature measured on the outer divertor target plate near the separatrix decreases till below 5 eV or even lower, but that of the farther outer divertor target plate does not change obviously; and the CIII and the Hα emissions at the plasma edge decrease as expected, but the Hα emission near the X-point increases. These results reflects some interesting characteristics, which needs to be studied by further modeling and experiments.

Power deposition measurements in deuterium and helium discharges in JET MKIIGB divertor by IR-thermography

One of the outstanding problems for a next step fusion device is the handling of the power fluxes into the divertor, in particular regarding fast transient heat deposition by edge localised modes (ELMs). For a next step fusion device, such as ITER, type-I ELMy H-Mode is the reference discharge and therefore of particular interest. At JET a thermography system with high time resolution is able to resolve temperature evolution during and between ELM periods; here, the analysis is focussed so far on type-I ELMy H-Mode discharges with ELM frequencies of 10–35 Hz. Influences from surface layers on the target tiles are investigated in special discharges and taken into account. Results about the distribution of ELM and inter-ELM power deposition on the inner and outer divertor target plates are presented. Additionally, the characteristic ELM power deposition time (or temperature rise time) and its dependence on pedestal parameters are studied. The thermal impact due to ELMs for ITER is calculated together with predictions for relative ELM midplane losses and heat flux evolution on the divertor target. The predicted values based on conservative assumptions exceed the material limits.