Strike Point Research Articles

Surface features of strongly heated bulk tungsten divertor plates at ASDEX Upgrade

• Horizontal cracks and humps at the surface were observed at ASDEX Upgrade solid tungsten divertor plates. • Recrystallization and grain growth is found at these locations. • High thermal stress due to temperature gradients is discussed to produce these cracks. • He bubble formation in tungsten is discusses as source of the surface humps. Solid tungsten tiles exposed at the outer strike point module of the ASDEX Upgrade divertor show cracks parallel to surface in the bulk and humps at the surface. At ASDEX Upgrade these horizontal cracks were observed for the first time. About 10 % of all tiles exhibit protrusions originated by horizontal cracks about 3-5 mm below the surface. The position of this damage coincidence with the location of highest deposited energy, which varies for the different tiles due to the mounting. The tungsten at the locations of the horizontal cracks exhibits recrystallization and grain growth, presumably due to the worse thermal contact caused by the horizontal cracks. Although, the operation of AUG was not hampered by these damages, these tiles had to be exchanged preventing progression of these damages. High thermal stress due to temperature gradients is discussed to originate these cracks. Additionally, humps are found at the surface at the position of high heat flux. Their occurrence seems to be related to the increased number of high power discharges. Focused ion beam prepared cross sections indicate pores and cavities in and below these humps. This finding is discussed in the view of He bubble formation in tungsten.

Spectroscopic camera analysis of the roles of molecularly assisted reaction chains during detachment in JET L-mode plasmas

The roles of the molecularly assisted ionization (MAI), recombination (MAR) and dissociation (MAD) reaction chains with respect to the purely atomic ionization and recombination processes were studied experimentally during detachment in low-confinement mode (L-mode) plasmas in JET with the help of experimentally inferred divertor plasma and neutral conditions, extracted previously from filtered camera observations of deuterium Balmer emission, and the reaction coefficients provided by the ADAS, AMJUEL and H2VIBR atomic and molecular databases. The direct contribution of MAI and MAR in the outer divertor particle balance was found to be inferior to the electron–atom ionization (EAI) and electron–ion recombination (EIR). Near the outer strike point, a strong atom source due to the D 2 + -driven MAD was, however, observed to correlate with the onset of detachment at outer strike point temperatures of T e,osp = 0 . 9 –2.0 eV via increased plasma-neutral interactions before the increasing dominance of EIR at T e,osp < 0.9 eV, followed by increasing degree of detachment. The analysis was supported by predictions from EDGE2D-EIRENE simulations which were in qualitative agreement with the experimental observations. • Roles of MAI, MAR and MAD in detachment studied by camera analysis of Balmer emission in JET. • Global particle balance found to be dominated by purely atomic ionization and recombination. • MAD found to have vital role in onset of detachment as atom source for plasma-neutral interactions.

Compatibility of large ELM control and stable partial detachment with neon/argon seeding in EAST

It is necessary to achieve simultaneous exhaust of excessive transient and steady-state heat fluxes on the divertor target for the divertor protection in the future fusion reactors. The sustained large ELM control and stable partial detachment have been achieved concurrently with argon (Ar) or neon (Ne) seeding in EAST. With Ne seeding, the large ELMs with frequency f ELM ∼ 100 Hz disappear and a stable ELM-free state with H 98,y2 > 1 is maintained. Meanwhile, the electron temperature T et around the lower outer strike point decreases from more than 70 eV during the large ELM burst to less than 5 eV in the stable ELM-free phase. In addition, a slight improvement of plasma confinement is observed in the partially detached state, mainly attributed to the increased electron density n e and ion temperature T i in the core region. In the pedestal region, the density gradient and the electron temperature show subtle variation. The effective charge number Z eff increases significantly after Ne seeding, leading to a decrease in the edge bootstrap current and the pedestal pressure gradient, and thus the stabilization of ELMs. With Ar seeding, the large ELMs are also suppressed at first, but soon transit to type-III ELMs with a high f ELM ∼ 1 kHz, highly correlated with the energy confinement degradation. The steady-state and transient heat fluxes on the divertor can be both well reduced with Ar/Ne seeding in EAST.

Simulation of Cloud-to-Ground Lightning Strikes to Wind Turbines Considering Polarity Effect Based on an Improved Stochastic Lightning Model

The accurate determination of the annual lightning flash number to a wind turbine is essential for designing proper lightning protection measures. However, the method to assess the lightning risk of wind turbines recommended by the IEC is not associated with the actual lightning attachment process. Additionally, there is little research on positive cloud-to-ground (+CG) lightning. In this study, a lightning risk assessment method correlated with wind turbines on the basis of an improved stochastic lightning model is proposed. Based on the model, the influence of the lightning current amplitude, wind turbine heights and blade rotations on lightning strike risk are quantitatively analyzed. Moreover, the development and distribution characteristics of negative cloud-to-ground (−CG) lightning and +CG lightning are discussed. Finally, a more accurate calculation method of lightning strike risk is proposed considering the polarity effect. The results show that the effect of blade rotations on lightning risk cannot be ignored when the lightning current is large or when the wind turbine is tall. The −CG lightning has more bifurcation, but the dispersity of its development path and the lightning strike point are both small. The +CG lightning has scattered lightning strike points, and the development paths are tortuous, and. According to the verification calculation, the results calculated in this paper are in better agreement with the observation data than the results calculated by the IEC (International Electrotechnical Commission) recommended method.

Influence of photovoltaic support on lightning transient under direct lightning strike

• A new transient circuit model for calculating the transient response of PV support is developed. • The transient overvoltage caused by the mode of air-termination rod and lightning strike point are investigated. • The influences of additional leg, tile angle and equipotential bonding have been studied. • Four types of PV supports are chosen and their lightning transient responses under direct lightning strike are comprehensively investigated. Due to the large-scale installation of photovoltaic (PV) plants in open areas, PV plants is exposed to lightning strike at a high risk. The influence of PV support on lightning transient under direct lightning strike is investigated in this paper. Firstly, the PV support composed of multiple branches is classified into vertical conductor, horizontal conductor and oblique conductor. Their wave impedances are individually derived. Considering the air-termination rod and earthing system, four types of PV supports are chosen and their lightning transient responses under direct lightning strike are comprehensively studied. It suggests that the transient response is heavily dependent on the number of grounding legs. The greater the grounding legs, the lower the overvoltage. The equipotential bonding provides additional routes for the discharge of lightning current. Its presence would minimize the voltage but increase the risk for the entire PV system. The additional auxiliary leg also plays a role. The non-isolated air-termination rod is suggested install at the symmetrical center of the PV support from the perspective of discharging the lightning current.

Misalignment of magnetic field in DIII-D assessed by post-mortem analysis of divertor targets

We assess the toroidal magnetic field B t asymmetry in DIII-D due to a misalignment of the toroidal field coils with respect to the poloidal magnetic field coils and vacuum vessel. The peak-to-peak variation of the divertor strike point (SP) radius is measured to be 1 cm, with an n = 1 toroidal pattern. We use the centre of a narrow carbon deposition band on tungsten-coated divertor tiles just inside the outer strike point (OSP) as a proxy for the divertor SP location. The band occurred in a series of reverse B t discharges with the OSP positioned on the divertor inserts due to strong E × B drift transport of C from the inner to the outer SP through the private flux region. The variation in band radius (and hence the magnetic SP) is a (4.89 ± 0.31) mm shift toward (310 ± 4)° toroidal direction. These measurements agree well with previous measurements of the 3D magnetic field distribution (Luxon 2003 Nucl. Fusion 43 1813), simulations performed by the mafot field line integration code, and recent Langmuir probe measurements in the small-angle-slot (SAS) divertor (Watkins et al 2019 Nucl. Mater. Energy 18 46). Comparison of these measurements in the SAS divertor also indicates that there is the possibility of a tilt (in conjunction with the shift) of the B t coil field of (0.04 ± 0.07)° towards the toroidal angle of (215 ± 25)°. Previous measurements suggested a field misalignment of (4.6 ± 0.3) mm in the 270° toroidal direction, and a tilt of (0.06 ± 0.02)° toward the 114° toroidal direction, which is similar to the results reported here. These studies will be important for better understanding the radial variation of the toroidal strike line in DIII-D, for designing the new generation of SAS divertor, and for developing an understanding of the impact of error fields on tokamaks with tightly baffled slot divertors.

Stochastic fluctuation and transport of tokamak edge plasmas with the resonant magnetic perturbation field

We present that a statistical method known as the complexity–entropy analysis is useful to characterize a state of plasma turbulence and flux in the resonant magnetic perturbation (RMP) edge localized mode (ELM) control experiment. The stochastic pedestal top temperature fluctuation in the RMP ELM suppression phase is distinguished from the chaotic fluctuation in the natural ELM-free phase. It is discussed that the stochastic temperature fluctuation can be originated from the narrow layer of the field penetration on the pedestal top. The forced magnetic island can emit the resonant drift wave of comparable sizes (relatively low-k) in the RMP ELM suppression phase, and it can result in the generation of stochastic higher wavenumber fluctuations coupled to tangled fields around the island. The analysis of the ion saturation current measurement around the major outer striking point on the divertor shows that it also becomes more stochastic as the stronger plasma response to the RMP field is expected.

Impact of pedestal density gradient and collisionality on ELM dynamics

BOUT++ turbulence simulations are conducted to capture the underlying physics of small ELM characteristics achieved by increasing separatrix density via controlling strike points from vertical to horizontal divertor plates for three EAST discharges. BOUT++ linear simulations show that the most unstable modes change from high-n ideal ballooning modes to intermediate-n peeling–ballooning modes and eventually to peeling–ballooning stable plasmas in the pedestal. Nonlinear simulations show that the fluctuation is saturated at a high level for the lowest separatrix density. The ELM size decreases with increasing separatrix density, until the fraction of this energy lost during the ELM crash becomes less than 1% of the pedestal stored energy, leading to small ELMs. Simulations indicate that small ELMs can be triggered either by the marginally peeling–ballooning instability near the peak pressure gradient position inside the pedestal or by a local instability in the pedestal foot with a larger separatrix density gradient. The pedestal collisionality scan for type-I ELMs with steep pedestal density gradient shows that both linear growth rate and ELM size decrease with increasing collisionality. On the contrary, the pedestal collisionality and pedestal density width scan with a weak pedestal density gradient indicate small ELMs can either be triggered by a high-n ballooning mode or by a low-n peeling mode in a low collisionality region 0.04–0.1. The simulations indicate the weaker the linear unstable modes near marginal stability with small linear growth rate, the lower nonlinearly saturated fluctuation intensity and the smaller turbulence spreading from the linear unstable zone to stable zone in the nonlinear saturation phase, leading to small ELMs.

Toroidal modeling of 3D perturbations generated by current filaments in scrape-off layer in tokamak with biased divertor targets

The divertor biasing technique offers a promising alternative to control the edge localized mode (ELM) as well as the divertor heat load in tokamaks, as compared to the resonant magnetic perturbation (RMP) generated by magnetic coils. The linear resistive MHD code MARS-F (Liu et al 2000 Phys. Plasmas 7 3681) is employed to study the plasma response to the n = 2 (n is the toroidal mode number) field perturbation in HL-2A, produced by the divertor biasing current filaments flowing in the scrape-off layer region. The response field due to biasing currents is compared with the RMP field utilized for ELM control in HL-2A. The strength of the radial resonant field perturbation, produced by the biasing currents of 100 A level, is found to be comparable with RMP generated by several kA of ELM control coil currents for the reference plasma. The plasma normal displacement near the X-point and the associated neoclassical toroidal viscosity torque are also computed to be similar between these two techniques. The modeling results thus strongly suggest that the biasing technique can be applied to control ELMs. Moreover, the biasing currents produce field perturbations, including the plasma response, that are localized more near the plasma edge (compared to the RMP counterpart), thus reducing the chance of mode locking associated with core perturbations. Particle orbit tracing also reveals that the biasing current produced magnetic perturbation tends to widen the heat deposition region and induce the strike point splitting of the ion saturation flow on the outer divertor surface, consistent with experimental observations in HL-2A. These toroidal modeling results confirm the possibility of ELM control and plasma exhaust solution by the divertor biasing technique.

Modeling of the impact of neon seeding on the detachment in EAST by SOLPS-ITER

In the EAST radiative divertor experiments, stable detachment and divertor protection have been achieved with mixed impurity gas seeding. Experiments indicate that neon (Ne) is an appropriate external impurity for plasma detachment in EAST. In this work, the physical mechanism of plasma detachment with Ne seeding in EAST is studied by using the SOLPS-ITER code package. The electron temperature Tet on the upper outer divertor target decreases to below 10 eV with Ne seeding, which is mainly due to the power loss and upstream power drop caused by the impurity radiation. For Tet &amp;lt; 10 eV, the significant drop of the poloidal saturation ion current js,p occurs around the upper outer strike point. The simulation results show that the js,p drop is mainly driven by the rise of the momentum loss and the upstream pressure loss. The increase in the momentum loss is mainly caused by the plasma–neutral interactions in the region from the X-point to the upper outer divertor target. The upstream pressure loss around the X-point is related to the decrease in the upstream electron temperature Teu, which originates from the increased radiation loss with Ne seeding.

Simulation study of the influence of upstream density and power in the scrape-off layer on the double-peaked density profile at the divertor target

The double-peaked distribution of particle deposition at divertor targets has been observed in various tokamaks, and is considered a potential approach for mitigating divertor particles and heat load in future fusion reactors. Recently, the systematical analysis of the double-peaked distribution behavior during EAST experiments shows that the appearance of the double-peaked profile is related to the line-average density and heating power. In order to understand general trends and related mechanisms, the influences of the upstream density (n e,sep) and power in the scrape-off layer (P SOL) on the double-peaked density profile are investigated by SOLPS-ITER simulations with full drifts and currents. It is found that the n e peak near the strike point is mainly contributed by the strong ionization source close to the target, and the n e peak in the far-SOL region is caused by the synergetic effects of poloidal and radial E × B drifts along the SOL. The double-peaked distribution is affected by the P SOL and impurity seeding by increasing or decreasing the whole profile of the electron temperature at the target (T et). When the peak value of T et (T et,peak) is fixed, the density peak in the far-SOL is increased for higher n e,sep by reducing the T et in the far-SOL region on the lower-field side under unfavorable B T and by the upstream-extended ionization source due to the geometry effect on the high-field side under favorable B T. Statistical analysis of the simulated results shows that the scaling expression of the peak ratio is ∼. In addition to the upper boundary found in the analysis of EAST experiments, a lower boundary of the region where the double-peaked feature appears on the P SOL-n e,sep plane is identified by simulations and preliminarily confirmed according to the measurements in several EAST discharges.

E × B flow driven electron temperature bifurcation in a closed slot divertor with ion B × ∇B away from the X-point in the DIII-D tokamak

An electron temperature bifurcation is observed in the small angle slot divertor, which has been developed to enhance neutral cooling across the divertor target by coupling a closed slot structure with appropriate target shaping. Experiments in the DIII-D tokamak and associated SOLPS-ITER modeling with full drifts find a strong interplay between drifts and divertor geometry on divertor dissipation. The coupling of divertor geometry and drift flows can strongly affect the path towards divertor detachment onset as the plasma density is raised. With the strike point on the inner slanted surface and ion B × ∇B away from the magnetic X-point, bifurcative transitions were observed with sharp decrease of T e towards detachment onset both experimentally and computationally. This differs from the situation for the open divertor where the T e cliff was only observed for ion B × ∇B towards the X-point. SOLPS-ITER modeling with full drifts demonstrates that the magnitude of the E × B drift flow is comparable with the main plasma flow. The reversal of both the poloidal and radial E × B flows near the strike point leads to rapid density accumulation right near the separatrix, which results in bifurcative step transition of divertor conditions with cold plasma across the entire divertor target plate. These results indicate that the interplay between geometry and drifts should be fully taken into account in future fusion reactor divertor designs.

Novel concept suppressing plasma heat pulses in a tokamak by fast divertor sweeping

One of the remaining challenges in magnetic thermonuclear fusion is survival of the heat shield protecting the tokamak reactor vessel against excessive plasma heat fluxes. Unmitigated high confinement edge localized mode (ELM) is a regular heat pulse damaging the heat shield. We suggest a novel concept of magnetic sweeping of the plasma contact strike point fast and far enough in order to spread this heat pulse. We demonstrate feasibility of a dedicated copper coil in a resonant circuit, including the induced currents and power electronics. We predict the DEMO ELM properties, simulate heat conduction, 3D particles motion and magnetic fields of the plasma and coil in COMSOL Multiphysics and Matlab. The dominant system parameter is voltage, feasible 18 kV yields 1 kHz sweeping frequency, suppressing the ELM-induced surface temperature rise by a factor of 3. Multiplied by other known mitigation concepts, ELMs might be mitigated enough to ensure safe operation of DEMO.

Modelling of tungsten impurity edge transport and screening for different divertor conditions in EAST

Tungsten (W) transport and screening in the edge plasma are investigated for EAST high dissipative divertor conditions. By combining the 1D impurity fluid model (1DImpFM) and the two-point model formatting (2PMF), W screening is proved to be enhanced for high upstream plasma density conditions, mainly because the impurity temperature gradient velocity decreases with the increase of the upstream plasma density. Based on dedicated EAST density ramp-up experiments, 2D simulations of W erosion and transport are carried out for different levels of dissipative divertor conditions by using the SOLPS-DIVIMP code package, and the modeling results are benchmarked with the 1DImpFM analytic model. The prompt-redeposition, the divertor screening, and the main SOL screening are quantitatively analyzed. For detached divertor conditions, the increase in the W ionization length reduces the prompt redeposition rate, but both the divertor screening and SOL screening are reinforced. The 1DImpFM can interpret well the W leakage in the near separatrix region; however, the 2D simulations suggest that the impurity pressure gradient force which is neglected by the 1DImpFM plays an important role, especially in the far-SOL region. With the divertor condition varied from the high-recycling regime to the deep detachment regime, the W source moves from the near strike point region to the far SOL, and thus makes the W transport in the far SOL more important. Therefore, the impurity pressure gradient force cannot be neglected for edge W transport analysis, especially for the detached divertor conditions.

Dependence of particle and power dissipation on divertor geometry and plasma shaping in DIII-D small-angle-slot divertor

• The compact SAS divertor in DIII-D provides a good testbed for understanding the effects of a tightly closed divertor on particle and power dissipation, and for application to core-edge integration solutions. • A longer outer leg facilitates the achievement of divertor dissipation in the SAS divertor, while a shorter leg leads to higher divertor temperatures and requires higher upstream densities to achieve the same level of divertor detachment. • The divertor conditions strongly depend on the divertor target shaping and Bt direction. • In-slot gas puffing can reduce detachment onset density by about 10% when the outer strike point is placed at the inner slanted surface. • Completed divertor detachment and a high confinement core with normalized energy confinement factor H 98 >1.0 can be simultanesouly achieved with the SAS divertor, demonstrating the benefit of a closed divertor for exploration of core-edge integration. Dedicated experiments in DIII-D find that magnetic shaping and divertor target geometry significantly affect the divertor plasma conditions and divertor detachment process in the small-angle-slot (SAS) divertor. The compact SAS divertor in DIII-D provides a good testbed for understanding the effects of a tightly closed divertor on particle and power dissipation, and for application to core-edge integration solutions. A longer outer leg facilitates the achievement of divertor dissipation in the SAS divertor, while a shorter leg leads to higher electron temperatures near the divertor target plate and requires higher upstream densities to achieve the same level of divertor detachment. In addition, with the ion B × ∇ B drift away from the SAS divertor and the outer strike point (OSP) near the outer corner, the target temperature is lower for a particular upstream density than with the OSP on a slanted or flat surface, leading to lower heat flux even when the particle flux remains similar. In contrast, with the ion B × ∇ B drift into the SAS divertor, a strike point at the inner slanted surface exhibits a lower upstream density to achieve divertor detachment than a strike point either at the outer corner or the outer slanted target. Experimental results and SOLPS-ITER simulations with full drifts suggest the strong interplay between drift flows and the neutral distribution resulted from target shaping. Furthermore, in-slot gas puffing has been shown to achieve global divertor detachment with an onset density about 10% lower than that using main-chamber gas puffing when the outer strike point is placed at the inner slanted surface. Corresponding modelling reveals that the local gas puffing enhances the neutral ionization which potentially facilitates the achievement of divertor dissipation. However, such improvement diminishes when the strike point is at the outer corner, which also indicates the geometric dependence on divertor performance in the SAS divertor. Even with different strike point locations, complete divertor detachment with very low particle and heat fluxes at the divertor targets and a high confinement core with normalized energy confinement factor H 98 >1.0 can be simultanesouly achieved with the SAS divertor with ion B × ∇ B drift into SAS divertor, demonstrating the benefit of a closed divertor for exploration of core-edge integration.

Influence of surface morphology on erosion of plasma-facing components in H-mode plasmas of ASDEX Upgrade

• Net erosion of plasma-facing materials investigated at the outer strike-point region of the ASDEX Upgrade divertor during H-mode discharges with small and frequent ELMs. • Increasing surface roughness reduces net erosion but less than what is the case in l -mode. • Net-erosion rates are generally 2–5 times higher in H-mode than in l -mode. • Exposure in H-mode conditions results in strong local variations in the poloidal and toroidal erosion/deposition profiles. • Both erosion and deposition require detailed analyses at the microscopic scale and the deposition patterns may be drastically different for heavy and light impurities. • Local plasma parameters have a huge impact on PFC erosion rates and surface morphology and its temporal evolution have to be taken into account for quantitative estimates of erosion rates and PFC lifetime under reactor-relevant conditions. Net erosion of plasma-facing materials was investigated at the low-field-side (outer) strike-point area of the ASDEX Upgrade (AUG) divertor during H-mode discharges with small and frequent ELMs. To this end, Au and Mo marker samples with different surface morphologies and geometries were exposed to plasmas using the DIM-II divertor manipulator. The results were compared to existing erosion and deposition patterns from various L- and H-mode experiments, in the latter case the main difference was the size and frequency of the ELMs. We noticed that increasing surface roughness reduces net erosion but less than what is the case in L-mode. On the other hand, net-erosion rates in H-mode are generally 2–5 times higher than the corresponding L-mode values, in addition to which exposure in H-mode conditions results in strong local variations in the poloidal and toroidal erosion/deposition profiles. The latter observation we associate with the large migration length, on the order of several cm, of the eroded material, resulting in strong competition between erosion and re-deposition processes especially at poloidal distances > 50 mm from the strike point. Considerable net erosion was measured throughout the analysed poloidal region unlike in L-mode where the main erosion peak occurs in the vicinity of the strike point. We attribute this qualitative difference to the slow decay lengths of the plasma flux and electron temperature in the applied H-mode scenario. Both erosion and deposition require detailed analyses at the microscopic scale and the deposition patterns may be drastically different for heavy and light impurities. Generally, the rougher the surface the more material will accumulate on locally shadowed regions behind protruding surface features. However, rough surfaces also exhibit more non-uniformities in the quality or even integrity of marker coatings produced on them, thus complicating the analyses of the experimental data. We conclude that local plasma parameters have a huge impact on the PFC erosion rates and, besides incident plasma flux, surface morphology and its temporal evolution have to be taken into account for quantitative estimates of erosion rates and PFC lifetime under reactor-relevant conditions.

First snowflake divertor experiments in MAST-U tokamak

First snowflake (SF) divertor experiments in the MAST-U tokamak demonstrated steady-state snowflake-plus divertor configurations in 450 kA ohmic L-mode plasmas. The SF divertor configuration features a second poloidal field (PF) null in the divertor region close by or overlapping with the main X-point. The resulting low PF region and two additional divertor legs (strike points) may lead to additional power and particle flux sharing via a hypothesized convective cell, and increased plasma-wetted area and radiation. The free-boundary Grad–Shafranov equilibrium code FIESTA was used to design SF configurations with several inter-null distances and orientations. In the experiment, the SF configurations with inter-null distances 0.13–0.20 m and lasting 0.2–0.3 s were obtained. Parallel connection lengths between the midplane and the outer strike point in the SF configurations (evaluated at field lines 1–2 mm from the separatrix in the midplane) were 25–30 m, higher than in the standard divertor (20–25 m) or the Super-X divertor (25 m). Diagnostic measurements highlighted salient SF features. The infra-red video bolometer diagnostic showed that the radiated power peaking in the PF null region was not as pronounces as in the standard divertor. Divertor ion fluxes measured by target Langmuir probes showed increased ion flux in the plate region where a secondary SF strike point landed, concomitantly with the SF formation. These measurements may suggest that some particle and heat redistribution was taking place in the convective SF zone. The first SF experiments provide a basis for future SF studies in MAST-U tokamak with higher input power, improved plasma control and diagnostic measurements, to be compared with the modeling predictions of plasma convective SF mixing and lower density strike point detachment threshold. • First snowflake (SF) divertor experiments in the MAST-U tokamak demonstrated steady-state snowflake-plus divertor configurations with inter-null distances 0.13–0.20 m in 450 kA ohmic L-mode plasmas. • The free-boundary Grad–Shafranov equilibrium code FIESTA was used to design SF configurations with several inter-null distances and orientations. • Divertor geometry (i.e., parallel connection lengths, divertor flux expansion and area expansion) in the SF-plus divertor fvorably comared to the standard divertor • Divertor measurements with the infra-red video bolometer diagnostic and target Langmuir probes may suggest that some particle and heat redistribution took place in the convective SF zone.

Evaluation of deuterium recycling properties by fueling termination in the EAST superconducting tokamak

• Hydrogen recycling properties have been investigated by evaluating the density decay time after the fueling termination. • A repeat of discharges diminishes the wall conditioning effect and enhances hydrogen recycling. • Approach of the strike point position to the pumping slot enhances the particle exhaust capability. • The hydrogen recycling coefficient of the LHCD discharge is increased during stop of fueling, although such increase appears not to be seen in the ohmic discharges. Deuterium recycling properties have been investigated in the EAST superconducting tokamak by evaluating the density decay time after the fueling termination. The density decay time of the latter discharge of the experiment of a day is ∼2.9 s, which is more than three times longer than that of the beginning discharge (∼0.88s), indicating that a repeat of discharges diminishes the wall conditioning effect by lithium coating. However, the difference in deuterium recycling among three magnetic configurations (lower single null, double null, and upper single null) is unclear. The density decay times are 3.3 s and 4.1 s when the strike point (SP) positions are located on the horizontal and vertical plates of the lower outer divertor, showing that the particle exhaust capability becomes stronger as the SP position approaches the pumping slot. Furthermore, the lower hybrid current drive (LHCD) power enhances deuterium recycling since the density decay times of high (∼2.7 MW) and low (∼0.8 MW) power LHCD discharges are 4.3 s and 1.2 s, respectively. The density decay time in the LHCD discharge linearly increases with an increase in a period of exponential fitting, implying that the recycling coefficient increases during the stop of fueling. However, the density decay time in the ohmic discharge is independent of the fitting time.

Validation of SOLPS-ITER simulations with kinetic, fluid, and hybrid neutral models for JET-ILW low-confinement mode plasmas

For JET L-mode plasmas in low-recycling conditions (electron temperature at the outer strike point, T e,ot ≳ 30 eV ), SOLPS-ITER simulations agree within the error bars for the experimental profiles at the low-field side (LFS) divertor target. The peak Balmer- α (D α ) emission in the LFS divertor agrees within the error bars of the KS3 filterscope diagnostic, but is approximately 30% lower than the peak value of the KT1 spectrometer. Simulations have been performed with fluid, kinetic, and hybrid models for the neutrals. The large fluid-kinetic discrepancies of more than a factor 2 are successfully corrected by using a hybrid fluid-kinetic approach, for which kinetic atoms are transferred to the fluid population when the local Knudsen number of the atom becomes smaller than a user-defined transition Knudsen number K n t . The hybrid-kinetic discrepancies are limited to a few % for K n t ≤ 100 . When increasing the upstream density to high-recycling conditions, at the onset of detachment ( T e,ot ≈ 5 eV ), the simulations predict more than a factor 2 lower peak ion saturation current to the LFS divertor than the experiments. Also the D α emission is underpredicted with approximately a factor 2. For these high-recycling conditions, the fluid-kinetic discrepancies are limited to maximum 50%, which are again corrected by using the hybrid approach. • SOLPS-ITER simulations agree within the statistical error bars for low-recycling conditions. • There is a factor 2 difference for high-recycling conditions. • Hybrid approach successfully corrects for fluid-kinetic errors. • Approximately a factor 6 speed-up of hybrid approach for high-recycling conditions.

Investigation of particle exhaust from EAST divertor

Particle control is one of the key issues for steady-state tokamak operation. The density decay time is used to evaluate the capability of the divertor to exhaust particle with all fueling turn off. Experiments of the EAST divertor's ability to exhaust helium particles were also performed during the EAST helium plasma operation. The experimental results show that the EAST divertor is less capable of exhaust helium particles than deuterium particles. And similar to deuterium, the helium particle exhaust performance is sensitivity on the different strike point location. During 2021 experiment campaign, a new lower tungsten divertor has been developed and installed on EAST, the particle exhaust capacity of the new divertor has also been investigated during this campaign.