SOLPS-ITER drift modelling of JET Ne and N-seeded H-modes

Rozhansky, V.; Veselova, I.; Senichenkov, I.; Giroud, C.; Pitts, R.A.; Wiesen, S.; Voskoboynikov, S.

doi:10.1016/j.nme.2021.101030

Cited by 15 publications

(20 citation statements)

References 19 publications

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“…This work also reinforces the argument that such effects are very pronounced in medium size tokamaks like DIII-D and AUG [29][30][31] due to the smaller size of the divertor compared to larger machines such as JET and ITER [32,33]. In future large devices drifts still have important effect, though maybe not as dramatic.…”

Section: Summary and Discussionsupporting

confidence: 80%

“…Those effects will likely reduce the impact on the pedestal characteristics widening the operation window for Ne seeding as radiative impurity. The results presented in this work are well in agreement with impurity seeding studies performed for AUG and ITER [29,30,32,33]. Impurities with higher ionization potential leak more efficiently out of the divertor.…”

Section: Impurity Enrichment Factorssupporting

confidence: 88%

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Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

et al. 2022

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A comparative study of nitrogen versus neon has been carried out to analyze the impact of the two radiative species on power dissipation, SOL impurity distribution, divertor and pedestal characteristics. The experimental results show that N remains compressed in the divertor, thereby providing high radiative losses without affecting the pedestal profiles and displacing carbon as dominant radiator. Neon, instead, radiates more upstream than N thus reducing the power flux through the separatrix leading to a reduced ELM frequency and compression in the divertor. A significant amount of neon is measured in the plasma core leading to a steeper density gradient. The different behaviour between the two impurities is confirmed by SOLPS-ITER modelling which for the first time at DIII-D includes multiple impurity species and a treatment of full drifts, currents and neutral-neutral collisions. The impurity transport in the SOL is studied in terms of the parallel momentum balance showing that N is mostly retained in the divertor whereas Ne leaks out consistent with its higher ionization potential and longer mean free path. This is also in agreement with the enrichment factor calculations which indicate lower divertor enrichment for neon. The strong ionization source characterizing the SAS divertor causes a reversal of the main ions and impurity flows. The flow reversal together with plasma drifts and the effect of the thermal force contribute significantly in the shift of the impurity stagnation point affecting impurity leakage. This work provides a demonstration of the impurity leakage mechanism in a closed divertor structure and the consequent impact on pedestal. Since carbon is an intrinsic radiator at DIII-D, in this paper we have also demonstrated the different role of carbon in the N vs Ne seeded cases both in the experiments and in the numerical modeling. Carbon contributes more when neon seeding is injected compared to when nitrogen is used. Finally, the results highlight the importance of accompanying experimental studies with numerical modelling of plasma flows, drifts and ionization profile to determine the details of the SOL impurity transport as the latter may vary with changes in divertor regime and geometry. In the cases presented here, plasma drifts and flow reversal caused by high level of closure in the slot upper divertor at DIII-D play an important role in the underlined mechanism.

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Section: Summary and Discussionsupporting

confidence: 80%

Section: Impurity Enrichment Factorssupporting

confidence: 88%

Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

et al. 2022

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“…For ITER, Ne looks more preferable than N due to N chemical activity. At the same time, the SOLPS‐ITER simulations with full account of drifts and currents reproduce the experiments on AUG [ 23,31,33 ] and JET, [ 39 ] and are used to study operational regimes of ITER edge plasma. [ 40–42 ] Thus, it is reasonable to study the details of impurity transport in SOLPS‐ITER simulation results for three machines, paying attention to the effect of machine size, magnetic field value, separatrix power, core plasma temperature, etc.…”

Section: Role Of Drifts and Currents In Divertor Plasmamentioning

confidence: 99%

“…On JET and ITER, this means that both N and Ne are suitable for the radiation power dissipation. [26,[39][40][41]…”

Section: Role Of Tokamak Size On Target Shielding By Impuritymentioning

confidence: 99%

Detached regime with highly radiating X‐point: Physics and modelling

Senichenkov

Rozhansky

Shtyrkhunov

et al. 2022

Contributions to Plasma Physics

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A possibility to operate with fully detached targets and highly radiating X-point was recently demonstrated on ASDEX Upgrade, JET, and DIII-D with intensive impurity seeding (N, Ne, Ar) and feedback control. Notable feature of such a regime is the radiated power fraction of up to 90% of discharge power inside the separatrix without a confinement degradation, or even with confinement improvement (in terms of H-factor), which leads to a full detachment of both targets. Therefore, this regime might be attractive for the next generation reactor scale tokamaks like DEMO or CFETR, where most of the power should be radiated. In the present report, the recent achievements in the understanding of such regime are reviewed. The effectiveness of divertor target shielding by different impurity gases is discussed, paying attention to its first ionization potential. SOLPS-ITER modelling results illustrating the impurity flow pattern are presented, and the importance of drift flows is demonstrated. The effect of the machine size on the effectiveness of the divertor shielding by impurity radiation is discussed. It is demonstrated that in bigger machines impurity radiation is better kept in the divertor and divertor asymmetry is less pronounced. As the seeding rate increases, both targets fully detach, and neutrals start to penetrate inside the separatrix above the X-point, leading to the plasma cooling there and to the formation of an observable localized radiation spot in the X-point vicinity.The cold zone above the X-point thus behaves as an energy sink like a divertor in conventional regime, so that the perpendicular decay length of perpendicular turbulent energy flow appears to be of the order of 𝜆 q of conventional divertor.During the formation of the radiating X-point a potential hill/well (depending on the direction of B × ∇B drift) is formed on perturbed closed flux surfaces, and the corresponding E × B drift vortex fluxes appear to give major contribution to the particle balance. SOLPS-ITER simulations show that the radial electric field significantly deviates from neoclassical formula and may even change sign (become positive). Thus, the zone of maximal poloidal rotation shear may shift inwards, which might be responsible for the inward shift of the transport barrier and for the confinement improvement. This effect is different to the discussed extension of peeling-ballooning stability boundary by intensive impurity injection, which

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“…For AUG and JET, the modelling input parameters are inspired by existing experimental results, but do not attempt to match a particular discharge. Instead, the full run databases comprising results obtained in previous numerical studies (AUG [4,8] and JET [9,10]) over the past 4 years are presented and analysed in this paper. Cross-field heat transport coefficients are chosen to match the typical SOL inter-ELM, heat flux widths, λ q observed on these devices.…”

Section: Introductionmentioning

confidence: 99%

Multi-machine SOLPS-ITER comparison of impurity seeded H-mode radiative divertor regimes with metal walls

Rozhansky¹,

Senichenkov²,

Veselova³

et al. 2021

Nucl. Fusion

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SOLPS-ITER modelling databases of three tokamaks—ASDEX-Upgrade, JET and ITER with fluid drifts activated are compared to understand the dependence of edge plasma performance on machine size and other global parameters. Two medium Z extrinsic radiating impurity species (Ne and N) are considered. It is demonstrated that N is better kept in the divertor region than Ne in semi-detached and detached divertor conditions due to smaller first ionization potential (FIP effect). Together with the fact that Ne radiates more efficiently at higher plasma temperatures, this leads to an increase in the efficiency of Ne for divertor heat load control with increasing machine size. In larger machines such as JET and ITER Ne can be as efficient a radiator as N while for ASDEX-Upgrade Ne easily leads to radiation from the pedestal and loss of H-mode stability. The relative roles of various physical effects are compared for the three tokamaks based on both whole databases and in more details for chosen semi-detached regimes with comparable fraction of radiated power. It is shown that for smaller machines drift effects are more significant and divertor asymmetries more pronounced.

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SOLPS-ITER drift modelling of JET Ne and N-seeded H-modes

Cited by 15 publications

References 19 publications

Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

Impurity leakage and radiative cooling in the first nitrogen and neon seeding study in the closed DIII-D SAS configuration

Detached regime with highly radiating X‐point: Physics and modelling

Multi-machine SOLPS-ITER comparison of impurity seeded H-mode radiative divertor regimes with metal walls

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