Investigating the impact of the molecular charge-exchange rate on detached SOLPS-ITER simulations

Plasma-molecular interactions generate molecular ions which react with the plasma and contribute to detachment through molecular activated recombination (MAR), reducing the ion target flux, and molecular activated dissociation (MAD), both of which create excited atoms. Hydrogenic emission from these atoms have been detected experimentally in detached TCV, JET and MAST-U deuterium plasmas. The TCV findings, however, were in disagreement with SOLPS-ITER simulations for deuterium indicating a molecular ion density ($D_2^+$) that was insufficient to lead to significant hydrogenic emission, which was attributed to underestimates of the molecular charge exchange rate ($D_2 + D^+ \rightarrow D_2^+ + D$) for deuterium (obtained by rescaling the hydrogen rates by their isotope mass). In this work, we have performed new SOLPS-ITER simulations with the default rate setup and a modified rate setup where ion isotope mass rescaling was disabled. This increased the $D_2^+$ content by $> \times 100$. By disabling ion isotope mass rescaling: 1) the total ion sinks are more than doubled due to the inclusion of MAR; 2) the additional MAR causes the ion target flux to roll-over during detachment; 3) the total $D\alpha$ emission in the divertor increases during deep detachment by roughly a factor four; 4) the neutral atom density in the divertor is doubled due to MAD, leading to a 50\% increase in neutral pressure; 5) total hydrogenic power loss is increased by up to 60\% due to MAD. These differences result in an improved agreement between the experiment and the simulations in terms of spectroscopic measurements, ion source/sink inferences and the occurrence of an ion target flux roll-over

Identification of the primary processes that lead to the drop in divertor target ion current at detachment in TCV

Using SOLPS-ITER we model a TCV conventional divertor discharge density ramp to understand the role of various processes in the loss of target ion current. We find that recombination is not a strong contributor to the rollover of the target ion current at detachment. In contrast, the divertor ion source appears to play a central role in magnitude (the source of most of the ion target current) and time, apparently dropping during the density ramps due to a drop in power available for ionization

Physics research on the TCV tokamak facility: from conventional to alternative scenarios and beyond

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device’s unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power ‘starvation’ reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in–out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added

Modélisation des disruptions déclenchées par injection massive de gaz dans les plasmas de tokama

Les disruptions du plasma sont des phénomènes se produisant dans les tokamaks et qui entraînent une perte totale du confinement du plasma et la fin de la décharge. Ces disruptions sont des phénomènes rapides et violents et peuvent endommager les murs du tokamak ainsi que sa structure si elles ne sont pas contrôlées. Un système de mitigation des disruptions est donc indispensable pour ITER afin de réduire les forces électromagnétiques, mitiger les charges thermiques et éviter les électrons runaways générés par les disruptions du plasma. Remplir tous ces objectifs fait du design de ce système une tâche difficile, pour laquelle un apport conséquent de l’expérience et de la modélisation est nécessaire. Nous présentons dans cette thèse des résultats de modélisation sur l’amortissement des disruptions par injection massive de gaz, qui est une des méthodes principales envisagées sur ITER pour le système de mitigation. Premièrement, un modèle issu des premiers principes pour décrire le transport des neutres dans un plasma est donné et est appliqué à l’étude de l’interaction entre l’injection massive de gaz et le plasma. Les principaux mécanismes en jeu sont décrits et étudiés. L’échange de charge entre les neutres et les ions du plasma est isolée comme jouant un rôle majeur dans cette dynamique. Ensuite, le code 3D de Magnétohydrodynamique non linéaire JOREK est appliqué à l’étude des disruptions déclenchées par injection massive de gaz. Un intérêt particulier est porté sur la phase de quench thermique et les phénomènes MHD qui le déclenchent. Les résultats obtenus avec ce code sont comparés avec les expériences effectuées sur le tokamak JET.Plasma disruptions are events occuring in tokamaks which result in the total loss of the plasma confinement and the end of the discharge. These disruptions are rapid and violent events and they can damage the tokamak walls and its structure if they are not controlled. A Disruption Mitigation System (DMS) is thus mandatory in ITER in order to reduce electromagnetic forces, mitigate heat loads and avoid Runaway Electrons (RE) generated by plasma disruptions. These combined objectives make the design of the DMS a complex and challenging task, for which substantial input from both experiments and modeling is needed. We present here modeling results on disruption mitigation by Massive Gas Injection (MGI), which is one of the main methods considered for the DMS of ITER. First, a model which stems from first principles is given for the tranport of neutrals in a plasma and applied to the study of the interaction of the MGI with the plasma. Main mechanisms responsible for the penetration of the neutral gas are described and studied. Charge-exchange processes between the neutrals and the ions of the plasma is found to play a major role. Then, the 3D non linear MHD code JOREK is applied to the study of MGI-triggered disruptions with a particular focus on the thermal quench phase and the MHD events which are responsible for it. The simulation results are compared to experiments done on the JET tokamak

Comparison between MAST-U conventional and Super-X configurations through SOLPS-ITER modelling

MAST-U has recently started operating with a Super-X divertor, designed to increase total flux expansion and neutral trapping, both predicted through simple analytic models and SOLPS calculations to reduce the plasma and impurity density detachment thresholds. In this study, utilising the SOLPS-ITER code, we are quantifying the possible gain allowed by the MAST-U Super-X and neutral baffling geometry, in terms of access to detachment. We show that a significant reduction of the upstream density detachment threshold (up to a factor 1.6) could be achieved in MAST-U, for the Super-X, as opposed to conventional divertor geometry, mainly through an increased total flux expansion, neutral trapping being found very similar between the different configurations. We also show that variations of the strike-point angle are complex to interpret in such a tightly baffled geometry, and that a case in which the target normal points more towards the separatrix does not necessarily imply a lower detachment threshold. As in previous calculations for TCV, we quantify the role of neutral effects through developing and applying a quantitative definition of neutral trapping

Drift effects in SOLPS-ITER simulations for the TCV divertor upgrade

This contribution reports on the progress towards including drift effects in SOLPS-ITER simulations to assess power distribution and detachment onset in the light of the 2019 TCV divertor upgrade [1]. The installation of in-vessel gas baffles (Figure 1a) is predicted to increase the divertor neutral density by a factor ∼ 5 and therefore facilitates the access to detachment [2]. The conditions for the onset of detachment at each strike point depend on the power entering each divertor leg and thus simulations must correctly include the power distribution between the inner and outer divertor that is greatly affected by scrape-off layer drifts. Surprisingly, the sharing of particles and heat between the targets in reversed field conditions is found to be shifted towards the outer target. This counter-intuitive flowpattern is due to an electric potential well below the X-point that occurs in high density reversed field simulations

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

Runaway beam studies during disruptions at JET-ILW

Équipe 107 : Physique des plasmas chaudsInternational audienceRunaway electrons (RE) during disruptions are a concern for future tokamaks including ITER with its metallic wall. Although RE are rare in spontaneous disruptions with the JET ITER-like Wall (JET-ILW), RE beams up to 380 kA were obtained using massive injection (MGI) of argon in JET-ILW divertor discharges. Entry points into the RE domain defined by operational parameters (toroidal field, argon fraction in MGI) are unchanged but higher RE currents have been obtained inside the JET-ILW MGI-generated RE domain when compared to JET-C. This might be due to the influence of the metallic wall on the current quench plasma. Temperatures of 900 degrees C have been observed following RE impacts on beryllium tiles. Heat deposition depth of similar to 2 mm has to be assumed to match the tile cooling time. 3D simulations of the RE energy deposition using the ENDEP/MEMOS codes show that material melting is unlikely with 100 kA RE beams

Runaway electron beam generation and mitigation during disruptions at JET-ILW

Équipe 107 : Physique des plasmas chaudsInternational audienceDisruptions are a major operational concern for next generation tokamaks, including ITER. They may generate excessive heat loads on plasma facing components, large electromagnetic forces in the machine structures and several MA of multi-MeV runaway electrons. A more complete understanding of the runaway generation processes and methods to suppress them is necessary to ensure safe and reliable operation of future tokamaks. Runaway electrons were studied at JET-ILW showing that their generation dependencies (accelerating electric field, avalanche critical field, toroidal field, MHD fluctuations) are in agreement with current theories. In addition, vertical stability plays a key role in long runaway beam formation. Energies up to 20 MeV are observed. Mitigation of an incoming runaway electron beam triggered by massive argon injection was found to be feasible provided that the injection takes place early enough in the disruption process. However, suppressing an already accelerated runaway electron beam in the MA range was found to be difficult even with injections of more than 2 kPa.m(3) high-Z gases such as krypton or xenon. This may be due to the presence of a cold background plasma weakly coupled to the runaway electron beam which prevents neutrals from penetrating in the electron beam core. Following unsuccessful mitigation attempts, runaway electron impacts on beryllium plasma-facing components were observed, showing localized melting with toroidal asymmetries

The JOREK non-linear extended MHD code and applications to large-scale instabilities and their control in magnetically confined fusion plasmas

International audienceJOREK is a massively parallel fully implicit non-linear extended magneto-hydrodynamic(MHD) code for realistic tokamak X-point plasmas. It has become a widely used versatile simulation code for studying large-scale plasma instabilities and their control and is continuously developed in an international community with strong involvements in the European fusion research programme and ITER organization. This article gives a comprehensive overview of the physics models implemented, numerical methods applied for solving the equations and physics studies performed with the code. A dedicated section highlights some of the verification work done for the code. A hierarchy of different physics models is available including a free boundary and resistive wall extension and hybridkinetic-fluid models. The code allows for flux-surface aligned iso-parametric finite element grids in single and double X-point plasmas which can be extended to the true physical walls and uses a robust fully implicit time stepping. Particular focus is laid on plasma edge and scrape-off layer (SOL) physics as well as disruption related phenomena. Among the key results obtained with JOREK regarding plasma edge and SOL, are deep insights into the dynamics of edge localized modes (ELMs), ELM cycles, and ELM control by resonant magnetic perturbations, pellet injection, as well as by vertical magnetic kicks. Also ELM free regimes, detachment physics, the generation and transport of impurities during an ELM, and electrostatic turbulence in the pedestal region are investigated. Regarding disruptions, the focus is on the dynamics of the thermal quench (TQ) and current quench triggered by massive gas injection and shattered pellet injection, runaway electron (RE) dynamics as well as the RE interaction with MHD modes, and vertical displacement events. Also the seeding and suppression of tearing modes (TMs), the dynamics of naturally occurring TQs triggered by locked modes, and radiative collapses are being studied