Scrape Off Layer (SOL) transport and filamentary characteristics in high density tokamak regimes

A detailed cross-device investigation on the role of filamentary dynamics in high density regimes has been performed within the EUROfusion framework comparing ASDEX Upgrade (AUG) and TCV tokamaks. Both devices have run density ramp experiments at different levels of plasma current, keeping toroidal field or q<sub>95</sub> constant in order to disentangle the role of parallel connection length and the current. During the scan at constant toroidal field, in both devices SOL profiles tend to develop a clear Scrape Off Layer (SOL) density shoulder at lower edge density whenever current is reduced. The different current behavior is substantially reconciled in terms of edge density normalized to Greenwald fraction. During the scan at constant q<sub>95</sub> AUG exhibits a similar behaviour whereas in TCV no signature of upstream profile modification has been observed at lower level of currents. Thelatter behaviour has been ascribed to the lack of target density roll-over. The relation between upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob-size and SOL density e-folding length is found independent of the plasma current, with a clear increase of blob-size with edge density normalized to Greenwald fraction observed. ASDEX Upgrade has also explored the filamentary behaviour in H-Mode. The experiments on AUG focused on the role of neutrals, performing discharges with and without the cryogenic pumps, highlighting how large neutral pressure not only in the divertor but at the midplane is needed in order to develop a H-Mode SOL profile shoulder in AUG.

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

Hugo De Oliveira, Alexander Karpushov, Benoît Labit, Roberto Maurizio, Holger Reimerdes, Christian Gabriel Theiler, Cedric Kar-Wai Tsui, Kevin Henricus Annemarie Verhaegh, Nicola Vianello

A detailed cross-device investigation on the role of filamentary dynamics in high-density regimes has been performed within the EUROfusion framework, comparing the ASDEX Upgrade (AUG) and TCV tokamaks. Both devices run density ramp experiments at different levels of plasma current, keeping the toroidal field or q(95) constant in order to disentangle the role of the parallel connection length and the current. During the scan at a constant toroidal field, in both devices the scrape-off layer (SOL) profiles tend to develop a clear SOL density shoulder at a lower edge density whenever the current is reduced. Different current behaviour is substantially reconciled in terms of the edge density normalized to the Greenwald fraction. During the scan at constant q(95) AUG exhibits similar behaviour, whereas in TCV no upstream profile modification signature has been observed at lower current levels. The latter behaviour has been ascribed to the lack of target density rollover. The relation between the upstream density profile modification and detachment condition has been investigated. For both devices the relation between blob size and the SOL density e-folding length is found independent of the plasma current, with the observation of a clear increase in blob size and the edge density normalized to a Greenwald fraction. ASDEX Upgrade has also explored filamentary behaviour in the H-mode. The experiments in AUG have focused on the role of neutrals, performing discharges with and without cryogenic pumps, highlighting how high neutral pressure, not only in the divertor but also at the midplane, is needed in order to develop an H-mode SOL profile shoulder in AUG.

2020

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks-A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

Stefano Coda, Joan Decker, Basil Duval, Yves Martin, Giovanni Tardini

This paper presents a survey of the experiments on runaway electrons (RE) carried out recently in frames of EUROFusion Consortium in different tokamaks: COMPASS, ASDEX-Upgrade, TCV and JET. Massive gas injection (MGI) has been used in different scenarios for RE generation in small and medium-sized tokamaks to elaborate the most efficient and reliable ones for future RE experiments. New data on RE generated at disruptions in COMPASS and ASDEX-Upgrade was collected and added to the JET database. Different accessible parameters of disruptions, such as current quench rate, conversion rate of plasma current into runaways, etc have been analysed for each tokamak and compared to JET data. It was shown, that tokamaks with larger geometrical sizes provide the wider limits for spatial and temporal variation of plasma parameters during disruptions, thus extending the parameter space for RE generation. The second part of experiments was dedicated to study of RE generation in stationary discharges in COMPASS, TCV and JET. Injection of Ne/Ar have been used to mock-up the JET MGI runaway suppression experiments. Secondary RE avalanching was identified and quantified for the first time in the TCV tokamak in RE generating discharges after massive Ne injection. Simulations of the primary RE generation and secondary avalanching dynamics in stationary discharges has demonstrated that RE current fraction created via avalanching could achieve up to 70-75% of the total plasma current in TCV. Relaxations which are reminiscent the phenomena associated to the kinetic instability driven by RE have been detected in RE discharges in TCV. Macroscopic parameters of RE dominating discharges in TCV before and after onset of the instability fit well to the empirical instability criterion, which was established in the early tokamaks and examined by results of recent numerical simulations.

Iop Publishing Ltd2018

Mirko Wensing

Thermonuclear fusion of light atoms is the primary energy source of stars, such as our Sun, that led to the emergence of life on Earth. However, its economic exploitation as a virtually unlimited and clean energy source is yet to be developed. One of the most promising designs for a nuclear fusion reactor is the tokamak, a toroidal device that uses strong magnetic fields to contain matter so it may be heated to fusion-relevant temperatures of

\sim 10^8\;^\circ

C. A substantial fraction of the heating power, required to maintain fusion-relevant plasma conditions, is transported across magnetic flux surfaces and channeled through a thin region of open magnetic field lines that surround the confined zone, the scrape-off layer, towards the plasma wall interface at the divertor targets. If left unmitigated, anticipated target heat fluxes in current reactor designs exceed the material limit and raise the need for an accurate predictive model of heat transport and power dissipation in the plasma edge to achieve a viable reactor configuration. This doctoral thesis is devoted to further understanding of divertor power exhaust, a key challenge on the way towards fusion energy, and contributes to the validation of such a predictive model, the SOLPS-ITER code. This work employs stringent comparisons between simulation results and experiments on the TCV tokamak to validate this model, where possible, and/or indicate remaining problems. This thesis presents the first scrape-off layer simulations that fully account for drifts, currents, carbon impurities and kinetic neutrals, for the TCV tokamak, and thereby provides unprecedented insight into drift-driven transport and plasma-neutral interaction within its divertor. In an initial stage, the SOLPS-ITER code was used to predict the effectiveness of divertor gas baffles to guide the first baffled TCV campaign. The experimental assessment in 2019 confirmed the essential simulation predictions. Drift simulations identify the diamagnetic current as the dominant cross-field contribution to the divertor charge balance. The resulting characteristics of target current profiles are tested and confirmed in TCV measurements. It is demonstrated, for the first time, that such electric currents give rise to a potential well below the magnetic X-point for the unfavorable magnetic field direction for H-mode access in highly resistive, i.e. low temperature, divertors. The simulation result is supported by reduced analytic models that highlight the underlying physics. The prediction is tested and confirmed in TCV experiments following novel diagnostic capabilities. The presence of such a potential well is shown to substantially reshape the divertor transport in detached divertor conditions. The simulations identify the

E\times B

-drift as the dominant radial transport channel for particles, heat and momentum in the divertor. A stringent comparison of simulation results to a wide set of edge-relevant diagnostics yields typically excellent qualitative agreement, but quantitative differences remain: the simulations conclude a colder and denser divertor plasma state with an overestimated neutral pressure. The identified shortcomings in this approach will contribute to further development towards a predictive divertor model of present day, and future fusion devices.

EPFL2021

Mirko Wensing

\sim 10^8\;^\circ

E\times B

EPFL2021

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks-A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

Stefano Coda, Joan Decker, Basil Duval, Yves Martin, Giovanni Tardini

Iop Publishing Ltd2018

Scrape Off Layer (SOL) transport and filamentary characteristics in high density tokamak regimes

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks-A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET

Drift-related transport and plasma-neutral interaction in the TCV divertor

Scrape-off layer transport and filament characteristics in high-density tokamak regimes

Drift-related transport and plasma-neutral interaction in the TCV divertor

Comparison of runaway electron generation parameters in small, medium-sized and large tokamaks-A survey of experiments in COMPASS, TCV, ASDEX-Upgrade and JET