Physical mechanism behind and access to the I-mode confinement regime in tokamaks

Mänz, P.; Happel, T.; Stroth, U.; Eich, T.; Silvagni, D.

doi:10.1088/1741-4326/ab9e17

Cited by 25 publications

(34 citation statements)

References 42 publications

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“…According to the simulations in reference [44], a PRE can be triggered when the plasma beta around the separatrix becomes large enough to induce radial magnetic incoherent fluctuations. These perturbations disturb the parallel electron dynamic which is central for the formation of the WCM in those simulations [45] and which is stabilizing for interchange effects. As a consequence, the WCM becomes interchange unstable and the associated enhanced transport relaxes edge gradients causing a PRE.…”

Section: Discussion On the Pre Triggering Instabilitymentioning

confidence: 94%

I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

et al. 2022

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In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light on their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks – Alcator C-Mod and ASDEX Upgrade – is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, a growing oscillating precursor before the PRE onset is observed in the region close to the separatrix in both devices, and a discussion on a possible triggering mechanism is outlined. The PRE relative energy loss from the confined region is found to increase with decreasing pedestal top collisionality ν* ped. Similarly, also the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing ν* ped. Based on these relations, the PRE relative energy loss in future devices such as DEMO and ARC is estimated. Finally, the divertor peak energy fluence due to the PRE is measured on each device. Those values are then compared to the model introduced in [1] for type-I ELMs. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections of the PRE divertor energy fluence to DEMO and ARC.

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Section: Discussion On the Pre Triggering Instabilitymentioning

confidence: 94%

I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

et al. 2022

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“…[43], a PRE can be triggered when the plasma beta around the separatrix becomes large enough to induce radial magnetic incoherent fluctuations. These perturbations disturb the parallel electron dynamic which is central for the formation of the WCM in those simulations [44] and which is stabilizing for interchange effects. As a consequence, the WCM becomes interchange unstable and the associated enhanced transport relaxes edge gradients causing a PRE.…”

Section: Discussion On the Pre-triggering Instabilitymentioning

confidence: 94%

I-mode pedestal relaxation events at ASDEX Upgrade

et al. 2020

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In some conditions, I-mode plasmas can feature pedestal relaxation events (PREs) that transiently enhance the energy reaching the divertor target plates. To shed light into their appearance, characteristics and energy reaching the divertor targets, a comparative study between two tokamaks -Alcator C-Mod and ASDEX Upgrade -is carried out. It is found that PREs appear only in a subset of I-mode discharges, mainly when the plasma is close to the H-mode transition. Also, the nature of the triggering instability is discussed by comparing measurements close to the separatrix in both devices. The PRE relative energy loss from the confined region increases with decreasing pedestal top collisionality ν * ped . In addition, the relative electron temperature drop at the pedestal top, which is related to the conductive energy loss, rises with decreasing ν * ped . Finally, the peak parallel energy fluence due to the PRE measured on the divertor in both devices is compared to the model introduced in [1] for type-I ELMs. The model is shown to provide an upper boundary for PRE energy fluence data, while a lower boundary is found by dividing the model by three. These two boundaries are used to make projections to future devices such as DEMO and ARC.

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“…A physical picture of the I-mode was derived from simulations using the gyro-fluid code GEMR [55]. Based on the experimentally justified assumption that at the separatrix the ion temperature is higher than the electron temperature and the ion temperature gradient is shallower than that of the electrons, the strong drive of the ITG turbulence is removed.…”

Section: Improved Confinement Modementioning

confidence: 99%

“…The simulations reproduce the intermittent transport related to the WCM and also clarify the origin of the I-mode operation window in heating power, magnetic field strength, and collisionality. The fact that a transport barrier forms only in the electron heat channel is attributed to the dissipation of the electron temperature fluctuations by parallel heat conductivity at low collisionalities [55].…”

Section: Improved Confinement Modementioning

confidence: 99%

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

Stroth

Team²

2022

Nucl. Fusion

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An overview of recent results obtained at the tokamak ASDEX Upgrade (AUG) is given. A work flow for predictive profile modelling of AUG discharges was established which is able to reproduce experimental H-mode plasma profiles based on engineering parameters only. In the plasma center, theoretical predictions on plasma current redistribution by a dynamo effect were confirmed experimentally. For core transport, the stabilizing effect of fast ion distributions on turbulent transport is shown to be important to explain the core isotope effect and improves the description of hollow low-Z impurity profiles. The L–H power threshold of hydrogen plasmas is not affected by small helium admixtures and it increases continuously from the deuterium to the hydrogen level when the hydrogen concentration is raised from 0 to 100%. One focus of recent campaigns was the search for a fusion relevant integrated plasma scenario without large edge localised modes (ELMs). Results from six different ELM-free confinement regimes are compared with respect to reactor relevance: ELM suppression by magnetic perturbation coils could be attributed to toroidally asymmetric turbulent fluctuations in the vicinity of the separatrix. Stable improved confinement mode plasma phases with a detached inner divertor were obtained using a feedback control of the plasma β. The enhanced D α H-mode regime was extended to higher heating power by feedback controlled radiative cooling with argon. The quasi-coherent exhaust regime was developed into an integrated scenario at high heating power and energy confinement, with a detached divertor and without large ELMs. Small ELMs close to the separatrix lead to peeling-ballooning stability and quasi continuous power exhaust. Helium beam density fluctuation measurements confirm that transport close to the separatrix is important to achieve the different ELM-free regimes. Based on separatrix plasma parameters and interchange-drift-Alfvén turbulence, an analytic model was derived that reproduces the experimentally found important operational boundaries of the density limit and between L- and H-mode confinement. Feedback control for the X-point radiator (XPR) position was established as an important element for divertor detachment control. Stable and detached ELM-free phases with H-mode confinement quality were obtained when the XPR was moved 10 cm above the X-point. Investigations of the plasma in the future flexible snow-flake divertor of AUG by means of first SOLPS-ITER simulations with drifts activated predict beneficial detachment properties and the activation of an additional strike point by the drifts.

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Physical mechanism behind and access to the I-mode confinement regime in tokamaks

Cited by 25 publications

References 42 publications

I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

I-mode pedestal relaxation events in the Alcator C-Mod and ASDEX Upgrade tokamaks

I-mode pedestal relaxation events at ASDEX Upgrade

Progress from ASDEX Upgrade experiments in preparing the physics basis of ITER operation and DEMO scenario development

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