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H-mode transition and pedestal studies in fusion plasmas

Online event

Location

Zoom meeting

Overview

Scientific discussion meeting oganised by Professor Eun-jin Kim and Dr Yasmin Andrew.

Tokamak à Configuration Variable, TCV, Swiss Plasma Centre, EPFL. Credits: A. Herzog

Understanding the high confinement (H-mode) transition and the plasma pedestal remain critical topics for the successful operation of future magnetic fusion devices. This meeting will provide a valuable opportunity for focused discussions on the cross-machine approach (experimentally and theoretically) to further our understanding of H-mode access and exit, providing a forum for the exchange of specialist knowledge and co-operation.

Meeting papers will be published in a future issue of Philosophical Transactions of the Royal Society A.

Poster session

This event will feature a poster session. The posters will be selected by the organisers of the meeting. If you are interested in submitting a poster for consideration please send a title, list of authors and a 200-word abstract, in the third person with no references nor pictures, to scientific.meetings@royalsociety.org with subject line ‘H-mode transition and pedestal studies in fusion plasmas: poster submission’ by 15 December 2021. Please note the poster presenter should submit a request to attend the meeting before they send us a poster abstract. 

Attending the event

This meeting is intended for researchers in relevant fields.

  • Free to attend
  • Advance registration essential
  • Live subtitles will be available

Enquiries: contact the Scientific Programmes team.

Event organisers

Select an organiser for more information

Schedule of talks

17 January

Session 1 07:00-12:15

8 talks Show detail Hide detail

Chairs

Professor Eun-jin Kim, Coventry University, UK

07:00-07:45 Title to be confirmed

Professor TS Hahm, Seoul National University, South Korea

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07:45-08:30 Study of internal transport barriers based on flux driven global toroidal system

Professor Yasuaki Kishimoto, Kyoto University, Japan

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08:30-09:00

09:00-09:45 Turbulence, cross-phase and zonal flow evolution during confinement transitions

Dr Carlos Hidalgo, Laboratorio Nacional de Fusión, CIEMAT, Spain

Abstract

The two main concepts for magnetically confined fusion plasmas are the tokamak and the stellarator. The flexibility and external control of the stellarator configuration in combination with unique diagnostic capabilities convert stellarators into ideal systems for the study of the relation between magnetic topology, electric fields and confinement transitions. This talk addresses advances in the characterisation of plasma transport, with emphasis on the physics of radial electric fields and transport control in the TJ-II stellarator. The presenter pays particular attention to the experimental evolution of fluctuation levels, the cross phase between fluctuating variables and zonal flows, revealing the simultaneous spatiotemporal evolution of these quantities, during spontaneous and biasing induced transitions. Causality detection techniques provide a deeper understanding of the interaction between the various fluctuating quantities. The experimental results described here, using unique diagnostic capacities in combination with advanced analysis tools, provide further physics understanding of confinement bifurcations driven by radial electric fields in stellarators, complementing well-known empirical approaches in tokamaks. It is concluded that there are different paths to achieve confinement bifurcations, and each path might correspond to different conditions (ie power / density threshold) for accessing the H-mode in fusion plasmas.

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09:45-10:30 Dynamical properties of the radial electric field at the tokamak plasma edge

Dr Yanick Sarazin, CEA, IRFM, France

Abstract

It is well admitted that the shear of the radial electric field Er plays a key role in triggering and sustaining the edge transport barrier of H-mode tokamak plasmas. Here the researchers first report, just inside the separatrix of WEST plasmas, on the formation of a deeper well of Er when operating in lower single null as compared to upper single null. These results obtained close to the L-H power threshold are consistent with the idea that the magnetic drift pointing towards the X-point favors the L-H transition. Second they unravel – by means of flux-driven simulations with the gyrokinetic code GYSELA encompassing core, edge and a simplified modelling of the scrape-off-layer in limiter configuration – how both the Reynolds stress and more critically its diamagnetic counterpart play a key role in the buildup of an Er well in the L-mode edge of tokamak plasmas. Leading to a mild steepening of the temperature profile at the edge, these results shed light on the possible dynamics at play in the L- to H-mode transition. Finally, a 1-dimensional nonlinear model allows one to explore the competition of the Reynolds stress, diamagnetic stress and neoclassical poloidal flow damping in the formation of a sheared Er profile.

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10:30-11:15 L-H transition studies at JET

Dr Emilia R Solano, Laboratorio Nacional de Fusión, CIEMAT, Spain

Abstract

The talk presents results from various dedicated L-H transition studies at JET-ILW. In particular Dr Solano reports changes in the value of the density at which the L-H transition power threshold is minimised as a function of plasma species and plasma shape. The researchers have obtained results in H, D, T, He and various mixtures. In He and D plasmas, Doppler reflectometer measurements of perpendicular velocity (related to the radial electric field profile) in the plasma edge indicate that there is no critical value of the radial electric field value or the shear of the vExB rotation before the transition. More importantly, during the L-mode phase, while the input power is being increased up to the L-H power threshold, there is no evidence of evolution of the Er profile.

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11:15-11:30 Break

11:30-12:15 Poster session

18 January

Session 2 14:30-19:30

7 talks Show detail Hide detail

Chairs

Dr Yasmin Andrew, Imperial College London, UK

14:30-15:15 Hidden variables in the L2H transition and the impact of the Scrape-Off-Layer

Dr Hendrik Meyer, United Kingdom Atomic Energy Authority, D3 Culham Science Centre, UK

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15:15-16:00 Transitions and pedestals in the I-mode confinement regime

Dr Amanda Hubbard, MIT Plasma Science and Fusion Center, USA

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16:00-16:45 Electric Field Shear as the order parameter for the edge plasma: from L → H transition to density limit

Professor Patrick H Diamond, University of California, San Diego, USA

Abstract

Please view the abstract for the talk here (PDF).

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16:45-17:15 Break

17:15-18:00 Turbulence characteristics and flow dynamics impacts on the H-mode transition

Dr Zheng Yan, University of Wisconsin – Madison, USA

Abstract

Please view the abstract for the talk here (PDF).

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18:00-18:45 Title to be confirmed

18:45-19:30 Improving H-mode access in ITER: connecting ‘microscopic‘ L-H transition physics to the power threshold scaling

Dr Lothar Schmitz, University of California Los Angeles, USA

Abstract

Please view the abstract for the talk here (PDF).

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19 January

Session 3 09:00-13:30

8 talks Show detail Hide detail

Chairs

Dr Carlos Hidalgo, Laboratorio Nacional de Fusión, CIEMAT, Spain

09:00-09:45 The road to pedestal tailoring at ASDEX Upgrade

Professor Elisabeth Wolfrum, Max Planck Institute for Plasmaphysics, Germany

Abstract

This work gives an overview of recent investigations at ASDEX Upgrade that show our current understanding of the transport mechanisms in the pedestal and how transport and stability in this narrow region can be influenced. For electron heat transport a constant temperature gradient length hints towards a local small scale turbulent transport mechanism. The ion heat transport is close to neoclassical values, however in some cases this only holds in the central part of the pedestal with deviations at the pedestal top and foot. The shape and position of the edge density profile are key to both stability and transport and remains the parameter which can be most varied in the pedestal. In the team's search for a scenario without large edge localised modes, ballooning modes can be driven unstable at the pedestal foot. Careful balance of the drive and stabilising terms allows the pedestal to be tailored such that the global peeling-ballooning stability limit is not breached. Another globally stable regime is achieved with strong nitrogen seeding, leading to the formation of an X-point radiator. These two ELM-free regimes are important research topics for the extrapolation to larger devices.

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09:45-10:30 Influence of the plasma configuration on the L-H transition in TCV

Dr Yves Martin, EPFL - Swiss Plasma Center, Switzerland

Abstract

The H-mode, with its improved confinement characteristics, is considered as the operational regime in the baseline scenario for ITER. The heating power flowing through the plasma separatrix is considered to be the main parameter that controls the access to the H-mode. Thorough analyses of international databases revealed an overall dependence of the required power on the plasma density, toroidal magnetic field and plasma size. However, the data fits the obtained scalings with a large scatter, indicating the importance of other parameters. The TCV tokamak, with its extreme plasma shaping capability is perfectly suited to explore the accessibility of the H-mode as a function of the plasma configuration. In particular, the impact of the plasma-wall distance on the threshold power has been intensively studied and showed a decrease of the threshold power by up to a factor of two when reducing the X-point height by three quarters. In a similar way, changes in the flux expansion or other parameters describing the magnetic field configuration in the divertor region also induce significant changes in the required power. In contrast, the installation of baffles separating the divertor region from the main plasma does not impact the access to the H-mode. 

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10:30-11:15 The self-organisation nature of a sheared plasma flow as it transitions between states

Dr Bogdan Teaca, Coventry University, UK

Abstract

In tokamak plasmas, sheared flows perpendicular to the driving temperature gradients can strongly stabilise linear modes, leading to a subcritical configuration. For this bistable system, changes in the amplitude of fluctuations can lead to a transition between quiescent and turbulent states. At the transition between these two states, we find an unstable submanifold colloquially called 'the edge of chaos’. The type of structures supported on the edge of chaos impacts the transition and the self-organisation nature of sheared plasma flows.

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11:15-11:45 Break

11:45-12:30 Comparing pedestal structure in JET-ILW H-mode plasmas with a model for stiff ETG turbulent heat transport

Dr Anthony Field, Culham Centre for Fusion Energy, UK

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12:30-13:00 Access to the High Confinement Mode on ST40

Dr Yasmin Andrew, Imperial College London, UK

Abstract

Please view the abstract for the talk here (PDF). 

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13:00-13:30 New statistical method for the L-H transition

Professor Eun-jin Kim, Coventry University, UK

Abstract

The need for a proper statistical theory for understanding fusion plasmas has grown significantly, with experiments and simulations revealing ample evidence for non-Gaussian fluctuations, anomalous transport, or intermittency. The latter questions the validity of the mean-field-type theory based on small Gaussian fluctuations, necessitating the calculation of an entire probability density function (PDF). In this paper, the researchers show the importance of intermittency and time-dependent PDF approach in the Low-to-High confinement mode (L-H) transition. 

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13:30-13:35 Closing remarks

H-mode transition and pedestal studies in fusion plasmas

17 – 19 January 2022

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