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Overview

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

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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 provided 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. Recordings of the presentations will be available on this page soon. Please note the times on the programme are in GMT. 

This meeting has taken place.  

Enquiries: contact the Scientific Programmes team.

Organisers

Schedule


Chair

07:00-07:05
Welcome by the Royal Society and the organisers
07:05-07:50
ExB Vortex Dynamics in a Magnetic island

Abstract

E x B shearing rate associated with vortex flow inside a macroscopic magnetic island is investigated. Due to the elongation of the MI and incompressibility of the E x B flow, the shearing rate near X-points is much lower than that near the mid-plane (x-axis of the local Cartesian coordinate) of the MI on the same flux surface.  This calculation of E x B shearing profile and, in particular, minimal E x B shear near the X-points [TS Hahm et al, Phys Plasmas 28, 022302 (2021)] is consistent with the recent experimental finding that turbulence tends to spread into an MI through regions around the X-points [K Ida et al, Phys Rev Lett 120, 245001 (2018)]. In addition, the researchers show that the vortex tends to be better sustained in a large MI, while toroidicity induced precession can break up the quasi-helical symmetry of the vortex, leading to more complicated flow pattern in a long term around a relatively thin MI [GJ Choi and TS Hahm, submitted to Phys Rev Lett (2022)].

Speakers

07:50-08:35
Study of internal transport barriers based on flux driven global toroidal system

Abstract

Transport barriers (TBs), either those produced in edges (H-mode) or inside (ITBs), are crucial for achieving high performance plasmas. Here, the researchers study the origin for the formation of ITBs which have been found in magnetic-shear free toroidal plasmas in both nearly flat and/or reversed safety factor profiles where the eigen-value is not determined from the magnetic structure due to the luck of resonance. Based on the global electromagnetic gyro-kinetic simulation in finite beta-value plasmas, the researchers present a new law to reveal the higher order counterpart as a discrete set of independent dispersion groups by regrouping toroidal mode numbers so as the non-resonant free energy to be sequenced according to the mismatch from the resonance. The regrouped dispersion leads to a new class of turbulence expressed by quasi-independent hierarchized spectra due to the selective energy transfer among them. They also present a flux driven full-f gyro-kinetic simulation as an example of the self-consistent formation of ITBs with heat source and sink by exciting different ion and electron modes, ie ITGs and TEMs, incorporated with ion and electron heating utilising the spatial dimension in magnetic-shear free toroidal plasmas.

Speakers

08:35-09:00
Break
09:00-09:45
Turbulence, cross-phase and zonal flow evolution during confinement transitions

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.

Speakers

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

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.

Speakers

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

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.

Speakers

11:15-11:30
Break
11:30-12:15
Poster session

Chair

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

Abstract

Speakers

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

Abstract

Speakers

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

Abstract

Speakers

16:45-17:15
Break
17:15-18:00
Turbulence characteristics and flow dynamics impacts on the H-mode transition

Abstract

Speakers

18:00-18:45
Pedestal turbulence during ELM’ing and ELM-free H-Mode plasmas

Abstract

A range of multi-field and multi-scale broadband turbulence and coherent instabilities are driven by the steep pressure gradients in the edge pedestal of ELM’ing and ELM-free or ELM-suppressed H-mode plasmas. Experimental characterisation of these fluctuations is therefore crucial to understanding, modeling, and projecting the pedestal structure to fusion reactors. Fluctuations in ELM’ing plasmas undergo a quasi-periodic cycle with fluctuations initially increasing rapidly during the inter-ELM cycle and then increasing more slowly during a longer quasi-steady phase. Pedestal fluctuations exhibit unique behaviour in ELM-suppressed scenarios. Application of Resonant Magnetic Perturbations causes an increase in broadband turbulence just inside the pedestal region that leads to changes in local density and ExB shear, suggesting that the turbulence responds directly to the RMP application. During Quiescent H-modes, which have exceptionally high edge ExB shearing rates, an Edge Harmonic Oscillation is driven that causes quasi-steady particle transport and exhibits a radially sheared structure and appears to replace the role of ELMs. Wide-pedestal QH-modes, in contrast, exhibit broadband turbulence that appear key to its increased transport and enhanced global confinement properties. Comprehensive 2D measurements of pedestal fluctuations will be presented, along with diagnostic and data analysis techniques that are under development.

Speakers

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

Abstract

Speakers


Chair

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

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.

Speakers

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

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. 

Speakers

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

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.

Speakers

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

Abstract

Speakers

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

Abstract

Speakers

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

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. 

Speakers

13:30-13:35
Closing remarks