Edge Pedestal Region Research Articles

Quantitative density fluctuation measurements utilizing quadrature reflectometers on DIII-D

Fixed-frequency reflectometers utilizing quadrature phase detection are employed on DIII-D to study both coherent and turbulent density fluctuations. For coherent fluctuations, the reconstructed phase information successfully identifies MHD/coherent mode activity, e.g. tearing modes, compressional Alfvén eigenmodes, etc. For an m = 3/n = 2 tearing mode, a basic 1D phase screen model is applied to infer fluctuation levels, and the result is consistent with the beam emission spectroscopy (BES) measurement. For turbulent fluctuation studies, a 2D full-wave code (Valeo E.J., Kramer G.J. and Nazikian R. 2002 Plasma Phys. Control. Fusion 44 L1) is applied to interpret reflectometry data obtained in the H-mode edge pedestal region to deduce density fluctuation levels. The measurement is simulated using realistic geometry, plasma conditions and antenna patterns. Comparison of the fluctuation level deduced from the 2D code with BES measurement shows reasonable agreement.

Transport studies in improved H-mode at ASDEX Upgrade

High confinement and stability are obtained simultaneously in stationary conditions in improved H-mode discharges at ASDEX Upgrade. The improved H-mode discharges are typically composed of two different phases: ‘lower heating phase’, where H98(y, 2) is similar to standard H-modes (H98(y, 2) ∼ 1), and ‘fully developed improved H-mode phase’, where H98(y, 2) is higher than standard H-modes (H98(y, 2) up to 1.4). In this paper, the confinement physics is studied by comparing these two different phases using ASTRA simulations. The results are compared with experimental observations. Firstly, the time evolution of the q-profile is simulated with ASTRA using experimental kinetic profiles and compared with experimental measurements. Secondly, the two different phases are compared by power balance analyses and predictive modelling using the Weiland model with ASTRA. Ion temperature gradient lengths and turbulence levels measured by reflectometry are compared between the two phases. Lastly, the roles of density peaking and pedestal pressure in confinement improvement are discussed. The transport analyses using the ASTRA code and experimental observations show that the fully developed improved H-mode phase is very similar to the lower heating phase in terms of core heat transport. With respect to the confinement improvement, it is suggested that the increase of the edge pedestal pressure plays an important role. Enhancement of the radial electric field at the edge is believed to be closely linked with confinement improvement at the edge pedestal region in the fully developed phase.

Far SOL ELM ion energies in JET

There is an increasing body of evidence that the energy lost from diverted tokamak plasmas due to edge localized mode (ELM) activity may not be confined solely to deposition on divertor components. Plasma-facing surfaces in the main confinement chamber also appear to intercept significant fluxes. Whilst this is of no practical consequence for the operation of present day facilities, concerns are being raised over the possible impact on future devices, in which ELMs carrying higher energies are expected. A key parameter required in this assessment is the energy transported by ions in the ELM as it moves through the scrape-off layer (SOL). This contribution presents the first known direct experimental demonstration that ELM events can convect ions with considerable energies to regions in the far SOL. These measurements, obtained on the JET tokamak with an ion energy analyser probe, are combined with a recently developed SOL transient model to show that the ions can, indeed, reach first limiting surfaces with energies that are a considerable fraction (∼50%) of those found in the H-mode edge pedestal region. This experiment–theory comparison supports a picture of the ELM in which filaments of hot plasma originating in the pedestal region dissipate energy primarily through parallel losses to the divertor targets during their radial propagation across the SOL.

Development of burning plasma and advanced scenarios in the DIII-D tokamak

Significant progress in the development of burning plasma scenarios, steady-state scenarios at high fusion performance and basic tokamak physics has been made by the DIII-D team. Discharges similar to the ITER baseline scenario have demonstrated normalized fusion performance nearly 50% higher than the value specified for Q = 10 in ITER reference scenario, under stationary conditions. Discharges have also been demonstrated in DIII-D with enhanced performance under stationary conditions that project to Q ∼ 10 for longer than 1 h in ITER at reduced current, if such a mode of operation can be realized in ITER. Proof of high fusion performance with full noninductive operation has been obtained. Underlying this work are studies validating approaches to confinement extrapolation, disruption avoidance and mitigation, tritium retention, edge localized mode avoidance and operation above the no-wall pressure limit. In addition, the unique capabilities of the DIII-D facility have advanced studies of the sawtooth instability with unprecedented time and space resolution, threshold behaviour in the electron heat transport, rotation in plasmas in the absence of external torque, measurements in the edge pedestal region and plasma fuelling. Understanding these phenomena at a fundamental level contributes to development and ultimately the optimization of tokamak scenarios.

An Understanding of H-Mode Pedestal Instabilities in the DIII-D Tokamak

The experimental and modeling results on H-mode edge-localized mode (ELM) instabilities from the DIII-D tokamak project are reviewed. This work has led to the conclusion that the most common type of ELM, called Type I, is triggered by a coupled peeling-ballooning instability driven by the pressure gradient and current density in the H-mode edge pedestal region. Good agreement is found between theoretically predicted stability boundaries and toroidal mode numbers for this instability and experimental observations of edge pedestal parameters and ELM amplitude and frequency as a function of discharge shape and edge-region collisionality. The range of toroidal mode numbers for which there is access to a second stability regime is shown to play an important role. This model of H-mode edge stability has been used to predict the pedestal parameters for ITER and FIRE.

High resolution edge density measurements in ASDEX upgrade H-mode discharges with broadband reflectometry

In high-confinement (H-mode) plasmas with edge localized modes (ELMs), the energy stored at the edge pedestal region directly affects the global plasma confinement. The measurement of the edge pedestal evolution and the study of the collapse mechanism of the pedestal structure due to ELMs is, therefore, of great importance to understand plasma confinement. In Axially Symmetric Divertor Experiment (ASDEX) Upgrade, information about the edge density profile has been obtained in H-mode plasmas both with Thomson scattering and by combining lithium beam and interferometry measurements. However, information around the edge pedestal region is not routinely available. Here we use microwave reflectometry to obtain high resolution measurements of the edge transport barrier and pedestal region, namely the pedestal position and density, the pedestal width, and the edge density gradient. The evolution of the pedestal density is obtained automatically with an algorithm applied directly to the group delay. The other parameters are derived from the corresponding density profiles. Measurements performed in the presence of type I ELMs reveal that the edge density profile modifications occur in the same time scale of the Dα radiation at the divertor. The collapse and recovery of the pedestal structure during each ELM is resolved. The values of the pedestal width (around 4–5 cm) measured in-between ELMs are similar to those obtained previously in similar ASDEX Upgrade discharges. Also, the evolution of the pedestal density given by reflectometry is found to be in good agreement with the changes of the line integrated density measured by the interferometer.

An edge pedestal model based on transport and atomic physics

A model is presented for the calculation of the characteristic scale lengths from transport considerations in the edge pedestal region of high confinement (H-mode) plasmas. The model is based on the requirements of heat and particle removal through the edge. Atomic physics effects on edge density and temperature gradient scale lengths are taken into account. An empirical fit for the width of the edge pedestal transport barrier is employed. Model problem calculations indicate that the model predicts the magnitudes and some trends of characteristic gradient scale lengths observed in current experiments.