Highlights 2016
Research news from the division Plasma Edge and Wall
Poster prize for E2M PhD student Alexander von Müller
Alexander von Müller from IPP’s E2M division has been awarded a "PhD poster prize" during the 29th SOFT (Symposium on Fusion Technology) which has been held in Prague from September 05 to 09.
The prize has been conferred during the abovementioned conference by the international organising committee and aimed at doctoral candidates who presented their research work in the form of a poster presentation.
His contribution with the title "Melt infiltrated W-Cu composites as advanced heat sink materials for plasma facing components of future nuclear fusion devices" dealt with tungsten-copper composites, a class of materials that could be beneficial with respect to applications in plasma facing components of future magnetic confinement nuclear fusion devices.
Due to the bombardment with particles emerging from the plasma, such components have to withstand intense loads during operation, especially high heat fluxes which have to be removed continuously and reliably.
The image on the left shows two micrographs of tungsten-copper composite materials that are currently being developed at IPP within the Plasma Component Interaction working group.
Evolution of the Edge Velocity Shear at the L-H-Transition
It is widely accepted that a velocity shear is responsible for the suppression of the edge turbulence, thus leading to the transition to the high confinement mode, the L-H transition. The flow is driven through a radial electric field. However, the origin and the evolution of it is still debated. It may be generated by turbulent stresses or neoclassical processes via the main ion pressure gradient. By spectroscopic means, the radial ExB velocity shear has been investigated at various magnetic field strengths, different electron densities, and in both hydrogen and deuterium plasmas. For all the cases, a threshold in the maximum flow velocity, a proxy for the shear, has been found at the H-mode onset. Moreover, the fast dynamics in the flow and the ion profiles were compared with a time resolution of 100 µs during different phases of the L–H transition. The ion pressure gradient is found to be the dominant contribution to the flow. Therefore, turbulence induced zonal flows can play a role only on timescales shorter than 100 µs. The experimental findings obtained in this work reveal the fundamental role of grad(pi) in the L–H transition physics.
For this work Marco Cavedon was awarded a PhD degree at the Technical University of Munich.
http://mediatum.ub.tum.de/doc/1295389/1295389.pdf
Intermittent turbulent fluctuations in ASDEX Upgrade I-mode plasmas
The I-mode confinement regime is characterized by steep edge profile gradients in the electron and ion temperatures, but not in the density. Hence, energy transport behaves like in H-mode while particle confinement is L-mode like. Most probably connected to this peculiar behavior is a quasi-coherent feature in the fluctuation characteristics called the weakly coherent mode (WCM), which resides in the plasma edge. It is unclear what the underlying instability of the WCM is and how density and temperature fluctuations can be decoupled.
Scientists from different research institutes in Germany, France and Portugal have recently joined forces to look in more detail into the I-mode on the ASDEX Upgrade tokamak. The main diagnostics used were Doppler and conventional reflectometry. In this effort, strong fluctuation amplitude bursts have been found in the I-mode edge plasma. The structures appear only in I-mode, and they become stronger with increasing confinement quality. They have higher amplitudes than the L-mode fluctuations (see figure) and are strongly intermittent. The central result is the demonstration that the density fluctuation bursts are linked to the WCM. This is a hint that they might play a role in inhibiting the density profile to steepen in I-mode. These results can be looked up in one of the "most read" recent articles in Nuclear Fusion, which is also highlighted by a "LabTalk".
T. Happel et al., Nucl. Fusion 56, 064004 (2016)
dx.doi.org/10.1088/0029-5515/56/6/064004
Power loads in snow flake divertors
Most of the high-power tokamaks worldwide investigate the single-null configuration (left figure), which is also foreseen for ITER. In such a configuration the targets and in particular the outer one are exposed to very high power fluxes that may limit the life-time of the corresponding plasma-facing components. In continuation to previous collaborations with the Swiss experiment TCV we further investigated as an alternative configuration the so called Snowflake (SF) divertor (right figure). It is characterized by a magnetic configuration, where two null points are localized close to each other. In a recently published article [1] the properties of the SF divertor in dependence of the radial distance of the null points were studied systematically by means of the transport code EMC3-EIRENE. It was found that the power load at the outer targets can be reduced significantly in a snowflake divertor as illustrated by the red diagrams in the figures.
These results fed into plans for the installation of further poloidal field coils in ASDEX Upgrade that would allow an experimental study of snowflake and other configurations in the upper divertor under high heating power conditions.
[1] T Lunt et al. 2016 Plasma Phys. and Control. Fusion 58 045027
Interaction of Deuterium Plasma with pre-nitrided Tungsten Surfaces
Experiments in the tokamak ASDEX Upgrade have shown that the addition of nitrogen to the plasma reduces the power flux to the wall and at the same time improves the plasma performance. However, nitrogen from the plasma is implanted into the tungsten surfaces and becomes chemically bound. This bound nitrogen influences plasma-surface-interaction processes and hydrogen retention in the wall material.
The interaction of nitrogen-containing tungsten surfaces (WNx) with hydrogen plasmas and their influence on hydrogen retention was investigated in laboratory experiments by Gao Liang in the frame of his PhD thesis at IPP.
Confocal laser scanning microscope image of a tungsten surface after deuterium implantation. The right-hand side of the sample was covered with a 100 nm thick WNx layer prior to D implantation. The pure tungsten surface (left) shows strong blistering (formation of gas-filled cavities underneath the surface), while no blisters occur on the WNx-covered half (right).
One important result is the finding that nitrogen implantation into tungsten strongly suppresses hydrogen diffusion.
At 300 K the diffusion of H in WNx is negligible and at 600 K it is still much lower than in pure W.
Additionally he developed a refined method to determine the deuterium depth profile at surfaces with significantly improved depth resolution.
With this work L. Gao graduated at the Ruhr University Bochum in January 2016.
L. Gao et al., Nuclear Fusion 56, 016004 (2016),
http://dx.doi.org/10.1088/0029-5515/56/1/016004
L. Gao et al., Physica Scripta T159, 014023 (2014),
http://dx.doi.org/10.1088/0031-8949/2014/T159/014023