Highlights 2015

IPP Garching plays host to world experts for workshop on H-mode physics

The Max-Planck-Institute for Plasma Physics (IPP) hosted the 15^th International workshop on H-mode physics and transport barriers that was held from October 19 to 21, 2015, in Garching, Germany. The biannual workshop was the latest in a series of such workshops which have been held around the world, e.g. in Fukuoka in Japan (2013), Oxford in the UK (2011), and Princeton in the US (2009).

The workshop concentrates on research topics which are undergoing intensive current investigation with high relevance to the specific area of transport barriers in tokamaks. The international advisory committee, consisting of members from China, Europe, Japan, Russia, South Korea and the US, selects the relevant topics and invites one scientist per topic to present an up-to-date overview of this topic, including experimental and theoretical results from all over the world. Not only distinguished scientists but also early career researchers were selected, who all lead lively discussions following their overview talks.

The conference attendees numbered 95 participants from 15 countries (43 Europe, 26 US, 26 Asia) with almost 80 contributions in the form of poster presentations. The topics ranged from turbulence in transport barriers to the effect of impurities on the stability of the edge transport barrier. The purpose of the workshop is to provide a platform for detailed discussions, to stimulate exchange of knowledge and to initiate cooperation where promising. The contributions to the conference, including the review talks, will be collected in the form of peer-reviewed papers and published in a special issue of Nuclear Fusion.

Elisabeth Wolfrum & Mike Dunne

Experimental validation of a filament transport model in turbulent magnetized plasmas

Elongated structures also called “filaments” or “blobs” are known to be the dominant mechanism of transport between the tokamak confined plasma and the main wall, the region known as Scrape-Off Layer (SOL). In many tokamaks, as the plasma density increases, a transition to enhanced perpendicular transport and the development of a “density shoulder” in the SOL are observed leading to enhanced interaction of the plasma with the wall. Hence, understanding this phenomenon is needed to predict the particle and heat fluxes to plasma facing components in a future fusion reactor.

Analytical models predict a change of propagation regime as the electron-ion collision frequency increases, leading to larger and faster filaments and to substantially higher transport. By analyzing Langmuir probe data from the ASDEX Upgrade and JET tokamaks, the formation of larger blobs and the development of a density shoulder in the SOL could now be related to the plasma conditions in the divertor summarized in an effective collisionality Λ_div. The physical model describes this transition as a switch in the filament dynamics from a typical sheath limited regime to an inertial regime in which the filament becomes disconnected from the divertor plates and both size and velocity of filaments can increase. The figure depicts this transition in terms of the density fall-off length λ_n in the SOL which substantially increases when Λ_div increases above one reflecting the formation of the density shoulder. This work was now accepted for publication (D. Carralero et al, Phys. Rev. Letter 2015).

Virtual Institute on Turbulence Research was successful in mid-term review

The E2M led Helmholtz Virtual Institute (VI) on "Plasma dynamical processes and turbulence studies using advanced microwave diagnostics" was awarded the highest grade "A" in June this year during a mid-term review by the HGF. This means the VI will continue to be funded for the next 2 to 3 years without further conditions. Under the umbrella of the VI several international fusion laboratories and universities have been brought together to investigate critical issues in the study of turbulent plasma transport in fusion devices.

A particular emphasis, or target, of the VI is the coupling of experiment and theory to advance the understanding of plasma turbulence and transport. In the frame of the VI, 13 PhD students are engaged on various research topics, as well as the installation and operation of numerous new microwave-based diagnostics on ASDEX Upgrade, in close collaboration with our research partners. The figure below shows a recent high temporal resolution result on the evolution of the density profile during the L-H transition using the ultra-fast swept reflectometer installed and operated on ASDEX Upgrade by a joint PhD student from CEA and IPP. Further information can be found here:

Nucl. Fusion 55 (2015) 083027 - http://stacks.iop.org/0029-5515/55/083027

PFMC-15 Poster Prize for Gerd Meisl

Gerd Meisl from IPP’s E2M division was awarded the poster prize sponsored by Physica Scripta at the PFMC-15 conference (15th International Conference on Plasma-Facing Materials and Components for Fusion Applications). PFMC-15 took place from May 18 to 22 in Aix-en-Provence.

The Physica Scripta Poster Prize is dedicated to young researches (PhDs or PostDocs) “in recognition of the quality of work presented at the PFMC-15”.
In the poster with the title „Simulating the nitrogen migration in Be/W tokamaks with WallDYN“ he investigates the impurity transport and the corresponding nitrogen deposition in nitrogen-seeded plasmas. Due to chemical interactions of nitrogen with tungsten and beryllium the plasma-surface-interaction processes in nitrogen-seeded plasma are  much more complex than those in comparable noble-gas-seeded plasmas.
The picture shows the local surface distribution of nitrogen retention in a simulated anticipated ITER discharge. High nitrogen retention (red and brown areas) occurs in areas of high beryllium deposition. The dominating process is co-deposition of nitrogen with beryllium.

From 2005 to 2010 a total of 60 prototypes for the actively cooled divertor targets of Wendelstein 7-X were tested. In this period of time a production technology fulfilling the requirements of long pulse operation was developed in cooperation with Plansee SE. The prototypes of the pre-series have confirmed the robustness of the finally selected design and technology. Prototypes specified for long pulse operation at 10 MW/m² were successfully loaded at 30 MW/m² in the high heat flux test facility GLADIS.

Owing to its two ion sources of 1 MW power each, its 8 m³ vacuum chamber and powerful water cooling, GLADIS offers the possibility to investigate small scale as well as full scale objects loaded with extremely high heat fluxes.

From 2011 to early 2015, 86 of the 970 delivered components were evaluated. For this purpose all CFC (carbon fibre composite) tiles were loaded with 100 cycles of 10 seconds duration at 10 MW/m². The temperature increase of the CFC tiles caused by the thermo-mechanical load was evaluated statistically. It was possible to show that in spite of the seemingly small tested fraction the probability of an undetected defective CFC tiles is negligibly low (<1×10^-10)

Future investigations for the divertor of Wendelstein 7-X will focus on diagnostics for the infrared thermography as well as on tests of full divertor modules.

Impact of plasma-wall interaction on the nitrogen transport in fusion experiments

Controlling the plasma-wall interaction is one of the greatest challenges on the way to a fusion reactor. Experiments in the tokamak ASDEX Upgrade posed an important step to overcome this challenge: It could be shown that the admission of nitrogen to the plasma reduces the power flux to the wall and at the same time improves the plasma performance. However, it was also realized that nitrogen from the plasma is implanted into the tungsten wall surfaces of ASDEX Upgrade and becomes chemically bound in the tungsten. The figure illustrates the manifold interactions of nitrogen with a tungsten surface.

Through the combination of laboratory experiments, computer simulations and dedicated ASDEX Upgrade experiments it was possible for the first time to describe this interaction quantitatively. The underlying physical picture is shown in the figure: The implantation of nitrogen into tungsten induces the formation of tungsten nitride. As nitrogen does not diffuse into the bulk material, the surface can only store a limited amount of nitrogen and excess nitrogen escapes back into the plasma. The combination of this newly established surface model with plasma transport calculations in the code WallDYN correctly describes the nitrogen balance in ASDEX Upgrade. The simulations reveal that the nitrogen balance is dominated by the storage of nitrogen in the surface.

With this work Gerd Meisl graduated at the TU München in January 2015.

G. Meisl et al., New J. Phys. 16, 093018 (2014),
http://dx.doi.org/10.1088/1367-2630/16/9/093018

G. Meisl et al., J. Nucl. Mater. 463 (2015) 668,
http://dx.doi.org/ 10.1016/j.jnucmat.2014.10.031

Experimental investigations and numerical modeling of divertor detachment in H-Mode Plasmas in ASDEX Upgrade

In future fusion power plants high power fluxes pose a threat to the wall materials, especially in the divertor. It is thought that detachment of the plasma from the divertor target is necessary to avoid excessive heat loads to and material erosion of the divertor target plates.

Strong detachment has been demonstrated for the first time in the all-tungsten ASDEX Upgrade tokamak in fusion-relevant H-mode plasmas. The deliberate seeding of nitrogen provokes additional line radiation that cools the divertor plasma and provides access to detachment. The observed radiation distribution exhibits a strong concentration above the X-point (s. figure), which indicates low temperatures and significant plasma parameter variations on closed field lines.

The detached plasma has been successfully modeled with the SOLPS-code. Assuming an increased plasma transport perpendicular to the magnetic field lines, most experimental measurements could be reproduced.

With this work Felix Reimold obtained his PhD degree on 08.01.2015 at the TU München.

Reimold et al., Nuclear Fusion 55 3 (2015) 033004
Reimold et al., Journal of Nuclear Materials Submitted (2015)

EUROfusion Fellowships

Dr. Matthias Willensdorfer has been working as a postdoctoral researcher at IPP since 2013 and is responsible for the ECE diagnostic, which measures the electron temperature. Previously, he completed both his diploma and PhD theses as a guest scientist at IPP and was in charge of the Lithium beam diagnostic.  In 2013 he finished his PhD thesis at the Technical University of Vienna entitled "Temporal behavior of the plasma edge density throughout the L-H transition in ASDEX Upgrade". Within the EUROfusion fellowship Dr. Matthias Willensdorfer will work on the impact of external magnetic perturbations and 3D effects on plasma transport.

Dr. Mike Dunne has been at IPP since 2010, first as a guest researcher from the University College Cork where in 2013 he defenced his PhD thesis entitled "Inter-ELM edge current density profiles on the ASDEX Upgrade tokamak". Since July 2013 he has been employed as a postdoctoral researcher at IPP, working in the pedestal edge physics group of E2M. His work under the fusion fellowship will focus on interpretive and predictive stability calculations in nitrogen seeded and pellet fuelled discharges on ASDEX Upgrade.

Dr. Gergely Papp graduated in 2013 from a joint PhD programme between Chalmers University (Sweden) and Budapest University of Technology (Hungary). He started working at IPP-Garching as a postdoc within the Max-Planck/Princeton Center later that year. His topic under the EUROfusion fellowship (starting mid-2015) is the self-consistent modelling (including experimental validation) of runaway electron dynamics in tokamak disruptions.