Plasma Surface and Divertor Physics

Prof. Ulrich Stroth

Research Field

We represent at the TUM the wide field of plasma physics in research and
teaching. The focus of our work lies on magnetized high-temperature plasmas as
they exist in fusion experiments as well as in the universe. Of special interest
for our research are processes relevant for developing a future energy source
based on fusion reactions in magnetically confined plasmas. The experimental
work is mainly carried out on the tokamak experiment ASDEX Upgrade located at
the MPI for Plasma Physik on this campus but also on other international
research facilities such as JET and, in the future, Iter. Of particular interest
for our research are non-linear plasma processes, which govern plasma
instabilities, turbulence and turbulent transport, the numerical description of
transport processes, as well as the physics of plasma-wall interaction. For our
experimental investigations we use optical diagnostics and different types of
electrostatic probes. The experimental data we compare with results from leading
simulation codes. Investigations on large fusion devices are rounded off by
studies of model systems on smaller laboratory experiments.

Address/Contact

James-Franck-Str. 1
85748 Garching b. München
+49 89 289 12491
Fax: +49 89 289 14474

Members of the Research Group

Professors

Staff

Teaching

Course with Participations of Group Members

Offers for Theses in the Group

Assessment of neutral density evaluation codes for fusion plasmas

The analysis of experimental impurity densities in fusion plasmas via charge exchange recombination spectroscopy requires an accurate knowledge of the neutral density populations in the plasma. At the ASDEX Upgrade tokamak there are several codes available to calculate these neutral densities, each of which approaches the problem differently. Generally, the neutral densities produced by the codes agree quite well. However, in certain parameter ranges discrepancies have been observed. The objective of this Bachelor’s work is to explore and compare the parameter dependencies of the neutral densities calculated via the different codes. The observed differences are to be quantified and documented and the resultant effect on the calculated impurity densities evaluated.

Contact: Dr. Rachael McDermott

suitable as
  • Bachelor’s Thesis Physics
Supervisor: Ulrich Stroth
Characterization of limit-cycle oscillations with small amplitudes in ASDEX Upgrade plasmas

In the edge of magnetized fusion plasmas, self-generated transport barriers occur which lead to desired confinement improvements if a critical heating power is exceeded. Slightly below the critical heating power threshold, a plasma state is observed featuring limit-cycle oscillations with small amplitudes in magnetic signals. In order to identify the physical origin of this quasi-oscillatory state and the relation to improved confinement regimes, a characterization by means of different plasma diagnostics has to be performed. This work includes participation in new experiments at the ASDEX Upgrade Tokamak located at the Max Planck Institute for Plasma Physics and software development for data analysis in Python.

Contact: Dr. Gregor Birkenmeier

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Concentration dependent interdiffusion of iron and tungsten

In this thesis, the interdiffusion of iron and tungsten will be investigated. This work is part of the larger effort to assess the suitability of reduced-activation ferritic martensitic (RAFM) steel for use in recessed areas of the first wall of a future fusion power plant. The lifetime of this material is limited by the erosion due to impinging energetic particles from the plasma. Erosion rates are critically influenced by interdiffusion of the steel constituents. Knowledge of interdiffusion rates is therefore critical for predictions of the lifetime of RAFM steel as a first-wall material. Tungsten layers will be deposited on iron substrates by magnetron sputtering and heated in vacuo to activate interdiffusion. The resulting elemental depth profiles will be assessed by means of Rutherford backscattering spectrometry as well as energy-dispersive X-ray emission. Interdiffusion coefficients of iron and tungsten will be calculated from these depth profiles.

Contact: Dr. Martin Oberkofler

suitable as
  • Master’s Thesis Condensed Matter Physics
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Development and implementation of an average Zeff measurement for ASDEX Upgrade plasmas

In fusion plasmas the parameter “Zeff” is the average ion charge and a measure of the cleanliness of the plasma. Although it is a quantity of interest to almost all physics analyses, it is often not well known and, at the ASDEX Upgrade (AUG) tokamak, there is no routine measurement of Zeff available. There are two standard approaches to determining Zeff. First, the densities of dominant impurities in the plasma can be individually measured and then combined to calculate Zeff. And second, the background Bremstrahlung radiation of the plasma, in combination with the measured electron temperature and density profiles, can be used to gain information on Zeff. The objective of this Master’s work is to first identify measurements of the background plasma radiation that are dominated by Bremstrahlung emission and to use these to provide a routine measurement of the average Zeff value in AUG. These values are to be benchmarked against the Zeff values obtained by directly measuring the impurity densities in the plasma and are to be incorporated into standard analysis routines used at AUG.

Contact: Dr. Athina Kappatou

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Dust injection into the Tokamak ASDEX Upgrade to study impurity transport

The presence of dust (micro particles from the wall material) is a safety issue for future fusion reactors like ITER. Another aspect of dust in fusion devices is its ability to penetrate into the core plasma and affect the plasma performance by impurity radiation. This process may be important for droplets produced by arcing on high–Z materials as tungsten. To study the penetration probability of dust, a dedicated experiment at ASDEX Upgrade is planned. A dust injector, provided by Korean collaborators, will be installed at a manipulator system at the midplane of ASDEX Upgrade to shoot pre-characterised micro particles into the plasma. The trajectories of these will be studied using fast cameras and spectroscopy. The main tasks of the master thesis will be the commissioning and characterisation of the injector in the laboratory, assistance for the ASDEX Upgrade experiments and evaluation of the data, especially the fast camera movies, in cooperation with other research groups.

Contact: Dr. Volker Rohde

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Investigation of the detached divertor operation at the ASDEX Upgrade tokamak

The operation of future fusion reactors requires a significant reduction of the power flux from the hot plasma to the material components in the divertor. In a tokamak the magnetically confined plasma is separated from the region of plasma wall contact. With sufficient power dissipation, e.g. by atomic line radiation, the plasma in this region can be cooled down leading to a so-called detached condition, where the power flux to the wall is significantly reduced. In the thesis, the temporal behavior and stability of the detached state will be analyzed in order to understand its impact on the main plasma and plasma-wall interaction in future devices. Detachment will be characterized by analyzing experimental measurements and by comparison to simplified models. During the thesis one learns about divertor plasmas, various plasma diagnostics, and how to develop analysis tools for the interpretation of the experimental data.

Contact: Dr. Matthias Bernert

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Non-local turbulent plasma transport in fusion plasmas

The understanding of particle and energy transport in magnetically confined plasmas is one of the key topics in fusion research. In general both collisional and turbulent transport are described in terms of Fick’s law through a diffusion coefficient and a local temperature or density gradient. There exist observations, however, where transport at one location depends on the gradients at a different position. Such a non-local description would dramatically change our physical understanding of transport in fusion plasmas. In this thesis, turbulence measurements from different microwave diagnostics on the ASDEX Upgrade tokamak will be analyzed and changes in the fluctuation characteristics will be correlated with profile changes. The objective of the thesis is to search for non-local phenomena in the ASDEX Upgrade plasma.

Contact: Prof. Ulrich Stroth

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Numerical study of plasma transport in the edge of the ASDEX Upgrade tokamak plasma

In tokamak plasmas, the region between the last closed magnetic surface and the material walls, known as scrape-off layer, is of special interest for particle and power exhaust. In the scrape-off layer, the magnetic field lines end on the target plates of the divertor and understanding the partition of transport parallel and perpendicular to the magnetic field is of great importance for predicting the power loads on the material boundary of fusion plasmas. As the plasma density is increased, a regime transition is observed leading to increased perpendicular transport and a density shoulder in the SOL. In order to investigate this transition, a 3D transport code (EMC3) will be used to reproduce experimental data from the ASDEX Upgrade tokamak and a simple 1D model is to be developed to understand the elementary physical processes leading to this transition.

Contact: Dr. Daniel Carralero

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Phase velocities of turbulent fluctuations in the plasma edge

The turbulent phase velocity is the propagation velocity of turbulent fluctuations and is one of the main features to determine the underlying instability. Turbulence measurements by reflectometry on the ASDEX Upgrade tokamak yield rather small phase velocities compared to theoretical expectations. In this thesis a detailed survey of phase velocities is to be carried out based on turbulence simulations at different plasma parameters. To characterize the different turbulent regimes, different techniques to estimate the phase velocity are applied and results will be compared with experiment. The outcome of this thesis will help to experimentally identify different instabilities in the edge plasma of ASDEX Upgrade.

Contact: Dr. Peter Manz

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth
Visible spectral lines of highly charged tungsten ions in fusion plasmas

The plasma facing components of the ASDEX Upgrade tokamak are made of tungsten. Tungsten can end up in the plasma by sputtering processes. However, high tungsten concentration in the core of a fusion plasma should be avoided, as it radiates strongly, leading to significant energy losses. Therefore, understanding the behavior of tungsten is fundamental for the experiments in fusion devices with tungsten as a wall material. Visible spectroscopy offers the opportunity to study highly charged tungsten ions in a fusion plasma. Specifically, previously unknown tungsten lines were identified in the wavelength range of the HeII line at 468.6 nm. The work involves the interpretation and characterisation of these emission lines, providing a valuable tool for tungsten studies.

Contact: Dr. Athina Kappatou

suitable as
  • Bachelor’s Thesis Physics
Supervisor: Ulrich Stroth
X-point Effect on Plasma Filament Dynamics

Turbulence in the region between the confined plasma and the material walls, called scape-off layer, is dominated by filamentary structures. The propagation of these structures is rather well understood in simple geometry. In diverted magnetic geometries like in the experiment ASDEX Upgrade a significant fraction of the exhaust power to the material surfaces is channeled through a point of null poloidal magnetic field called the X-point. In this work, the influence of the X-point on the dynamics of filamentary structures is studied with numerical simulations. Special emphasis is given to rarely studied regions of the diverted plasma as the high field side or the private flux region. This work is possible due to the recent development of a flux-coordinate independent code called GRILLIX.

Contact: Dr. Peter Manz

suitable as
  • Master’s Thesis Nuclear, Particle, and Astrophysics
  • Master’s Thesis Applied and Engineering Physics
Supervisor: Ulrich Stroth

Current and Finished Theses in the Group

Entstehung von Plasmafilamenten am Plasmarand
Abschlussarbeit im Masterstudiengang Physik (Kern-, Teilchen- und Astrophysik)
Themensteller(in): Ulrich Stroth

Condensed Matter

When atoms interact things can get interesting. Fundamental research on the underlying properties of materials and nanostructures and exploration of the potential they provide for applications.