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Plasma-Wall Interaction in Linear Plasma Devices

New challenges

The interaction between the plasma and the wall materials in a fusion reactor is a key factor determining the lifetime of the wall components and thus the overall cost-effectiveness of the facility. Both the ITER experiment currently under construction and DEMO, the first demonstration reactor, will bring about particular challenges.

  • Far greater particle and heat flux densities on the first wall than in present-day facilities, with both steady-state and transient loads.
  • Much longer pulse lengths and ultimately steady-state operation, which means that the total amount of plasma particles (“fluence”) and heat on the wall components will be substantially higher than in current confinement experiments, leading to huge amounts of eroded wall material.
  • Exposure of the first wall to fast fusion neutrons produced in the deuterium–tritium fusion process, resulting in damage to materials due to lattice defects and the formation of helium in nuclear reactions.
  • Synergetic effects resulting from the interplay of neutron damage and particle and thermal loads.

Whereas sone important aspects of Plasma-Wall Interaction – such as energy extraction and the interplay between material erosion and transport into the plasma – take place on short time scales, several fundamental issues and challenges are linked to much longer time scales and high fluences. These include the redeposition of eroded wall material in layers, the change in the material’s surface morphology and microstructure (in the case of the latter, through the influence of the neutrons in particular), and material fatigue as a result of the large number of transient effects during extensive operating phases in a reactor.

While such conditions are not achieved in current tokamak experiments, stellarators such as Wendelstein 7-X in principle facilitate much longer pulse lengths (up to 30 minutes in future with a cooled divertor in Wendelstein 7-X). For this reason, stellarators are important experiments for issues surrounding plasma-wall interaction.

In order to study the long-term effects of plasma-wall interaction and characterize wall materials today, however, test facilities specializing in plasma-wall interaction are required. Linear plasma devices such as PSI-2 at Jülich offer such opportunities.

Yet the objectives of these studies extend far beyond applications in nuclear fusion: extreme loads and high operating temperatures are also characteristic of other branches of energy research, and hydrogen retention in materials is of major importance. In our investigations at Jülich, therefore, basic processes are characterized on a microscopic level in order to describe them using suitable models and thus apply them to other processes. In addition, we develop methods for analysing the processes in material loads (e.g. the resulting change in surface properties, or hydrogen inventory) which can either be applied during the loads (in situ diagnostics) or directly afterwards, without removing the materials from the system (in vacuo diagnostics).

PSI-2 at Jülich

In PSI-2, a stationary plasma is produced in a low-pressure, high-current arc discharge between a heated, cylindrical cathode consisting of lanthanum hexaboride and a hollow anode made of molybdenum. Directed by an axial magnetic field, the plasma flows from the source region through a pumping stage into the exposure area. This is where the material samples are exposed to the PSI-2 plasma.

Der Plasmagenerator PSI-2

The PSI-2 linear plasma device at Jülich.

Using a linear manipulator, the material samples are fed in through the target exchange and analysis chamber, which can be separated from the PSI-2 vessel by means of vacuum gate valves. The material samples are withdrawn into this chamber after being exposed to the plasma in order to investigate the influence of the plasma on surface composition and hydrogen inventory without removing the material from the vacuum.

Kopf des Targetmanipulators in der Targetwechselkammer von PSI-2

Target manipulator head in the target exchange chamber of PSI-2.

Laser-based methods such as laser-induced desorption and laser-induced plasma spectroscopy are the principal methods used for analysing material surfaces at PSI-2.

An important factor in the material tests is the ability to use laser radiation to apply transient heat pulses to the surface in addition to plasma exposure, in order to investigate the synergistic effects of particle and thermal loads. The PSI-2 laser allows the edge-localized modes occurring in fusion devices to be applied at the highest pulse rates.

Summary of PSI-2's key operating parameters:

Plasma flux density on material samplesMax. 1023m-2s-1
Plasma fluence per day of experimentMax. 5·1027m-2
Plasma diameter6·10-2 m
Power flux densityUp to 2 MWm-2 (with electrical biasing of the material samples)
Transient power flux densitiesMax. 3 GWm-2 (Laser energy 40 J/ pulse length 1 ms)
Energy of incident ionsUp to 200 eV (with electrical biasing)
Stationary magnet field0.1 T in the target exposure area

Current investigations at PSI-2 address aspects such as the physical processes involved in hydrogen retention in tungsten (notably the influence of surface morphology and of impurities such as argon and helium in the plasma), the damage thresholds of tungsten when exposed to plasma and simultaneous transient thermal loads, tungsten erosion in typical fusion conditions, and the characterization of low-activation steel (EUROFER) as a material for the first wall in a fusion reactor.

Wolframexposition in PSI-2

Tungsten exposure in PSI-2.

Furthermore, in linear plasma devices such as PSI-2, the properties of particularly cold divertor plasmas – as are envisaged in fusion devices like ITER – can also be investigated in order to optimize the lifetime of the wall components exposed to the highest thermal loads (the divertor baffles). These investigations are particularly important for refining numerical codes such as B2-Eirene. As well as plasma discharges in which deuterium is used as the injection gas, other gases can also be used, for example helium or argon. Plasmas of mixed composition can also be adjusted.

Blaues Argon-Plasma in PSI-2

The first argon plasma in PSI-2.

Kaltes Deuterium-Plasma in PSI-2

Cold deuterium plasma in PSI-2.

Influence of neutrons on the behaviour of wall materials

The fast neutrons produced in the deuterium–tritium fusion reaction damage the wall material by bringing about the formation of defects in the lattice thus permanently changing its microstructure and the transmutation of lattice atoms. As a result, the thermal, chemical, and physical properties of the wall materials are permanently changed, with fundamental impacts on the materials’ resistance to thermal loads and erosion. The defects and gaps in the lattice produced by the neutrons also increase the hydrogen inventory, which in fusion devices adjusts itself to the first wall on the basis of the permanent particle flux. Because tritium is used as a fuel in a fusion reactor, limiting the wall inventory is an important aspect of ensuring safe reactor operation.

Although damage to wall materials in fusion reactors can be simulated with high-energy ion beams, there are significant differences in the depth distribution of lattice damage and in the energy spectrum of the atoms knocked out of the lattice structure. For this reason, materials research for fusion energy also uses material samples already damaged in research reactors by neutrons from nuclear fission processes. These come close to the damage pattern produced by fast fusion neutrons, although transmutation processes do not occur to the same extent.

The lifetime of the wall components in a fusion reactor and the deposition of hydrogen are thus essentially determined by the coming together of plasma, thermal loads (both steady-state and transient), and damage to the material by neutrons. The simultaneous interplay of these three load elements will only become a reality in a fusion reactor.

In order to investigate these fundamental material issues today, however, preparations are under way at Forschungszentrum Jülich for an experiment on the exposure of wall materials damaged by neutrons in a linear plasma device with simultaneous exposure to transient thermal loads. This device, known as JULE-PSI, is under construction at Forschungszentrum Jülich’s Hot Materials Laboratory (HML) and will open up unique opportunities for characterizing wall materials for fusion.

The Hot Materials Laboratory and the JULE-PSI linear plasma device

Forschungszentrum Jülich's Hot Materials Laboratory (HML) provides an infrastructure for investigating low-level radioactive materials in a controlled area, and also has an additional section with three Hot Cells. Plans are currently in progress for an additional Hot Cell, in which a new linear plasma device (JULE-PSI) modelled on PSI-2 will be built. This device will also have a target exchange and analysis chamber in which the material samples can be investigated in vacuo with plasma and laser radiation following load tests.

The shielded area provided by the Hot Cells will make it possible to handle neutron-irradiated material samples from research reactors and, in future, from ITER as well. Moreover, work can also be carried out on toxic substances such as beryllium.

In particular, the concepts for handling the material samples and the diagnostics for analysing both the plasma and the properties of the material samples after exposure to loads were developed on the basis of the technologies tested at PSI-2.

In order to optimize the load profile of the plasma jet (to ensure greater homogeneity), the geometry of the plasma source, in particular the shape of the cathode, will be modified for JULE-PSI – instead of a hollow cylinder, a planar cathode will be used. A new design for the vacuum vessel will considerably broaden the options for observing the material samples during exposure in comparison to PSI-2. A laser for producing transient thermal loads (pulse energy 100 J, pulse length 1 ms) will also be used in JULE-PSI.

The construction and commissioning of JULE-PSI will initially take place outside the controlled area of HML, with construction work to begin in the second half of 2014. Following the necessary consolidation of the HML infrastructure to facilitate JULE-PSI operation (i.e. the construction of a new Hot Cell), JULE-PSI is scheduled to be moved to HML and put into operation there in late 2016.

Konstruktionszeichnung JULE-PSI

Design drawing JULE-PSI

Contact

Prof. Bernhard Unterberg

Tel. +49 2461 61-4803
Fax +49 2461 61-2660
b.unterberg@fz-juelich.de


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