Plasma surface interactions - Engineering

Scientific programme

The mission of the Plasma Surface Interactions – Engineering group is to explore the behaviour of materials under extreme heat and particle fluxes. The understanding and control of plasma-surface interaction effects is for example crucial for the design of materials able to withstand the harsh conditions expected in a fusion reactor. Additionally, the non-equilibrium conditions induced by the bombardment by extreme fluxes of low-energy particles also opens a novel route for the synthesis of advanced nanostructured materials. Those two lines of research form the basis of the scientific program of the PSI-E group. In particular the following points are actively studied:

 

  • Surface evolution under extreme ion irradiation
  • Diffusion/retention of hydrogen species under non-equilibrium irradiation conditions
  • Interplay between stationary and transient effects on metal surface damage
  • Power exhaust of liquid metals and prospects for future reactors
  • Development of novel plasma processing techniques

The research is carried out in collaboration with a large number of groups as illustrated in figure 1.



Powerful linear plasma generators, such as the in-house Pilot-PSI and Magnum-PSI devices operated at the FOM Institute DIFFER, provide a cost-effective approach to the fundamental understanding of plasma-surface interactions, with a good access to the plasma-material interaction zone for diagnostics and featuring flexible sample manipulation. Those devices are worldwide unique in their abilities to reproduce and even exceed the heat (>30MW.m-2) and particle fluxes (up to 1025m-2s-1) expected in the divertor of a fusion reactor. In addition, combined steady-state/pulsed operations are possible to study the effects of transient heat loads on a plasma-facing surface, similar to those expected during so called Edge-Localized Modes (ELMs). Heat loads in excess of 1GW.m-2 can be generated with a repetition rate of 10 Hz, and pulse duration of about 1 ms. In addition, a smaller scale experiment, Nano-PSI, provides a versatile platform to study ion irradiation-induced nano-structuring of surfaces.

Staff


Name

Position

E-mail

Gregory De Temmerman Group leader G [dot] C [dot] detemmerman [te] differ [dot] nl
P.A. Zeijlmans van Emmichoven Senior scientist P [dot] A [dot] Zeijlmans [te] differ [dot] nl
Thomas Morgan Postdoc T [dot] W [dot] morgan [te] differ [dot] nl
Sébastien Bardin Postdoc S [dot] Bardin [te] differ [dot] nl
Hennie van der Meiden Research engineer H [dot] J [dot] vanderMeiden [te] differ [dot] nl
Miranda van den Berg Senior research engineer m [dot] a [dot] vandenberg [te] differ [dot] nl
Kirill Bystrov PhD student k [dot] bystrov [te] differ [dot] nl
Irem Tanyeli PhD student i [dot] tanyeli [te] differ [dot] nl
Rianne 't Hoen PhD student M [dot] H [dot] J [dot] tHoen [te] differ [dot] nl
Long Cheng Guest L [dot] Cheng [te] differ [dot] nl
Stein van Eden Master student g [dot] g [dot] vaneden [te] differ [dot] nl
Dirk van den Bekerom Master student d [dot] c [dot] m [dot] vandenbekerom [te] differ [dot] nl
Stein van Eden Master student g [dot] g [dot] vaneden [te] differ [dot] nl
Wenjia Han Guest W [dot] Han [te] differ [dot] nl

Highlights

Development of a pulsed plasma source for ELM-simulation experiments.

Edge Localized Modes (ELMs) are a major concern for the lifetime of the divertor plasma-facing materials (PFMs) in ITER. The very high localized heat fluxes will lead to material erosion, melting and vaporization. A new experimental setup has been developed for ELM simulation experiments with relevant plasma conditions. For this purpose, the plasma source of Pilot-PSI has been modified to allow for transient heat and particle pulses superimposed on the steady-state plasma. The high flux plasma is generated by the cascaded arc plasma source which is powered by a current regulated power supply. In parallel, a capacitor bank (8400μF, 4.2 kJ) is connected to the plasma source and discharged in the plasma source to transiently increase the input power (fig. 1a). This results in a transient increase of the electron density and temperature. The plasma source was modified to accommodate the high heat fluxes generated during such pulses. Peak discharge currents of about 14 kA have been generated, corresponding to a peak input power in the plasma source of about 5.5 MW. The plasma source can be operated in a pulsed mode with a variety of gases (e.g. Ar, H, D, He, N) as well as with gas mixtures. Peak surface heat fluxes of up to 1 GW.m-2 have been generated with a pulse duration of about 0.5-1 ms (up to 1MJ.m-2)- as illustrated in fig. 1b. The shape and the duration of the pulse can be adapted to the needs. In addition, a pulsed bias system has been developed to vary the ion energy during the pulse.


Figure 1: (a) Schematic overview of Pilot-PSI with the pulsed source system, (b) evolution of the peak heat flux to the target surface as a function of the peak input power in the plasma sources for different gases.

Carbon erosion and re-deposition

The issue of gross and net erosion of carbon under ITER relevant plasma conditions remains an open question, especially the importance of local re-deposition and the structure of the deposits formed under such conditions. It has been observed that thick co-deposits are readily formed on the surface of graphite targets exposed to ITER-relevant hydrogen and mixed hydrogen/argon plasmas (ne ~ 1020 m-3, Te ~ 1 eV) in Pilot-PSI. These co-deposits consist of cauliflower-like dust particles and, surprisingly, accumulate in the region exposed to the peak particle and heat flux. Observations show that increasing the ion energy in hydrogen plasma prompted formation of large (D > 30 μm) dust particles, which are not detected on the surface of floating targets (Fig. 2). Addition of argon into the hydrogen plasma beam, on the other hand, shifted the particle size distribution towards smaller values (Dmax ≤ 10 μm). Co-deposits formed during CH4 injection experiments are similar to that observed on plasma-exposed graphite surfaces.

 



 
  
Figure 2: Scanning electron microscope images of the plasma exposed surfaces of negatively biased (a) and floating (b) carbon target. Corresponding size distributions of the formed co-deposits are shown on the right.

Formation of helium-induced nanostructure on high-Z metals

Helium bombardment of tungsten surfaces can lead to strong microstructural changes such as dislocation loops, helium holes and bubbles, and formation of a fibre-formed nanostructure (also referred to as fuzz). The occurrence of those effects is strongly dependent on the surface temperature. The formation of helium-induced nanostructures on molybdenum and tungsten surfaces has been studied in Pilot-PSI. Tungsten and molybdenum samples were exposed to high fluxes (around 1024m-2s-2) of pure helium plasmas in the temperature range 600-2400 C. Figure 3 illustrates the different morphologies observed by scanning electron microscopy after plasma exposure for tungsten (a and b-b is a cross-section image of a) and molybdenum (c and d). The structure size varies along the surface, the maximum size is observed in the middle of the plasma beam (highest temperature and flux) and is the smallest towards the edge (lower temperature and fluxes). The growth kinetics of the molybdenum nanostructure appears similar to that of the tungsten nanostructure.

 

Figure 3: Scanning electron microscope images of tungsten (a and b) and molybdenum surfaces (c and d) after exposure to a high flux helium plasma with a surface temperature higher than 1000C

Nano-structured metal oxides for solar water splitting

One of the main challenges in developing highly efficient nanostructured photo-electrodes is to achieve good control over the desired morphology and good electrical conductivity. An efficient plasma-processing technique has been developed to form porous structures in tungsten substrates. Low-energy helium ion irradiation is used to modify the surface. After an optimized two-step annealing procedure, the mesoporous tungsten transforms into photoactive monoclinic WO3. The excellent control over the feature size and good contact between the crystallites with the plasma technique offers an exciting new synthesis route for nanostructured materials for e.g. solar water splitting.


 

Publications

View the complete list of publications of our group via Scopus. Or you can download the citations.txt file in the bottom of this page.



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