Plasma surface interactions - Experimental

Scientific programme

The mission of the PSI-E group is to perform basic plasma-surface interaction research needed for the design validation and performance predictions of the plasma-facing components of future fusion devices.
The central experiment will be the high ion flux (>10²⁴ m^-2s^-1), high power (10 MW/m²) linear steady-state plasma generator Magnum-PSI, operating in a magnetic field of 3 T (superconducting magnet). This worldwide unique experiment is designed to reach the parameter range of plasma in front of the high-flux plasma facing components of the next step fusion reactor ITER. This will allow the fundamental study of very pressing problems associated with the erosion and migration of materials, fuel retention, dust production and the effects of transient high heat loads.
A pilot experiment, Pilot-PSI, has been set up in which the hydrogen plasma source is developed to the level that it can produce the fluxes given above. A cascaded arc is used as the plasma source. The plasma beam in the Pilot-PSI device can be confined by a magnetic field of up to 1.6 T for short pulses.

Personnel 

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 steady-state plasma conditions and transient heat/particle source. 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 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 (D_max ≤ 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 fibreform nanostructure (also referred to as fuzz). The occurrence of those effects is strongly dependent on the surface temperature. The formation of helium-induced nanostructure 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

Recent publications:

M.A. van den Berg, S. Brons, O.G. Kruijt, J. Scholten, R. Pasquet, P.H.M. Smeets, B. Schweer, G. De Temmerman, The target for the new plasma/wall experiment Magnum-PSI, Fusion Eng. Des. 86 (9-11), 1745-1748 (2011)

J. Rapp, G. De Temmerman, G.J. van Rooij, P.A. Zeijlmans van Emmichoven, A.W. Kleyn, Plasma-facing materials for fusion reactors at FOM-Rijnhuizen, Rom. J. Phys. 56 (S), 30-35 (2011)

K. Bystrov, J. Westerhout, M.Matveeva, A. Litnovsky, L. Marot, E. Zoethout, G. De Temmerman, Erosion yields of carbon under various plasma conditions in Pilot-PSI, J. Nucl. Mater. 415 (1S), S149-S152 (2011)

J.J. Zielinski, R. Al, H. van der Meiden, W. Melissen, J. Rapp, G. De Temmerman, Production and characterization of transient heat and particle pulses in Pilot-PSI, J. Nucl. Mater. 415 (1S), S70-S73 (2011)

G. De Temmerman, J.J. Zielinski, S. van Diepen, L. Marot, M. Price, ELM simulation experiments on Pilot-PSI using simultaneous high flux plasma and transient heat/particle source, Nucl. Fusion 51 (7), 073008 (2011)

G. De Temmerman, J. Dodson, J. Linke, S. Lisgo, G. Pintsuk, S. Porro, G. Scarsbrook, Thermal shock resistance of thick boron-doped diamond under extreme heat loads, Nucl. Fusion 51 (5), 052001 (2011)

O.Lischtschenko, K. Bystrov, G. De Temmerman, J. Howard, R.J.E. Jaspers, R.König, Density measurements using coherence imaging spectroscopy based on Stark broadening, Rev. Sci. Instrum. 81 (10), 10E521 (2010)

S. Porro, G. De Temmerman, D.A. MacLaren, S. Lisgo, D.L. Rudakov, J. Westerhout, M. Wiora, P. John, I. Villalpando, J.I.B. Wilson, Surface analysis of CVD diamond exposed to fusion plasma, Diamond Relat. Mater. 19, 818-823 (2010)

G. De Temmerman, J.J. Zielinski, H.J. van der Meiden, W. Melissen, and J. Rapp, Production of high transient heat and particle fluxes in a linear plasma device, Appl. Phys. Lett. 97, 081502 (2010)

International collaborations

• Trilateral Euregio Cluster (TEC) with partners Ecole Royale Militaire, Brussels, Belgium and Institut fuer Energieforschung-4, Forschungszentrum Juelich, Juelich, Germany

• University of Basel, Switzerland

• Heriot-Watt University, Scotland

• CCFE Culham, UK

• CIEMAT, Spain

• University of California at San Diego, USA

• CNRS - Universite de Provence, Marseille, France

• Princeton Plasma Physics Laboratory, USA