10 January 2013
Hit a material with an intense enough plasma pulse, and it starts protecting itself by ejecting a protective blanket in front of it. That result from PhD candidate Jakub Zieliński might be excellent news for future fusion reactors such as ITER, where so-called ELM eruptions will expose the wall to gigawatts of power per square meter. At DIFFER, Jakub Zieliński created the first lab experiment capable of mimicking ITER's ELMs. Zieliński will defend his thesis, which describes the new pulsed plasma source, on Monday 14 January 2013 at Eindhoven University of Technology.
Interior of DIFFER's experiment for plasma surface interaction Pilot-PSI
Plasma eruptions in ITER
ITER is designed to show the technical feasibility of fusion as en energy source. It is the first reactor of its kind designed to produce more power from fusion than is needed to run the device. A critical issue for the success of fusion as a sustainable energy source is the lifetime of the reactor wall; the exhaust or divertor of a fusion reactor is subjected to a continuous bombardment of heat and fast particles. On top of that, intense eruptions called Edge Localized Modes or ELMs can increase the power density by a factor 100. In the future fusion reactor ITER, the heat and particle flow in load steady state and during ELMs will be orders of magnitude higher than in present-day fusion experiments. The mitigated ELMs are expected to deposit up to a gigawatt of power per square meter on the exhaust.
To properly study the plasma surface interaction under ITER conditions, DIFFER has developed the two experiments Pilot-PSI and its big brother Magnum-PSI. These are amongst very few laboratory experiments in the world that can create the steady state plasma near ITER's exhaust. During his PhD research, Zieliński developed a pulsed plasma source for Pilot-PSI, which can spike the power density in the plasma just as in real ELMs. The resulting setup is the first in the world that can mimic all plasma conditions near the wall of ITER in an accessible laboratory experiment.
Cartoon of Pilot-PSI: cover image for Jakub Zieliński's PhD thesis "A high power pulsed plasma system for material testing under simultaneous continuous and transient loads" - copyright: J.J. Zieliński
With the new pulsed plasma system, Zieliński and colleagues discovered a totally unexpected and beneficial behaviour of the reactor wall material tungsten exposed to ELMs. During a large power pulse of the plasma, the wall temperature can start to decrease while the pulse is still underway, thereby protecting the material from a part of the incoming energy. The team attribute this to self-shielding: one possibility is that hydrogen absorbed in the surface is released by the start of the ELM and forms a buffer before the wall material. Another option is that ions from the plasma scatter back from the surface and shield the rest of the pulse. More research is needed to find out how this effect works in detail and what implication it will have for the ITER wall materials.
Zieliński: "If the plasma surface interactions and materials issues are not dealt with properly, the life time of the strike zones, i.e. the strips of material that take most of the power, could be days rather than years."