6 May 2013
With a unique addition, FOM institute DIFFER's plasma experiment Magnum-PSI has qualified itself to perform material tests for the international fusion project ITER. At the reactor exhaust, the ITER wall will be exposed not only to a continuous bombardment of charged particles, but also to short, repeating miniature solar flares. Magnum-PSI is the first laboratory setup in the world capable of simulating these circumstances, by producing both the constant dense plasma flow and the ten to hundredfold more intense transient events. The two processes might cause the wall material to grow brittle through recrystallization. Together with two German institutes, DIFFER is now investigating how ITER's planned wall material tungsten will hold up under the combination of the two exposure types.
No other part of ITER will be stressed as severely as the reactor's exhaust, or divertor. A fusion reactor produces energy by fusing hydrogen isotopes to form helium in a hot, charged gas or plasma. Strong magnets isolate the plasma from the reactor wall, except at the exhaust where the helium leaves the reactor and the plasma comes into direct contact with the wall material. The intention is to use the metal tungsten to cover the exhaust because of its excellent heat-resistant properties.
Artificial solar flares
Apart from the continuous bombardment of charged particles, the ITER exhaust also faces short, sudden energy pulses from the edge of the plasma. These so-called Edge Localised Modes or ELMs are the fusion reactor equivalent of solar flares. "In Magnum-PSI we could already create the steady state plasma conditions expected at the ITER divertor", explains Magnum-PSI's team leader dr. Greg De Temmerman. "At the end of 2012 we augmented the machine with a capacitor bank which now allows us to boost the power of the plasma source for a fraction of a second, many times per second in a row."
DIFFER's new experiment Magnum-PSI can create the intense plasma conditions expected at the exhaust of the future fusion reactor ITER. Credit: DIFFER / Bram Lamers
The same plasma pulses occurs at the wall of a fusion reactor: ELMs follow on each other's heels in a train of energy discharges. Using its pulsating source, Magnum-PSI can simulate ELMs twice the size of those in existing fusion reactors. This makes the experiment uniquely suited to test materials for future fusion reactors, thinks De Temmerman: "We are now testing realistic tungsten wall elements in Magnum-PSI, to check whether the material will grow brittle under the combined load of steady state plasma exposure and ELM discharges in ITER." In the experiments so far, Magnum-PSI has exposed tungsten elements to 17.400 sequential ELM discharges with a peak power of 10 MW per square meter. Each artificial solar flare causes the surface temperature of the tungsten to rise and by about 300 degrees Celsius in a layer of a few nanometers thick.
|Surface temperature of a tungsten element during exposure to a series of plasma pulses on top of the steady state plasma in Magnum-PSI. These artificial solar flares (ELMs) will also develop in the fusion reactor ITER. Each ELM lasts up to a millisecond and reaches a peak power density of 10 MW/m2. During an ELM, the surface temperature rises by approximately 300 degrees Celsius.|
Dutch-German multi machine cooperation
The tests of the ITER wall material are performed in close collaboration with two fellow research institutes in Germany. The Forschungszentrum Jülich exposes tungsten elements to ELM-like high heat fluxes with its electron cannon JUDITH, while the Max-Planck-Institut für Plasmaphysik uses its experiment GLADIS to determine the defects of exposure to the steady state load only. De Temmerman: "This way we can study all the individual effects on the tungsten in isolation. In Magnum-PSI, we're applying the two types of exposure simultaneously to investigate synergistic effects. Data-collection is ongoing, but the results so far are spectacular. We're 17.400 artificial ELMs in, and we're reaching unprecedented areas in terms of particle loading."
The ITER Organization will take the final decision on the material for the reactor exhaust this summer.
Artist's concept of the ITER fusion reactor under construction in Cadarache in southern France by a collaboration of 34 countries. Credit: ITER Organization
In the international fusion project ITER, science and industry are joining forces to show the technical feasibility of fusion as an energy source. ITER was designed to produce ten times more power from fusion than the reactor consumes itself: 500 MW versus 50 MW. Seven partners (the countries in the E.U., the U.S., Russia, Japan, China, India and South-Korea) are constructing ITER in Cadarache in southern France. The total budget for the ITER construction is 15 billion euros and the first plasma is planned for 2020.