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Mission possible

4 Dec 2009 - Sabina Griffith
Installation of the TBM mock-up enclosure into a DIII-D horizontal port. The enclosure was designed by the DIII-D engineer in the foreground, Amy Bozek.

EUROFER 97 steel is a candidate structural material for future fusion power reactors, as well as for the Test Blanket Modules (TBMs) to be tested in ITER. EUROFER 97 can withstand the high heat and neutron fluxes in a fusion reactor. But it is also ferromagnetic and will produce stray magnetic fields that will interact with the magnetic fields of the tokamak. It is expected to increase the localized ripple in the toroidal magnetic field which could enhance energetic particle losses and reduce thermal energy and particle confinement.

"Estimations of the effect varied from negligible to significant," says Joe Snipes, Senior Scientific Officer for Integrated Scenarios, "and the effects on confinement are very hard to calculate." This is why at last year's MHD workshop in Austin, Texas, Joe proposed an experiment that would seek to prove whether the fears were justified or not. It was the team at the DIII-D Tokamak situated near San Diego, California, who reached out to assist ITER in sorting out the issue.

One year later, the results are on the table and they are "pretty encouraging," as Joe expresses it. "We do see an impact from the TBM's magnetic fields, but it is not by far what we feared it would be. Across a wide range of plasma conditions, plasma rotation is reduced by 10 to 50 percent, but particle and energy confinement are unaffected under some conditions and only reach 15 to 20 percent reduction at the highest levels of localized ripple, which were three to five times what the TBMs in ITER will produce. Based on these initial results, we expect the TBMs will have little effect on ITER."

In order to mimic the situation on ITER, the DIII-D team did not simulate a full TBM support structure, but instead used a special electromagnet that generated the magnetic field that would have been induced in the ferromagnetic steel.

"DIII-D was the ideal test-bed to run this experiment," says Joe. "Thanks to Tony Taylor and his team and of course to all the scientists and engineers from around the world that assisted." And for the history books he adds: "This solid effort would certainly not have happened without President Obama's Stimulus Package."

The 20th Meeting of the Test Blanket Working Group in Aix-en-Provence.
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