Bathing in silver
19 Sep 2016
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R.A.
Only the emptiest and most remote regions of outer space are colder than the ITER magnets. Extreme cold, cold that is just above absolute zero, is needed for achieving superconductivity, the physical state that allows electricity to flow through a conductor without encountering resistance.
A real-size mockup for a large section of vacuum vessel thermal shield is undergoing the step-by-step process of silver coating at SFA Engineering Corp in Changwon (Korea).
Technical and economic benefits of superconductivity are considerable: superconducting magnets do not heat up or consume electrical power¹; they carry higher current and, as a consequence, produce stronger magnetic field. Magnet superconductivity is a sine qua non condition for the development of commercial fusion.
The ITER toroidal field and poloidal field coils, all 7,000 tonnes of them, are cooled by circulating liquid helium at 4.5 K (minus -269 °C) inside of the cable-in-conduit conductors. In order to maintain this extremely cold temperature, the magnetic system must be completely insulated from all sources of heat—whether originating from inside the machine or from the outside environment.
There are three ways heat can be transferred from one body to another: conduction through contact; convection through a fluid (air); or radiation by way of electromagnetic waves. To prevent the transfer of heat from the environment by conduction and convection, the entire tokamak is enclosed in a giant vacuum chamber that acts like a "thermos"—the 30-metre high, 30-metre in diameter cryostat.
Situated within the cryostat vacuum, on supports that are insulated against thermal transfer, the tokamak's toroidal and poloidal field coils are immune to conduction and convection. But they can be exposed to heat radiating from any surface that happens to be warmer—and considering their intensely cold temperature, this means just about everything else.
The solution for protecting the coils from thermal radiation comes in the form of a 10- to 20-millimetre-thick barrier that is actively cooled with gaseous helium at 80 K (minus 193 °C). The thermal shield—850 tonnes of stainless steel—will closely encase the tokamak's magnetic system.
Welding underway on a thermal shield lower port section. The 850-ton thermal shield is made of 600 individual components that range from a few hundred kilos to approximately 10 tonnes.
The thermal shield is actually two components in one—a barrier that stands between the vacuum vessel and the magnets (called the vacuum vessel thermal shield) and another between the cryostat and the magnets (called the cryostat thermal shield). Both must be made "opaque" to thermal radiation; this is achieved by coating all thermal shield surfaces with a material that radiates as little heat is as possible.
One of most efficient "low-emissivity" materials happens to be ... silver.
"It will take some five tonnes of silver²," explains Nam Il Her, the ITER technical responsible officer for the thermal shield. "We will be depositing, by way of electroplating, a 5- to 10-micrometre-thick layer on each of the 600 parts that make up the thermal shield—a total surface of nearly 2,000 m²."
The fabrication of the thermal shield, part of Korea's contributions to the project, is underway now in Changwon at SFA Engineering Corp. The complex fabrication sequence for silver coating—requiring a succession of 11 different baths—is undergoing process qualification on real-size component mockups.
"The thermal shield is not necessarily the component that comes to mind when one thinks about ITER," says Germán Perez-Pichel, a former Monaco Fellow now working as a mechanical engineer in the ITER Vessel Section/Division. "From the outside, it just looks like panels and tubes. But dealing with such a large and heavy—yet thin—component is extremely challenging: tolerances are minimal, clearances with other systems are very tight and the silver coating has to be just perfect..."
The first thermal shield sector should be delivered to ITER in mid-2018 to be preassembled with a vacuum vessel sector and two corresponding toroidal field coils. This complex pre-assembly operation will require five to six months.
¹ Superconducting cables have negligible electrical resistance. The consequence is that no thermal losses occur when current is circulated inside the cables. When current is established in the magnets, very limited additional power supply is necessary to compensate the connection losses.
² Five tonnes of silver will be required in the electroplating baths. The mass of silver that will coat the thermal shield panels is estimated at just under 800 kg total.