Challenges facing high-field tokamaks

Abstract

David Kramer wrote an interesting report on high-magnetic-field fusion devices for the August 2018 issue of Physics Today (page 25). The high magnetic field certainly does shrink the device’s plasma volume, but high magnetic field is a double-edged sword. It has significant disadvantages. The story points out only one: the increased pressure on the field coils.Another disadvantage is that as one shrinks the device, the neutron wall loading increases. Take SPARC, the tokamak being developed by Commonwealth Fusion Systems (CFS). It has 1/70 the volume of ITER, the international prototype fusion energy reactor, but 10 times the power density. Whereas ITER hopes to achieve 500 MW of neutron power, SPARC hopes to achieve about 70 MW. If one assumes surface area scales as the 2/3 power of volume, SPARC’s surface area is about 1/17 of ITER’s. Hence SPARC, a small experimental device, will have about 2.5 times ITER’s wall loading of about 1 MW/m2! The problem will only get worse as CFS moves to devices like the ARC (affordable, robust, compact) reactor, which will produce commercially interesting amounts of power. Wall loading is a big issue, not a minor detail, in fusion physics.In addition, whereas the plasma scales to smaller size with increasing magnetic field, the fusion blanket does not. No matter what the magnetic field, the blanket has to prevent leakage of uncharged neutrons out the other end. The minimum blanket thickness I have seen is about one and a half meters thick. The blanket alone dictates some minimum size for a power-producing fusion device. It is difficult to see how shrinking the minor radius to below a meter buys you very much if the blanket thickness is one and a half meters. That could be a problem especially for the Tokamak Energy device, a spherical tokamak, which relies on a thin center post that must remain superconducting in the presence of an intense neutron flux.To me, the most important advantage of using high-temperature superconductors (HTS), whether at 5 T or 10 T, is a point Kramer mentions in passing at the end. Namely, the magnets could be disassembled and reassembled rather easily. Since I first heard of tokamaks a half century ago, the story has always been that because of the interlocking coil arrangement, one could not do maintenance on them. The new HTS magnets, in one fell swoop, may have solved that issue. To me, that is the really big deal. Section:ChooseTop of page <<CITING ARTICLES© 2019 American Institute of Physics.