PPPL QUASAR Stellerator: A concept on the path to fusion energy

QUASAR stellerator design. Credit: PPPL

Completion of a promising experimental facility at the U.S. Department of Energy's Princeton Plasma Laboratory (PPPL) could advance the development of fusion as a clean and abundant source of energy for generating electricity, according to a PPPL paper published this month in the journal IEEE Transactions on Plasma Science.

The facility, called the Quasi-Axisymmetric Stellarator Research (QUASAR) experiment, represents the first of a new class of fusion reactors based on the innovative theory of quasi-axisymmetry, which makes it possible to design a magnetic bottle that combines the advantages of the stellarator with the more widely used tokamak design.

Experiments in QUASAR would test this theory.

Construction of QUASAR, originally known as the National Compact Stellarator Experiment (NCSE), was begun in 2004 and halted in 2008 when costs exceeded projections after some 80 percent of the machine's major components had been built or procured.

George "Hutch" Neilson

"This type of facility must have a place on the roadmap to fusion," said physicist George "Hutch" Neilson, the head of the Advanced Projects Department at PPPL.

Both stellarators and tokamaks use magnetic fields to control the hot, charged plasma gas that fuels fusion reactions.

While tokamaks put electric current into the plasma to complete the magnetic confinement and hold the gas together, stellarators don't require such a current to keep the plasma bottled up.

Stellarators rely instead on twisting, or 3D, magnetic fields to contain the plasma in a controlled "steady state."

Stellarator plasmas thus run little risk of disrupting or falling apart as can happen in tokamaks if the internal current abruptly shuts off.

ITER: the world's largest Tokamak

Developing systems to suppress or mitigate such disruptions is a challenge that builders of tokamaks like ITER, the international fusion experiment under construction in France, must face.

Stellarators had been the main line of fusion development in the 1950s and early 1960s before taking a back seat to tokamaks, whose symmetrical, doughnut-shaped magnetic field geometry produced good plasma confinement and proved easier to create.

But breakthroughs in computing and physics understanding have revitalized interest in the twisty, cruller-shaped stellarator design and made it the subject of major experiments in Japan and Germany.

PPPL developed the QUASAR facility with both stellarators and tokamaks in mind. Tokamaks produce magnetic fields and a plasma shape that are the same all the way around the axis of the machine—a feature known as "axisymmetry." QUASAR is symmetrical too, but in a different way.

While QUASAR was designed to produce a twisting and curving magnetic field, the strength of that field varies gently as in a tokamak, hence the name "quasi-symmetry" (QS) for the design.

This property of the field strength was to produce plasma confinement properties identical to those of tokamaks.

"If the predicted near-equivalence in the confinement physics can be validated experimentally," Neilson said, "then the development of the QS line may be able to continue as essentially a '3D tokamak.'"

More information: Neilson, G.H.; Gates, D.A.; Heitzenroeder, P.J.; Breslau, J.; Prager, S.C.; Stevenson, T.; Titus, P.; Williams, M.D.; Zarnstorff, M.C., "Next Steps in Quasi-Axisymmetric Stellarator Research," Plasma Science, IEEE Transactions on , vol.42, no.3, pp.489,494, March 2014. DOI: 10.1109/TPS.2014.2298870

Road to Fusion: ITER blanket technology approved

The ITER machine with its 440 blanket modules.

The design of the ITER blanket system, a crucial technology on the way to fusion power, has been approved and is now ready to proceed to the manufacturing stage.

"The development and validation of the final design of the ITER blanket and first wall technology is a major achievement on our way to deuterium-tritium operation—the main goal of the ITER project," says Rene Raffray, in charge of the blanket for the ITER Organization.

"We are looking at a first-of-a-kind fusion blanket which will operate in a first-of-a-kind fusion experimental reactor."

The ITER blanket system provides the physical boundary for the plasma and contributes to the thermal and nuclear shielding of the vacuum vessel and the external machine components such as the superconducting magnets operating in the range of 4 Kelvin (-269°C).

Directly facing the ultra-hot plasma and having to cope with large electromagnetic forces, while interacting with major systems and other components, the blanket is arguably the most critical and technically challenging component in ITER.

The blanket consists of 440 individual modules covering a surface of 600 m2, with more than 180 design variants depending on the segments' position inside the vacuum vessel and their functionality.

Each module consists of a shield block and first wall, together measuring 1 x 1.5 metres and weighing up to 4.5 tons—dimensions that not only demand sophisticated remote handling in view of maintenance requirements during deuterium-tritium operation, but also an approach to attaching the modules which is far from trivial when considering the enormous electromagnetic forces.

The first wall is made out of shaped "fingers." These fingers are individually attached to a poloidal beam, the structural backbone of each first wall panel through which the cooling water will be distributed.

Each module consists of a shield block and first wall, together measuring 1 x 1.5 metres and weighing up to 4.5 tons.

Depending on their position inside the vacuum vessel, these panels are subject to different heat fluxes.

Two different kinds of panels have been developed: a normal heat flux panel designed for heat fluxes of up to 2 MW/m2 and an enhanced heat flux panel designed for heat fluxes of up to 4.7 MW/m2.

The enhanced heat flux panels are located in areas of the vacuum vessel with greater plasma-wall interaction and they make use of the hyper-vapotron technology, which is similar to that used for the divertor dome elements.

All panels are designed for up to 15,000 full power cycles and are planned to be replaced at least once during ITER's lifetime. A sophisticated R&D program is currently under way in Japan for the development of remote handling tools to dismantle and precisely re-position the panels.

The ITER blanket system provides the physical boundary for the plasma.

Due to the high heat deposition expected during plasma operation—the blanket is designed to take a maximum thermal load of 736 MW—ITER will be the first fusion device with an actively cooled blanket.

The cooling water is fed to and from the shield blocks through manifolds and branch pipes.

Furthermore, the modules have to provide passage for the multiple plasma diagnostic technologies, for the viewing systems, and for the plasma heating systems.

Because of its low plasma-contamination properties, beryllium has been chosen as the element to cover the first wall.

Other materials used for the blanket system are CuCrZr (Copper, Chromium, Zirconium) for the heat sink, ITER-grade steel 316L(N)-IG for the steel structure, Inconel 818 for the bolts and cartridges, an aluminium-bronze alloy for the pads that will buffer the electro-mechanical loads acting on the segments, and alumina for the insulating layer.

The procurement of the 440 shield blocks is equally shared between China and Korea.The first wall panels will be manufactured by Europe (50%), Russia (40%) and China (10%). Russia will, in addition, provide the flexible supports, the key pads and the electrical straps.

The preparation of the Procurement Arrangements will now be launched leading to the fabrication hand-over starting at the end of this year.

The assembly of the ITER blanket system is scheduled for the second assembly phase of the ITER machine starting in May 2021 and lasting until August 2022.