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MIT, MA, USA

Man-Made Stars: Evaluating Structural
Integrity in High Performance Nuclear
Fusion Machines for Power Generation

MIT Plasma Science and Fusion Center researchers use numerical simulation to evaluate and
optimize the proposed design of the Advanced Divertor experiment — a compact nuclear fusion
machine that packs full-scale reactor power into an R&D testbed.

by JENNIFER SEGUI

Nuclear fusion occurs naturally in the    a toroidal or donut-shaped vacuum           FIGURE 1. Schematic of the proposed ADX
core of the sun, releasing enormous       vessel and maintained at high pressure      tokamak from MIT PSFC.
amounts of radiant energy as mass         to produce a dense plasma with high
is lost when hydrogen nuclei fuse         likelihood of collision. External magnetic  con gurations, where the divertor is
together to form larger helium atoms.                                                 a component that serves as the power
We observe this energy here on Earth        elds con ne and control the plasma        exhaust system for removing fusion
as sunlight, despite being on average     in a manner analogous to the intense        ashes from a tokamak. When ions
nearly 93 million miles away.             gravitational elds at the sun’s core,       escape con nement by the magnetic
                                          thus producing nuclear fusion.
   Demonstrating the feasibility of                                                     elds that control the plasma, the
hydrogen fusion as a clean, safe, and        “Recent advances in high                 divertor collects and guides them out of
practically limitless source of energy    temperature superconductors could           the vessel.
has been the primary objective of         allow us to design a tokamak operating
over 50 years of international research   at higher magnetic elds, increasing            The modular vessel must not only
efforts. At MIT the concept of a very     the performance of the plasma to            withstand the high heat uxes and
high magnetic eld approach to fusion      reactor levels,” explains Jeffrey Doody,    magnetic elds needed to produce
has been the primary focus of research.   a mechanical engineer at PSFC. “The         nuclear fusion, but also survive plasma
At the MIT Plasma Science and Fusion      research focus then switches from           disruptions, which are another source
Center (PSFC), experiment, leading-edge   improving the performance of the            of stress in the vacuum vessel shell
theory, and numerical simulation are      plasma to the support systems in            generated when the plasma collapses.
combined to identify and understand       the tokamak.”
the science and technology that can                                                      “To evaluate the proposed ADX
make fusion energy available sooner.         Using numerical simulation, Doody        vessel design, we perform numerical
                                          and his colleagues are designing the
   The Advanced Divertor eXperiment       ADX structure to sustain reactor-
(ADX) is a nuclear fusion experiment,     level heat uxes and magnetic elds,
and more speci cally a tokamak,           making it a suitable testbed for power
proposed by researchers at the PSFC       exhaust systems and plasma-material
to provide heat uxes, densities, and      interactions to support the development
temperatures similar to what we expect    of next-stage fusion machines.
to have in a fusion reactor, though with
only short plasma discharges              ð SURVIVING PLASMA
(see Figure 1).                           DISTRIBUTIONS

   In a tokamak, temperatures in excess   The proposed design for the ADX
of 150 million degrees Celsius cause      vacuum vessel is innovative in that it is
electrons to separate from nuclei,        comprised of ve separate axisymmetric
forming a fully-ionized superheated       shells, as shown in Figure 2, instead of
plasma from gaseous hydrogen fuel.        a single cylinder. The modular design
The core plasma is contained within       makes it possible to swap out magnetic
                                          coils and test different divertor

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