  In the era of big science, countries allocate big research and development
budgets to large scientific facilities that boost collaboration and research
capability. A nuclear fusion device called the "tokamak" is a source of great
interest for many countries because it ideally generates sustainable energy
expected to solve the energy crisis in the future. Here, to explore the
scientific effects of tokamaks, we map a country's research capability in
nuclear fusion research with normalized revealed comparative advantage on five
topical clusters -- material, plasma, device, diagnostics, and simulation --
detected through a dynamic topic model. Our approach captures not only the
growth of China, India, and the Republic of Korea but also the decline of
Canada, Japan, Sweden, and the Netherlands. Time points of their rise and fall
are related to tokamak operation, highlighting the importance of large
facilities in big science. The gravity model points out that two countries
collaborate less in device, diagnostics, and plasma research if they have
comparative advantages in different topics. This relation is a unique feature
of nuclear fusion compared to other science fields. Our results can be used and
extended when building national policies for big science.
