On the fusion triple product and fusion power gain of tokamak pilot plants and reactors

[摘要] The energy confinement time of tokamak plasmas scales positively with plasma size and so it is generally expected that the fusion triple product,nTτ _E, will also increase with size, and this has been part of the motivation for building devices of increasing size including ITER. Heren ,T , andτ _E are the ion density, ion temperature and energy confinement time respectively. However, tokamak plasmas are subject to operational limits and two important limits are a density limit and a beta limit. We show that when these limits are taken into account,nTτ _E becomes almost independent of size; rather it depends mainly on the fusion power,P _fus. In consequence, the fusion power gain,Q _fus, a parameter closely linked tonTτ _E is also independent of size. Hence,P _fus andQ _fus, two parameters of critical importance in reactor design, are actually tightly coupled. Further, we find thatnTτ _E is inversely dependent on the normalised beta,β _N; an unexpected result that tends to favour lower power reactors. Our findings imply that the minimum power to achieve fusion reactor conditions is driven mainly by physics considerations, especially energy confinement, while the minimum device size is driven by technology and engineering considerations. Through dedicated R&D and parallel developments in other fields, the technology and engineering aspects are evolving in a direction to make smaller devices feasible.

[关键词] tokamaks;pilot plants;fusion reactors;steady state operation