Impact of the aspect ratio on tokamak reactor design: Application to demo using the 0-dimensional ScaLa model

Abstract : The impact of the aspect ratio R/a (major over minor radius) on the design of tokamak reactors is addressed by means of the generic and comprehensive 0-dimensional ScaLa model for tokamak dimensioning. The European DEMO (fusion gain Q=40 and power P$_{fus}$=2.037GW) is taken as the target. Three scaling laws are considered for the energy confinement time, namely IPB98(y,2) used as the ITER reference, the electrostatic DS03 law and the recently upgraded ITPA20 one. First, it appears that the increase of the energy confinement time with A found in two of the scaling laws does not necessarily result in an advantage of working at large A, cross-dependencies leading to sometimes counter-intuitiveyet realtrends. Second, the opposite trends with respect to a scan in A which is observed regarding the product of the plasma volume times the square of the magnetic fieldused as a proxy for the costdoes not provide a clear answer as to whether small or large aspect ratio tokamaks should be favoured. This mild conclusion pushes for consolidating experimental databases with tokamaks exhibiting aspect ratios departing from the common 3 value.