On Heat Loading, Novel Divertors, and Fusion Reactors

It is shown that the limited thermal power handling capacity of the standard divertors (used in current as well as projected tokamaks) forces extremely high (∼ 95%) radiation fractions fRad in tokamak fusion reactors [1–3] with heating powers considerably larger than ITER-FEAT [4]. Independent of how the radiation may be apportioned between the scrape off layer (SOL) and the core, these enormous values of fRad have profound and deleterious consequences on the core confinement and stability to the extent that a high power hypothetical fusion reactor operating with the standard divertor (SD) is not likely to meet the daunting confinement requirements. Even operation in modes that have an internal transport barriers (ITBs) [5, 6] is not expected to lead to a dependable fusion power reactor with acceptable economics. The core confinement and stability problems caused by high fRad are shown to be adequately addressed by X-Divertors (XD) which, through a flaring of the field lines near the divertor plates, considerably enhance the divertor thermal capacity. The use of this new class of divertors will lower the bar on confinement sufficiently that confinement at the level of the routinely found H-mode [7] could be enough for a fusion reactor. A possible class of experiments that could lay the foundation for an efficient and attractive path to practical fusion power is suggested.