Reactor similarity for plasma-material interactions in scaled-down tokamaks as the basis for the Vulcan conceptual design

摘要

Dimensionless parameter scaling techniques are a powerful tool in the study of complex physical systems, especially in tokamak fusion experiments where the cost of full-size devices is high. It is proposed that dimensionless similarity be used to study in a small-scale device the coupled issues of the scrape-off layer (SOL) plasma, plasma-material interactions (PMI), and the plasma-facing material (PFM) response expected in a tokamak fusion reactor. Complete similarity is not possible in a reduced-size device. In addition, "hard" technological limits on the achievable magnetic field and peak heat flux, as well as the necessity to produce non-inductive scenarios, must be taken into account. A practical approach is advocated, in which the most important dimensionless parameters are matched to a reactor in the reduced-size device, while relaxing those parameters which are far from a threshold in behavior. "Hard" technological limits are avoided, so that the reduced-size device is technologically feasible. A criticism on these grounds is offered of the "P/R" model, in which the ratio of power crossing the last closed flux surface (LCFS), P, to the device major radius, R, is held constant. A new set of scaling rules, referred to as the "P/S" scaling (where S is the LCFS area) or the "PMI" scaling, is proposed: (ⅰ) non-inductive, steady-state operation; (ⅱ) P is scaled with R~2 so that LCFS areal power flux PjS is constant; (ⅲ) magnetic field B constant; (ⅳ) geometry (elongation, safety factor q_*, etc.) constant; (ⅴ) volume-averaged core density scaled as n ≈n~R~(-2/7); and (ⅵ) ambient wall material temperature T_(w,0) constant. It is shown that these scaling rules provide fidelity to reactor conditions in the divertor of the reduced-size device, allowing for reliable extrapolation of the behavior of the coupled SOL/PM1/PFM system from the reduced-size device to a reactor. The P/S scaling is used as the basis for the Vulcan tokamak conceptual design.