Speaker
Description
Boronization, a process involving coating of the plasma facing components (PFCs) with boron (B) either by glow discharge (GDB) or solid boron injection (SBI), is an established way of improving operation of tokamaks and stellarators due to medium and high-Z impurity level reduction. However, a quantitative prediction of the amount of B required on next-step fusion devices such as ITER or a Fusion Power Plant (FPP), while critical, remains elusive. We put forward a 0-D heuristic model to interpret and predict the evolution of wall conditions in response to boronization. The model assumes that B introduced in the vessel passivates with plasma operation, expressed as cumulative energy injected in the machine, as a proxy for particle and energy fluence to the machine PFCs, and that only non-passivated, “active”, B is responsible for wall improvements. The evolution of active B amount is governed by a 0-D continuity equation with an effective passivation rate ε [MJ], representing the effect of passivation mechanisms, such as erosion of deposited layers, migration of B to shaded areas, saturation of retention capacity, oxidation or implantation of carbon. The wall improvements are expressed in terms of indicators such as brightness of specific impurity lines, radiative losses, neutral pressure or wall fueling, whose changes are assumed to depend on the active B through a response function, defined by the parameter μ [mg], the mass of active B required to saturate an indicator’s response to an increase of active B. The model is applied to different experimental DIII-D datasets where wall conditioning indicators are tracked throughout a series of plasmas, including plasmas following GDB or with phases of SBI. With an optimized choice of the parameters ε and μ, the model reproduces the experimental evolution of wall conditioning indicators reasonably well. Notably, different ε and μ are needed to describe indicators associated with different physics mechanisms. This allows us to account for significant differences in the experimentally observed evolution of indicators of wall fueling (plasma density, main chamber pressure, characterized by a prompt, relatively short-lived response to B injection) compared to indicators of impurity retention (oxygen or nitrogen brightness at breakdown, characterized by a saturated, lasting response). The values of ε and μ determined for various indicators and datasets can be used to inform and quantify the applicability of boronization to other conditions and devices.
Work supported by the U.S. DOE under DE-AC02-09CH11466 and DE-FC02-04ER54698.