Plasma-wall self-organization in magnetic fusion. (16th December 2021)

• Record Type: Journal Article
• Authors: Escande, D.F.; Sattin, F.; Zanca, P.
• Is Part Of: Nuclear fusion. Volume 62:Number 2(2022)
• Publication Date: 2021-12-16
• Subjects: plasma-wall interaction -- self-organization -- sputtering -- density limit -- radiation; Nuclear fusion -- Periodicals; 621.48405
• DOI: 10.1088/1741-4326/ac3c87
• Languages: English
• ISSNs: 0029-5515

Abstract

Abstract: This paper introduces the concept of plasma-wall self-organization (PWSO) in magnetic fusion. The basic idea is the existence of a time delay in the feedback loop relating radiation and impurity production on divertor plates. Both a zero and a one-dimensional description of PWSO are provided. They lead to an iterative equation whose equilibrium fixed point is unstable above some threshold. This threshold corresponds to a radiative density limit, which can be reached for a ratio of total radiated power to total input power as low as 1/2. When detachment develops and physical sputtering dominates, this limit is progressively pushed to very high values if the radiation of non-plate impurities stays low. Therefore, PWSO comes with two basins for this organization: the usual one with a density limit, and a new one with density freedom, in particular for machines using high-Z materials. Two basins of attraction of PWSO are shown to exist for the tokamak during start-up, with a high density one leading to this freedom. This basin might be reached by a proper tailoring of ECRH assisted ohmic start-up in present middle-size tokamaks, mimicking present stellarator start-up. In view of the impressive tokamak DEMO wall load challenge, it is worth considering and checking this possibility, which comes with that of more margins for ITER and of smaller reactors.