PLASMA-WALL SELF-ORGANIZATION IN MAGNETIC FUSION

The paper introduces the theory of plasma-wall self-organization (PWSO) in magnetic fusion and its first experimental check. The basic idea is the existence of a time delay in the feedback loop relating radiation and impurity production on divertor or limiter targets. Both a zero and a one-dimensional description of PWSO at flat-top 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-target 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. Two basins of attraction of PWSO are shown to be possible for the tokamak during start-up, with the 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 using high-Z materials, by 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 fulfilling more easily Lawson criterion, and consequently more margins for ITER and smaller reactors. Recent experimental data of the J-TEXT tokamak with Ohmic or ECRH-assisted ohmic start-up confirm that cleaner plasmas provide higher density limits. These limits agree with those predicted from the 0D and 1D PWSO models