Wall conditioning in fusion devices and its influence on plasma performance

In magnetically confined fusion plasmas a strong nonlinear connection exists between the processes occurring at the wall, the properties of the plasma edge, and those of the main plasma. The ion- and electron temperature profiles, the power flow in the plasma edge and its radiated fraction (radiative edge) the concentration and penetration depth of neutral hydrogen atoms and of plasma impurities into the scrape-off-Iayer (SOL) are examples for such strongly interlinked quantities. Via complex mechanisms they modify energy and particle transport, plasma confinement, and the performance of the discharge. High performance implies maximizing thefusion tripie product n₈T₈T₄ of deuteron and triton density n₈, ion temperature T₈ and energy confinement time T₄ in a thermal plasma. Hence it is important to achieve high densities, avoid the cooling of the plasma by excessive radiation of power, avoid dilution of the triton and deuterons by impurities, and to promote high energy confinement, consistent with the requirements of energy removal and helium pumping. The appropriate architecture of the plasma edge appears to be essential in order to obtain thermonuclear fusion plasmas with high performance. The edge plasma itself is strongly influenced by the plasma-wall interaction. These plasma-surface interaction processes have been reviewed e. g. by McCracken and Stott 1. Detailed descriptions of various mechanisms and diagnostic aspects can be found in ref. 2. Recent reviews of plasma edge phenomena have been published by Stangebyand McCracken 3, and by various authors in ref. 4. Atoms and ions from the plasma have a strong interaction with matter. Their range is limited to typically several 10 nm from the surface even in low-Ztarget materials (Be or C) with small stopping powers. It is this thin layer, the near-surface region, where plasma-surface interactions take place and where all the basic processes like sputtering, chemical erosion, trapping, reflection etc. occur. These processes may induce the release of wall material and thus introduce plasma impurities or have an effect on the hydrogenic particle balance (hydrogen recycling). It is obvious, that a modification of this near surface region e. g. by discharge cleaning or by depositing thin films of appropriate chemical composition and physical structure will bring about major changes in the plasma-surface-interaction processes and may thus allow an active control of phenomena critical to the plasma and its performance. Although only partially understood today, plausible interrelations of plasma-surface-interactions and different high confinement modes from the point of view of plasma theory were discussed in arecent review paper by. . .