Interchange Driven Turbulence and Bellan Instability in Tokamak Scrape-off Layer

We present description of an electrostatic turbulent model ESEL and explain the interchange mechanism leading to turbulence generation in tokamak scrape-off layer. After introducing theory of Bellan instability, we discuss threshold for particle detrapping from turbulent structures (blobs) due to their finite Larmor orbits in case of turbulence simulated by the ESEL model. Since plasma potential as well as floating potential are available from the simulation, it is possible to evaluate error made by using experimentally obtained floating potential in simulations of particle diffusion. We also show that due to bipolar nature of blobs the outflow of particles from the blob caused by Bellan instability can be, in certain conditions, asymmetric and localized mainly into its upper and inner boundary. Introduction Nowadays, tokamak is the most promising device for using thermonuclear fusion as a profitable source of energy. However, there is still a lot of problems that limit its operability, one of them being an incomplete understanding of particle and energy transport in tokamak boundary. Cross-field fluxes that are originating in the region of confined plasma carry outwards significant amounts of energy, thus featuring very high risk of damaging tokamak first wall, divertor target and other plasma facing components. In the same time, impurities released by these components spread through the boundary region into main plasma, cooling it down by their radiation and decreasing the fusion rate due to dilution of the fuel. It is generally agreed that anomalous radial transport of plasma particles, that is much larger than expected by taking into account collisional diffusion only, is closely connected with existence of turbulent structures called blobs. These blobs were, in form of long-living correlated structures, experimentally detected in scrape-off-layer (SOL) of various fusion devices [see e.g. Martines et al., 2002; Zweben et al., 2004; Terry et al., 2005]. As will be described later, these blobs and the turbulence itself originate at the vicinity of last closed flux surface (LCFS). Simultaneously with blobs that are transporting hot and dense plasma outwards, there may emerge also so called holes, i.e. low-density and low-temperature structures moving inwards into the region of confined plasma. Appearance of turbulent structures significantly alternates transport of individual particles in SOL, either by convection of trapped particles [see e.g. Naulin et al., 1999], particle heating by potential fluctuations or by enhancing their diffusive, or even superdiffusive behaviour [Krĺın et al., 1999]. As shown in Pánek et al. [2005], the resulting effect of these mechanisms can strongly differ between different particle species. Consequently, transport of plasma particles and plasma impurities can have a completely different character. Particles with higher ratio of mass and charge are less likely trapped and convected, but their movement can more easily transit into superdiffusive state. This effect, first explored in Bellan [1993] and called Bellan instability, is in the main focus of interest of our work. In this paper we present first results of estimations of rate of Bellan instability in turbulent potential numerically simulated by a 2D fluid turbulent code ESEL [Garcia et al., 2005a]. Since understanding of evolution and origin of turbulent structures in SOL is crucial for understanding of Bellan instability, a short review of interchange instability and the ESEL model is given in first two sections. Then, an estimation of stochasticity threshold in ESEL simulations is described. In the last section we present a new mechanism that could be responsible for asymmetric particle fluxes in the turbulent region.