Journal of Nuclear Materials 196-198 (1992) 476-480 North-Holland jnurnalof nuclear materials Transport analysis of divertor plasma in JT-60U Katsuhiro Shimizu, Kiyoshi Itami, Hirotaka Kubo, Nobuyuki Asakura and Michiya Shimada Department of Fusion Plasma Research, Naka Fusion Research Establishment, Japan Atomic Energy Research Institute, Mukoyama, Naka-Machi, Naka-Gun, Ibaraki-Ken 311-01, Japan A simple divertor model has been developed in order to analyze the divertor transport based on experimental data. The fluid equations are solved numerically in one dimension along the magnetic field lines. In contrast to the conventional divertor code, the boundary conditions are given by experimentally determined plasma parameters at the divertor plate, which are measured by Langmuir probes. The neutral particle transport is simulated in two dimensions by a Monte Carlo code. The interaction between the plasma and the neutral particles is solved self-consistently using an iterative procedure; the plasma parameters converge in a few iterations. The little computational time needed facilitates a systematic analysis of the divertor transport. By applying this model to the JT-60U divertor plasmas, the particle confinement time in the main plasma and the heat diffusivityin the scrape-off layer are evaluated. I. Introduction Understanding of the divertor transport is very im- portant to predict the behavior of particles and impuri- ties in future tokamaks. A number of divertor codes has been developed to analyze divertor characteristics. In most elaborate divertor codes [1-5], the fluid equa- tions and neutral particle transport are treated in two dimensions. These codes require much labour for de- velopment and take much computational time. Their applications have been restricted to few cases because of the complexity of handling these codes. A simple divertor model has been developed to systematically investigate the divertor transport based on experimental data. The flow along the magnetic fields dominates the diffusion across the magnetic fields so that the transport behavior of the divertor plasma can be treated in one dimension. The fluid equations along the magnetic field are solved with the boundary conditions of the plasma parameter at the divertor plate (nd, Td), which are measured by Langmuir probe array. On the other hand, the behavior of neutral particles must be treated in two dimensions. The neu- tral particle transport is simulated in the MHD equilib- rium with a real wall geometry by a Monte Carlo code [6]. The interaction between the plasma and neutral particles is solved iteratively; the plasma parameters converge in a few iterations. The little computational time needed facilitates a systematic analysis of the divertor transport. This model also provides a method to evaluate the particle confinement time in the main plasma, %, and the heat diffusivity in the scrape-off layer, X • These transport properties are very important to predict the possibility of a cold and dense divertor plasma in future tokamaks. These parameters, however, such as the anomalous cross field diffusion coefficients, parti- cle and heat fluxes flowing into the divertor region, must be assumed in the conventional predictive trans- port code. This simple analysis model enables us to evaluate these transport parameters based on experi- mental data. By applying the model, the divertor char- acteristics in JT-60 upgrade have been investigated. 2. Simple divertor model We consider the plasma parameters in the divertor region including a part of the scrape-off layer which is upstream of the separatrix point. The steady state transport equations which describe the divertor plasma are simplified by using the following assumptions: - the ion temperature is equal to the electron temper- ature, - the effect of the cross field transport is negligible. The simplified transport equations along the field lines are given by V~. (n~) = S~, (1) VII" (nmivii 2) + VII(2nT) = Sp, (2) 1 2 __ Vil'qll = VII"{(5T + ]mivii )nvi I KIIVIIT } = S E, (3) where Sn, Sp and S E are the particle, momentum, and energy source, respectively, due to the interaction with the recycling neutrals; n and T are the density and temperature, respectively; VII is the flow velocity paral- lel to the field lines; m i is the ion mass; KII= KoT5/2 is the classical electron heat conductivity along the field lines. For the definition of the differential operator Vii- 0022-3115/92/$05.00 1992 - Elsevier Science Publishers B.V. All rights reserved