M. Okamoto

N. Nakajima

N. Ohyabu

S. Murakami

W.X. Wang

National Institute for Fusion Science (NIFS-Repository)

§ 16. Monte Carlo Scrape-off Layer Plasma Wang,W.X. (Grad. Uni. Advanced Studies), Okamoto,M., Nakajima,N., Murakami,S., Ohyabu,N. We concentrate on the Scrape-off Layer (SOL) region between the potential sheaths formed in front of two divertor plates. As-suming that the plasma in the SOL is uni-form due to fast streaming of particles along open magnetic field lines, we propose a kinetic SOL model, taking essential physics processes into account. The distribution function fa( v, t) evolves only in velocity space as follows 8fa(v, t) C I . ot = a+ SOUrces Sinks, (1) where Ca is Coulomb collision operator. The cross-field plasma transport from the core into the SOL and secondary electron emission from the plates are treated as sources, and parti-cle and energy sink effects due to the absorb-ing plates are effectively modelled through the magnetic connection length L between two di-vertor plates. In the source terms, the conduc-tive energy flux from the confinement region into the SOL is effectively modelled for the first time via randomly exchanging the source par-ticles and the SOL particles. The conductive energy flux, which is independent of cross-field particle flux in determining the SOL structure, together with the convective energy flux intro-duced by the particle source term, ensures in-tegrated balance of energy flowing to the SOL and energy lost to the divertor plates. Coulomb collisions are incorporated by the nonlinear Monte Carlo operator[1] which can accurately simulate the collision effects even when the SOL plasma deviates far from a Maxwellian distribution. Instead of solving Passion's equa-tion, the total potential drop between the SOL plasma and the plate is consistently determined by employing charge neutrality constraint. The model Fokker-Planck equation ( 1) is solved via particle Monte Carlo simulation under charge neutrality constraint and zero-current condi-154 tion. A steady state SOL plasma and total potential drop are obtained in terms of total particle flux and input power across the sep-aratrix, the temperature of main plasma near the separatrix, secondary electron emission co-efficient, and the SOL size. The collisional SOL plasma is studied by us-ing the particle Monte Carlo simulation. Fig-ure 1 shows (a) the normalized total potential drop e<I> /Te and (b) the electron sheath energy transmission factor 1 e as functions of secondary electron emission coefficient Oe. The numerical values of e<I> /Te and le are in close agreement with theoretical predictions[2); the validity of the model is confirmed. Within the present framework, the model can be extended to include other effects, such as neutral recycling and impurity flux, by in-troducing additional source terms. E-. .......... 4.0 -o& C1) I I 0.0 a Simulation (a) D. Theory I 0.5 0 1::. 1.0 ... 15.0 ~------------., .s (,) - 0 Simulation r! c i s ~ 10.0 -{:. ~ Gl a Dol c: ~ (,) 5.0 -Cl r.;j Reference ATbeory I aTbeory II I 0.0 Fig.1 (b) I 0.5 1.0 o. 1) Wang,vV.X., Okamoto,M., Nakajima,N. and Murakami,S., NIFS-345, March 1995. 2) Stangeby,P.W., Phys. Fluids 27 (1984) 682. 

Monte Carlo Scrape-off Layer Plasma

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