M. Okamoto

N. Nakajima

S. Murakmni

W.X. Wang

National Institute for Fusion Science (NIFS-Repository)

§19. Electron-ion Collisions in SOL Simulation \Vang,\iV.X., Okamoto,:Lvf., Nakajirna,N., Murakmni,S. The cooling effect of second electrons enlit-ted fron1 target plates has been investigated with regard to the high ten1perature divertor operation[! ,2]. In the previous sirnulation, we neglected electron-ion collisions for nun1erical sirnplicity. The rarnifications of neglecting e-i collisions and the influence of e-i collisions re-rnain to be clarified. If 1i ~ Te, as is usually case in the scrape-off layer (SOL), the transfer of n1on1entum frorn ions to electrons and energy exchange between ions and electrons occur in the tirne T:i f'.J Jrni/'rneTii (Tii: ion equilibration tirne). In the situation of previous sirnulations of col-lisional SOL plasma, T:i f'.J 25L/(2vt;11) (L: magnetic connection length between two diver-tor plates; Vtill: ion parallel thennal velocity; L/(2vtill): ion transit tirne). This observation in1plies that, even in a collisional SOL plasrna, the mornenturn transfer frorn ions to electrons and energy exchange between ions and elec-trons due to i-e collisions are not significant compared to ion n1omenturn and energy sinks due to the plates. Hence e-i collisions rnay have little effect on the ion behavior in the SOL. On the other hand, the transfer of rnon1entum from electrons to ions occurs in a tin1e Tei of the same order as the electron-electron mon1entum transfer time Tee· Thus, in the SOL e-i colli-sions n1ay have an effect on electron behavior as important as e-e collisions do. The conse-quences of neglecting the n1ornentum transfer frorn electrons to ions are not explicitly known. To study the effect of e-i collisions, our Monte Carlo sirnulation has been extended to include unlike-particle collisions. The l\1onte Carlo n1odel solves the steady state problen1 by initial value nwthods. Since the ion behavior of the SOL is not sensitive to e-i collisions, we start the calculation for electrons including e-i collisions after a steady state of ions is reached so as to reduce con1puting tin1e. 200 The principle quantities fron1 the sirnulations with and without e-i collisions are con1pared in table 1 for both collisional and collision-less SOL plasrna (the secondary electron ernis-sion coefficient takes 0.8 in the sirnulations). The rnost interested quantity is electron en-ergy transn1ission factor le which measures the cooling effect of secondary electrons. The e-i collisions indeed have son1e influence on l e es-pecially in collisionless regin1e. However, al-though in collisionless regin1e le is reduced when e-i collisions are included, its value is still smaller than that of collisional regirne. This re-sult still supports our previous conclusion fron1 the sirnulations neglecting e-i collisions: in col-lisionless SOL plasrna the cooling effect of sec-ondary electrons becornes weak and the tern-perature can be rnaintained to be high. Table 1. Principle quantities fron1 the simulations with and without e-i collisions Collisional case ( L = 2450m) n ~ ~ e~ le n _. T. T. -T,. without e-i 0.94 0.35 0.36 1.78 12.5 with e-i 10.1 0.36 0.37 1.84 12.1 Collisionless case (L = 122.5rn) n ~ ~ e~ l e n_. T. T.. -T., without e-i 0.79 0.34 0.67 0.95 7.4 with e-i 0.75 0.30 0.58 0.96 9.5 (n: SOL plasn1a density; q): total potential drop between SOL and plate; re: electron energy trans-mission factor; Te(l_,ll): electron (perpendicular, parallel) ten1perature; Ts: source temperature; ns: the density used for nonnalization) Reference 1) Wang,W.X., Okamoto,M., Nakajima,N., Murakarni ,S. and Ohyabu,N., Nucl. Fusion 36 (1996) 1633. 2) Wang,W.X., Okamoto,M., Nakajirna,N., Murakarni,S. and Ohyabu,N., acool'ing Effect of Secondary Electrons in the High Temperature Divertor Operation", NIFS-Report No. 480 . 

Electron-ion Collisions in SOL Simulation

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