M. Okamoto

N. Nakajima

N. Ohyabu

S. Murakami

W.X. Wang

National Institute for Fusion Science (NIFS-Repository)

§ 17. Cooling Effect of Secondary Electrons rn High Temperature Divertor Operation Wang,W.X. (Grad. Uni. Advanced Studies), Okamoto,M., Nakajima,N., Murakami,S., Ohyabu,N. Recently high temperature divertor plasma operation has been proposed to improve the energy confinement of core plasma in toroidal devices[1]. One of key physical issues there is the influence of secondary electrons emitted from the divertor plate, which tend to cool the scrape-off layer (SOL) plasma. While main-taining a high edge temperature is critical to achieve the improvement of the energy confine-ment. Since particle recycling is controlled to be a low level by efficient pumping, the cool-ing due to atomic processes associated with the particle recycling is not significant. On the other hand, secondary electrons emitted from the plates due to the impact of plasma on the target are inevitable and become an essential cooling source for the SOL plasma. By using particle Monte Carlo simulation[2], we have investigated the SOL plasma over a wide range of Coulomb collisionality with fo-cus on the cooling effect of secondary electrons in collisionless regime, which is importantly rel-evant to the high temperature divertor opera-tion. In collisionless SOL plasma, the electron distribution function shows a strong depar-ture from Maxwellian. The low collisionality and non-Maxwellian feature affect the trans-port properties of the SOL plasma, and result in the total potential drop <I> between the SOL and plate and electron sheath energy transmis-sion factor le differing significantly from the analytical estimates based on Maxwellian elec-tron assumption. Figure 1 shows the depen-dences of (a) e<I> /Tell and (b) le on collisional-ity L/(2A.e) where L is the parallel dimension of the SOL and Ae is electron mean-free-path in the parallel direction. The secondary elec-tron emission coefficient be takes 0, 0.5, and 0.8. The potential drop -e<I> /Tell increases with collisionality and decreases with increasing the secondary emission. It has been found that le is reduced as decreasing collisionality and that in the presence of strong secondary emission its value in collisionless regime is considerably smaller than in collisional regime. This fact indicates that in collisionless SOL plasma the cooling effect of secondary electrons becomes weak and the temperature can be maintained to be high (the electron equilibrium tempera-ture is inversely proportional to !e)· The nu-merical study here provides a positive evidence that the high temperature divertor operation may be possible. T E-- 0 6.=0.0 ......... (a) >& v 4.0 .... a 6.=0.5 I A 6.=0.8 -; 0 :;: = 0 0 4) .,J 0 0 0 c.. 0 "t1 2.0 - A A 4) A .!::! -; ~ s ,_, 0 z d ,, d 0.0 10-3 to-~ to-• t0° to• Collisionality L/(2A.) ... 15.0 .3 r 0 6.=0.0 (b) <> • ~ Ill. a 6.=0.5 A c ~ 0 ~ . A 60=0.8 ~ 10.0 r-c . .. A t-~ 0 ... 0 0 5.0 d - 0 Jjil c 8 0 0 !; <> 0 ii3 ~ 0.0 " "' "' t0-3 to-• to-• t0° to• Collisionali ty L/(2A.) Fig.1 Reference 1) Ohyabu,N., Watanabe,T., et al., Nucl. Fusion 34, (1994) 387. 2) Wang,W.X., Okamoto,M., Nakajima,N., Murakami,S . and Ohyabu,N. , "A Monte Carlo Model for Velocity Space Effects in Low Recycling Scrape-off Layer Plasmas ", to be published, Nucl. Fusion. 155 

Cooling Effect of Secondary Electrons in High Temperature Divertor Operation

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