H." Suzuki

K. Mori

M. Takagi

N. "Ohno

N. Ezumi

S. Takamura

National Institute for Fusion Science (NIFS-Repository)

§ 16. Density Dependence of Plasma Detach-ment in Simulated Gas Divertor Experi-ments of the TPD-1 Device Ohno, N., Ezumi, N., Mori, K., Takagi, M. Takamura, S. (School of Engineering, Nagoya University) Suzuki, H. It is one of the most critical requirements tore-duce the heat load to the divertor plate in the next generation fusion devices such as LHD, intended to have long pulse and steady state operation. Dy-namic gas target divertor is one of the most impor-tant candidates14>. We have already done simu-lated gas target di vertor experiments in the TPD-I device to observe the plasma detachment clearly5>, in which a strong reduction of plasma pressure and heat load to the target plate was ob-served. In this reports, we present further experi-mental results to indicate strong dependence of plasma detachments on plasma density n . Gas target experiments are performedeby feed-ing a secondary helium gas into the plasma test region. Figure 1 shows the dependence of the floating potential vf of the target plate on the gas pressure P. At higher ne, Vc is decreasing with an increase in P, which is corresponding to decreas-ing the electron temperature T in front of the tar-e get .A hove P "' 2 Pa, the detached plasma was ob-served. On the other hand, at ne -0.2x1019m-3 , we have not observed the detached plasma even after feeding the secondary gas up to P "' 3 Pa. These experimental results indicate that there is a thresh-old of n e for a generation of the plasma detach-ment. In order to discuss the density dependence of the detachment, we consider an energy balance of electrons. Electrons mainly lose their energy through ionization, radiation and energy ex-change between electrons and ions. Figure 2 shows _n e's dependence of the energy exchange time -r;et T normalized by the plasma confinement ti~e -r . Here, -rP is determined by L I 0.2C8, where L ts a fength of plasma column and C is ion sound velocity. It is found that at n > l.Oxi019m-3 ,~ is e T small enough that electrons can transport their en-ergy to ions, whose energy is lost by a charge ex-change process between ions and neutral particles. A simple estimation shows that the energy loss mentioned above is more effective than the 122 ionizaiton and radiation loss in a high density plasma. A decrease in T leads to smaller value of . e r\ , which enhances the electron energy loss . This positive feed back is thought to be a reason · why there is the plasma density threshold for the detachment. Reference [1]W. L. Hsu et. al., Phys. Rev. Lett. 49(1982) 1001. [2]L. Schmitz et al. J. Nucl. Mater. 176&177(1990) 522. [3]L. Schmitz et al. J. NucL Mater. 196-198 (1992) 841. --[4]L. Schmitz et al. Proc. of 20th EPS Conf. on Controlled Fusion and Plasma Physics l:ZC (1993) 751. [5] N. Ohno et al. J. Nucl. Mater. 220-222 ( 1995) 279. ,..... > ..._ -C1) -20 -«S a. -C1) 12A, c: -40 0.2 x 1Q19m-3 .... «S I I--.... > ~ ~~~~~~~~.-~~~~~ 0 1 2 3 Secondary Gas Pressure (Pa] Fig. 1 Dependence of the floating potential Vt on a gas pressure P. 10° ~a. 10-1 ..... ~ 10-2 10~ 10""' 103) "e (m-3) Fig. 2 Dependence of the energy exchange time ~ T normalized by the plasma confine-ment time -r on n . p e 

Density Dependence of Plasma Detachment in Simulated Gas Divertor Experiments of the TPD-I Device

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