H." Suzuki

K. Mori

M. Takagi

N. "Ohno

N. Ezumi

S. Takamura

National Institute for Fusion Science (NIFS-Repository)

§13. Density Threshold of Plasma Detachment in Gas Target Experiments 150 Ohno, N., Ezumi, N., Mori, K., Takagi, M., Takamura, S. ( School of Engineering, Nagoya University) Suzuki, H. The simulated gas target divertor experiment has been performed to investigate fundamental physics of plasma detachment in the linear plasma device, TPD-1, which has a high heat flux and high density plasma in steady state1l. The existence of density threshold for plasma detachment was observed in our experiment. It is found the electron-ion temperature relaxation process is a key to deterimin the density dependence of the plasma detachment2J. The inset in fig. 1 shows the schematic of experimental arrangement. When the plasma is attached, the plasma is terminated by the water-cooled target plate, located at a distance 87 em from the orifice. Neutral pressure P mea-n sured with the capacitance manometer at 33cm away from the orifice, can be controlled up to 25 mTorr. Then, T e e and space potential Vs are measured by two fast-scanned Langmuir probes deginated "downstream" and "up-stream," installed at a distance 20 em and 53 em from the target plate, respectively. Figure 1 shows the dependence of plasma pressure P, which is estimated as n T by meaning n and T at differ-e e e e ent positions along the magnetic field line, on the neutral pressure Pn. Furthermore, we note that the plasma pres-sure P is much larger than the neutral pressure P n, in which the plasma detachment is observed. At a high plasma density neO- 1.3x1019 m-3, the downstream plasma pressure is found to be decreasing more dramatically with an increase in Pn than that at upstream, as shown in fig. 1(a). On the other hand, at neO- 2.0x1018 m-3 'pis gradu-ally decreasing at both positions. Figure 1 (c) shows the ratio of P' s at downstream and upstream, which indicates that there is a steep plasma pressure drop only in the high plasma density neO ~ 1.3x1019 m-3• Figure 2 shows neo's de-pendence of the neutral pressure: Pdetach at which plasma detachment occurs. As decreasing n , the threshold value e of ?detach is found to be rapidly increased, and below ne-1.6x101Sm-3, plasma detachment is not observed in our ex-perimental condition. This result indicates that there is the plasma density threshold for a generation of the plasma detachment. 128 neutral feeding • 100 targ~latel t .._ 0 f-- down up .s 50 scanned probe ~ :::J en en ~ 0 0.. co E en co 0::: 10 0 1 0.8 0. 0.6 :::J 0.. -c 0.4 :;: 0 1:J 0.. 0.2 0 0 10 20 Neutral Gas Pressure [mTorr] Fig. 1 Dependence of plasma pressure on neutral gas pressure Pn. (a) neO- 1.3 X 1019 m-3 '(b) neO -2.0 x 10 18 m-3 and (c) the ratio of the plasma pres-sures measured at upstream and downstream. 1 I I ' 30""" \ -,. 'i::' \ .._ ,.... -0 \ I-.s \ Detachment 20 r-' ~· -..c (.) • '. ~ ~ ..,. ..... .,.. Q) .... -"0 ,.... .---a.. Attachment 10 r- -I I I I 0 4 8 12 neo [1 Q18 m-3] Fig. 2 Threshold values of the neutral pressure for the plasma detachment as a function of nco· Reference 1) N. Ohno et al., J. Nucl. Mater. 220-222 ( 1995) 279. 2) N.Ohno et al., Contri. Plasma Phys. to be published. 

Density Threshold of Plasma Detachment in Gas Target Experiments

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