A. Tsushima

K. Hanada

M. Ishiguro

M. Sakamoto

T. Tawaraya

National Institute for Fusion Science (NIFS-Repository)

To measure the boundary plasma of a sphericaltokamak QUEST, a probe head, as shown in Fig. 1, wasattached at the top of a fast scanning system and movedback and forth in a distance of about 500 mm horizon-tally during about 1 s. On the probe head, there wereﬁve cylindrical electrodes with diameters of 1 mm andlengths of 2 mm: three electrodes (1, 2, 3) were set inarray on an insulator mound and two electrodes (A, B)were set aside and paired for a Mach probe. One of thethree electrodes (3) in array on the insulator mound wasmade of molybdenum, the other four electrodes (1, 2, A,B) were made of tungsten.Fig. 1: Top view and side view of probe head with ﬁvecylindrical electrodes: three electrodes (1, 2, 3) were setin array on an insulator mound and two electrodes (A,B) were set aside; Four electrodes (1, 2, A, B) were madeof tungsten and one electrode (3) was made of molybde-num.Since the eﬀective coeﬃcient of secondary electronemission by electron impact depends on a material, weexpect that the diﬀerence between the probe signal fromthe tungsten electrode and that from the molybdenumelectrode indicates the eﬀect of secondary electron emis-sion. Then, using the tungsten and molybdenum elec-trodes (2 and 3) on the insulator mound, probe charac-teristics were obtained in a boundary plasma at a lim-iter shadow. Relations between ln Ie and Vp are shownin Fig. 2 by dots. The top and bottom ﬁgures are theresults using the tungsten and molybdenum electrodes,respectively. Here, Ie is electron current and Vp is biasapplied to each electrode. Slopes of solid lines indicateelectron temperatures Tef of fast electron components,Tef = 26 eV and Tef = 24 eV for the tungsten andmolybdenum electrodes, respectively; and those of bro-ken curves indicate electron temperatures Tes of slowelectron components, Tes = 8.7 eV and Tes = 7.4 eVfor the tungsten and molybdenum electrodes, respec-tively. The corresponding electron temperatures for thetwo electrodes almost agree with each other, and den-sity ratios between fast and slow electron components,(nef/nes)’s, from the two electrodes agree with eachother, if the reduction of the electron current by 1−δ¯(Te)is assumed. Here, δ¯(Te) is the eﬀective coeﬃcient of sec-ondary electron emission due to a Maxweiilan electronwith an temperature Te and is represented byδ¯(Te) = δm(2.72)2∫ ∞0ξ e−ξ−2√(kBTe/Em)ξ dξ (1)with δm = 1.36 and Em = 650 eV for the tungsten elec-trode and δm = 1.25 and Em = 375 eV for the molyb-denum electrode.1) These results suggest that the eﬀectof secondary electron emission can be considered usingδ¯(Te).We are now evaluating the ion ﬂow velocity ofthe QUEST boundary plasma with the inﬂuence of sec-ondary electron emission.10-1100101102 0  10  20  30  40  50  60I (mA)Vs-Vb (V)IeIec10-1100101102 0  10  20  30  40  50  60I (mA)Vs-Vb (V)IeIecFig. 2: Semilogarithmic plots of Ie are depicted by dots,where the top and bottom ﬁgures are from measurementsof tungsten and molybdenum electrodes, respectively. Asolid line ﬁts the dots for Vs − Vb > 30 V. Crosses areobtained by subtracting the solid line from the dots. Abroken line ﬁts the crosses for Vs − Vb < 20 V.1) Tawaraya, T., Tsushima, A., and Yoshimura, S.:Proc. Plasma Conference 2011 (Kanazawa, Japan,2011) 22P172-B.523§56. Probe Measurement under the Influence of Fast Electron in QUEST Boundary PlasmaTsushima, A., Tawaraya, T. (Yokohama Na. Univ.),  Sakamoto, M. (Univ. Tsukuba),  Ishiguro, M., Hanada, K. (Kyushu Univ.)

Probe Measurement under the Influence of Fast Electron in QUEST Boundary Plasma

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