A." Sagara

H. Sugai

H. Suzuki

K. "Nakamura

N. Matsunami

National Institute for Fusion Science (NIFS-Repository)

§ 13. In-situ Measurements of SecondaryElectron Emission Coefficient in Plasma-Surface InteractionNakamura, K., Suzuki, H. (Chubu University)Matsunami, N., Sugai, H. (Nagoya University)Sagara, A.High energy particles such as neutrals, ions andphotons produced in a fusion reactor are exhausted throughdiverter area, then a large heat load is applied to the diverterplate. In this meaning, there are strong plasma-surfaceinteraction on a surface of the diverter. As one of theinteractions, there is secondary electron emission, and itssecondary electron emission coefficient (SEEC) has beenmeasured for various target materials and incident ions.However, in most cases, such a measurement has beencarried out with beam experiments in a ultra high vacuumenvironment for a pressure lower than 10.6 Torr which ismuch different from actual plasma environment. Therefore,new measurement technique is necessary to obtain the actualsecondary electron emission coefficient in the plasmaenvironment. Recently, a scintillation-based novel techniquefor high energy secondary electron measurements in PIlI hasbeen developed [1]. In this paper, we extend the techniqueby replacing the scintillation detectior with a semiconductordiode detector for more precise measurements, enabling oneto obtain incident ion flux as well as ion-induced secondaryelectron emission coefficient during the process. Themeasured ion flux is also compared with Bohm flux.Figure 1 shows schematic diagram of theexperimental apparatus. One tum loop antenna is set in a 35-cm-diam. and 50-cm-Iong cylindrical stainless steel chamber,and a 13.56 MHz inductively-coupled plasma is generated atargon pressures of 1.3 Pa by supplying the antenna with RFpowers up to 600 W. Under typical conditions, the electrondensity is 1.Ixl017 m-3 and the electron temperature is 2.3eV. 10 Jls-Iong high voltage pulses up to 6 kVare applied toa spherical copper target inserted into the plasma with arepetition rate of 10 pps. In order to measure thesecondary electrons, a thermoelectrically-cooled Si-PINdiode are used. As showin in Fig. 2, when the high-energyelectrons are incident on the diode detector, hole-electronpairs are created. Since an inverse bias is applied to thedetector, the resultant charges are extracted to a chargeamplifier whose output voltage is proportional to the totalamount of the created charges. The secondary electroncurrent Ise obtained by differentiating the integrated outputvoltage enables one to discriminate high energy electrons(>2 keY) from background low-energy electrons in theplasma. The absolute calibration revealed that the detectorhas a sensitivity of mA/mm2 with a fast response timeshorter than Jls.Immediately after applying the target pulsevoltage Vt of -6 kVat t=0, the target current It flows with apeak component for 0<t<2 Jls followed by a constantcomponent (t>3 Ils). The secondary electron current Isemeasured by the diode detector also has a waveform similarto It. However the peak of Ise is significantly lower than thatof It because Ise obtained by the high energy electronmeasurement essentially does not include displacementcurrent. Therefore the present technique is available for theselective measurement of the secondary electron current.Taking account ofthe detector sensitivity, the solid angle ofthe detector, and the spatial profile of the plasma densityaround the target, a current ratio of It/Ise gives the SEEC yduring the ion implantation as by It! Ise= 1+ y-l.Simultaneously the incident ion flux of 3.5x1021 cm-2s·1 isobtained, and this value shows a good agreement with theBohm flux of 3.8x1021 cm-2s-1 estimated from the measuredplasma parameters.References[1] K. Nakamura et al: Plasma Sources Sci. & Tech. 6, 86(1997).SecondaryElectronDetectorr=---, ---+-H-- PlasmaAbsorptionProbeFig. 1 Experimental apparatusFig. 2 Detection of high energy secondary electrons89

In-situ Measurements of Secondary Electron Emission Coefficient in Plasma Surface Interaction

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