H." Zushi

K. "Nagaoka

M. Osakabe

T. Ryokai

Y. Mahira

Y. Takeiri

National Institute for Fusion Science (NIFS-Repository)

1) Y. Mahira, H. Zushi, K. Nagaoka, M. Osakabe, etal., Kyushu-Okinawa-Yamaguchi sectional meeting ofJapanese Society of Plasma Fusion Research, OitaUniv. 17-18 Dec. 2011.20 ..."IS ';?10 ,g30S.-. - ...... - .. _- ..... - .. ??????????????????????????????????????????????????300 (a ... - - ........ _ 25250? 200..." ISOf? 100soFig. 1: Schecatic of QUEST and the hybrid probe sys-tem.Energetic particle confinement is one of the key is-sues for steady state operation scenario in the thermonu-clear fusion research, because the loss of energetic par-ticle could give serious damage to the first wall in thevacuum vessel. In particular, the energetic particles ac-celerated by RF heating in ST (spherical torus device)tends to distribute outside of the bulk plasma. Recently,the localized loss of such energetic particles was observedon the first wall. The purpose of this collaboration is in-vestigation of energetic electron behaviors and its lossmechanism to the wall in ST.In order to observe energetic particle spatial dis-tribution, a movable hybrid probe has been installed inQUEST (see Fig. 1). The hybrid probe is a directionalprobe to measure particle flux density and heat flux den-sity. The sheath thermocouple is used as a electrode ofan electrostatic probe. Therefore the particle flux den-sity and the heat flux density can be observed simulta-neously with high spatial resolution. The probe drivesystem has a two drive mechanism. The probe positioncan be changed by motor drive during the shot intervalof the plasma operation, which is called as slow drivesystem. The probe position can be also changed duringa plasma discharge using a compressed air drive system,called as a fast drive system. The heat flux was inves-tigated for the first time, and the time evolution of theelectrode temperature is shown in Fig. 2(a). The heatflux density is estimated bywhere p is mass density, C is specific heat, V and S arevolume and surface area of the electrode, respectively.The loss of heat flux from the electrode is neglected inthis experiment, because the time scale of heat loss ismuch longer than the input flux, which can be seen bythe gradient of the temperature increase and decrease intime (see Fig. 2(a)). The profile of the heat flux densityobserved with the hybrid probe is shown in Fig. 2(b).In this experiment, the last closed flux surface (LCFS)is located at the R = 860 mm, and the heat flux out-side of the LCFS is considered to be mainly determinedby energetic electrons. It is observed that the energeticelectrons distribute 115 mm far from LCFS. The orbitcalculation of energetic electrons with the energy of 10keV was carried out. The detailed comparison of the en-ergetic electron orbit with the observation was discussedby Mahira 1).(1)" (b)III10"'5? 4 1li II???.:850 900 9SO 1000 1100 1150Rlmm]Fig. 2: (a) The time evolution of temperature increaseof probe head. (b) The radial profile of power densitymeasured by thermal probe.516§49. High-energy Particle Loss and its Influence on Plasma-wall Interaction in QUESTNagaoka, K., Osakabe, M., Takeiri, Y., Mahira, Y. (Kyushu Univ.), Ryokai, T. (Kyushu Univ.), Zushi, H. (Kyushu Univ.)

High-energy Particle Loss and its Influence on Plasma-wall Interaction in QUEST

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