s. Masuzaki

National Institute for Fusion Science (NIFS-Repository)

§44. Divertor Plasma Properties in LHD Masuzaki, s. Edge plasma control using the divertor for improvement of the core plasma perfonnance is one of the major experi-mental goals in LHD. In the first and second experimental cycles, all experiments were perfonned under the open "natu-ral" helical divertor (HD) configuration. One of the charac-teristics of HD configuration is the existence of the ergodic region surrounding the core plasma and the region with multiple thin curved layers surrounding the ergodic region. I ) The effects of this magnetic structure to the confinement and divertor function must be obvious. For divertor plasma measurement, a Langmuir probe array (21ch), and a thermocouple array (28ch) were set on the divertor plate which is located at the torus inboard side of the poloidal planes, where the magnetic surfaces are hori-zontally elongated. Two divertor legs can be detected by this arrays(see Fig.l). The spatial resolution of the Langmuir probe array was designed to be 5 mm at the fi nest part to detect the narrow divertor channels. Profiles of the divertor particle flux (ldiv), electron density (ne,div) and temperature (Te,div) were measured for various discharges. Figure 2 shows a typical I div profile and a magnetic field connection length profile obtained by calculation. Horizontal axis is the position along the probe array. Two peaks of I div were observed, and their positions are agree with the divertor traces, that is, long magnetic field lines' positions. The same results were obtained in the discharges with other Rax. It is found that the width of r div profile is insensitive to plasma parameters, and is strongly restricted by the magnetic field structure. This agrees with previous calculation results of magnetic field tracing2) qualitatively. The peak value of ldiv profile is typically 1 x I Onm-2S-I. This is roughly corresponding to the outward flux from core plasma, that is, N/'tp. For the case of N=<ne> V=3x l019(nY 3)x30(m3)=9xI02o and 'tp=O.I(s), N/'tp is 9xI021 (s-I). When divertor wet area is assumed to be 1.6 m2 from the profiie of ldiv, then particle flux to divertor is 1.6xlOn (s-I). Electron temperature in divertor plasma is typically 20e V. Therefore, the heat flux to divertor is estimated to be 360 kW, with the assumptions of Ti,div=Te,div. This is about a half value of ra-diation power. This initial experimental phase, Langmuir probe measurement was perfonned only one location in non axis symmetric HD configulation, thus detailed quantitative study of energy and particle balance is difficult. It is neces-sary to measure the plasma parameters at other typical loca-tions simultaneously. Figure 3 shows the Te,div as a function of Te,edge (p= I) measured with YAG Thomson scattering system, and it in-dicates that Te,edge is about 10 times higher than Te.div. It is also shown that Te,div dependence on Te,edge changes at Te,edge = - 170eY. In the higher Te,edge region, the rising rate of Te,div becomes small. This critical Te,edge is also related to density decay phenomenon2). In the discharge of Te.edge < -170 e V, no density decay was observed. The data in the high Te,edge region were obtained during density decay phase. This tendency is also observed in the discharge with B t = 2.5 T. One possibility of the reduced Te,div rising is that dur-ing density decay phase, the energy loss in ergodic region increases due to ionization, radiation and charge exchange. 48 Temperature pedestal observed at p=O.9 is possibly relat-ing with this result. \) Ohyabu, N., et a1.: Nuclear Fusion, Vo1.34, No.3 (1994)384. 2) Motojima, 0., et al.: Physics of Plasmas, Vo1.6, No.5 (\999) 1843. LHD rnter I. #1 Inner diag. port Boundary of Ergodic-Region Fig.} Cross-section view(perpelldicular to helical coils at inboard side). ....-.. CJ) N E -N N 0 T"" --#5834 @t=O.9-1.0 101 2 10° E ~ 1 10-1-1 () OL-~~~--~--~--~--WU~Uf~ 10-2 -200 -150 -1 00 -50 0 50 100 150 200 Probe position (mm) Fig. 2 Aprc~file (~ftlle particle flux to the dibertor plate(rdil') and connection length of tnagnetic field lines on which Langmuir probe and thermo-couple arrays are set. (#5834: H:rdrogen discharge, NBI co and clr. injection) 40 35 30 :;- 25 ~ > 20 :.0 ai r-15 10 • ~-r~~ • Bt = 2.5 T 5 <0 g 0 Bt = 1.5 T ~.c- •• .,. 0 0 100 200 300 400 T e,edge (e V) Fig. 3. Te.cliv as afunction (~fTe.edRlp=l) 

Divertor Plasma Properties in LHD

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)