K. Takiyama

K." Sato

T. "Oda

Y. V. Ralchenko

National Institute for Fusion Science (NIFS-Repository)

§31. C V and C VI Spectrum at the Edge Plasma in LHD Oda, T. (Fac. Eng., Hiroshima Kokusai Gakuin Univ.), Ral~henko, Y. V. (Weizmann Institute of Science), Takiyama, K. (Fac. Eng., Hiroshima Univ.), Sato, K. VUV spectrum of C V ( 1s-2p, 40.2 A and 40.7 A) and C VI ( ls-2p, 33.7 A) from the edge plasma in LHD has been observed and analyzed. A line of sight ( L.S. ) of VUV spectrometer was settled in three different directions on the shot-to-shot basis : (1) L.S. crosses only the peripheral plasma region ( p-v. 1) avoiding the strongly emitting layer of C V and C VI., (2) L.S. crosses the C V and C VI emitting layer ( P 'V 0.8), (3) L.S. crosses the center of the plasma ( p,., 0). The measurements were made in a series of nominally identical discharges (Bt = 2.75 T, Rax = 3.6 m, 11e 6x1019m-3) . A gross variation in C V intercombination I resonance intensity ratio ( called the G-ratio) was observed at around the last closed magnetic flux surface. Compared with normally expected ratio of 0.6 - 0.7, observed G-ratio reaches to 1. (Fig. 1.) To be exact, G-ratio is defined as (x+y+z)/w, where w is the resonance line at 40.26 A ( 1 s2 1 So -1s2p 1PJ), y is the intercombination line at 40.73 A ( 1 s2 1 So - 1 s2p 3P 1 ), x is the forbidden line (1 s2 1 S0 -1s2p 3P2) and z is the intercombination line at 41.47 A (ls2 1So - 1s2s 3S1). For carbon ion, the x line is blended with y but its intensity is negligibly small, and the z line is quenched by electron collisions at 11e > 1o t2 -3 S h G . . em . o, t e -ratio : y/w ts expected to be depend mainly on electron temperature T e because of the difference of electron impact excitation cross-section between singlet and triplet state. Thus the G-ratio is a decreasing function of Te, becoming larger than 1 below Te.-v 50 eV. Collisional Radiative Model (CRM) calculations were carried out assuming several ion aboundance t . . N(CS+) C4+ ra lOS m IN( ). Though the G-ratio depends also on elec.,'tron density 11e , 11e could be a supplementary parameter in the case of edge plasma condition ( Ile"' 1012 - 1013 cm-3). We assumed the ne of 1013 cm-3 244 in CRM calculation. In simple CRM, the G·ratio of 0.9 gives electron temperature of 40""" 10 eV . This T e at LCFS is rather low value compared with the one from Thomson Scattering Measurements. We suspect additional important processes for populating mechanism in triplet state of C4+. Firstly, charge-exchange ( C5+ + H ) process was taken into account in CRM. As the result of calculation, it was found that the charge-exchange process may affect the G-ratio and increase its value. However, it was found that the higher excited levels of C4+ ion are more sensitive to the charge-exchange process than the G-ratio. For example, C V line ( 1s2 1S0 - 1s3p 1P1 ) at 34.97 A becomes so intense, when the G-rado is affected with the charge-exchange process. In our experiments, such a strong C V line from the higher excited levels has not been observed. The principal reason for the high G-ratio will be attributed to other recombination processes ( dielectronic or radiative recombination). 0.0 0.2 0.4 0.6 0.8 1.0 1.2 RHO Fig. l. G-ratio as a function of normalized minor radius 

C V and C VI Spectrum at the Edge Plasma in LHD

Abstract

Similar works

Full text

Available Versions

National Institute for Fusion Science (NIFS-Repository)