G. Matsunaga

Group CHS

K. Toi

M. Takechi

National Institute for Fusion Science (NIFS-Repository)

§8. Observation of Global Alfven Eigenmodes Destabilized in the CHS Plasmas with Finite Magnetic Shear Takechi, M., Matsunaga, G. (Dep. Energy Eng. Science, Nagoya Univ.), Toi, K., CHS Group We are studying toroidal Alfven eigenmodes (T AEs) in the CHS heliotron/torsatron, focusing on the destabilization condition and internal structure of TAEs with low toroidal mode numbers, n= 1 and 2. In CHS plasma, T AE gap can be formed due to its moderate magnetic shear. On the other hand global Alfven eigenmodes (GAEs) are easily destabilized in the W7-AS stellarator with very low magnetic shear. Recent-ly, two kinds of GAEs have been observed even in the CHS plasma. One is n= 1 GAE generated by the transition from the n=1 TAE in a co-NBI heated plasma and the other the n=O GAE in a counter-NBI heated plasma. Figure 1 (a) shows the time evolution of the magnetic fluctuations in the CHS plasma heated with co-NBI where the magnetic axis position of the vacuum field is Rax = 0.949 m and the toroidal field Bt = 0.9 T. The mode of the frequency f-180kHz is observed from t-30ms. This mode is the n= 1 T AE related to the m=2 and m=3 coupling and disappears at t-65ms. Then, n=2 T AE split into multiple peaks is destabilized. The fluctuations of f>200kHz are higher harmonics of the T AE. The n= 1 modes appear again from t-90ms in the fre-quency range of f<90kHz just below the n=2 TAE with multi-ple peaks and in the range of f-150kHz. Figure 1 (c) shows the T AE gap structure calculated by a simple dispersion rela-tion for a large-aspect-ratio tokamak equilibrium at t=50ms. As seen from Fig. 1 (c) the observed frequency of n=1 mode (indicated by the horizontal line) lies near the lower bound of the n= l,m=2/3 T AE gap. Figure 1 (d) shows the T AE gap structure at t= 120ms. The n= l ,m=2/3 T AE gap does not exist at the time. As seen from this figure, the higher frequency of n= 1 mode observed at t= 120ms lies near the minimum of the n= lIm=3 Alfven spectrum, and the lower frequency of n= 1 mode lies near the maximum of n= lIm=2. These n= 1 modes are thought to be n= lIm=3 and n= lIm=2 GAEs. The rotation-al transform increases with the increase in the plasma current. Therefore, the lIq=O.4 surface of n= 1, m=2/3 disappears in the plasma, and T AE cannot exist there. After that, GAE gap produced by the n=lIm=2 and n=lIm-3 continua is expanded due to the increase in the plasma current. The GAEs would be destabilized within the gap. Figure 2 (a) shows the time evolution of the magnetic fluctuations in the CHS plasma heated with counter-NBI, where Rax = 0.974 m and Bt = 0.9 T. Two n=O modes are ob-served. The figure 2 (c) shows the T AE gap structure at t= lOOms. Because n=O, no T AE gap is formed. The Figure 268 shows the observed higher frequency of the mode lies near the minimum value of the n=O/m= 1 Alfven spectrum which is indicated as fMIN in Fig. 2 (a). The value of fMIN is cal-culated using ne(O), where the impurity effect is taken into account. The calculated frequency fMIN agrees well with the observed frequency of the n=O mode. Therefore the mod 250r---~-'--~~~----------r----' 'N 2OO l: ~ >-g 150 ~ [ u.. 100 500~~~~~~~------~~~~--~--S 400 ~. 300~ ........... _ _ ...... g' 200 ~ 1 00 I----D-~____,f__~+I F:===:=;"'-~~~ 2 m=3 , . 0 w... ................... ___ s-__ -.jl...:ll c.. ...... ..r;;;;"",-.&. .. """"_ ...... ~ 0.0 0.2 0.4 P 0.6 0.8 0.0 0.2 0.4 P 0.6 0.8 1.0 Fig.l (a) Observed magnetic fluctuations ofn=1 GAEs. (b) The electron density and the plasma current. (c)(d) Alfven spectra for n= I at t=50ms and t= 120ms, respecti vel y. 300 -'N ~25O i 200 ::J ¥ 150 " LL 100 (a) , 5O~--~--------~----------~--~ _ 2 r-----:-----:------+--------:::--_,O M 'E ~ 1 52 )( (b) Ic·O '--.......................... _...0..-______ 1_ . ______________ ........ 10 40 60 80 100 120 140 160 TIme (ms) -. 500 fill :c 400 (c) ...lIIi ~. 300 ~ 200 r==:.--------'I ~ 100 ~ O~~~~~~~~~ 0.0 0.2 0.4 0.6 0.8 1.0 P Fig.2 (a) Observed magnetic fluctuations of n=O GAE. (b) The electron density and the plasma current. (c) Alfven spectra for n=O at t=IOOms. 

Observation of Global Alfv?n Eigenmodes Destabilized in the CHS Plasmas with Finite Magnetic Shear

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