Neoclassical tearing modes in TFTR reversed shear experiments
- Princeton Plasma Physics Lab., NJ (United States); and others
Coherent MHD modes are often observed in reversed magnetic shear experiments in TFTR. The majority of the modes have toroidal mode number n = 2 and poloidal mode number m = 5. They appear in the high power heating phase on both the reversed shear (RS) and enhanced reversed shear (ERS) plasmas. Tearing parity is observed from the local ECE fluctuation measurement. Namely, the {delta}T{sub e} changes phase across the mode rational surface. The measured q profile from MSE polarimetry shows that the mode is located {approximately} 10 - 15 cm outside the minimum q (or q{sub min}). The radial width of the mode is about 25 cm, which extends to or beyond q{sub min}. Due to the high m number, the magnetic island size calculated using the q profile and edge magnetic coil measurements is small, i.e., w < 5 - 6 cm, or w/a < 5 - 6%. The stability of the classical current-driven tearing mode is estimated using the analytic formula: It is found that for typical TFTR RS/ERS plasmas, {Delta}{prime} is negative (stable) to m/n = 5/2 modes in both the negative and positive shear regions. On the other hand, neoclassical {del}p-driven tearing theory, which has been proved to provide a very good description of the observed island evolution in TFTR supershot, gives a consistent physical picture in agreement with the observation. According to the theory, where L{sub q} = (q{prime}/q){sup -1}, L{sub p} = -(p{prime}/p){sup -1}, {epsilon} = r/R, {beta}{sub p} is the poloidal {beta} and k{sub 2} is a constant of order unity. In the reversed shear region, L{sub q} < 0, so the neoclassical term is strongly stabilizing.
- DOE Contract Number:
- AC02-76CH03073
- OSTI ID:
- 489517
- Report Number(s):
- CONF-960354--
- Country of Publication:
- United States
- Language:
- English
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