K. Ida

T. Kobuchi

National Institute for Fusion Science (NIFS-Repository)

§16. The Effect of Net Toroidal Current on Shafranov Sh ift Kobuchi, T., Ida, K It is important to investigate the effect of the magnetic axis shift on confinement properties. A net toroidal current affects the MHD stability in heliotronltorsatron configurations. The effect of direction of the net toroidal currents on the local ideal MHD stability or Mercier criterion for plasma in the heliotronltorsatron configuration of the LHD have been investigated by Ichiguchi et aJl). In this calculation. the toroidal current in the direction opposite to equivalent plasma current is expected to cause the inward magnetic axis shifts in low beta value. In order to investigate the effect of toroidal current on magnetic axis shifts, neutral beam current drive (NBCD) experiment was carried out with H2 and neon puff, the vacuum magnetic axis of !?axv = 3.60 m. the toroidal field of B.. = 1.5 T and the magnetic configuration of K = 1.02. The NB drives net current of -110 kA in the direction to anti-parallel to equilibrium plasma current. The central electron density, 11e(O). measured by FIR interferometer and the stored energy, w". estimated by diamagnetic loops increase up to 5 X 1019 m-3 and 310 kJ. respectively. Figure I (a) shows the magnetic axis shift measured in this experiment. which is plotted as a function of net toroidal current. Figure shows that the magnetic axis shifts inward O.IOr;-,-~-~--~~ (.) o from position of the magnetic axis in vacuum magnetic fiux surface at !?axv = 3.60 m as the magnitude of toroidal plasma current increase in counter direction. The plasma for this experiment is in the line averaged electron density <11e> range of 0.3-0.S x 1019 m 3 and w" range of70-ISO kJ. In order to compare the experimental results with equilibrium calculation, the magnetic axis shifts are calculated with VMEC code in finite beta, <~> = 0.24 % and 0.34 % for different profiles of toroidal plasma current as seen in Fig. I (b). The electron density and temperature profiles used In this calculation are 11e - (l-pS) and To - (l.p2). respectively. In LHD the typical current profile is expected to be JI - (l _p2) by current diffusion calculation. When the net toroidal current profile is JI and .h profile, the magnetic axis keep shifting inward as the magnitude of toroidal current is increased. This trend is consistent with the measurements. On the other hand, in .h profile, the magnetic axis start to shift outward at large toroidal current in the region Jp ~ -40 kAff due to lower the rotational transform as seen in Fig. I (c). The NBCD experiment demonstrates that the magnetic axis shift can be reduced by driving the toroidal plasma current in the direction anti-parallel to equivalent plasma current (counter injection) without increase of the Shafranov shift due to lower the rotational transform. I) K. IChiguchi et a!. Nuc!. Fusion 33 (1993) 4S1 0.4 0.6 0.8 1.0 0.8 1.0 P p Fig. 1 (a) The shifts of magnetic axis measured by soft x-ray CeD camera (symbols) and calculated with VMEC code (lines) for three type of net current profiles in finite beta as a function of the net toroidal current (b) net toroidal current profiles using theoretical calculation (c) radial profiles of rotational transform fa; typical (JJ - (I.pl!)). broad (J, - (I·P'Y) and peaked (J3 - (1·pl!Y) current profiles with Jp ~ ·50 kA/l'ln <P> ~ 0.34 %. 20 

The Effect of Net Toroidal Current on Shafranov Shift

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