Zonal flows in the geodesic acoustic mode frequency range in the JIPP T-IIU tokamak plasmas

Abstract

Large potential oscillations were detected in JIPPT-IIU tokamak plasmas in a wide range of plasma cross-sections in measurements using a multi-sample-volume heavy ion beam probe. These oscillations have large amplitudes reaching a few hundreds of volts and their frequencies are in the range of the geodesic acoustic mode (GAM). They are found over a wide range of plasma cross-sections and commonly have m = 0 structures. As they were Fourier analysed, it was found that the central frequency is higher in the core of the plasma and lower in the edge of the plasma. These observations agree with the properties of theoretically predicted GAM oscillations. It was also found that the frequency spectrum is peaked in the core and broad in the edge, which may have something to do with damping mechanisms of the GAM. The phase relation between the density and the electric field fluctuations was studied extensively in terms of the cross-correlation function. The level of the density fluctuation was low as it should be, and the expected 90° phase difference was found in a limited radial domain.

Extension of geodesic acoustic mode theory to helical systems

“…In the JIPP-T-II U device, a low density Ohmic plasma was used where Te Ͼ 3Ti and thus the damping rate is estimated to be very small. 22 This feature resembles to that of the ion acoustic mode, where frequency is determined by electron temperature and therefore the phase velocity of the wave can be higher than the thermal speed of ions. The GAM was found only in the region Ͼ 0.85 in the DIII-D experiment.…”

“…22 Thus the magnetic field has a single dominant Fourier component, i.e., toroidal ripple ␦ m=1,n=0 ͑ ͒ ϳ ͑2 ͒ = 2͑r/R͒.…”

“…The existence of zonal flows including GAM has been recently demonstrated in tokamak experiments in DIII-D͑Doublet-III-D͒ 19 by use of beam emission spectroscopy ͑BES͒, and in TEXT͑Texas Experimental Tokamak͒, 20,21 JIPP-TII U͑Japanese Institute of Plasma Physics Tokamak͒, 22,23 and JFT-2M͑Japan Atomic Energy Research Institute Fusion Torus 24 by use of heavy ion beam probe ͑HIBP͒, respectively. Similar oscillations of potential were found also in the helical device CHS͑Compact Helical System͒, 25 in which the mode structure has been clearly elucidated by use of a dual HIBP.…”

Heavy Ion Beam Probe, Present Status and Future Development

“…2(b) and 2(c) [23][24][25][26]. The power source (HVPS) for the electrostatic acceleration up to 500 keV is originally developed for electron microscopes.…”

“…The shape of SVs is like a rhombus and the longer diagonal of the rhombus is about 5 mm, although its dimensions and shape are different on the position of the steps. These values are listed on Table 1 [24]. Figure 2 shows the time trace of the density and potential at A4 (core area, r/a p 0.2), and C2 (outer area, r/a p 0.8) of the low-density OH plasmas.…”