Electrostatic fluctuations at the edge of the CT-6B tokamak

Abstract

Langmuir probes and a Mach probe were used for measuring edge fluctuations and parallel plasma flow, respectively, on the CT-6B tokamak. A maximum radial gradient in parallel flow was observed to be located roughly at the maximum Er shear layer (or, namely, the poloidal velocity shear layer). Er shear is found to be very possibly consistent with a regulating effect on edge turbulence features. Non-linear analysis indicates that non-linear phase coupling in turbulence may be influenced by Er shear, or that there exist coherent structures induced by strong Er shear, and the phase coupling processes show an intermittent character. The results provide some new observations for a better understanding of the basic mechanisms of edge turbulence.

Effect of poloidal velocity shear on the edge fluctuations in the SINP tokamak

“…Figure 4 shows the radial profiles of the fluctuation levels. The radial profile of the normalized density fluctuation ñ/n is very different from that of the normalized potential fluctuations φf /T e , indicating a deviation from the Boltzmann relationship, as observed on other tokamaks also [19,24]. The temperature fluctuation level Te /T e is 20-35% and is not negligible, as assumed by several authors previously.…”

“…The propagation vector k θ (and so the phase velocity in the laboratory frame) is affected by the E r × B rotation and also reverses sign at r/a ≈ 1. A strong shear dV E×B /dr in the poloidal velocity exists and appears to increase in the SOL, contrary to the observations of localized shear layer on other tokamaks [9,19]. We therefore take the region from r/a ≈ 0.95 to ≈1.05 (where dV E×B /dr > 0.5 × 10 6 s −1 ) as the shear region.…”

“…Though fluctuations are seen to be suppressed during such transitions, there is growing evidence that the drop in the fluctuation level is not the only cause of the reduction in transport. The modification of the phase angle between the density/pressure and potential fluctuations also plays a significant role as shown by numerical simulation [15], analytical calculations [16,17] and experiments [18][19][20]. The general result is that the cross-phase is as strongly (or sometimes even more strongly) suppressed by shear as the amplitude, although there has been some theoretical result to the contrary [21].…”

Investigation of Poloidal Phase Velocity of Floating Potential Fluctuations in the IR-T1 Tokamak Edge Plasma

“…This phenomena is also observed in other tokamaks and stellarators. [4,5,6] With MATLAB software and Fourier Transform (FFT) technique, two-point cross-correlation between two probes signals in poloidal direction, is calculated [7,8,9]. The poloidal phase velocity is determined with ratio of mutual distance of two poloidally separated probes (d) over the time delay of the signal appearing at these two probes (t), = ⁄ .…”

The plasma current profile during current reversal in AC operation of the CT-6B tokamak

“…Its major radius is 45 cm and the minor radius of the plasma is 12.5 cm. In normal tokamak discharges [10], the toroidal field is 6-13 kG, the plasma current is 10-30 kA and the plasma duration is 40-50 ms.…”