Formation and Sustainment of Current Hole in JT-60U
T. Fujita, Y. Miura, T. Suzuki, S. Ide, H. Takenaga, T. Oikawa, A. V. Chankin, Y. Sakamoto, Y. Koide, T. Hatae, O. Naito, A. Isayama, Y. Kamada, K. Hamamatsu, H. Shirai and N. Hayashi
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
Abstract.
A stable tokamak plasma with nearly zero toroidal current in the central region (a "current hole") has been sustained for several seconds in the JT-60U tokamak for the first time [1]. In a typical discharge with q95 ~ 5, the radius of the current hole extended up to ~40% of plasma minor radius. The absolute value of safety factor at the axis q(0) was estimated to be greater than 70. The current hole with a normalized radius of > 0.25 was maintained for 4-5 s (~10τE) without any global MHD instabilities. Though the temperature and density profiles were extremely flat in the current hole, internal transport barriers (ITBs) were formed outside the current hole, and high temperature plasmas with Ti(0) > 10 keV were confined in the current hole by ITBs. These observations imply the possibility of stable operation of tokamaks with no toroidal current at the axis.
When q(0) =100, the banana width of thermal electrons is calculated to be several cm and hence EC wave injection into the current hole should generate peaked profiles of heat deposition and driven current. However, expected changes in Te(r) or j(r) were not observed inside the current hole. This indicates that either q(0) is much higher than 100 or large anomalous transport exists and hence the electron confinement is absent in the current hole.
It was observed that j(0) started to decrease and the current hole was fully formed during the increase of off-axis bootstrap current in the ITB layer. This indicates that the decrease in the toroidal electric field Eφ(ρ), which is caused by the increase of the off-axis non-inductive current through the Faraday law, is a cause of the formation of the current hole. This process was also involved in the formation of the current hole in JET where the lower hybrid current drive supplied the off-axis non-inductive current [2].
The above process can lead to a negative j(0) in principle, but no significant negative j(0) has been observed so far. This fact implies that the decrease in j(0) stops when it becomes zero. Furthermore, no current is generated with ECCD or NBCD in the current hole. These observations of clamping j(0) at zero level suggest that the current hole is not a result of a transient zeroEφ(ρ) near the axis, but rather of some kind of self-organized structure. One possibility is that Eφ(0) is negative but j(0) cannot be negative for some reason and is therefore kept zero. Recently, a resistive kink instability was proposed to explain the absence of negative j(0) [3], but this is unlikely to be the case in JT-60U where no MHD instabilities are observed.
[1] T. Fujita et al., Phys. Rev. Lett. 87, 245001 (2001).
[2] N. C. Hawkes et al., Phys. Rev. Lett. 87, 115001 (2001).
[3] G. T. A. Huysmans et al., Phys. Rev. Lett. 87, 245002 (2001).