Formation of current hole and its effects on ITB structures in JT-60U reversed shear plasmas
T. Fujita, T. Oikawa, T. Suzuki, Y. Sakamoto, T. Takizuka, S. Ide, A. Isayama, Y. Koide, T. Hatae, O. Naito, H. Shirai
Japan Atomic Energy Research Institute, Naka Fusion Research Establishment Naka-machi, Naka-gun, Ibaraki-ken 311-0193 Japan
Abstract.
A reversed shear (RS) configuration with internal transport barriers (ITBs) has been intensively studied as a candidate of advanced tokamak concept in which a high beta, high confinement and large bootstrap current fraction are required. The high performance RS plasmas in JT-60U [1] are characterized by box-type ITBs [2] with large radii and by a very high safety factor on axis, q(0). However, the exact value of q(0) was unknown because of insufficient accuracy of motional Stark effect (MSE) diagnostics, possible effects of radial electric field on MSE measurements and inability for our equilibrium code to deal with a high q(0) equilibrium.
Recently, these problems have been resolved by improvements on our MSE measurement system and equilibrium code and it has been observed that a region with nearly zero toroidal current or a "current hole" is formed and sustained stably for several seconds in a central region of JT-60U RS plasmas. The current hole is established by a negatively induced toroidal electric field through the increase of off-axis bootstrap current due to the heating starting at a low plasma current. The decrease of current density stops when it becomes nearly zero and the radius of current hole expands after that. It seems that the central current density can not become negative. In an extreme case, the radius of current hole extended up to ~35% of plasma minor radius. In this case, the central current density was estimated less than 8% of current density averaged over the plasma cross section; q(0) was larger than 50 or was more than ten times as high as the surface value of q, q95 (~5).
Since the confinement is poor in the current hole with little poloidal magnetic field, the density and temperature profiles become flat there. The flat regions, however, extend to larger radii even with taking into account of a banana width of thermal ions. This indicates that the radius of flat region or the location of ITB "shoulder" is not simply determined by a q profile. On the other hand, since the poloidal magnetic field is very small near the current hole, a local poloidal beta and a local bootstrap current fraction can be large even with a small pressure gradient. This in turn may restrict the pressure gradient and affect the ITB structure to keep the equilibrium. A precise profile measurement using a fast major radius scan has been attempted to evaluate the small pressure gradient inside the ITB shoulder. The results of equilibrium analysis based on these measurements will be reported at the meeting.
[1] T. Fujita et al., Nucl. Fusion 39 (1999) 1627.
[2] H. Shirai et al., Nucl. Fusion 39 (1999) 1713.