Internal Transport Barrier for Electrons in JT-60U Reversed Shear Discharges

Abstract

A new type of internal transport barrier (ITB) has been observed in JT-60U reversed shear discharges. It accompanies clear electron temperature and density pedestals and significant reduction of effective thermal diffusivities of electrons and ions (x eff e and x eff i ); x eff e sharply drops by a factor of 20 within 5 cm while x eff i is smaller than the conventional neoclassical value by a factor of 4 or more. The ratio of ion temperature to electron temperature was less than 1.5 inside the ITB. The ITB lies in the negative shear region and extended beyond 60% of the plasma minor radius. High density plasmas with high confinement have been obtained with the formation of ITB. [S0031-9007 (97)02736-1] PACS numbers: 52.55.Fa, 52.25.FiA reversed shear configuration, which has negative magnetic shear in the inner region and positive magnetic shear in the outer region, has been proposed as an advanced tokamak operation [1][2][3]. The magnetic shear is defined as ͑r͞q͒dq͞dr, where q is the safety factor and r is the volume-averaged minor radius. The reversed shear configuration has a possibility of economical steadystate tokamak reactor with high b, good confinement, and a large bootstrap current fraction. Here, b is the ratio of plasma pressure to magnetic field pressure. In experiments, reversed shear configurations were formed by pellet injection [4], initial current ramp with neutral beam (NB) heating [5-7], off-axis lower hybrid current drive (LHCD) [8], off-axis electron cyclotron heating (ECH) [9], and so on, and confinement improvement or formation of transport barrier in the negative shear region was observed. Furthermore, internal transport barriers were observed for the ion temperature in JT-60U high b p plasmas [10] (b p is the ratio of plasma pressure to poloidal magnetic field pressure), which had monotonic q profiles [11].In previous beam heating experiments [5,7,10], the reduction of the ion thermal diffusivity ͑x i ͒ or peaking of ion temperature ͑T i ͒ profile is reported but the reduction of electron thermal diffusivity ͑x e ͒ is not clear. In JET pellet enhanced performance mode, the reduction of x e is not known since the electron and ion power balances or x i and x e are not separated [12]. Clear reduction of x e in the negative shear region is reported in LHCD or ECH experiments [8,9]. However, it should be noted that the reduction of x e in these experiments is caused mainly by off-axis power deposition of lower hybrid or electron cyclotron waves rather than the spatial change of electron temperature ͑T e ͒ gradient. The inward electron heat flux (negative x e ) was observed in DIII-D off-axis ECH experiments [13], which is considered to be due to the profile resilience. This profile resilience effect may have some contribution to the reduction of x e in these experiments. Though the fast wave current drive experiments with reversed shear in indicates the reduction of x e with on-axis heating, x e decreases continuously toward the axis and the effect of negative magnetic shear on x e profile ...

Damping of toroidal ion temperature gradient modes

“…The reductions of the growth rate due to the negative magnetic shear and due to the nonzero k are confirmed, which are consistent with the internal transport barrier ͑ITB͒ formation observed in the large tokamaks with the negative magnetic shear and sheared rotation. [20][21][22] However, more elaborate investigation of the ITB formation is done by considering simultaneously several other effects, which are not included in this work, such as negative shear stabilization of the trapped-electron mode and stabilization of the ITG mode due to the Shafranov shift resulting from the increase of the safety factor in the core region for the negative shear case. 26 …”

“…[20][21][22] However, the results in these works only showed the dependence of positive growth rates on the magnetic shear. In Fig.…”

Experimental study of the bifurcation nature of the electrostatic potential of a toroidal helical plasma

“…Some of the patterns are quite important for fusion application owing to its association with formation of transport barrier. Among new achievements of internal transport barriers in tokamaks, [29][30][31][32][33][34] the internal transport barrier for electron thermal energy is confirmed for the first time in the CHS as the toroidal helical plasmas. The internal transport barrier is associated with the bifurcation nature of the radial electric field.…”

Dynamic procedure for filtered gyrokinetic simulations

“…the magnetic shearŝ. The effects of the latter on plasma microturbulence has been the subject of numerous experimental [19][20][21][22][23] , theoretical 24 , as well as numerical [25][26][27][28] studies. In this context, it was also found that negative magnetic shear can help improve the plasma confinement in a tokamak by decreasing the level of turbulence.…”