JT-60U MONTHLY SUMMARY

January 2008

  The following experiments were carried out in this month.

(1) Toroidal rotation and momentum transport
Beam perturbation techniques with perpendicular NBs were applied in H-mode plasmas with low torque input and without ITB in order to investigate the characteristics of momentum transport and toroidal rotation. The toroidal momentum diffusivity (χφ) and the convection velocity (Vconv) increased with increasing heating power over the whole radius. The ratio of χφi at r/a=0.5 was about 1-3, here χi is the ion heat diffusivity. The inward convection velocity (-Vconv) at r/a=0.5 increased with increasing χφ, and the value of Vconvφ was around -2 m-1.

(2) Dynamics of type-I ELM
The dynamics of edge impurity ions during a Type-I ELM cycle for different rotating plasmas, such as time evolution and radial perturbation to the ion temperature (Ti), toroidal (VT) and poloidal (VP) rotation including radial electric field (Er), were investigated. The absolute value of VT decreased but did not reach zero by each ELM for both co- and counter-rotating plasmas. On the other hand, change in edge VP due to an ELM was in the ion-diamagnetic direction. It is noted that the change in VT and VP both contributes to weakening Er.

(3) Confinement at high density with SMBI
Confnement degraded with SMBI as well as gas-puffing, while it was kept constant with shallow pellets from the high-field side (HFS). The change in confinement is closely related with a strong core-edge linkage. The pedestal pressure was enhanced with the HFS pellet injections with keeping clear ITBs. However, in the SMBI case, the pedestal pressure was the same level as that in the standard ELMy H-mode plasmas. Time behavior of Ti was investigated for understanding mechanisms of the confinement degradation with SMBI. The edge Ti quickly decreased and cold pulse propagated toward the central region. The Ti decrease had its maximum value at r/a~0.8 just after the SMBI pulse. Therefore, SMBI could directly affect the plasma parameters at r/a~0.8, although light from SMBI mainly emitted outside the separatrix. The Ti time evolution was well reproduced using the power balance χi estimated before the SMBI pulse.

(4) Development of full current drive plasma
FullCD plasma was developed using NBCD, LHCD and bootstrap simultaneously at Ip=0.8 MA, Bt=2.3 T, q95=5.8, βN=1.6, βp=1.5, and bootstrap current fraction fBS=0.5. Weak magnetic shear was sustained by off-axis LHCD and NBCD with bootstrap current for about 2 s (1.5 times the current relaxation time). The period was limited by the start of LH notching due to arcing interlock of LHRF system. The current profile that has qmin>2 was free from low-q MHD (sawtooth and m/n=3/2 or 2/1 NTM), and was fully relaxed at the end of the sustainment.

(5) Effects of ECH on internal transport barrier of ion temperature and electron density
Ti- and ne-ITBs degraded during central ECH in weak shear plasmas with Ip~1 MA and BT=2-3.7 T. Although the electron heat flux was increased with ECH, Te gradient was almost constant. This stiffness was ascribed to the increase in χe probably due to enhancement of short-spatial-scale fluctuations. At the same time, χi also increased, indicating existence of clear relation between electron heat transport dominated by short-spatial-scale fluctuations and ion heat transport dominated by long-spatial-scale fluctuations. The density fluctuation level in the frequency range of ITG did not increase with ECH. However, correlation length tended to be longer with ECH.