In the JT-60 upgrade, the operation of divertor plasmas with high triangularity configuration was performed for improving the plasma performance since 1995. For this high triangularity operation, the plasma shaping capability, the vertical field coil was divided into two blocks and controlled independently. One is used for the plasma radial position control, and the other is dedicated for the triangularity control. The power supply was modified step by step according to increasing the plasma current. Followings are major modification items: (1) bus-bar switches for changing the combination of the poloidal field coils and the power supplies, (2) rearrangement of thyristor converters and (3) modification of the control system. As a result, the triangularity has been increased from the original value of ~ 0.1 to ~ 0.6 at the plasma current of 1 MA and ~ 0.4 at 2 MA.

Although the plasma performance was much improved, some issues on the plasma control were found.

(a) Control of the horizontal field coil current
An over-current fault in the thyristor converters for the horizontal field coil was often observed in conjunction with MHD instabilities. The oscillations detected with magnetic sensors produces high frequency components in the current control reference through the feedback control system. Since the converter banks are connected in anti-parallel so as to supply bi-directional DC currents, the circulating current through the converters increases to the excessive level by the natural commutation characteristics of the thyristor converter. Hence, we have introduced a new control algorithm using a derivative limiter to overcome this problem.

(b) X-point control
Damage of the in-vessel components like divertor tiles was caused by faults in the divertor separatrix line control. When the inner leg entered a gap between the rows of the divertor armor tiles and the first wall, energetic particles stroke the cooling pipes for the tiles. Thus, the pipes melted due to the excessive heat load and nitrogen gas leaked out. This means that the radial position control of the X-point is required for keeping the hitting points within the divertor plates in addition to the control of X-point height. Then, we introduced it instead of the averaged triangularity control.

The results of the issues and their measures in the operation with the new divertor will be presented in this conference.