Tungsten control in type-I ELMy H-mode plasmas on EAST

The first experimental investigation of the tungsten behavior in ELMy H-mode plasmas with co-/counter neutral beam injection (NBI) and unfavorable/favorable B-t was performed on EAST. Tungsten was found to accumulate easily in ELMy H-mode plasma with co-NBI heating and unfavorable B-t. Thus, in this case the tungsten concentration can exceed 10(-4), resulting in degradation of the plasma confinement and periodic H-L transitions. To reduce the tungsten concentration in steady-state type-I ELMy H-mode operation, counter-NBI is applied to modify the density and temperature and brake the plasma toroidal rotation. The applied counter-NBI decreases the PHZ + E-r inward pinch velocity and reverses the direction of neoclassical inward convection, thus decreasing the tungsten concentration from similar to 7 x 10(-5) to similar to 2 x 10(-5) in type-I ELMy H-mode plasma with favorable B-t. A comparison of the effects of different B-t directions on the tungsten behavior also shows that favorable B-t is beneficial for reducing the tungsten concentration in the core plasma. These results imply that counter-NBI with favorable B-t can effectively prevent tungsten accumulation and expand the operating window for exploring steady-state type-I ELMy H-mode operation of EAST.

Automated summary

The experimental investigation on tungsten behavior in ELMy H-mode plasmas with different NBI directions and B-t orientations at EAST showed that tungsten accumulation can be reduced by counter-NBI with favorable B-t, thus improving plasma performance and extending the operational window.