Control of hydrogen content and fuel recycling for long pulse high performance plasma operation in EAST

Control of fuel recycling and hydrogen content in the Experimental Advanced Superconducting Tokamak (EAST) using various methods is studied for high-power long pulse high confinement mode (H-mode) plasma operation. The results show that long duration first wall baking and discharge cleaning in EAST provides the necessary clean vacuum environment with a high ultimate vacuum of 3.6 × 10^-6 Pa and low outgassing rate of ∼1.5 × 10^-4 Pa m³ s^-1 for plasma operation. In-vessel cryopumps are found to provide an efficient method for particle recycling control in the divertor region to decrease global recycling coefficient (R _global) from ∼1.0 to ∼0.8 during ohmic heating plasmas, and it provides a high particle exhausting rate of 10²⁰-10²¹ D-atoms s^-1 in high-power plasma operations. Long duration silicon coating is found to be more powerful than baking and discharge cleaning, which reduces the H/(H + D) ratio gradually to a value as low as 8%, and lithium coating is more effective and more efficient than silicon coating, to further reduce the H/(H + D) ratio to 3%. Real-time lithium powder injection is a novel method for recycling control, which reduces R _global from 0.94 to 0.82 under lithium coating wall conditioning. By the combination and optimization of the above methods, H-mode plasmas with low hydrogen content and low fuel recycling have been achieved in EAST and have been extended gradually to 101 s in 2017. Recycling flux has even decreased gradually during 101 s H-mode plasmas due to increased first wall temperature and the resulting increased Li-II emission. These results provide valuable references on hydrogen content and fuel recycling control for long pulse H-mode plasmas of up to 400-1000 s with high-power heating in EAST and future fusion devices.