Particle control in long-pulse H-mode discharges of JT-60U

Nakano, Tomohide; Kubo, Hirotaka; Asakura, Nobuyuki; Takenaga, Hidenobu; Oyama, Naoyuki; Kawashima, Hisato; Fujimoto, Kayoko

Particle control is one of the key issues for steady-state operation in fusion reactors. In short discharges, the first wall absorbs hydrogen particles and it works as a pump (wall pumping). The wall pumping is effective to control the plasma density. However, in future tokamak devices, wall retention increases in a long discharge. Then, it is expected that the wall retention is saturated and the wall pumping does not work. Therefore, for steady-state operation, particle control by active divertor pumping without the wall pumping should be established. In JT-60U, long-pulse high-density H-mode plasmas have been sustained by divertor pumping under global wall saturation. The energy confinement and ELM activity can be sustained, while wall pumping does not work and even outgas appears. The outgas is attributed to increase in the divertor-plate temperature. On the other hand, wall pumping continues for longer than 20 s in high-density (ne 0.8 nGW) discharges with an X-point MARFE and detached divertor plasma. X-point MARFEs have also been controlled by the divertor pumping. As a result, the electron density control by the divertor pumping and the gas puffing is effective under the wall saturation. However, it should be noted that the wall-pumping rate changes to maintain the electron density against the control. The change in the wall pumping suggests dynamic equilibrium between particle flux and desorption as an important wall-pumping mechanism.