Characteristics of double-peaked particle deposition at divertor target plates in the EAST tokamak

Abstract

Increasing the heat and particle deposition on the divertor target plates is an effective approach for reducing the extremely high heat load. Future tokamak fusion devices are expected to have a deposition width on the order of a millimeter. Recently, a double-peaked distribution (DPD) pattern of particle deposition at the divertor target plates has been observed in both deuterium and helium plasma discharges in the Experimental Advanced Superconducting Tokamak, which can significantly spread the deposition width of heat and particle fluxes, and clearly shows a toroidal symmetric distribution. It has been found that the DPD behavior occurs not only in plasma discharges with lower hybrid wave (LHW) heating, but also with electron cyclotron resonance heating or neutral beam injection (NBI) heating alone. In addition, the DPD shows an obvious in–out asymmetry between the divertor target plates, which strongly depends on the toroidal field direction, i.e. it distinctly appears on the outer divertor plate in unfavorable B t conditions, while shifting to the inner divertor plate under favorable B t conditions. Meanwhile, the transition between the single-peaked distribution and the DPD is normally correlated with the line-averaged plasma density and the auxiliary heating power. The statistical results for the pure LHW heating discharge scheme show that the DPD behavior is significantly affected by both the plasma density and the heating power, i.e. the lower the ratio of the plasma density to the heating power, the more likely it is that DPD behavior will occur. The critical boundary lines between the DPD and non-DPD patterns are obtained with the line expressions ∼ 2P LHW,abs + 1.4 and ∼ 3.33P LHW,abs + 1 for the unfavorable and favorable B t cases, respectively. Finally, the underlying physics is also discussed, suggesting that the electric field drifts and the local sheath boundary condition may exert a strong influence on the DPD.

Role of E × B drift in double-peak density distribution for the new lower tungsten divertor with unfavorable B t on EAST

“…The basic features are also consistent with Γ Ne scanning. Therefore, the seeding rate or input power scan qualitatively reproduces the evolution of the double-peak density profile observed in the density-or power-scan experiments carried out in DIII-D [5] and EAST [11].…”

“…In this paper, we carry out an analysis on the formation of the double-peak density profile with a focus on an unfavorable B t configuration. The simulation qualitatively reproduces the dependences of the doublepeak profile on heating power and density observed in experiments [5,11]. The results demonstrate that the formation of a doubly peaked density profile requires two conditions: (1) buildup of a broad high-density region by radial particle transfers via E p × B drift; (2) creation of a strong enough particle sink by reversed particle flux (pointing away from the target) originated from E r × B drift.…”

“…Furthermore, the reduced 2D model proposed in [10] showed that poloidal E × B drift flow in the open-field-line plasma plays an essential role in the formation of the double-peak density profile and acts as a bridge connecting the downstream divertor and the upstream plasmas. The main features of double-peak particle profiles observed in EAST experiments are summarized in [11]. The doubly peaked profile for SN configurations appears at the outboard divertor plate for unfavorable B t and at the inboard divertor plate for favorable B t (this applies to both the upper tungsten divertor and the previous lower graphite divertor).…”

“…Detached or partially detached divertor operations with acceptable levels of heat flux density and plasma temperature are required to facilitate steady-state scenarios within foreseen technologies for plasma-facing components. Plasma distributions and the associated convective and conductive transport in the divertor and scrape-off layer (SOL) are crucial in affecting the way of approaching divertor detachment [4][5][6][7].the Doubly peaked density and particle flux profiles have repeatedly been observed in many tokamaks, including but not limited to JET [8,9], DIII-D [5,10], and EAST [11]. The double-peak distribution can influence the plasma-wetted area where heat flux is deposited, and affect the way of divertor detachment.…”

“…This type of double-peak particle distribution is toroidally symmetric and can be observed in both deuterium and helium plasma discharges. It does not depend on heating methods, divertor target materials, plasma current (450-600 kA), nor toroidal magnetic field magnitude (2.0-2.5 T) [11]. Clarifying the detailed physical mechanism related to the formation of the double-peak pattern is beneficial to understanding the plasma distributions in edge plasmas dominated by complicated conductive and convective transport.…”