Plasma performance improvement with favourable B t relative to unfavourable B t in RF-heated H-mode plasmas in EAST

Self Cite

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.

Section: Effects Of the Heating Power And Plasma Density On The Dpd B...mentioning

confidence: 87%

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

Wang

et al. 2021

Self Cite

“…with the ion B × ∇B drift direction away from the active X-point, as well as the change of λ q with B T direction in ASDEX-Upgrade [43] and TCV [16], have been experimentally observed. Besides, experimental studies [41,44] on EAST revealed that plasmas with favorable B T tend to have steeper pedestal density gradients compared to plasmas with unfavorable B T , which has also been observed on DIII-D [40]. It is desirable to unfold the implications of these magnetic-field-direction dependent phenomena for λ q .…”

Section: Introductionmentioning

confidence: 90%

“…To reduce the uncertainty involved in the extrapolation of present scaling law to ITER and other future tokamak devices, it is critical to develop the fullest possible theoretical understanding of the sensitivities of λ q to plasma conditions. So far, experiments and simulations have been performed to investigate the effects of the magnetic field direction on L-H transition power threshold [33][34][35][36], divertor asymmetries [37,38], detachment [39], pedestal structure [40], core plasma performances [41] and so on. Recently, the flattened density profile, i.e.…”

Section: Introductionmentioning

confidence: 99%

“…To sum up, motivated by the highly accepted HD model [23], which provides excellent agreement with experimental data in terms of both absolute value and scaling of λ q (a key physics parameter for tokamaks) [11,12], many 2D edge plasma simulations [26][27][28][29][30] have been performed to capture the underlying mechanisms. So far, influences of the magnetic field direction on the formation of edge plasma profiles have been experimentally observed on many tokamaks (including DIII-D [40,42], ASDEX-Upgrade [43], TCV [16], EAST [41,44] and so on), but have not been considered in both the HD model and related 2D edge plasma simulations. For further insights into λ q , a series of numerical simulations are performed by using the comprehensive 2D edge plasma simulation code, SOLPS-ITER, based on a generic magnetic equilibrium of EAST, focusing on the drift-driven transport, especially the impact of magnetic field direction, results of which will be presented in this paper.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

SOLPS-ITER analysis of drift effects on plasma profiles in the EAST scrape-off layer

Wang¹,

Liang²,

Zha³

et al. 2022

Self Cite

Drift effects on the plasma profiles in the scrape-off layer (SOL) of the experimental advanced superconducting tokamak (EAST) have been numerically investigated using the comprehensive 2D edge modeling package, SOLPS-ITER, based on a generic magnetic-equilibrium with lower single null (LSN) configuration. SOL particle diffusivity (DSOL) has been scanned from large (1.0 m2/s) to extremely low (0.02 m2/s) to gradually highlight the role of the drift-based neoclassical mechanisms in the radial particle transport. To address the impact of magnetic-field-direction on the drift-driven transport, plasma profiles, flows and currents in the SOL of EAST plasmas with the toroidal magnetic field (BT) direction favorable and unfavorable for H-mode access, i.e. with the ion B×B drift pointing towards and away from the active X-point, are simulated and analyzed. Results demonstrated that the drift-driven transport, considered as the key process in the formation of SOL plasma profiles, is magnetic-field-direction dependent and thus SOL flows and currents as well as the SOL widths can be obviously affected by the direction of drifts. With BT changed from the favorable direction to the unfavorable one, the flattening of density radial profile as well as the increase of power decay length, in the SOL, can be achieved and can be further enhanced as the weight of turbulent transport (i.e. DSOL) gets reduced, due to the increased contribution of ion parallel viscosity to the radial ion flow. Particularly, with DSOL ≤ 0.05 m2/s in the simulations, the dominant role of drift-based neoclassical mechanisms in the radial particle transport will lead to the formation of the so-called edge density-shelf in plasmas with unfavorable BT. Power scrape-off width in plasmas with unfavorable-BT is much insensitive to the turbulent transport level and can remain to be relatively high even when DSOL has been decreased to be extremely low. Due to the compressing/widening effect of the drift-driven inward/outward radial particle flow, the simulated power scrape-off width exhibits an in-out asymmetry, which is also magnetic-field-direction dependent. This work represents a step towards deeper understanding of physics mechanisms determining SOL widths in EAST.

“…The I-mode operation regime on EAST [12] has been widely achieved in D plasma in the unfavorable B T divertor configuration (B × ∇B directed away from the active X-point), where the L-H transition threshold power is significantly higher and the energy confinement is much worse than those at the favorable configuration [13]. The access to the I-mode regime in D plasma on EAST was found to be independent from the heating method and wall material.…”

Section: Introductionmentioning

confidence: 98%

I-mode operation in helium plasma with pure radio frequency wave heating and ITER-like tungsten divertor on EAST

Zhang¹,

Gong²,

Qian³

et al. 2021

Self Cite

The characteristics of I-mode operation in helium (He) plasma with radio-frequency (RF) wave heating and ITER-like tungsten divertor have been investigated for the first time on EAST, under the favorable B T configuration. Stationary I-mode in He plasma lasting for ∼5.4 s was achieved at I p = 0.5 MA, B T = 2.4 T with a total injected RF power of 3.5 MW, along with the appearance of the 30–90 kHz weakly coherent mode and the ∼13 kHz edge temperature ring oscillation. The pulse length was only limited by the heating power duration. He concentration was found to play a critical role in I-mode operation, as L–I transition threshold power was increased and the energy confinement was degraded with increasing He concentration. The experimental investigation result on energy confinement and divertor heat flux between I- and H-mode plasma was also given at a similar He concentration.