Estimation of tungsten production from the upper divertor in EAST during edge localized modes

During edge localized modes(ELMs), the sheath evolution in front of the Experimental Advanced Superconducting Tokamak(EAST) upper divertor is studied to estimate the sputtered tungsten(W) atoms from the divertor target. A large potential drop across the sheath is formed during ELMs by compared with inter-ELMs, and the maximum of sheath potential drop can exceed one thousand of e V in current EAST operation. Due to the enhancement of the sheath potential drop during ELMs, the W physical sputtering yield from the deuterium(D) ions and the impurity ions on the upper divertor target is found to be significant. It is established that the sputtered W yield during ELMs is at least higher by an order of magnitude than inter-ELMs, and D ions and carbon(C) ions are the main ions governing the W production for the current H-mode with ELMs discharges. With increase in the pedestal electron temperature, the maximum of the D and C ion impact energy during ELMs shows a nearly linear increase, and the D ions have sufficient impact energy to cause the strong W physical sputtering. As a consequence, the D ions may dominate the sputtered W flux from the divertor target when the C concentration is controlled less than one percent for the higher heating power H-mode with ELM discharges in near future.

Edge localized modes suppression via edge E × B velocity shear induced by RF sheath of ion cyclotron resonance heating in EAST

The control of large edge localized modes(ELMs) is a critical issue for the successful operation of future burning plasma devices,such as the international thermonuclear experimental reactor(ITER) and China fusion engineering test reactor(CFETR). In this paper, we present a new active and effective means of ELM suppression using ion cyclotron resonant heating(ICRH) on the experimental advanced superconducting tokamak(EAST). We obtained the key role of the external E × B velocity shear near the pedestal top and the scrape-off-layer(SOL) induced by the RF sheath potential of ICRH in ELM suppression. The experimental results showed a positive correlation between the RF sheath and the E × B shear rate in SOL. BOUT++ simulations indicate that increased E × B velocity shear rates in the pedestal and SOL regions promote ELM suppression;thereby, supporting the experimental observations on EAST. These findings suggest a new simple approach to access the ELM suppressed regimes in plasma with low torque input as ITER baseline discharges.

Investigation of lower hybrid current drive during H-mode in EAST tokamak

H-mode discharges with lower hybrid current drive （LHCD） alone are achieved in EAST divertor plasma over a wide parameter range. These H-mode discharges are characterized by a sudden drop in Da emission and a spontaneous rise in main plasma density. Good lower hybrid （LH） coupling during H-mode is obtained by putting the plasma close to the antenna and by injecting D2 gas from a pipe near the grill mouse. The analysis of lower hybrid current drive properties shows that the LH deposition profile shifts off axis during H-mode, and current drive （CD） efficiency decreases due to the increase in density. Modeling results of H-mode discharges with a general ray trueing code GENRAY are reported.

Observation of chaotic ELMs in HL-2A tokamak

The high confinement mode （H-mode） operation is recently obtained in HL-2A divertor configuration, the corresponding edge localized mode （ELM） is recognized as being of type III. Time intervals in ELM time series are analysed to obtain the information about the ELM process. Signatures of unstable periodic orbits （UPOs） are detected, which are indicators of chaos and may be used to control the big ELM events.

Morphological and structural damage investigation of nanostructured molybdenum fuzzy surface after pulsed plasma bombardment

Steady high-flux helium(He)plasma with energy ranging from 50 eV to 90 eV is used to fabricate a fiber-form nanostructure called fuzz on a polycrystalline molybdenum(Mo)surface.Enhanced hydrogen(H)pulsed plasma in a wide power density range of 12 MW/m^(2)-35 MW/m^(2)is subsequently used to bombard the fuzzy Mo,thereby simulating the damage of edge localized mode(ELM)to fuzz.The comparisons of surface morphologies,crystalline structures,and optical reflectivity between the original Mo and the Mo treated with various He^(+)energy and transient power densities are performed.With the increase of He ion energy,the Mo nano-fuzz evolved density is enlarged due to the decrease of filament diameter and optical reflectivity.The fuzz-enhanced He release should be the consequence of crystalline growth and the lattice shrinkage inside the Mo-irradiated layers(^(2)00 nm).The fuzz induced by lower energy experiences more severe melting damage and dust release under the condition of the identical transient H plasma-bombardment.The H and He are less likely to be trapped due to aggravated melting evidenced by the enhanced crystalline size and distinct lattice shrinkage.As the transient power density rises,the thermal effect is enhanced,thereby causing the fuzz melting loss to aggravate and finally to completely disappear when the power density exceeds 21 MW/m^(2).Irreversible grain expansion results in huge tensile stress,leading to the observable brittle cracking.The effects of transient thermal load and He ion energy play a crucial role in etching Mo fuzz during ELM transient events.

Properties of plasma radiation during discharges with improved confinement on HL-2A Tokamak

In the recent experiment on the HL-2A tokamak, two types of improved confinement regimes have been achieved in different configurations. One is the improved confinement regime in limiter configuration during electron cyclotron resonant heating （ECRH）, characterized by a sharp decrease in Hα emission accompanied by an increase in the total radiation of plasma, the line averaged electron density and the stored energy of plasma. The other is high confinement regime （H-mode） in divertor configuration during a combination of ECRH and Neutral beam injection （NBI） heating, characterized with edge localized modes （ELMs） besides the features mentioned above. The ELMs are found to be localized on the plasma edge （r/a ≥ 0.8）, causing average losses of particles and stored energy in the ranges of about 1 3% and 3-5% respectively during a single ELM event. So far, the ELMs observed in the HL-2A are type Ⅲ ELMs with low amplitude and high repetition frequency in a range from 200 Hz to 350 Hz. An investigation of the radiated power density profiles shows that radiative cooling effect plays a significant role in the transition back to the L-mode and the triggering of ELM events.