Luo Guangnan

Radiative Divertor Plasma Behavior in L-and H-Mode Discharges with Argon Injection in EAST

Introducing strong radiative impurities into divertor plasmas has been considered as an important way to mitigate the peak heat load at the divertor target plate for ITER, and will be employed in EAST for high power long pulse operations. To this end, radiative divertor experiments were explored under both low (L) and high (H) - mode confinement regimes, for the first time in EAST, with the injection of argon and its mixture (25% Ar in D2). The Ar injection greatly reduced particle and heat fluxes to the divertor in L-mode discharges, achieving nearly complete detached divertor plasma regimes for both single null (SN) and double null (DN) configurations, without increasing the core impurity content. In particular, the peak heat flux was reduced by a factor of ~6, significantly reducing the intrinsic in-out divertor asymmetry for DN, as seen by both the new infra-red camera and the Langmuir probes at the divertor target. Promising results have also been obtained in the H-modes with argon seeding, demonstrating a significant increase in the frequency and decrease in the amplitude of the edge localized modes (ELMs), thus reducing both particle and heat loads caused by the ELMs. This will be further explored in the next experimental campaign with increasing heating power for long pulse operations.

Molecular Dynamics Simulations of Deposition and Damage on Tungsten Plasma-Facing Materials by Tungsten Dust

Classical molecular dynamics has been used to study the interactions between tungsten (W) plasma-facing materials (PFMs) and dust grains. The impact velocity of dust grains is in the range from 324 m/s to 3240 m/s. The main effect of dust grains with low impact velocity is deposition. However, a material surface can be damaged by high velocity dust grains. The cumulative damage of impacting dust grains has also been take into account. When the impact velocity is low, no significant damage is detected but a porous firm forms on the surface. Serious damage can be produced on PFMs if the impact velocity is high.

Study of Scrape-Off-Layer Width in Ohmic and Lower Hybrid Wave Heated Double-Null Divertor Plasma in EAST

Edge profiles in Ohmic and lower hybrid (LH) wave heated discharges in EAST are presented. A comparison of the measured profiles is made with those from the theoretical prediction for the scrape-off-layer (SOL) width. The edge plasma parameters are diagnosed by a triple probe divertor diagnostic system and fast reciprocating probes at the outer mid-plane. The experimental results show that the SOL width of double-null (DN) divertor plasmas in EAST appears to exhibit a negative dependence on the power crossing the separatrix, which is consistent with the collisional SOL scalings of JET and Alcator C-Mod. This will provide useful information for extrapolation to the ITER SOL width scaling for power deposition.

Fabrication of W/Cu Graded Heat-sink Materials by Electroless Plating and Powder Metallurgy

Abstract The W/Cu graded heat-sink materials with high density were fabricated by electroless plating and powder metallurgy. The microstructure, interface and fracture of the graded heat-sink materials were observed using the field emission scanning electron microscope (FESEM). The mechanical properties of the materials such as bending strength and hardness were determined. The results show that the structure of each layer is uniform and dense. The components of the cross-section present a graded distribution. There is no obvious interface among layers. The relative density of three-layer W/Cu graded heat-sink materials reaches 99.2%. The average values of microhardness HV of the radiating layer, the transitional layer and the sealing layer are 2000, 2100, 2400 MPa, respectively. It is indicated through the experimental results of bending strength that the bending strengths of the sealing layer and the radiating layer as the load-bearing surfaces are 428.5 and 480.7 MPa, respectively.

Preparation and Properties of W-15Cu Composite by Electroless Plating and Powder Metallurgy

Abstract W-15Cu composite samples were prepared by electroless plating (for composite powder) and powder metallurgy (for sintering samples). Starting W powder was activated by a chemical pretreatment first, and then the pretreated W powder was electroless plating to get Cu-cladded W powder, which was pressed into green compacts at different pressing pressures. Afterward, the composite was sintered into W-15Cu composite samples, and Cu-cladded W powder was characterized. The relative density, hardness, compressive strength and electrical conductivity of the composite samples were tested. The morphologies of the Cu-cladded W powder and the samples were investigated. Results show that the prepared powder has good compaction behavior with high purity, uniform particle size and a dense Cu cladding layer. W particles still remain sole ones and no obvious grain coarsening appears in the sintering samples, while 3D Cu network is all over the structure of the samples. The electrical conductivity and the compressive strength of the sample change with pressing pressure and sintering temperature.

Thermal Performance and Microstructure of Vacuum Plasma Sprayed Tungsten Coatings under Cyclic Heat Load

Abstract Tungsten coatings on copper substrate were prepared by vacuum plasma spraying (VPS). NiCrAl and W75Cu25 materials were selected as intermediate layers. The experiment result of cyclic heat load of electron beam with 5 MW/M 2 (heat flux) and 2 s pulse duration indicates that NiCrAl interlayer improves the thermal conduction of the coatings and reduces their thermal and residual stress. The specimen with W75Cu25 interlayer fails with a bad thermal performance because of fabrication problems. The microstructure characterization of the VPS-W coating shows that erosion crater and fine microcracks appear on the tungsten coating after cyclic heat load tests. And erosion crater has a coarse and loose microstructure. Local plastic deformation occurs in the coating because of thermal stress. Cracks originate from debonding of molten tungsten particles at high temperature. However, the crack propagation is constrained by local plastic deformation and pores in the tungsten coating.

Deuterium Retention in SiC-Coated Graphite Tiles of EAST

Fuel retention and recycling in plasma facing materials is a crucial issue for fusion devices, especially for the long pulse discharge devices. In this work, the deuterium retention and the surface erosion of SiC-coated graphite tiles exposed to EAST plasmas have been studied by post-mortem analyses, i.e., thermal desorption spectroscopy (TDS), secondary ion mass spectroscopy (SIMS) and scanning electron microscopy (SEM). The results show that the sample cut from the high field side (HF) tile has been intensely eroded due to deuterium bombardment on plasma facing surfaces in the initial phase of discharges and trapped highest amount of deuterium. Lower deuterium retention has been found in the inner divertor sample, which is presumably due to the particular exposure history in the 2010 spring campaign.

Temperature of the Limiter Surface Measured by IR Camera in HT-7 Tokamak

Temperature measurement by IR (infrared) camera was performed on HT-7 tokamak, particularly during long pulse discharges, during which the temperature of the hot spots on the belt limiter exceeded 1000°C. The heat load on the surface of the movable limiter could be obtained through ANSYS with the temperature measured by IR-camera. This work could be important for the temperature measurement and heat load study on the first wall of EAST device.