Numerical Investigation on the Ground Response of a Gob-Side Entry in an Extra-Thick Coal Seam

This study was aimed at the large deformation phenomenon of rock mass surrounding the gob-side entry driven in a 20 m extra-thick coal seam. Taking tailgate 8211 as the engineering background, a numerical investigation was employed to analyze the deformation law of the gob-side entry. The study results are as follows. (1) Because the immediate roof was composed of weak coal mass with a thickness of 17 m, the roof coal mass was vulnerable to fail with the effect of overlying strata pressure; thus, a visual subsidence of roof coal mass with a maximum convergence of 800 mm was observed in the field. (2) The bearing capacity of the coal pillar was significantly less than that of the panel rib, resulting in the pillar failing more easily under the ground pressure and then generating large-scale squeezing deformation. (3) The roof and panel rib were in a state of shear failure with a failure depth of about 5 m. The coal pillar was entirely in a state of plastic failure. (4) A support scheme including an asymmetric anchor beam truss, roof angle anchor cable, and anchor cable combination structure was proposed. The field work confirmed that this support scheme could efficiently control the deformation and failure of the rock mass surrounding the gob-side entry. This study provides the theoretical basis and technical support for the control of rocks surrounding the gob-side entry in similar conditions

Air contamination and its effect on deuterium desorption in liquid lithium

The clean wall condition in tokamak devices can be achieved by utilizing the active chemical property of lithium (Li). This also exposes Li to air contamination even when protected by inert gas. To determine whether air contamination affects the retention of hydrogen isotopes in Li, the desorption performances of deuterium (D2) in liquid Li with D/Li ratios of 4.1 mol.%, 2.5 mol.%, and 0.6 mol.% were compared. At a D/Li ratio as low as 0.6 mol.%, the results revealed a significant change in the desorption temperature of D2 by Li contaminants. The ratio of contaminants to D2 in Li was discovered to be important. In this regard, the influence of N2, O2, and H2O (accounting for 15 mol.% of D2) on the desorption performance of D2 in liquid Li was discussed. Both H2O and N2 preabsorbed in liquid Li could react with D2, leading to the changed desorption temperature of D2. Notably, the H2O present in Li consumed all of D2 via reaction, whereas the same amount of N2 as H2O could only react with a part of D2. However, the effect of O2 on the desorption temperature and the effect of impurities on the desorption amount of D2 were not obvious. The impacts of the findings of this work on the retention and extraction of hydrogen isotopes in Li may be further explored to prompt the application of Li in fusion devices

Study on corrosion behavior of China low activation ferritic/martensitic steel in static liquid lithium

Liquid lithium (Li) is a candidate material for the first wall and blanket coolants/breeders in fusion devices. Reduced activation ferritic/martensitic (RAFM) steels are also considered primary candidate structural materials for fusion reactors. Thus, the compatibility of RAFM steel with liquid Li is one of the key issues for liquid Li first walls and blankets. In this research, the corrosion behaviors of China low-activation ferritic/martensitic steel (CLF-1) in static liquid Li at 620 and 820 K under an argon atmosphere were investigated. After exposure to liquid Li, CLF-1 steel underwent slight changes in mass, while its corrosion resistance level remained at Grade 1. Due to the selective dissolution of C, Cr, Mn, and Fe and chemical reactions, grain boundary corrosion and corrosion particles of Cr carbides were observed on the corroded CLF-1 steel surface. However, the corrosion of CLF-1 steel under 820 K liquid Li was more severe than that at 620 K, and pitting corrosion occurred on the corroded CLF-1 steel surface. After exposure to 620 K of liquid Li, the hardness of the CLF-1 steel surface was the same as the initial hardness, while it decreased after exposure to 820 K of liquid Li. However, the tensile properties of CLF-1 steel showed no changes after exposure to liquid Li. Additionally, Ni and NixMny alloys were detected due to mass transfer from the 304 SS vessel to the CLF-1 surface. The results indicated that the corrosion resistance of CLF-1 steel is better than that of 304 SS and 316L SS in liquid Li

Active wall conditioning through boron powder injection compatible ELM control in EAST

Real-time wall conditioning and edge-localized mode (ELM) suppression through real-time boron powder injection into high-confinement mode discharges of the Experimental Advanced Superconducting Tokamak (EAST) have been investigated. The boron powder injection effectively conditioned the plasma-facing components, as shown by the reductions in low-Z and high-Z impurities and the global recycling level in the whole inner vessel, and suppressed the strong magnetohydrodynamic (MHD) activity in the plasma core. The improved wall conditions also resulted in an earlier transition from L- to H-mode. Boron powder injection also effectively suppressed ELM in the lower single null configuration with BT in the favorable direction while leaving the plasma density and stored energy largely unaffected

Experiments of continuously and stably flowing lithium limiter in EAST towards a solution for the power exhaust of future fusion devices

Liquid lithium (Li) can partly ameliorate lifetime and power-exhaust issues of plasma facing components (PFCs) by enabling a self-healing, self-replenishing surface with a reduced susceptibility to neutron damage in future fusion devices. To assess operational stability and heat-exhaust capability under tokamak exposure, two generations of continuously flowing liquid Li (FLiLi) limiters on the concept of a thin flowing Li film have been successfully designed and tested in high performance discharges in EAST. The design uses a circulating Li layer with a thickness of <0.1 mm and a flow rate ∼2 cm3s−1. In addition, the limiter employs a novel electro-magnetic pump to drive liquid Li flow from a collector at the bottom of the limiter into a distributor at its top. Free surface gravitational flow closes the loop for a continuously flowing liquid Li film on the wetted PFC. Here we summarize key FLiLi limiter development and experimental results in H-mode plasmas. Keywords: Limiter, Lithium, Plasma facing component, EAS

Injected mass deposition thresholds for lithium granule instigated triggering of edge localized modes on EAST

The ability of an injected lithium granule to promptly trigger an edge localized mode (ELM) has been established in multiple experiments. By horizontally injecting granules ranging in diameter from 200 microns to 1mm in diameter into the low field side of EAST H-mode discharges we have determined that granules with diameter > 600 microns are successful in triggering ELMs more than 95% of the time. It was also demonstrated that below 600 microns the triggering efficiency decreased roughly with granule size. Granules were radially injected from the outer midplane with velocities ~ 80 m/s into EAST upper single null discharges with an ITER like tungsten monoblock divertor. These granules were individually tracked throughout their injection cycle in order to determine their efficacy at triggering an ELM. For those granules of sufficient size, ELM triggering was a prompt response to granule injection. By simulating the granule injection with an experimentally benchmarked neutral gas shielding (NGS) model, the ablatant mass deposition required to promptly trigger an ELM is calculated and the fractional mass deposition is determined.readme, digital data file

ELM elimination with Li powder injection in EAST discharges using the tungsten upper divertor

We report the first successful use of lithium (Li) to eliminate edge-localized modes (ELMs) with tungsten divertor plasma-facing components in the EAST device. Li powder injected into the scrape-off layer of the tungsten upper divertor successfully eliminated ELMs for 3-5 sec in EAST. The ELM elimination became progressively more effective in consecutive discharges at constant lithium delivery rates, and the divertor D-alpha baseline emission was reduced, both signatures of improved wall conditioning. A modest decrease in stored energy and normalized energy confinement was also observed, but the confinement relative to H98 remained well above 1, extending the previous ELM elimination results via Li injection into the lower carbon divertor in EAST [J.S. Hu et al., Phys. Rev. Lett. 114 (2015) 055001]. These results can be compared with recent observations with lithium pellets in ASDEX-Upgrade that failed to mitigate ELMs [P.T. Lang et al., Nucl. Fusion 57 (2017) 016030], highlighting one comparative advantage of continuous powder injection for real-time ELM elimination.readme, digital data file

ELM frequency enhancement and discharge modification through lithium granule injection into EAST H-modes

The injection of impurity granules into fusion research discharges can serve as a catalyst for ELM events. For sufficiently low ELM frequencies, and granule sizes above a threshold, this can result in full control of the ELM cycle, referred to as ELM pacing. For this research, we extend the investigation to conditions where the natural ELM frequency is too high for ELM pacing to be realized. Utilizing multiple sizes of lithium granules and classifying their effects by granule size, we demonstrate that ELM mitigation through frequency multiplication can be used at ELM triggering rates that nominally make ELM pacing unrealizable. We find that above a size threshold, injected granules promptly trigger ELMs and commensurately enhance the ELM frequency . Below this threshold size, injection of an individual granule does not always lead to the prompt triggering of an ELM; however, collective ablation in the edge pedestal region does enhance the ELM frequency. Specifically, Li granules too small to individually trigger ELMs were injected into EAST H-mode discharges at frequencies up to 2.3 kHz; collectively the granules were observed to enhance the natural ELM frequency up to 620 Hz, resulting in a ~2.4x multiplication of the natural ELM frequency and a 50% decrease of the ELM size.readme and digitial data file