Search

A single, versatile fabrication method for producing microfluidic chips on poly(methyl methacrylate) (PMMA) substrates is described. First, we processed the microchannel using CO2 laser beam machining on the PMMA substrate. In this course the influence of laser power, scanning speed and focal distance on the processing quality of microchannel were investigated by orthogonal experiment. The experimental result shows that the greatest influence factor is focal distance. Based on the experiment, we found a set of optimal parameters to process the microchannel within the set parameters. Next, the substrate with the microchannels and the cover are combined using thermal bonding. Since the previous bonding technology has a low success rate, we have proposed a new bonding termed a pre-bonding technique for bonding chips. The principle of this technology is to first remove the material stress with low pressure, and then use high pressure bonding. Finally, the processing quality of the microchannel and the feasibility of the pre-bonding method were verified successfully by the mixed experiment. This study can help people pay attention to the effect of focal distance on the shape of microchannels when using CO2 laser beam machining to process microchannels. [ABSTRACT FROM AUTHOR]

Proppant transport and distribution in fractures is critical to evaluate the effectiveness of hydraulic fracturing of unconventional reservoirs such as shale and tight sandstone. This study reproduced the process of proppant transport in rough fractures by two fluid method (TFM) and investigated the wall-retardation effect on proppant migration and settlement. Based on our previous work, the solid–liquid drag model considering wall-retardation effect was further discussed by Lattice Boltzmann method (LBM). The characteristics of rough surfaces of shale and tight sandstone samples were analyzed, and the 4 m × 0.3 m (length × height) rough fracture models were constructed based on these characteristics. The proppant transport and distribution in smooth fractures, sheared fractures and joint fractures were compared, and then the effects of fracture morphology, aperture and mated state on the retarded proppant transport were explored. In smooth fractures, the effect of wall retardation will disappear when the eccentric factor is more than 2.5. In rough fractures, the wall-retardation effect on proppant transport is further enhanced by the unevenly distributed fracture aperture. Both vertical and horizontal velocity of proppant decrease in rough fractures, and this phenomenon is strengthened when the wave-shaped texture or stratification is vertical to the flow direction. The none-flow zones formed by the contact of fracture surfaces will even destroy the continuity of flow filed and block up the proppant transport. However, the morphology of rough fractures will produce little impact on proppant transport when the mean fracture aperture overs the critical fracture aperture. For sheared fractures, the critical fracture aperture is around 6 mm, while for joint fractures, the critical fracture aperture is much less than 4 mm. [ABSTRACT FROM AUTHOR]

Toroidal Alfvén eigenmodes (TAE) and ellipticity induced Alfvén eigenmodes (EAE) driven by fast electrons are observed for the first time in steady-state EAST plasmas heated by lower hybrid wave (LHW). For the analyzed discharge four modes appear after the injection of LHW. Comparing experimental observations with simulation, high frequency modes are located near and low frequency mode is near . The two positions are very close to where most of the heating power is deposited and population inversion occurs. Analysis of resonance condition between particles and waves indicates that barely-passing, deeply trapped and barely-trapped electrons can drive the mode. [ABSTRACT FROM AUTHOR]

Increasing the density in a tokamak is limited by the so-called density limit, which is generally performed as an appearance of disruption causing loss of plasma confinement, or a degradation of high confinement mode which could further lead to a H  →  L transition. The L-mode and H-mode density limit has been investigated in EAST tokamak. Experimental results suggest that density limits could be triggered by either edge cooling or excessive central radiation. The L-mode density limit disruption is generally triggered by edge cooling, which leads to the current profile shrinkage and then destabilizes a 2/1 tearing mode, ultimately resulting in a disruption. The L-mode density limit scaling agrees well with the Greenwald limit in EAST. The observed H-mode density limit in EAST is an operational-space limit with a value of . High density H-mode heated by neutral beam injection (NBI) and lower hybrid current drive (LHCD) are analyzed, respectively. The constancy of the edge density gradients in H-mode indicates a critical limit caused perhaps by e.g. ballooning induced transport. The maximum density is accessed at the H  →  L transition which is generally caused by the excessive core radiation due to high Z impurities (Fe, Cu). Operating at a high density () is favorable for suppressing the beam shine through NBI. High density H-mode up to could be sustained by 2 MW 4.6 GHz LHCD alone, and its current drive efficiency is studied. Statistics show that good control of impurities and recycling facilitate high density operation. With careful control of these factors, high density up to 0.93 stable H-mode operation was carried out heated by 1.7 MW LHCD and 1.9 MW ion cyclotron resonance heating with supersonic molecular beam injection fueling. [ABSTRACT FROM AUTHOR]