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SummaryThis paper presents a systematical review of the largest polymer flood project in the world, applied to multilayered, heterogeneous sandstone reservoirs in the giant Daqing Oilfield in China. First, reservoir and fluid characteristics are highlighted to understand the heterogeneity of the reservoir. Next, the project history is summarized, including laboratory studies, pilot tests, commercial tests, and fieldwide applications. Third, typical polymer flood performance and reservoir management measures are presented. Finally, key understandings and lessons learned from more than 50 years of experience are summarized.The La-Sa-Xing Field in the Daqing Field Complex contains three types of reservoir sands: Type I sand with high permeability, Type II sand with medium permeability, and Type III sand with low permeability. Polymer flood was studied in the laboratory in the mid 1960s, followed by small-scale pilots beginning in 1972 and industrial-scale pilots starting in 1993, all of which successfully reduced water cut and enhanced oil recovery. Fieldwide application commenced in 1996, targeting the Type I sand. With Type II sand being brought onstream in 2003, the project achieved a peak production of 253,000 BOPD in 2013. Polymer flood reduced water cut by 24.8%. Reservoir management measures, such as zonal injection, profile modification, hydraulic fracturing in low-permeability sand, and injection optimization, proved to be effective. Based on the water-cut performance, production can be divided into four stages: (1) water-cut decline, (2) low water cut, (3) rebound, and (4) water chase. Fit-for-purpose improved-oil-recovery measures were implemented for each stage to improve production performance.Key understandings and lessons learned include the following: (1) Polymer flood improves both sweeping and displacing efficiencies; (2) high interlayer permeability contrast leads to low incremental recovery; (3) variable well spacing should be adopted for different reservoir types; (4) adoption of large molecular weight (MW) and large slug size greatly enhances recovery; and (5) salt-resistant polymer is beneficial for produced water reinjection in Type II sand; (6) zonal injection increased swept reservoir zones by 9.8% and swept pay thickness by 10.3%; (7) profile modifications helped improve vertical conformance in injection wells and led to enhanced sweeping efficiency and extended low water-cut stage; and (8) optimization-recommended well spacing for Type I, Type II, and Type III sands is 10–15.5, 5.6–7.6, and 2.5–3.6 acres, respectively.In comparison with generally 6–8% incremental recovery by polymer flood in the industry, this project achieved an impressive incremental recovery of 12%, enhancing the oil recovery factor from 40% by primary recovery and waterflood to 52% stock tank oil initially in place (STOIIP). The progressive approach from laboratory experiments through pilots and finally to field application is a best practice for applying polymer flood fieldwide for a giant field such as the La-Sa-Xing Field.

This study explored the usefulness of genomic copy number variation sequencing (CNV-Seq) in the prenatal diagnosis of pregnant women.Based on prenatal diagnostic indications, CNV-Seq analysis was done in the samples from the 579 pregnant women of the 7 subgroups that included advanced maternal age (group A), high risk noninvasive prenatal test (NIPT) (group B), high risk Down's (Group C), abnormal ultrasound findings (Group D), adverse pregnancy history (Group E), chromosome abnormalities in couples (Group F), and the mixed group (Group G).A total of 57 (9.84%) cases have abnormal CNV-Seq results. Among them, 21 cases were aneuploid chromosomal number abnormalities (3.63%, 21/579), and 36 cases were CNV abnormalities (6.22%, 36/579), including 7 cases of pathogenic copy number alteration (pCNA) (1.21%, 7/579) and 29 cases variants of uncertain significance (VUS) (5.01%, 29/579). The total detection rates of abnormal CNV-Seq in Group G and Group B were 20.27% (15/74) and 15.91% (14/88), which were significantly higher than those in other groups (p0.05). Among 36 cases of abnormal CNV-Seq, 7 cases were chromosome fragment deletion or duplication, which were pathogenic CNV, and some rare chromosomal diseases were detected.Patients with a high risk of NIPT or multiple indications of prenatal diagnosis are highly suspected of chromosomal diseases. CNV-Seq is a useful tool for detecting chromosome abnormalities for prenatal diagnosis of pregnant women more accurately and provides more comprehensive information for prenatal diagnosis to reduce birth defects.

For disruptions without mitigation, a large amount of thermal energy and poloidal magnetic energy will be dissipated inside the vacuum vessel (VV). A slow current quench may be accompanied by a large halo current, while a fast current quench often causes large eddy current, which will result in electromagnetic force. At the same time, fast current quench will induce strong toroidal electric field, which will result in a large fraction of runaway current and the hitting of runaway beam on first wall. The disruption mitigation is essential for large scale tokamak. The existing methods to mitigate disruptions, such as massive gas injection and resonant magnetic perturbations, are aimed at increasing the runaway generation threshold or the lose rate of runaway electrons. It may not work for ITER with Ip = 15 MA operation. The root of runaway generation is the large toroidal electric field induced by fast current quench and the large avalanche factor with high plasma current. The reduction of toroidal electric field is favor for the runaway suppression. The magnetic energy transfer (MET) based on electromagnetic coupling for disruption mitigation has been proposed on J-TEXT. It has the advantage of transferring the magnetic energy to outside of vessel by the electromagnetic coupling. It accelerates the current quench (CQ) rate and reduces the toroidal electric field at the same time. The runaway current has been suppressed by the MET system on J-TEXT. The experimental results show that the MET can reduce the energy dissipated in the VV by 20 % through transferring of energy to outside of VV. The MET can increase the CQ rate about 50.7 % and decrease the loop voltage about 35.3 %. The MET provides a new idea to transfer the magnetic energy and to suppress runaway current for disruption mitigation in future devices.

Herein, we introduce a new strategy towards the cyclohexene oxidation reaction, which is driven by molecular oxygen with a cobalt(II)/amine acid complex as the catalyst without any solvents. The as-prepared products include cyclohexene oxide, 2-cyclohexene-1-ol and 2-cyclohexene-1-one. Under optimized conditions, the oxidation of cyclohexene with molecular oxygen yields a conversion rate of 82.5%. Notably, the catalytic oxidation system has the advantages of being clean, safe and operationally simple. Our experiment indicates that the oxidation follows a radical-chain mechanism rather than a catalytic mechanism, which reveals that 2-cyclohexene-1-hydroperoxide could play a key role in the free-radical chain reaction through a complicated but necessary product analysis.

T2 pure copper and 1060 aluminium alloy 10mm in thickness were joined by friction stir welding. The influence of fixed position and offset on the microstructure and hardness of joints was analysed. A joint having a good surface formation and a defect-free cross section was obtained when the copper was fixed on the advancing side and the offset was 3 mm. Layered structures, composite-like structures and clear copper side and nugget zone interface can be observed at the weld cross section. Under the combined action of heat and force, copper and aluminium inter-diffused and formed layered IMC at the interface. The layered IMC consists of Al2Cu and Al4Cu9 by EDS and TEM analysis. The hardness of the joint first increases and then decreases from Cu side to Al side, and the hardness of the offset of 3mm is the lowest.

A high spectra resolution X-ray imaging crystal spectrometer has been implemented on J-TEXT Tokamak for the measurements of Kα spectra of helium-like argon and its satellite lines. The wavelength range of Kα spectra of helium-like argon is from 3.9494 A to 3.9944 A that includes the resonance line w, intercombination lines x and y, forbidden line z and numerous satellite lines, referenced using standard Gabriel notation. In low-density discharge, the intensity of q, r satellite lines and z lines can be significantly enhanced by non-thermal electrons. Non-thermal electrons are produced due to the low plasma density. The high hard X-ray flux from NaI detector and significant downshift electron cyclotron emissions from energetic runaway electrons also indicated that there is a large population of runaway electrons in the low-density discharge. The non-thermal part of electrons can affect the excitation/transition equilibrium or ionization/recombination equilibrium. The q line is mainly produced by inner-shell excitation of lithium-like argon, and the r line is partially produced by inner-shell excitation of lithium-like argon and dielectronic recombination of helium-like argon.

The generation of runaway electrons (REs) during disruptions is a key issue for the safe operation of large tokamaks. For better design, a reliable scenario to suppress RE generation and for the investigation of RE generation during disruptions is highly essential. On J-TEXT, RE generation is strongly dependent on the pre-disruption electron density, toroidal magnetic fields (B T ) and magnetic perturbations. RE generation can be avoided in discharges with a low B T or a high electron density. For discharges with a high B T , a high electron density threshold is required to suppress RE generation. However, this threshold decreases with the application of resonant magnetic perturbations (RMP) which is applied before the thermal quench. The enhancement of magnetic perturbation increases the RE loss during disruptions, leading to robust runaway suppression in the discharges with a relatively low electron density. The electron density threshold required for RE suppression reduces with the increase of RMP strength and the m/n = 2/1 mode RMP is more efficient than the m/n = 3/1 mode RMP for the reduction of density threshold, where m and n are the poloidal and toroidal mode numbers, respectively. The NIMROD simulation is applied to investigate the transport of REs during disruptions, which indicates that the 2/1 mode RMP can create stronger magnetic perturbations during a disruption, resulting in a high loss ratio of RE seeds. All results provide evidence of the significant effect of RMP mode and amplitude on the electron density threshold for RE generation, which might give an insight into future large reactor tokamak operation with high electron densities.

The aim of the current study was to elucidate the role of miR-455-3p in the pathogenesis of esophageal squamous cell carcinoma (ESCC) and its prognostic value in patients with ESCC.Expression levels of miR-455-3p and FAM83F mRNA in ESCC tissues and adjacent normal tissues were detected by quantitative RT-PCR. The X2-test was used to assess miR-455-3p expression on clinicopathological parameters. The association with overall survival of patients was analyzed by Kaplan-Meier survival analysis. Cox's multivariate regression model was performed to identify independent prognostic factors of overall survival. The effect of miR-455-3p on proliferation was evaluated by kit-8 (CCK-8), and cell invasion was evaluated by transwell assays. The molecular target of miR-455-3p was identified using a computer algorithm and confirmed experimentally. Furthermore, the effect of miR-455-3p up-regulation on FAM83F expression was examined by Western blot.miRNA-455-3p was significantly increased in ESCC tissues and cell lines. Also, miR-455-3p expression was significantly associated with histological grade, lymph nodes metastasis and clinical stage (all p0.05). The patients with low miR-455-3p expression had shorter survival time than those with high miR-455-3p expression. Furthermore, univariate and multivariate analysis identified low miR-455-3p expression as an unfavorable prognostic factor for overall survival. Moreover, transfection with the miR-455-3p mimic enhanced the cell proliferation and invasion in ESCC cells. Luciferase reporter assays confirmed that miR-455-3p binding to the 3'-UTR regions of FAM83F inhibited the expression of FAM83F in ESCC cells. Western blot confirmed that overexpression of miR-455-3p resulted in down-regulation of FAM83F in ESCC cells.Our findings indicate that miR-455-3p plays an anti-oncogenic role in the development of ESCC by downregulation of FAM83F and could be an independent marker for predicting the clinical outcome of ESCC patients.

An x-ray imaging crystal spectrometer has been developed on joint Texas experimental tokamak for the measurement of electron and ion temperatures from the Kα spectra of helium-like argon and its satellite lines. A two-dimensional multi-wire proportional counter has been applied to detect the spectra. The electron and ion temperatures have been obtained from the Voigt fitting with the spectra of helium-like argon ions. The profiles of electron and ion temperatures show the dependence on electron density in ohmic plasmas.

Disruptions have the possibility of causing severe wall damage to large tokamaks like ITER. The mitigation of disruption damage is essential to the safe operation of a large-scale tokamak. The shattered pellet injection (SPI) technique, which is regarded as the primary injection method for ITER, presents several advantages relative to massive gas injection, including more rapid particle delivery, higher total particle assimilation, and more centrally peaked particle deposition. A dedicated argon SPI system that focuses on disruption mitigation and runaway current dissipation has been designed for the Joint Texas Experimental Tokamak (J-TEXT). A refrigerator is used to form a single argon pellet at around 64 K. The pellet will be shaped with a 5 mm diameter and a 1.5-10 mm length. Helium gas at room temperature will be used as a propellant gas for pellet acceleration. The pellet can be injected with a speed of 150-300 m/s. The time interval between injection cycles is about 8 min. The pellet will be shattered at the edge of the plasma and then injected into the core of plasma. The first experiments of SPI fast shutdown and runaway current dissipation have been performed.

Fast electron bremsstrahlung (FEB) emission during Ohmic discharge experiments on the Joint Texas Experimental Tokamak (J-TEXT) has been measured by a recently developed vertical multi-channel FEB diagnostic based on CdZnTe detectors. There are 5 sight lines to observe the vertical emission of fast electrons at the high-field side with a spatial resolution of 5 cm. The FEB emission in the energy range of 30-300 keV can be measured. The generation of fast electrons accelerated by loop voltage has been confirmed during the early phase of discharge by analyzing the signals of FEB emission. The runaway electron beam instabilities have been observed with the FEB diagnostic on J-TEXT.

Disruptions have the potential to cause severe damage to large tokamaks like ITER. The mitigation of disruption damage is one of the essential issues for the tokamak. Massive gas injection (MGI) is a technique in which large amounts of a noble gas are injected into the plasma in order to safely radiate the plasma energy evenly over the entire plasma-facing wall. However, the radiated energy during the disruption triggered by massive gas injection is found to be toroidally asymmetric. In order to investigate the spatial and temporal structures of the radiation asymmetry, the radiated power diagnostics for the J-TEXT tokamak have been upgraded. The multi-channel arrays of absolute extreme ultraviolet photodiodes have been upgraded at four different toroidal positions to investigate the radiation asymmetries during massive gas injection. It is found that the toroidal asymmetry is associated with plasma properties and MGI induced MHD activities.

Poly(4-vinylpyridine) (P4VP) is a widely studied polymer for applications in catalysis, humidity sensitive and antimicrobial materials due to its pyridine group exhibiting coordinative reactivity with transition metals. In this work, the non-covalent functionalization of single-walled carbon nanotubes (SWCNTs) with P4VP in CO2-expanded liquids (CXLs) is reported. It is found that P4VP stabilized SWCNTs show good dispersion in both organic solvent and aqueous solution (pH = 2). The ability to manipulate the dispersion state of CNTs in water with P4VP will likely benefit many biological applications, such as drug delivery and optical sensors. Furthermore, the structure and morphology of P4VP/SWCNTs composite are examined, with the focus on molecular weight of P4VP (MW-P4VP), the pressure of CXLs and the concen- tration of P4VP. It is amazing that the P4VP15470 wrapping patterns undergo a notable morphological evolution from dot- like crystals to bottle brush-like, then to compact kebab-like, and then to widely-spaced dotted kebab patterns by facile pressure tuning in the higher polymer concentration series. In other words, the CXLs method enables superior control of the P4VP crystallization patterns on SWCNTs. Meanwhile, the CXL-assisted P4VP crystal growth mechanism on SWCNT is investigated, and the dominating growth mechanism is attributed to 'size dependent soft epitaxy' in P4VP15470/SWCNTs composites. We believe these studies would reveal great potential for P4VP in building up functional structures in CXLs that are responsive to environmental stimuli.

This work reports the functionalization of multi-walled carbon nanotubes (MWCNTs) with crystalline poly(4- vinylpyridine) (P4VP) in CO2-expanded liquids (CXLs). The structure and morphology of MWCNT-induced polymer crys- tallization are examined, with the focus on molecular weight of P4VP (MW-P4VP), the pressure of CXLs and the concentration of P4VP. First, it is observed that the crystallization morphologies for the P4VP/MWCNTs composite with a low molecular weight P4VP (LMW-P4VP) matrix could be finely controlled in CXLs, and it is surprising to find that the P4VP8700 wrapping patterns undergo a morphological evolution from dot crystals to dotted helical wrappings, and then to dense helical patterns by facile pressure tuning under lower polymer concentration. In other words, the CXLs method enables superior control of the P4VP crystallization patternings on MWCNTs, particularly efficient for LMW-P4VP at lower polymer concentration. Meanwhile, the CXL-assisted P4VP crystal growth mechanism on MWCNT is investigated, and the dominating growth mechanism is attributed to 'normal epitaxy' at lower P4VP concentration rather than 'soft epi- taxy' at higher concentration. We believe that this work reports a new crystalline polymer wrapping approach in CXLs to noncovalent engineering of MWCNTs surfaces.

Increasing the plasma density is one of the key methods in achieving an efficient fusion reaction. High-density operation is one of the hot topics in tokamak plasmas. Density limit disruptions remain an important issue for safe operation. An effective density limit disruption prediction and avoidance system is the key to avoid density limit disruptions for long pulse steady state operations. An artificial neural network has been developed for the prediction of density limit disruptions on the J-TEXT tokamak. The neural network has been improved from a simple multi-layer design to a hybrid two-stage structure. The first stage is a custom network which uses time series diagnostics as inputs to predict plasma density, and the second stage is a three-layer feedforward neural network to predict the probability of density limit disruptions. It is found that hybrid neural network structure, combined with radiation profile information as an input can significantly improve the prediction performance, especially the average warning time (). In particular, the is eight times better than that in previous work (Wang et al 2016 Plasma Phys. Control. Fusion 58 055014) (from 5 ms to 40 ms). The success rate for density limit disruptive shots is above 90%, while, the false alarm rate for other shots is below 10%. Based on the density limit disruption prediction system and the real-time density feedback control system, the on-line density limit disruption avoidance system has been implemented on the J-TEXT tokamak.

A large number of runaway electrons (REs) with energies as high as several tens of mega-electron volt (MeV) may be generated during disruptions on a large-scale tokamak. The kinetic energy carried by REs is eventually deposited on the plasma-facing components, causing damage and posing a threat on the operation of the tokamak. The remaining magnetic energy following a thermal quench is significant on a large-scale tokamak. The conversion of magnetic energy to runaway kinetic energy will increase the threat of runaway electrons on the first wall. The magnetic energy dissipated inside the vacuum vessel (VV) equals the decrease of initial magnetic energy inside the VV plus the magnetic energy flowing into the VV during a disruption. Based on the estimated magnetic energy, the evolution of magnetic-kinetic energy conversion are analyzed through three periods in disruptions with a runaway current plateau.

The response of plasma toroidal rotation to the external resonant magnetic perturbations (RMP) has been investigated in Joint Texas Experimental Tokamak (J-TEXT) ohmic heating plasmas. For the J-TEXT’s plasmas without the application of RMP, the core toroidal rotation is in the counter-current direction while the edge rotation is near zero or slightly in the co-current direction. Both static RMP experiments and rotating RMP experiments have been applied to investigate the plasma toroidal rotation. The core toroidal rotation decreases to lower level with static RMP. At the same time, the edge rotation can spin to more than 20 km s−1 in co-current direction. On the other hand, the core plasma rotation can be slowed down or be accelerated with the rotating RMP. When the rotating RMP frequency is higher than mode frequency, the plasma rotation can be accelerated to the rotating RMP frequency. The plasma confinement is improved with high frequency rotating RMP. The plasma rotation is decelerated to the rotating RMP frequency when the rotating RMP frequency is lower than the mode frequency. The plasma confinement also degrades with low frequency rotating RMP.

The suppression of disruption-generated runaway electrons (REs) by supersonic molecular beam injection (SMBI) has been investigated on the J-TEXT tokamak. Experimental results demonstrate that the hydrogen injected by SMBI during plasma current flattop phase can provoke magnetic perturbations, which increase RE losses rapidly. The effective radial diffusion coefficient of REs due to SMBI is estimated as D r ≈ 16 m2 s−1. Based on this benefit, the SMBI has been used to explore the suppression of disruption-generated REs. In J-TEXT, RE current is created with rapid argon injection by a massive gas injection valve. It is found that hydrogen SMBI before disruption efficiently suppresses the generation of RE current.

The ARGO-YBJ experiment is currently under construction at the Yangbajing Cosmic Ray Laboratory (4300 m a.s.l.).The detector consists of a central carpet, 74 × 78 m2, made of a single layer of Resistive Plate Counters (RPCs), and surrounded by a partially instrumented guard ring for a total instrumented area of about 6700 m2. The digital read-out, performed by means of pick-up electrodes 6.7 × 62 cm2 (strip), allows to measure the charged particle number of small size air showers. The technique of counting the number of fired strips on the ARGO carpet corresponds to operate in the 10÷500 TeV energy region where both direct and indirect measurements on the primary cosmic radiation have been performed. Many composition models have been proposed by different experiments. In this work we discuss the ability of the ARGO detector to discriminate among some models.

The measurement of internal magnetic fluctuation is important for the study of transport in tokamak plasmas. The runaway electron transport induced by the sawtooth crash can be used to obtain the internal magnetic fluctuation. Inversed sawtooth-like activities on hard x-ray (HXR) fluxes following sawtooth activities were observed after the application of electrode biasing on J-TEXT tokamak. The runaway diffusion coefficient Dr is deduced to be about 30 m2/s according to the time delay of HXR flux peaks to the sawtooth crashes. The averaged value of normalized magnetic fluctuation in the discharges with electrode biasing was increased to the order of 1 × 10−4.

Runaway currents following disruptions have an important effect on the first wall in current tokamaks and will be more severe in next generation tokamaks. The behavior of runaway currents in massive gas injection (MGI) induced disruptions have been investigated in the J-TEXT tokamak. The cold front induced by the gas jet penetrates helically along field lines, preferentially toward the high field side and stops at a location near the q = 2 surface before the disruption. When the cold front reaches the q = 2 surface it initiates magnetohydrodynamic activities and results in disruption. It is found that the MGI of He or Ne results in runaway free shutdown in a large range of gas injections. Mixture injection of He and Ar (90% He and 10%Ar) consistently results in runaway free shutdown. A moderate amount of Ar injection could produce significant runaway current. The maximum runaway energy in the runaway plateau is estimated using a simplified model which neglects the drag forces and other energy loss mechanisms. The maximum runaway energy increases with decreasing runaway current. Imaging of the runaway beam using a soft x-ray array during the runaway current plateau indicates that the runaway beam is located in the center of the plasma. Resonant magnetic perturbation (RMP) is applied to reduce the runaway current successfully during the disruption phase in a small scale tokamak, J-TEXT. When the runaway current builds up, the application of RMP cannot decouple the runaway beam due to the lower sensitivity of the energetic runaway electrons to the magnetic perturbation.

The coupling of spin waves for a Heisenberg system with a finite size is studied. It is found that the final results are boundary condition dependent. For a periodic system, the finite-size effect weakens the coupling of spin waves and an additional term proportional to -T72//N appears with N, the system's size, in the expression for the number density of spin waves at temperature T. The free boundary greatly complicates the coupling of spin waves. There are three new kinds of scattering process: between spin waves parallel to the free surfaces of the system, between spin waves parallel to the surface and other waves, and between spin waves with the same components in the direction perpendicular to the free surface of the system. Unlike the others, the last effective interaction tends to depress the excitation of spin waves at low temperatures.

The suppression of runaways following disruptions is key for the safe operation of ITER. The massive gas injection (MGI) has been developed to mitigate heat loads, electromagnetic forces and runaway electrons (REs) during disruptions. However, MGI may not completely prevent the generation of REs during disruptions on ITER. Resonant magnetic perturbation (RMP) has been applied to suppress runaway generation during disruptions on several machines. It was found that strong RMP results in the enhancement of runaway production instead of runaway suppression on J-TEXT. The runaway current was about 50% pre-disruption plasma current in argon induced reference disruptions. With moderate RMP, the runway current decreased to below 30% pre-disruption plasma current. The runaway current plateaus reach 80% of the pre-disruptive current when strong RMP was applied. Strong RMP may induce large size magnetic islands that could confine more runaway seed during disruptions. This has important implications for runaway suppression on large machines.

Disruption mitigation is essential for the next generation of tokamaks. The prediction of plasma disruption is the key to disruption mitigation. A neural network combining eight input signals has been developed to predict the density limit disruptions on the J-TEXT tokamak. An optimized training method has been proposed which has improved the prediction performance. The network obtained has been tested on 64 disruption shots and 205 non-disruption shots. A successful alarm rate of 82.8% with a false alarm rate of 12.3% can be achieved at 4.8 ms prior to the current spike of the disruption. It indicates that more physical parameters than the current physical scaling should be considered for predicting the density limit. It was also found that the critical density for disruption can be predicted several tens of milliseconds in advance in most cases. Furthermore, if the network is used for real-time density feedback control, more than 95% of the density limit disruptions can be avoided by setting a proper threshold.

In order to mitigate the negative effects of the plasma disruption a massive gas injection (MGI) valve is designed for the joint Texas experimental tokamak. The MGI valve is based on the eddy-current repulsion mechanism. It has a fueling volume of 30 ml. The piston of the MGI valve is made by non-ferromagnetic material, so it can be installed close to the vacuum vessel which has a strong toroidal magnetic field. A diode is use to prevent current oscillation in the discharge circuit. The drive coil of the valve is installed outside the gas chamber. The opening characteristics and the gas flow of the MGI valve have been tested by a 60 l vacuum chamber. Owing to the large electromagnetic force the reaction time of the valve is shorter than 0.3 ms. Duration for the opening of the MGI valve is in the order of 10 ms.

The crystal structure of the high-${T}_{c}$ superconductor ${\mathrm{Ba}}_{2}$${\mathrm{YCu}}_{3}$${\mathrm{O}}_{\mathrm{x}}$ at 750 \ifmmode^\circ\else\textdegree\fi{}C was elucidated using Rietveld analysis of neutron powder diffraction data. We find that the phase is tetragonal at this temperature, with space group P4/mmm, and lattice constants a=3.8823 A\r{} and c=11.826 A\r{}. This is an oxygen deficient triperovskite, in which Ba and Y ions are ordered along the c axes in Ba-Y-Ba-Ba sequence. The occupation factors of the oxygen sites 4i, 2g, 2f are 0.97, 1.00, 0.33, respectively. The total number of the oxygen atoms per unit cell is 6.54.

The crystal structure of the high-T/sub c/ superconductor Ba/sub 2/YCu/sub 3/O/sub 7+//sub x/ at 124 and 71 K was refined by Rietveld analysis of neutron diffraction data. We found that at 124 and 71 K the structure is of the same space group Pmmm, but when the temperature is decreased and passes the critical temperature T/sub c/, 0.12 of the oxygen population on the one 1e site transfers to the 1b site. This remarkable change in the structure can be related to its superconductivity.

The crystal structure of the high-${T}_{c}$ superconductor ${\mathrm{Ba}}_{2}\mathrm{Y}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7+x}$ has been studied using neutron diffraction. This compound has an orthorhombic structure with space group Pmmm and lattice parameters $a=3.8132$ \AA{}, $b=3.8774$ \AA{}, and $c=11.653$ \AA{}. We found an occupancy of 0.1 for oxygen atoms at the $1b$ site and a total number of oxygen atom per unit cell of 7.1. The high quality of the sample used may be associated with the partial occupation at the $1b$ site by oxygen atoms.

Bakelite RPCs, assembled according to the ARGO design, have been operated in the high altitude Laboratory of YBJ using dedicated electronics to pick-up the streamer signal. Here we report on the results concerning absorbed current, single counting rate, efficiency and time resolution. Environmental data concerning the operating temperature inside the ARGO experimental hall are also reported. (C) 2003 Elsevier Science B.V. All rights reserved.

A resistive plate counters (RPCs) carpet of similar to50 m(2) has been put in operation in the Yangbajing Laboratory (Tibet, P.R. China) at 4300 in a.s.l., in order to study the RPCs performance at high altitude and the detector capability of imaging the EAS disc. This test has been performed in view of an enlarged use of RPCs for the ARGO-YBJ experiment. This experiment will be devoted to a wide range of fundamental issues in cosmic rays and astroparticle physics, including in particular gamma-ray astronomy and gamma-ray bursts physics at energies greater than or equal to 100 GeV. In this paper we present and discuss the procedures adopted to calibrate the detector and reconstruct the shower direction. Results concerning many shower features as the angular distribution, the density spectrum, the time profile of the shower front, are found well consistent with the expectation. (C) 2002 Elsevier Science B.V. All rights reserved.

The characteristics of a 50m(2) RPC carpet(prototype of YBJ-ARGO experiment)was analyzed using its test run data. A correction method of the systematic time error is suggested, and nonuniform azimuthal angle distribution possibly due to direction reconstruction error on an asymmetric carpet is reported.

An RPC carpet covering similar to 10(4) m(2) (ARGO-YBJ experiment) will be installed in the YangBaJing Laboratory (Tibet, People's Republic of China) at an altitude of 4300 m a.s.l. A test-module of similar to 50 m(2) has been put in operation in this laboratory and about 10(6) air shower events have been collected. The RPC performance at high altitude and the carpet capability of reconstructing the shower features are presented. (C) 2000 Elsevier Science B.V. All rights reserved.

ARGO-YBJ (Astrophysical Radiation with Ground-based Observatory at YangBaJing) is a detector optimized to study small size air showers. It consists of a layer of Resistive Plate Counters (RPCs) covering an area of similar to 6500 m(2) and will be located in the Yangbajing Laboratory (Tibet, China) at 4300 m a.s.l. ARGO-YBJ will be devoted to a wide range of fundamental issues in cosmic rays and astroparticle physics, including in particular gamma-ray astronomy and gamma-ray bursts physics in the range 10 GeV divided by 500 TeV. The sensitivity of ARGO-YBJ to detect high energy GRBs is presented.