Distributed fiber optic sensors for tunnel monitoring:A state-of-the-art review

Distributed fiber optic sensors(DFOSs)possess the capability to measure strain and temperature variations over long distances,demonstrating outstanding potential for monitoring underground infrastructure.This study presents a state-of-the-art review of the DFOS applications for monitoring and assessing the deformation behavior of typical tunnel infrastructure,including bored tunnels,conventional tunnels,as well as immersed and cut-and-cover tunnels.DFOS systems based on Brillouin and Rayleigh scattering principles are both considered.When implementing DFOS monitoring,the fiber optic cable can be primarily installed along transverse and longitudinal directions to(1)measure distributed strains by continuously adhering the fiber to the structure’s surface or embedding it in the lining,or(2)measure point displacements by spot-anchoring it on the lining surface.There are four critical aspects of DFOS monitoring,including proper selection of the sensing fiber,selection of the measuring principle for the specific application,design of an effective sensor layout,and establishment of robust field sensor instrumentation.These four issues are comprehensively discussed,and practical suggestions are provided for the implementation of DFOS in tunnel infrastructure monitoring.

Frost Resistance of Pervious Concrete Mixed with Waste Glass Powder

The contents of waste glass powder(WGP)(0%,10%,15%,20%,25%)and water-binder ratio(W/C)(0.24,0.26,0.28)were used as influencing factors,and the quality loss rate(Δm)and compressive strength loss rate(Δfc)were used as characterization parameters.The Ca/Si ratio and main element contents of C-S-H gels with different WGP content were investigated by energy dispersive spectrometry(EDS).The pore structure evolution characteristics of WGP composite cementing materials were investigated by low field nuclear magnetic resonance(NMR).UsingΔfc as the index of frost resistance degradation and Weibull function,the frost resistance degradation of glass doped pervious concrete(WGP-PC)was modeled.The results show that,with WGP,for the same number of cycles,Δm andΔfc decrease and increase with the increase of WGP.Under the same WGP content,Δm andΔfc decrease first and then increase with the increase of W/C.After 100 freeze-thaw cycles,the samples with WGP content of 20%and W/C of 0.26 have the best freeze-resistance.Microscopic tests show that with the increase of WGP content,the Ca/Si ratio of C-S-H gel decreases at first and then increases with the increase of WGP content.The extreme value of Ca/Si is 2.36 when WGP is added by 20%.The pore volume of hardened paste with 20%WGP content decreased by 18.6%compared with that of cement system without WGP.The overall compactness of the specimen was improved.On the basis of the test data,a life prediction model was established according to Weibull function.The experiment showed thatΔfc could be used as a durability degradation index,and the slope of the reliability curve became gentle after WGP was added,which reduced the damage degradation rate of PC.W/C was 0.26.It's about 5000 hours.

Measurement of Electron-Drift Velocity in Ar＋CH4 Mixtures Using Double-Grid Method

Based on analyzing the induced signals from the double-grids of an ionization chamber, the electron-drift time between the two grids is determined and the electron-drift velocity is derived. A waveform digitizer is employed to record pulses from the two grids of the ionization chamber. The electron-drift velocity is measured as a function of the reduced electric field E/p for eight different ratios of Ar＋CH4 mixtures. By analyzing the experimental data of this study, self-consistency of experimental data is achieved, and formulae for calculating electron-drift velocity in any ratio of Ar＋CH4 mixtures are obtained.

High Power THz Undulator Radiation from Linear Accelerator

A 30-MeV femto-second electron linac is built at the Shanghai Institute of Applied Physics, which can produce high power, coherent THz undulator radiation. We report the experimental facility and measurement of the power, frequency spectrum. First experiments show the averaged power at THz to be about 20mW.

Theoretical Design of a 104MHz Ladder Type IH-RFQ Accelerator

Beam dynamics and rf designs of a 104 MHz ladder type IH-RFQ （L-IH-RFQ） accelerator are finished at Peking University for the acceleration of 14C＋ from 40 keV to 500 keV. As a specific feature, the output beam energy spread is as low as 0.6% achieved with the internal discrete bunching method, which makes potential applications of RFQ feasible, such as accelerator mass spectrometry and ion implantation. Tolerances of the beam dynamics design are studied by means of changing the input beam parameters, and the results are quite satisfying. On the other hand, the L-IH-RFQ structure is employed, taking advantage of its mechanical stability and the absence of inter-electrode voltage asymmetry. Radio-frequency properties are studied and optimized for reducing power loss with Microwave Studio （MWS）. Tuning of the field flatness and frequency is investigated in principle.

Progress in AMS Measurement of 182Hf at CIAE

Accelerator mass spectrometry （AMS） is one of the most promising methods to detect minute amounts of 182Hf. However, the sensitivity of 5×10^-11 for ^182Hf/180Hf obtained previously by the AMS method at China Institute of Atomic Energy （CIAE） cannot meet the requirement of some applications. We present some new improvements of measurement method for AMS measurement of 182Hf at the CIAE HI？13 tandem accelerator system. As a result, a sensitivity of 1.0×10^-11 for 182Hf/180Hf is achieved.

Modeling the rail surface unevenness of a high-precision radio telescope

This study proposed a coarse-fine mixed model for describing the rail surface unevenness of an ultra-large fully steerable radio telescope （Qi Tai Telescope） with a diameter of 110 meters. The rail surface unevenness includes information on error arising from two different scales, i.e., the long-period- short-change and the short-period-long-change. Consequently, in this study an idea of a mixed model was proposed, in which trigonometric and fractal functions were, respectively, used to describe infor- mation on error from two scales. Key parameters were determined by using the least squares method and the wavelet transform method, and finally, a specific mathematical expression of the model was obtained by optimization. To validate the effectiveness of the new modeling method, the mixed model was then used to describe the rails of the Green Bank Telescope, the Large Millimeter Telescope, and a radio telescope in Miyun, Beijing. A comparative study revealed that the maximum error was less than 15 %, thus the result was superior to those of existing modeling methods.

Time-Resolved Measurement of Radiatively Heated Iron 2p-3d Transmission Spectra

An experimental measurement of radiatively heated iron plasma transmission spectra was performed on Shenguang II laser facility. In the measurement, the self？emission spectrum, the backlighting spectrum, and the absorption spectrum were imaged with a flat filed grating and recorded on a gated micro channel plate detector to obtain the time-resolved transmission spectra in the range 10-20 ？ （approximately 0.6-1.3 keV）. Experimental results are compared with the calculation results of an unsolved transition array （UTA） code. The time-dependent relative shift in the positions of the 2p-3d transmission array is interpreted in terms of the plasma temperature variations.

Proton Ratio of HL-2A Bucket Ion Source

For heating the tokamak plasma effectively, the ion source must be capable of producing ions with high proton ratio. The proton ratio, which is found to be more than 65.6% at the ion current of 19.6A with the extraction voltage of 39.6 k V, is measured with an image spectrograph by Doppler shift effect of Balmer-α-radiation spectrum emitted from fast hydrogen particles. The tendency of proton ratio with the ion density in experiment is almost the same as the mode devised by Zhang et al. Okumura et al. only gave the affection of the plasma volume and ion loss area on the proton ratio, but the relationship between the ion density in chamber and the proton ratio was not presented. We give the relationship.

Coverage Optimization of Field Observation Instrument Networking Based on an Improved ABC Algorithm

The severe conditions of cold and arid areas seriously affect the progress of data collection and analysis for field observation instruments.Therefore,this study adopted the modified artificial bee colony(ABC)algorithm to optimize the coverage of nodes and designed an energy-efficient node coverage optimization method.In the coverage optimization,the coverage rate and the number of working nodes are considered comprehensively,and the fitness value calculation is improved.The experimental results reveal that the modified ABC algorithm has better coverage optimization performance than the original ABC algorithm,genetic algorithm(GA),and particle swarm optimization(PSO)algorithm.