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This study introduces a laser scattering system to protect a low-speed aircraft. Scattering was selected to reduce the laser’s intensity targeting the sensor of an aircraft and simultaneously maintaining the functionality of aircraft optics. Mie scattering, known for effectively decreasing short-wave infrared light, was employed by utilizing water aerosols having a diameter of 1 to 5 μm. Experimental results regarding the decrease of the laser intensity via scattering confirmed that the theoretical and experimental values resulted in a similar decrease rate under static conditions. To validate the theoretical values, the path length, which the laser passing through water aerosols, was changed. To assess the system’s feasibility in flow conditions, a low-speed wind tunnel was employed to generate two flow speeds: 5.5 m/s and 17.6 m/s. Remarkably, the reduction of laser intensity was only affected by the path length, and was somewhat unaffected regardless of flow speed and the uniformity of the flow, only to the path length. In all cases, the initial laser intensity was set to 10 mW. Under static conditions, the intensity dropped to 8.21 mW, showing a decrease of 17.9%. In flow conditions of 5.5 m/s, 17.6 m/s, and in distorted flow, the laser intensity decreased by 18.3%, 18.1%, and 18% respectively. As a preliminary study, these results demonstrate the system’s capability to protect a low-speed aircraft targeted by lasers even under dynamic flow conditions, may suggest a possibility of providing a practical defence solution.

In this study, a combined test facility was developed using a combination of an arc-jet tunnel and a shock tunnel for aerothermodynamic testing. The performance validation of individual parts was performed, and results were obtained from the combined test. A small-scale Huels-type arc-jet tunnel was used to preheat the test model by aerodynamic heating before conducting the experiments in the shock tunnel to duplicate the hot surfaces of flight objects encountered during hypersonic flight. The high-enthalpy flow in the arc-jet tunnel provided a heat flux of 1.99±0.03 MW/m 2 for a flat-faced model of 10 mm diameters, and the flow condition of the shock tunnel used in this study simulated a Mach 5 flight at a pressure altitude of about 24 km. The two combined experiments employing different shape and material models were carried out to examine the effect of aerothermodynamic phenomena. In the first experiment, the effect of ablation-induced shape change on the fluid-structure was investigated using a cone model manufactured of AL6061 material. The effect of surface roughness on the fluid-structure was examined in the second experiment, which used a hemisphere model constructed of STS303 material. Although substantial findings could not be validated due to the limits of qualitative evaluations utilizing visualization methods, however preheating-related changes in surface roughness were found. As a follow-up study, a force measuring experiment based on the test procedures is being carried out at this facility utilizing a preheated model with an accelerometer. The performance and experimental results obtained using this integrated setup are discussed in detail, highlighting the potential of this combined hypersonic test facility.

Serial femtosecond crystallography (SFX) provides opportunities to observe the dynamics of macromolecules without causing radiation damage at room temperature. Although SFX provides a biologically more reliable crystal structure than provided by the existing synchrotron sources, there are limitations due to the consumption of many crystal samples. A viscous medium as a carrier matrix reduces the flow rate of the crystal sample from the injector, thereby dramatically reducing sample consumption. However, the currently available media cannot be applied to specific crystal samples owing to reactions between the viscous medium and crystal sample. The discovery and characterisation of a new delivery medium for SFX can further expand its use. Herein, we report the preparation of a polyacrylamide (PAM) injection matrix to determine the crystal structure with an X-ray free-electron laser. We obtained 11,936 and 22,213 indexed images using 0.5 mg lysozyme and 1.0 mg thermolysin, respectively. We determined the crystal structures of lysozyme and thermolysin delivered in PAM at 1.7 Å and 1.8 Å resolutions. The maximum background scattering from PAM was lower than monoolein, a commonly used viscous medium. Our results show that PAM can be used as a sample delivery media in SFX studies.

Three force measurement techniques in a shock tunnel, the free-flight, movable-support force balance, and stress-wave force balance techniques were employed, and each technique’s characteristics were assessed. For each force measurement technique, the system setup, data processing method, measurement uncertainties, and applied range of the test model size-flow establishment time were described in detail and compared. For a comparison and discussion, the drag coefficients of a circular pointed cone model with a semi-angle of 18.4° at a nominal freestream Mach number of 6 were measured. As a result, three force measurement techniques yield similar drag coefficients. However, the measurement uncertainties were increased in the order of the free-flight, the stress-wave force balance, and the movable-support force balance techniques. The main causes of the measurement uncertainties were the corner detection uncertainties for the free-flight techniques, and the propagation of the internal or external vibrations for the movable-support and stress-wave force balance techniques. To estimate the appropriate range of the test model size and flow establishment time for each technique’s application, the force measurement systems of the present work and the available literature were compared. As a result of comparative discussion, force measurement environments that can be advantageous for each technique are suggested.

A self-seeded X-ray free-electron laser (XFEL) is a promising approach to realize bright, fully coherent free-electron laser (FEL) sources in the hard X-ray domain that have been a long-standing issue with longitudinal coherence remaining challenging. At the Pohang Accelerator Laboratory XFEL, we have demonstrated a hard X-ray self-seeded XFEL with a peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1% bandwidth (BW)–1 at 9.7 keV. The bandwidth (0.19 eV) is about 1/70 times as wide (close to the Fourier transform limit) and the peak spectral brightness is 40 times higher than in self-amplified spontaneous emission (SASE), with substantial improvements in the stability of self-seeding and noticeably suppressed pedestal effects. We could reach an excellent self-seeding performance at a photon energy of 3.5 keV (lowest) and 14.6 keV (highest) with the same stability as the 9.7 keV self-seeding. The bandwidth of the 14.6 keV seeded FEL was 0.32 eV, and the peak brightness was 1.3 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1. We show that the use of seeded FEL pulses with higher reproducibility and a cleaner spectrum results in serial femtosecond crystallography data of superior quality compared with data collected using SASE mode.A hard X-ray self-seeded X-ray free-electron laser at the Pohang Accelerator Laboratory provides X-ray pulses with peak brightness of 3.2 × 1035 photons s–1 mm–2 mrad–2 0.1%BW–1 at 9.7 keV and a very small shot-to-shot electron energy jitter of 0.012%.

A three-component stress-wave force balance system for measuring lift, drag, and pitching moment of scramjet model in a shock tunnel was designed and tested under Mach 6 flow condition. To isolate the system from external disturbances, an aerodynamic shield was designed, vibration isolation mounts were used, and stress bars containing strain gauges were installed between the model and vibration isolation mounts. To increase signal-to-noise ratio of stress waves, brass was chosen for the stress bar, and scramjet model with no non-metallic parts was used. Dynamic calibration using an impact hammer and calibration lugs was conducted at eight calibration stations to obtain the system impulse response functions. A quarter-type Wheatstone bridge was selected to enhance the force recovery capability of the system. The designed system was applied to shock tunnel tests, and the mean values of the three-component forces and their coefficients were calculated within an effective test time.

We demonstrate, for the first time, time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the Fe K-edge using a self-seeded X-ray free-electron laser (XFEL) beam at the FXL endstation in Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL). Focusing on the application of self-seeded XFEL for time-resolved XANES, we show advantages in photon flux, measurement speed, and signal-to-noise ratio (S/N). Using the high-stability, narrow-bandwidth self-seeded mode, we achieved an incident X-ray bandwidth of approximately 0.71 eV and improved spectral purity compared to conventional self-amplified spontaneous emission (SASE) operation. A 50 mM aqueous solution of iron(II) tris(1,10-phenanthroline) dichloride was photo-excited by a 400 nm femtosecond laser, and ultrafast electronic dynamics were probed by synchronized XFEL pulses. The enhanced spectral purity and energy stability enabled clear detection of transient features obscured by SASE pulse bandwidth and energy jitter. The results highlight the clear benefits of the self-seeded XFEL source, showing a 25% improvement in signal-to-noise ratios compared to the SASE with double crystal monochromator (DCM) mode, particularly for time-resolved XANES experiments. This work lays the foundation for advanced exploration of chemical and biological dynamics with improved spectral accuracy in complex environments.

A cylinder-type driver tube staging valve is developed for the test time extension of a reflected-shock tube by delaying the expansion waves’ arrival at the test flow. By applying the staging valve, a low-acoustic-speed gas can be filled near the driver tube end wall without significant mixing with the driver gas. As a result, the expansion waves decelerate while passing through the low-acoustic-speed gas, and the test time could be further extended by delaying the expansion waves’ arrival at the test flow. The test time extension results are evaluated based on the history of pressure measurements at the driven tube end wall under two test flow conditions. The experimental details regarding the test time and the pressure over the test time are described. The experimental results and discussions indicate that the shock tube’s test time can be extended without significant flow disturbance.

This paper presents a new force measurement technique to investigate the effect of aerothermodynamic phenomena, particularly aerodynamic heating, on aerodynamic forces. This technique is successfully applied within a hypersonic combined test facility that integrates arc-jet and shock tunnel. A preheated model transport system is developed to axially immobilize the model during preheating, safe transport to the shock tunnel, and ensure quasi-free-body axial motion during force measurement tests. A comparative evaluation revealed a significant 11.2 % increase in drag coefficient for test models preheated in the arc-jet tunnel, followed by an 8.56 % decrease when cooled to room temperature. By closely comparing drag coefficients under three conditions (cold, hot, and cooled), this study analyzes the distinct effects of ablation-induced shape change and surface temperature on drag coefficients, respectively. This technique allows for a more realistic simulation of hypersonic flight conditions within ground test facilities.

Bluetooth Low Energy (BLE) is a short-range wireless communication technology aiming at low-cost and low-power communication. The performance evaluation of classical Bluetooth device discovery have been intensively studied using analytical modeling and simulative methods, but these techniques are not applicable to BLE, since BLE has a fundamental change in the design of the discovery mechanism, including the usage of three advertising channels. Recently, there several works have analyzed the topic of BLE device discovery, but these studies are still far from thorough. It is thus necessary to develop a new, accurate model for the BLE discovery process. In particular, the wide range settings of the parameters introduce lots of potential for BLE devices to customize their discovery performance. This motivates our study of modeling the BLE discovery process and performing intensive simulation. This paper is focused on building an analytical model to investigate the discovery probability, as well as the expected discovery latency, which are then validated via extensive experiments. Our analysis considers both continuous and discontinuous scanning modes. We analyze the sensitivity of these performance metrics to parameter settings to quantitatively examine to what extent parameters influence the performance metric of the discovery processes.