Abstract
Dynamic measurements of shock and detonation velocities are performed using long chirped fiber Bragg gratings (CFBGs). Such thin probes, with a diameter of typically 125 µm or even 80 µm can be directly inserted into high-explosive (HE) samples or simply glued laterally. During the detonation, the width of the optical spectrum is continuously reduced by the propagation of the wave-front, which physically shortens the CFBG. The light power reflected back shows a ramp-down type signal, from which the wave-front position is obtained as a function of time, thus yielding a detonation velocity profile. A calibration procedure was developed, with the support of optical simulations, to cancel out the optical spectrum distortions from the different optical components and to determine the wavelength-position transfer function of the CFBG. The fitted slopes of the X–T diagram give steady detonation velocity values which are in very good agreement with the classical measurements obtained from discrete electrical shorting pins (ESP). The main parameters influencing the uncertainties on the steady detonation velocity value measured by CFBG are discussed. To conclude, different HE experimental configurations tested at CEA (Commissariat à l’Energie Atomique et aux Energies Alternatives) are presented: bare cylindrical sticks, wedges for shock-to-detonation transitions (SDT), spheres, a cast-cured stick around a CFBG, and a detonation wave-front profile configuration.
Highlights
Chirped fiber Bragg gratings (CFBGs) are commonly used for continuous and dynamic detonation wave-front location measurements in high-explosive (HE) materials with recording rates higher than 10 MHz
The position of the detonation or shock wave-front along the fiber corresponds to a wavelength on the shock wave-front along the fiber corresponds to a wavelength on the chirped fiber Bragg gratings (CFBGs) reflected spectrum, and reflected spectrum, and it needs to be carefully calibrated beforehand
Just prior to any experiment, the chirp rate (CR) of the CFBGs mounted on the set-up are measured using a commercial Optical frequency-domain reflectometry (OFDR) from Luna Technologies (OBR 4600)
Summary
Chirped fiber Bragg gratings (CFBGs) are commonly used for continuous and dynamic detonation wave-front location measurements in high-explosive (HE) materials with recording rates higher than 10 MHz. The system was based on the Doppler effect of the reflected optical signal at the bow shock inside the fiber attached to the detonation wave-front. This technique requires a high-power laser and is very sensitive to the wave-front shape, which is the case when the wave-front velocity is higher than in silica. The position of the detonation or shock wave-front along the fiber corresponds to a wavelength on the shock wave-front along the fiber corresponds to a wavelength on the CFBG reflected spectrum, and reflected spectrum, and it needs to be carefully calibrated beforehand.
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