Chirped fiber Bragg grating (CFBG) detonation velocity sensor system block diagram.
Illustration of chirped fiber Bragg grating detonation velocity sensor system response functions. (a) CFBG spectrum is narrower than ASE light source spectrum. (b) Total reflected light intensity (R) varies linearly with grating length (L) and wavelength (λ) if both the source and return spectrums are idealized flat-tops. (c) A fiber adjacent to a detonating explosive will produce data that measures the detonation position (L) as a function of time.
Photograph of 8-channel chirped fiber Bragg grating detonation velocity detection system. Labeled components include: light sources (1) and (3); four-channel detector chassis (2) and (4); two-channel fiber spectrometer (5); personal computer (6). The recording digitizers are not shown.
Spectrum from an 85 mm chirped fiber Bragg grating. Bandwidth is Δλ = 30.3 nm.
Example of calibration method 1 showing successive 2 mm cuts of an 85 mm (Δλ = 18 nm) CFBG. In this the laser cut calibration method, a sacrificial CFBG is used. Plotted in (a) is a wire grid surface plot of grating spectrum versus wavelength and length that has been successively cut from beginning (long wavelength) to end (short wavelength) in 2 mm increments. Resulting measured voltage per cut position is shown in (b) as dots. The red solid line in (b) is a linear fit to data points and is used to unfold data from a detonation experiment by a mapping from voltage to length along the grating (slope is −0.011 V/mm).
Example of calibration method 2 by laser heat probe of an 85 mm (Δλ = 18 nm) CFBG. In the laser heat probe method the focused laser position is scanned along the length of the grating and to temporarily heat the grating. In the measured reflectance spectrum, a dip is formed at a given position by the laser heating shown in (a). The CFBG reflectance fully recovers when laser is turned off and the grating cools down. In (b), a collection of position points (blue squares) are used to numerically integrate the CFBG spectrum (black line) to compute the area under the spectrum and generate an interpolated look up table (LUT) (blue line) to be used in unfolding data from detonation experiments. In (b) the black line corresponds to bottom and left axes, and the blue symbols/line correspond to top and right axes.
Photograph of PBX 9502 rate stick assembly showing placement of diagnostics: two 85-mm-long CFBGs, 12 electrical pins, and the flat streak camera imaging facet.
Recorded CFBG data waveforms from PBX 9502 rate stick experiment: (a) raw data voltage versus time traces; (b) traces normalized and calibrated to length using LUT method of L vs. t data retrieval. A linear fit to the calibration corrected CFBG2 trace (green curve) in (b) yields a detonation front velocity (i.e., slope) of 7.477 mm/μs.
(a) Results of the PBX 9502 rate stick experiment showing a comparison of the CFBG data (red and black curves), the streak camera break out data (green curve), and the electrical pin data (blue crosses). The inset table is the extracted velocity from a linear least squares fit for all three measurements: CFBGs, electrical pins, and streak camera. The standard error values in the slope of the linear fits reported as velocity in the table are: ±0.0010, ±0.0011, ±0.0005, and ±0.021 mm/μs for CFBG1, CFBG2, streak camera, and pins, respectively. (b) Images of the static (top) and dynamic (bottom) streak camera measurement.
Experimental diagram of (a) assembly side view and (b) photo of tamping plate showing location of CFBGs in radial detonation front measurement in conventional HE (PBX 9501). The CFBGs are each 60 mm long and are separated by 120° in azimuth angle.
Recorded CFBG data waveforms from PBX 9501 radial detonation front experiment: (a) raw data voltage versus time traces; (b) traces normalized and calibrated to length using “laser cut” method of L vs. t data retrieval. A linear fit to the calibrated traces in (b) yields an average detonation front velocity (i.e., slope) of 8.75 mm/μs.
Polar view of experimental diagram showing placement of CFBGs for detonation front measurement in PBX 9501 along a curved meridian (line of longitude). The hemispheric shaped HE charge is contained in an outer metal case, and two 70-mm-long CFBGs are epoxied between the HE and case. Azimuth angle between the two CFBGs is 60°.
Recorded CFBG data waveforms from PBX 9501 detonation front travels along a meridian line (longitude) in hemisphere detonation front experiment: (a) raw data voltage versus time traces; (b) traces normalized and calibrated to length using “laser cut” method of L vs. t data retrieval. The green trace represents an extrapolated line tangent to a linear fit of the CFBG1 calibrated trace between 5 μs and 7 μs. It is used to demonstrate that the phase velocity is not linear.
Calculated velocity/slope fits to CFBG data from Fig. 13 show that curvature of hemispheric HE in experiment yields a phase velocity that increases with detonation front position as it propagates toward the equator.
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