Exploding bridgewire (EBW) detonators were invented during the Manhattan Project over 75 years ago. Initially developed for precise timing and reproducibility, they continue to be used in many applications. Despite widespread use and reliability, their mechanism for function remains controversial. They provide precision timing, yet their function is described in terms of a “lost time” accounting for nearly half of the function time. Buried in understanding the EBW function is the mystery of how an incoherent impulse such as powering a bridgewire yields the coherent energy output of a detonation. Even the general phenomena by which release of chemical energy in a crystalline organic explosive becomes associated with the sonic plane of a steady detonation wave remain uncertain. Here, we investigate the EBW function with a suite of diagnostics and show that stationary heating occurs during the “lost-time.” We use x-ray radiography to observe the propagation of a shock wave from bridgewire vaporization and establish that the origin of the radially emanating detonation wave is spatially separated from the initial shock. Utilizing the observed temperature as a boundary condition in our explosive response models yields a thermal ignition consistent with the “lost-time” and detonation location consistent with previous work. With these results, we define a direct thermal initiation mechanism for the EBW function consistent with previous integral observations and explain the displacement of initiation from the bridgewire burst in time and space.

Exploding Bridgewire Detonators (EBWs) utilize a low density pellet of a secondary explosive which is initiated by the vaporization of a metal wire (the bridgewire) subject to sudden high current.1–3 The initiating pellet is coupled to another explosive, pressed to higher density, such that a steady, reproducible full detonation is achieved at the end of the assembly. The fact that the bridgewire burst requires a high voltage and fast voltage ramp provides safety with respect to electrostatic discharge. The use of secondary high explosives provides safety with respect to thermal and mechanical disturbances. These safety features and the precise timing achievable have made EBWs desirable for many applications. Despite their highly reproducible behavior and widespread use, there remain questions about the mechanism by which they function with attribution to both shock4–7 and non-shock8–10 mechanisms. Previous studies have focused on the integral detonator response to characterize the function.4–7,9,11–13 These studies all document an unaccounted for time delay8,12 between the burst of the bridgewire and the output of the detonator. They also show an offset in the apparent location of the detonation from the bridgewire location. Based on known detonation velocities in the input and output pellets, EBW detonators exhibit a lag time of nearly ½ of the total function time, which is called the “lost-time” in the literature.4–7,9,11,12 The center of initiation measured from the detonator output is inferred to be approximately 1 mm above the location of the bridgewire.

In this work, we apply a suite of in-situ diagnostics to commercially available EBWs with low density initial pressing pellets composed of the secondary explosive pentaerythritol tetranitrate (PETN) coupled to high density acceptor pellets of the nitramine hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX). We report the direct observations of visible light, IR emission, density, and propagation velocity during the function of commercial RP-80 detonators. These observables show that the previously undiagnosed lost-time is the time for the bridgewire to cause a temperature ramp in the initial pressing input pellet (IP), leading to the thermal initiation of the IP at the location previously inferred to be the center of initiation. We use these observations and put forth a direct thermal initiation hypothesis for the mechanism of the function that is consistent with our observations as well as those of others found in the literature. This mechanism can also be considered in other initiation problems in secondary explosives and provides a way to tie an incoherent thermal input to a coherent detonation output.

The detonators used are commercial RP-80 and RP-503 detonators (Teledyne-Risi Corporation) with a thin transparent sleeve enabling visible imaging. They are composed of a gold bridgewire, an initial pressing of PETN powder at 0.88 g/cc, and an output pellet of 94% RDX/6% 461 binder pressed to 1.6 g/cc. The quoted function time for the RP-80 detonator is 2.65 μs + −0.125 μs. Figure 1 shows an x-ray image of an RP-80 detonator. The suite of diagnostics employed include 10 Mfps visible light emission imaging (Shimadzu HPV-X2) with an integration time of 50 ns. Image magnification was selected to allow full field imaging of the detonator and post function expansion and a spatial resolution of 150 μ/pixel. Experiments were run with single point visible light emission measurements collected with 200 μm core multimode fibers in contact with the detonator to provide a continuous time resolution of visible light emission. Infrared point measurements were performed using fiber optics in contact with the side of the PETN initial pressing and output coupled to an amplified InGaAs photodetector. A piezoelectric thin film sensor was placed at the top of the detonator to indicate the breakout time (or final function time) of the detonator. X-ray transmission images were taken using a flash x-ray source and a scintillator coupled camera. The flash x-ray source provided a single x-ray pulse with a duration of 50–60 ns and a peak energy of 150 kVp. The x-ray field of view and magnification were chosen to provide full field imaging of the detonator and provided images with approximately 50 μm per pixel. Both the x-ray absorption and the change in x-ray absorption of the organic explosive are low, making it critical to enhance the contrast by taking the ratio of dynamic and static images in order to measure the changes occurring. The technique of x-ray phase contrast imaging is more sensitive to these small changes and has been applied by several groups at the APS facility.14 In order to map out the evolution of the detonator function with x-ray radiography, multiple experiments were run with varying time delays between energizing the bridgewire and the x-ray flash. Timing fiducials for all the diagnostics were collected on a bank of oscilloscopes which also recorded voltage and current monitors from the bridgewire.9,15Figure 1 shows a schematic of the diagnostic suite applied.

FIG. 1.

Experimental schematic.

FIG. 1.

Experimental schematic.

Close modal

Figure 2 (Multimedia view) shows 50 ns frames extracted from the ultrafast visible imaging. The two lines of images are the views along the two symmetry axes with respect to the bridgewire direction. The frames in Fig. 2 begin when visible light first appears as a dim glow in the IP volume. The top row is the view along the bridgewire, showing an ellipse as it intercepts the clear plastic sleeve of the detonator perpendicular to the line of sight giving it an hourglass shape. In the bottom row, the detonation evolves as an elliptical shell growing from a volume 1 mm above the bridgewire with the long axis of the ellipse coming out of the page. We extract a position x and time t of the detonation front from the leading edge of the bright shell. The breakout time when the wave front reaches the top of the RDX pellet can be seen directly in the emission video.

FIG. 2.

Visible light images taken with 50 ns exposure at 100 ns intervals are shown. The image sequence begins at 2.85 μs in Fig. 4. The images begin at left and proceed to the right with time. The upper and lower images are obtained viewing simultaneously along or perpendicular to the wire axis, respectively. The contrast is the same for all images for comparison of intensity levels. Multimedia views: https://doi.org/10.1063/1.5088606.1; https://doi.org/10.1063/1.5088606.2

FIG. 2.

Visible light images taken with 50 ns exposure at 100 ns intervals are shown. The image sequence begins at 2.85 μs in Fig. 4. The images begin at left and proceed to the right with time. The upper and lower images are obtained viewing simultaneously along or perpendicular to the wire axis, respectively. The contrast is the same for all images for comparison of intensity levels. Multimedia views: https://doi.org/10.1063/1.5088606.1; https://doi.org/10.1063/1.5088606.2

Close modal

X-ray images taken at two different times along the same two orthogonal axes are shown in Fig. 3. Views orthogonal to the wire are shown on top, and the two metal posts of the bridge circuit can be seen. The left panels in the figure show x-ray transmission obtained during the detonator function with two different time delays from bridgewire burst (2.9 μs and 3.4 μs). The right panels show the ratio of the dynamic image to an initial static image, enhancing the change in transmission during the function. In the region of the pins where x-ray transmission through the high density metals is low, the change in transmission ratios shows black and white bands which are useful as position fiducials. The earliest x-ray images were obtained during current flow and bridgewire heating, but before vaporization, and showed no change in the PETN or RDX pellets. The images immediately after bridgewire burst such as those in Fig. 3 show a diffuse region of increased transmission in the PETN just above the bridgewire and a dark band where the gold from the bridgewire has moved into the PETN. In subsequent images, a curved dark band progresses through the detonator, marking the boundary between the unperturbed material above (ahead of) the wave and the material behind it. The band propagates across the PETN initial pressing and the RDX output pellet followed by a higher transmission region. From a series of experiments with frames at varying delays from wire vaporization, we can build up an x, t diagram for the edge in the x-ray images. These positions and times are plotted as open diamond and triangle symbols shown in Fig. 4. The image contrast is low due to the small change measured in a low density material. We hope to enhance the contrast using phase contrast techniques in future studies.

FIG. 3.

X-ray images taken along the same two orthogonal axes as Fig. 2 are shown (times 2.9 μs and 3.4 μs). Views orthogonal to the wire are shown on top, and the two metal posts of the bridge circuit can be seen. The left panels show direct x-ray, and the right panels show the ratio of the dynamic image to an initial static image, enhancing the change in transmission during the function. In all grey scale x-ray images, lighter grey indicates higher transmission/lower density. The PETN pellet is adjacent to the bridgewire at the interface and vertical spatial origin at y =0 mm. The RDX pellet, distinguishable in radiography by the darker transmission signal, begins at y =3.9 mm. The end of the RDX pellet and detonator is at y = 9 mm. The lighter transmission region above the RDX is air, and the train of bridgewire/PETN/RDX is bound by a transparent polycarbonate cap, also visible in the radiography.

FIG. 3.

X-ray images taken along the same two orthogonal axes as Fig. 2 are shown (times 2.9 μs and 3.4 μs). Views orthogonal to the wire are shown on top, and the two metal posts of the bridge circuit can be seen. The left panels show direct x-ray, and the right panels show the ratio of the dynamic image to an initial static image, enhancing the change in transmission during the function. In all grey scale x-ray images, lighter grey indicates higher transmission/lower density. The PETN pellet is adjacent to the bridgewire at the interface and vertical spatial origin at y =0 mm. The RDX pellet, distinguishable in radiography by the darker transmission signal, begins at y =3.9 mm. The end of the RDX pellet and detonator is at y = 9 mm. The lighter transmission region above the RDX is air, and the train of bridgewire/PETN/RDX is bound by a transparent polycarbonate cap, also visible in the radiography.

Close modal
FIG. 4.

External timing measurements. The voltage monitor for EBW voltage is shown as the solid grey line. The thick black vertical line rising at 4.3 μs shows a piezoelectric sensor monitor of breakout time at the top of the detonator. The open diamonds and triangles mark the beginning of each exposure for x-ray flash imaging and are plotted as the vertical position of the density wave. The solid black squares mark the beginning of each exposure for visible imaging and are plotted as the vertical position of the leading edge of the luminosity. The lines are fits to these x, t points and yield the velocities noted in the text. The solid blue line is the calculated temperature from the measured InGaAs voltage, and the overlaying black line is a calculated thermal trajectory applied to the PETN thermal decomposition model. The evolution of three representative species from the model output is shown in light blue, green, and purple and plotted as concentrations normalized to the initial uncompressed PETN density = 1.0.

FIG. 4.

External timing measurements. The voltage monitor for EBW voltage is shown as the solid grey line. The thick black vertical line rising at 4.3 μs shows a piezoelectric sensor monitor of breakout time at the top of the detonator. The open diamonds and triangles mark the beginning of each exposure for x-ray flash imaging and are plotted as the vertical position of the density wave. The solid black squares mark the beginning of each exposure for visible imaging and are plotted as the vertical position of the leading edge of the luminosity. The lines are fits to these x, t points and yield the velocities noted in the text. The solid blue line is the calculated temperature from the measured InGaAs voltage, and the overlaying black line is a calculated thermal trajectory applied to the PETN thermal decomposition model. The evolution of three representative species from the model output is shown in light blue, green, and purple and plotted as concentrations normalized to the initial uncompressed PETN density = 1.0.

Close modal

InGaAs photodiode coupled fiber optics was used to measure infrared emission as a function of time at 2 discrete locations. The initial infrared light emission is attributed to thermal Planck grey-body emission from the solid, unreacted explosive.16 The thermal emission begins at the time of the bridgewire burst and then rises for 1150 ns of “lost time.” We use previous measurements to calibrate T(V) and plot the measured temperature in Fig. 4.

The overall timing for all diagnostics is shown in Fig. 4. A fiducial trigger synchronizes the 2.5 kV voltage pulse to the bridgewire with all other diagnostics. The voltage to the bridgewire is recorded and is plotted as the solid grey line in Fig. 4. The bridgewire voltage remains high for approximately 1 μs before a large voltage transient is observed simultaneously with a dip in current. This has been observed by many and is attributed to wire vaporization.17 The thick black line in Fig. 4 shows the output of a piezoelectric sensor, monitoring the breakout time at the top of the detonator at time 4.3 μs. The open symbols mark the time and position of the density front from the x-ray images. The solid blue line shows the temperature rise calculated from the measured InGaAs intensity, and the black squares show x, t from the visible light emission images.

The expected function time can be calculated by measuring back in time from the observed breakout time of the detonator and assuming nominal detonation velocities of 8 km/s for the 1.6 g/cc RDX output pellet for a travel distance of 4 mm and 4.9 ± 0.2 km/s for the 0.88 g/cc initial pressing of PETN with a length of 5 mm. The total anticipated transit time for steady detonation is therefore 500 ns for the RDX plus and 1000 ns for the PETN. The measured function time for the detonator measured from bridgewire burst (vaporization) to output time is 2650 ns, leaving 1150 ns unaccounted for. This has been referred to previously as the detonator lost time.

The compendium of time sequence data in Fig. 4 has been divided into regions according to an interpretation of the diagnostics. In the EBW region, voltage is applied to the bridgewire which then melts/vaporizes and provides the energy input into the PETN IP.15,17–20 For nearly 1 μs after bridgewire burst, no visible light is observed, but thermal emission increases which we label the dark zone. Visible light emission begins as a diffuse glow from a volume we refer to as the initiation volume in analogy with the ignition volume observed in thermal explosion experiments.21–23 The location of this initiation volume is measured directly from the location of the center of the visible light emission approximately 1 mm above the bridgewire. This is consistent with the description by others of a “center of initiation” or “detonation kernel.”5,20,24,25 Both visible and thermal emission increase with time as reaction progresses but remain fixed in space thus the name “Eulerian” regime. At a time consistent with the end of the lost time, propagation of the luminous front begins and travels at 4.9 ± 0.2 km/s in the low density PETN initial pressing, consistent with a detonation velocity at this density. This is called the “Lagrangian 1” region as it is the first observation of reaction with a moving reference frame. The visible light intensity diminishes at the interface between the initial pressing and output pellets, and the velocity of the traveling reaction remains stable. The interface regime lasts approximately 100 ns and is followed by a second propagating reaction front which we call “Lagrangian 2.” The wave in this regime travels through the RDX output pellet with a velocity of 8.56 ± 0.4 km/s. The propagation of the dark band identified in x-ray transmission is first identified in the dark zone regime, significantly before emanation of the detonation luminosity. This density wave travels at ∼3 km/s, and a linear fit to x, t can be extrapolated to an origin on the bridgewire at a time during wire vaporization. This is shown as the dashed line in Fig. 4. The propagating luminosity begins later in time but travels faster. The two waves coalesce at the IP/output pellet interface. A counter-propagating wave is seen going back through the IP towards the bridgewire when the waves reach the output pellet. Finally, the wave front reaches the end of the RDX pellet and marks the breakout time for reaction.

We divide the EBW detonator function into impulse and response. The impulse from the bridgewire has been previously found to be the explosion of the bridgewire due to superheating above the melt point to generate rapid vaporization.17–20 The bridgewire vaporization provides the initial impulse to the PETN. The response of the PETN has been considered to be either a shock response5–7 or a non-shock deflagration to detonation (DDT) response.8,9 By directly observing the density, luminosity, and temperature, we see that the bridgewire burst launches a wave into the IP. The density wave it creates is observed to travel at 3 km/s. However, this does not directly initiate detonation. The propagating luminous chemistry does not begin with the density wave, but at a location remote from the bridgewire and delayed from the density wave. This indicates that the mechanism of the function is not a direct shock from the bridgewire burst. The DDT explanation is also inconsistent with our measurements with no deflagration wave being observed. There are a wave at shock velocity observed through density measurements and then a wave at detonation velocity observed through visible light emission, but no sub-sonic deflagration wave. What is observed is a temperature increase beginning from bridgewire burst and leading to initiation of a luminous wave traveling at detonation velocity from a region previously referred to as the “detonation kernel.”9,13 The luminous detonation wave emanates elliptically from the initiation volume, inconsistent with both direct shock and DDT mechanisms.

Our observations of lost time and center of initiation are in direct agreement with previous integral studies. However, the in-situ observations of density, temperature, and luminous chemistry suggest a different interpretation for the mechanism of the detonator function. To explain all the observations from this study and previous work by others, we propose a direct thermal initiation (DTI) mechanism of the EBW function. Our measurements show PETN heating beginning at bridgewire burst with temperature rising for ∼1 μs with a location centered 1 mm above the bridgewire. The heat ramp is a generated by the bridgewire burst, and PETN chemistry proceeds as the temperature increases 1 mm above the bridgewire due both to the location at which the bridgewire deposits its thermal energy after the burst and as an optimization of the heat generation from PETN exothermic chemistry and heat loss. The PETN chemistry model we use is an extension of the Henson-Smilowitz global chemistry model originally developed for HMX26 and expanded for TATB and PETN.27 All reaction rates and thermo-physical properties used are independently determined as for the HMX model. The inputs are the crystal density and thermal boundary condition, and the outputs are chemical progress, heating, and pressurization based on a product JWL EOS.28 Full details of the model will be discussed elsewhere. At the temperature of 1000 °C imparted by the vaporized gold bridgewire, our PETN chemistry model predicts an ignition time consistent with the detonator lost time observation.

The output of this model is shown as the solid black line in Fig. 4 overlaying the calculated temperature in blue. The time and location are consistent with observations of initiation volume in this experiment series. The initiation of the PETN exothermic chemistry from this model then serves as the input for a wave coalescence behavior due to increasing temperature and pressure.29 Wave coalescence for PETN subject to the calculated pressurization occurs within 150 μm and 110 ns, which is within 1 pixel and 1 frame in the measurements presented here. The prompt appearance of the luminous wave at the pellet surface and the measured velocity indicate that this coalescence is consistent with the location of detonation initiation. The DTI model is fully consistent with our current observations as well as previous observations of lost time and center of initiation.

Our observations of density loss (by radiography), elliptical initiation volume (by visible imaging), and temperature rise (by pyrometry) in the ∼1 μs following EBW vaporization are consistent with a non-shock thermal response pathway. We conclude from this study that the “lost time” of previous work is the time during which the PETN is self-heating to a thermal initiation. The location of initiation serves as the previously identified “detonation kernel.” It is determined by the balance between heating and loss in the PETN subsequent to wire vaporization. The thermal initiation in the low density PETN yields a reaction wave propagating at approximately 4.9 ± 0.2 km/s. Upon reaching the RDX interface, this wave generates a detonation wave with a run time of approximately 100–200 ns. A detonation wave then propagates through the RDX output pellet at a velocity of approximately 8.56 ± 0.4 km/s which is consistent with the expected detonation velocity for RDX pressed to 1.6 g/cc. Thus, the unknown lost-time and initiation offset are completely consistent with the response of the PETN to the thermal input observed from the bridgewire burst. Both of them solve the controversies of the EBW detonator function and provide a direct tie between thermal input and shock output.

This research was supported by Science Campaign 2, Engineering Campaign 6, and the Surety Program administered by Los Alamos National Laboratory.

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