A selection of commercially available high-power optical fibres have been characterised for radiation susceptibility in
Sandia’s Annular Core Research Reactor (ACRR). The fibres were subjected to a total gamma and neutron dose >2 Mrad(Si)
in a 7 ms pulse. The neutron fluence was >10<sup>15 </sup> n/cm<sup>2</sup>. Changes in the transmission characteristics of optical fibres carrying
high energy, short duration laser pulses (power densities of around 1.5 GW/cm<sup>2</sup>) were measured.
All fibres survived at least two consecutive radiation exposures, showing typical transient transmission losses of around 20%.
Post radiation exposure, the transmission characteristics returned to those of pristine fibres within one minute.
High power laser systems have a number of uses in a variety of scientific and defense applications, for example laser
induced breakdown spectroscopy (LIBS) or laser-triggered switches. In general, high power optical fibers are used to
deliver the laser energy from the source to the target in preference to free space beams. In certain cases, such as nuclear
reactors, these optical systems are expected to operate in ionizing radiation environments. In this paper, a variety of
modern, currently available commercial off-the-shelf (COTS) optical fiber designs have been assessed for successful
operation in the transient gamma radiation environment produced by the HERMES III accelerator at Sandia National
The performance of these fibers was evaluated for high (~1 MW) and low (<1 W) optical power transmission during
high dose rate, high total dose gamma irradiation. A significant reduction in low optical power transmission to 32% of
maximum was observed for low OH- content fibers, and 35% of maximum for high OH- fibers. The high OH- fibers were
observed to recover to 80% transmission within 1 μs and 100% transmission within 1 ms. High optical power
transmission losses followed generally similar trends to the low optical power transmission losses, though evidence for
an optical power dependent recovery was observed. For 10-20 mJ, 15 ns laser pulses, around 46% was transmitted
coincident with the radiation pulse, recovering to 70% transmission within 40 ns of the radiation pulse.
All fibers were observed to completely recover within a few minutes for high optical powers. High optical power
densities in excess of 1 GW/cm<sup>2</sup>
were successfully transmitted during the period of highest loss without any observed
damage to the optical fibers.
High-Speed Multi-Frame Laser Schlieren is used for visualization of a range of explosive and non-explosive events.
Schlieren is a well-known technique for visualizing shock phenomena in transparent media. Laser backlighting and a
framing camera allow for Schlieren images with very short (down to 5 ns) exposure times, band pass filtering to block
out explosive self-light, and 14 frames of a single explosive event.
This diagnostic has been applied to several explosive initiation events, such as exploding bridgewires (EBW), Exploding
Foil Initiators (EFI) (or slappers), Direct Optical Initiation (DOI), and ElectroStatic Discharge (ESD). Additionally, a
series of tests have been performed on "cut-back" detonators with varying initial pressing (IP) heights. We have also
used this Diagnostic to visualize a range of EBW, EFI, and DOI full-up detonators. The setup has also been used to
visualize a range of other explosive events, such as explosively driven metal shock experiments and explosively driven
microjets. Future applications to other explosive events such as boosters and IHE booster evaluation will be discussed.
Finite element codes (EPIC, CTH) have been used to analyze the schlieren images to determine likely boundary or initial
conditions to determine the temporal-spatial pressure profile across the output face of the detonator. These experiments
are part of a phased plan to understand the evolution of detonation in a detonator from initiation shock through run to
detonation to full detonation to transition to booster and booster detonation.
Los Alamos National Laboratory is currently designing a series of direct optically initiated (DOI) detonators. The primary purpose of this series of detonators is to achieve a level of safety in the face of unintentional initiation from an electrical source. The purpose of these experiments is to determine the minimum spotsize that will initiate the low density initial pressing in these laser detonators. With this information it is expected that a more robust optically initiated detonator can be designed and manufactured. Results from a series of experiments will be discussed. First a range of small core diameter fiber optics with varying energy injection levels will be tested to find the minimum energy level necessary to achieve reliable initiation. Second, a range of apertures will be employed to trim the spotsize down to a minimum size that will still maintain reliable initiation. This information will help to understand whether the initiation criteria for the DOI Laser Detonator are dominated by energy density, total energy or a combination of these criteria.
The initiation of explosives by laser illumination has been known for many years. In this paper we will discuss the development of a working detonator design that reduces the energy required for detonation in a low-density secondary explosive by vaporizing a thin metal coating. We present data on the development of the design for a workhorse laser detonator that provides enhanced safety over existing exploding bridgewire detonators (EBWs). Comparison of this laser initiated data to an exploding-bridgewire (EBW) provides insight into the mechanism of initiation of detonation in low-density PETN by the plasma source. A novel diagnostic technique to determine the run-distance to detonation also known as the apparent Center-of-Initiation (COI) will also be discussed.
Conference Committee Involvement (4)
Optical Technologies for Arming, Safing, Fuzing, and Firing VI
2 August 2010 | San Diego, California, United States
Optical Technologies for Arming, Safing, Fuzing, and Firing V
5 August 2009 | San Diego, California, United States
Optical Technologies for Arming, Safing, Fuzing, and Firing IV
13 August 2008 | San Diego, California, United States
Optical Technologies for Arming, Safing, Fuzing, and Firing III
29 August 2007 | San Diego, California, United States