Speaker
Description
Measurement of the strain waveforms is critical to improving SNS target performance and reliable lifetime, as well as to evaluating the efficacy of strain mitigation techniques such as the injection of helium gas into the mercury flow. As the measurements must take place in a very limited space and in the presence of intense electro-magnetic interference and ionizing radiation, fiber-optic sensors provide an optimum solution. However, off-the-shelf fiber sensors often could not survive the residual radiation dose (~ 10 kGy) experienced by the target before operation and they failed before any measurements of the response to the beam pulse were attempted. Recently, we developed a novel type of fiber-optic interferometric sensors which includes a sensing interferometer using high-radiation-tolerant fluorine-doped single-mode fibers and an all-fiber based, polarization-insensitive, phase-shifted optical demodulator to provide precise and fast signal processing. The sensing mechanism is based on the low-coherence interferometry technique and the signal demodulator uses an all-fiber based Faraday Michelson interferometer. The fiber sensors are installed in the interstitial space between the mercury target vessel and a water-cooled shroud and their outputs are sent to the demodulation system located in the target manipulator’s gallery through 70-ft long relay fiber cables.
The fiber-optic sensors have demonstrated excellent performance in the measurement of both slow and fast (up to MHz) strains in a very high radiation (108 ~ 109 Gy) conditions. Measurements have been conducted at different proton power levels from more than 20 fiber sensors located at different positions on the mercury vessel. In this talk, we will report measurement results particularly in the following aspects.
i) A systematic characterization of the response of the mercury target to the proton pulses with different power levels and as a function of locations on the target vessel.
ii) An in-situ verification of the efficacy of the small-bubble helium gas injection into the mercury flow. The strain measurement directly revealed the clear reduction of the strain magnitude by the gas injection and its dependence on the gas flow rate.
iii) The radiation-induced-attenuation in the optical fiber has been experimentally investigated at both short-term and long-term ranges.
iv) A radiation-induced volume expansion of the adhesive material and its effect on the strain measurement has been studied.
v) Use of the sensors to measure the strain response of solid targets made of tungsten and other materials to a proton pulse.
| Themes for the contribution | 7 Operation of targets and beam dumps: |
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