Spontaneous brillouin scattering quench diagnostics for large superconducting magnets

Large superconducting magnets used in fusion reactors, as well as other applications, need a diagnostic that can non-invasively measure the temperature and strain throughout the magnet in real-time. A new fiber optic sensor has been developed for these long-length superconducting magnets that simultaneously measures the temperature and strain based on spontaneous Brillouin scattering in an optical fiber. Using an extremely narrow (200 Hz) linewidth Brillouin laser with very low noise as a frequency shifted local oscillator, the frequency shift of spontaneous Brillouin scattered light was measured using heterodyne detection. A pulsed laser was used to probe the fiber using Optical Time Domain Reflectometry (OTDR) to define the spatial resolution. The spontaneous Brillouin frequency shift and linewidth as a function of temperature agree well with previous literature of stimulated Brillouin data from room temperature down to 4 K. Analyzing the frequency spectrum of the scattered light after an FFT gives the Brillouin frequency shift, linewidth. and intensity. For the first time, these parameters as a function of strain have been calibrated down to 4 K. Measuring these three parameters allow for simultaneously determining the temperature and strain in real-time throughout a fiber with a spatial resolution on the order of several meters. The accuracy of the temperature and strain measurements vary over temperature-strain space, but an accuracy of better than + 2 K and ± 100 Pe are possible throughout most of the calibrated temperature-strain space (4-298 K and 0-3500 p/g). In the area of interest for low-temperature superconducting magnets (4-25 K), the temperature accuracy is better than + 1 degree.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Nuclear Science and Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2008

Author dc:creator

• Mahar, Scott B

Advisor dc:contributor.advisor

• Joseph V. Minervini and Joel H. Schultz.

• Nuclear Science and Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/53258

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/53258