Capacitive Stress Gauges in Model Dipole Magnets
Capacitive transducers are used to measure mechanical stress in the windings of superconducting magnets. The transducer consists of a bonded laminate of alternating thin foils of stainless steel and high-strength polymer (polyimide). The thin, flat package is ideal for measuring the accumulation of Lorentz stress that develops in the windings when current is passed through the coil to generate magnetic field. All groups who have used these transducers have experienced problems with failure of the gauges during operation. The Texas A&M group is endeavoring to determine the causes of the failures and remedy them so that we can continue to use the transducers in our next model dipole. During the construction of the previous model dipole, called TAMU II, there were transducer failures at very high ramp rates. From the raw measurements many insightful observations can be made including stress magnitudes, distributions, and magnetic field variance as a function of those stresses. My goal is to improve upon current methods of transducer fabrication to yield more consistent response, more robust reliability, and provisions for other possible uses. I will also develop a new method for reducing error in calibration. A key element of my approach will be to develop fixtures for the fabrication that can produce more reliable bonding between the metal and polymer layers within the transducer. After some investigation into the causes of variability of these transducers, it was found that the more rigid construction methods did not alleviate many problems. The main success of the new construction method was a decreased failure rate. Calibration, however, was quite successful. Evidence from the data taken in this project points toward a converging value of weighted capacitance. Though the offsets to capacitance due to a heating cycle may be unpredictable, convergence of weighted capacitance values over several heat cycles can certainly reduce error in model magnet stress measurements.
Ragland, R. Blake (2009). Capacitive Stress Gauges in Model Dipole Magnets. Available electronically from