A Fiber Optics Sensor for Strain and Stress Measurements

2600 IEEE TRANSACTIONS ON

MAGNETICS, VOL. 30, NO. 4, JULY 1994

A Fiber Optics Sensor for Strain and Stress Measurements in Superconducting Accelerator Magnets

Johannes M. van Oort Superconducting Magnet Group, Accelerator and Fusion Research Division, Lawrence Berkeley Laboratory, 1 Cyclotron Road, Berkeley. CA 94720. USA

Herman H J. ten Kate Applied Superconductivity Centre, University of Twente, P.O. Box

217.7500

AE, Enschede, The Nethedands

Abstract - A novel cryogenic Interferometric fiber optics sensor

for the measurement of strain and stress in the coil windings of superconducting accelerator magnets is described. The sensor can

operate with two different readout sources,monochromatic laser light and white light respectively. The sensor head is built up as an extrinsic Fabry-Perot interferometer formed with two cleaved

fiber surfaces, and can be mounted in several configurations. When read with laser light, the sensor is an extremely sensitive

relative strain or temperature detector. When read with white light the absolute strain and pressure can be measured. Results are presented of tests in several configurations at 77 K and 4.2

K,

both for the relative and absolute readout method. Finally, the possible use for quench localization using the temperature sensitivity is described.

I. INTRODUCTION

This paper describes the use of the well known principle of interferometric strain sensors to measure stress and strain in accelerator magnets. In applying this principle to these magnets, several parameters are of influence on the sensor.

First, there is the high magnetic field ( up to 14 T in this

case >, second there is the low temperature ( down to 1.8 K )

and finally the high compressive stress the sensor has to withstand ( reaching up to 150 MPa ). A sensing technique

will be described that is independent of the magnetic field,

able to survive the high compressive stress, while still able to measure stress and strain with sufficient accuracy. The inherent sensitivity to temperature changes is compensated. Two

different sensing principles are utilized, the first using laser light providing an extremely sensitive relative measurement, the second using white light interferometry giving the possi- bility of an absolute readout as reference. Both these measure-

ments provide important information related to the design and the behaviour of superconducting accelerator magnets. The relative strain measurement can be used to detect small

Manuscript received September 20, 1993. This work was supported by the Director, Office of Energy Research, Office of High Energy and

Nuclear Physics, High Energy Physiesr Division, US. Department of Energy,

under contract No. DE-AC@3-76SFOOb98.

changes in strain during ramping, or to detect the initiation of a quench cad by mechanical disturbances in the coil windings. The absolute measurement can be used to measure prestress,

stress and strain during steady-state operation, and changes after

quench or ramping. A short overview of the theory is given, followed by a description of the sensor and the different ways

of incorporating it into the coil windings. Next, the experimental results are described, followed by a description of the foreseen

test in the 13 T accelerator dipole magnet currently under construction at LBL. Finally, some design considerations and future developments are discussed, especially the further

miniatu-

rization of the sensor using integrated optics.

11. DESCRIPTION OF THE FIBER-OPTIC SENSOR

The extrinsic Fabry-Perot fiber-optic interferometer is constructed by positioning two cleaved fiber surfaces close to each other. The fibers are cleaved using a standard cleaving

technique, generally yielding an almost perpendicular surface. The two surfaces form a so-called low finesse cavity, which

creates a sinusoidal signal at the detector. The Sensor head is shown in Fig. 1. The reflection from the first glass-air interface

and that from the second air-glass interface interfere in the lead fiber. This interference is dependent on the gap distance dSq.

and can be described in terms of a plane-wave approximation

Ul(X,z,t) = Ale‘iR)

i

U2(5 Z, t) = A2t?Jp?), (1)

with U, the reflection from the sensing mirror, U, the reflection from the reference mirror and cp the phase. The reflection

coefficient A, is approximated by

1

(2) t . ‘core

=

‘

r

r,, + 2dgW tan( sin-’ (NA)) ’

with r,, the fiber core radius, t the transmission coefficient of the air-glass interface and NA the numerical aperture of the single-mode fiber, given by