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BACKGROUND OF THE INVENTION
This invention relates to the detection of temperature transient conditions
in superconducting magnets and, more particularly, to the use of fiber
optics to detect temperature transient detection in superconducting
magnets.
It is a characteristic of superconducting magnets that the superconducting
wire forming the magnet coil may undergo a spontaneous quench condition,
i.e., change from a superconducting state to a normal state. A quench
condition is a very localized condition that results in a localized hot
spot from the sudden increase in wire resistance and concomitant
resistance heating. This localized heating can produce temperatures in
excess of 400-800 K. that cause failure in the wire insulation and
degradation of the superconductor critical current. Thus, it is highly
desirable to detect a temperature transient condition and apply rapid
heating to the entire magnet in a time scale commensurate with the onset
of the temperature transient, in the order of 100 msec.
Fiber optic-based systems have been used for temperature transient
detection. Fiber optic-based systems have many advantages over
conventional voltage monitor systems for quench detection, e.g., very low
thermal conductivity, high resistance to EMI interference (with resulting
low false trigger rates), high radiation resistance, reduced mechanical
complexity, high system reliability due to a reduced number of components,
and efficient multiplexing capabilities. O. Tsukamoto et al., "Detection
of Temperature Rise at 4.2 K. by Using a Dual-Core Optical Fiber--An
Optical Method to Detect a Quency of a Superconducting Magnet," 31
Advances in Cryogenic Engineering, Plenum House, New York (1986), pp.1269,
teaches an optical fiber system using a dual-core fiber. A temperature
rise of 1.0 K. at 4.2 K. is detected, equivalent to a quench in a
superconducting magnet. As taught by Tsukamoto, a laser light is split and
input to outer and core fibers. A temperature transient condition causes a
change in the optical length of the outer fiber, primarily from a change
in the refractive index, wherein interference fringes are formed when the
light signals from the inner and outer fibers are combined at the output
of the dual-core optical fiber. The dual-core fibers are, however,
somewhat difficult to incorporate into a magnet winding. The dual core
fiber also has a sensitivity limitation since both fibers are adjacent any
temperature transient event so both the outer and inner fiber are
responding to the same temperature change, albeit at different rates.
Further, dual-core fibers require doped fibers that are inherently
radiation sensitive and would not be useable in some important
applications, e.g., high energy particle accelerators, such as the
superconducting supercollider, and superconducting magnets for fusion
application, e.g. ITER.
Tsukamoto does teach that two single-mode fibers may be used in a
Mach-Zender arrangement to improve on the sensitivity of the dual fiber
arrangement. Then, one fiber provides a reference signal and the other
fiber a temperature signal. Tsukamoto notes, however, that there are
difficulties in placing this arrangement in a cryogenic region and in
making the system compact enough to be a quench detector. There is no
teaching about placing the reference and temperature sensor fibers on
superconducting magnets, but the statement on placement difficulties and
the teachings on the dual core fibers infer that the two fibers are placed
on the same magnet to subject both fibers to the same environment to
minimize noise
Accordingly, it is an object of the present invention to provide a
single-core optical fiber system for detecting temperature transients in
superconducting magnets, wherein only a single fiber is incorporated in a
magnet to limit the space required to incorporate the fiber sensor.
It is another object of the present invention to provide a high sensitivity
system using separated reference and sensor fibers for rapid detection of
temperature transients in superconducting magnets.
It is a further object of the present invention to detect temperature
transients in superconducting magnets using undoped optical fibers for
improved transmission stability in a high radiation environment.
One other object of the present invention is to provide a Mach-Zender type
arrangement with improved noise rejection.
An additional object of the present invention is to reduce the number of
detector fibers required in large systems.
Still another object of the present invention is to reduce the number of
false quench alarms arising from a large array of superconducting magnets.
Additional objects, advantages and novel features of the invention will be
set forth in part in the description which follows, and in part will
become apparent to those skilled in the art upon examination of the
following or may be learned by practice of the invention. The objects and
advantages of the invention may be realized and attained by means of the
instrumentalities and combinations particularly pointed out in the
appended claims.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the
purposes of the present invention, as embodied and broadly described
herein, the apparatus of this invention may comprise a system for
detecting a temperature transient in a superconducting magnet using a
signal optical fiber in functional proximity with the superconducting
magnet for responding to a temperature transient condition and a reference
optical fiber remote from the signal optical fiber. A coherent light
source outputs coherent light for input to the signal and reference
optical fibers. A phase modulator is functionally connected to the signal
optical fiber or the reference optical fiber to provide a phase shift of
about 90.degree. to the coherent light in the signal or reference optical
fiber. An interferometer then detects a relative phase difference in
coherent light output from the signal and reference optical fibers. A peak
sample and hold circuit is connected to feed back light output from the
signal or the reference fiber to the phase modulator to stabilize the
modulator. In one embodiment, a low pass filter provides feed back to the
phase modulator to stabilize the modulation while a high pass filter
passes temperature transient signals to a suitable analyzer, such as a
digital storage oscilloscope, alarm, or the like.
In another embodiment of the present invention, a fiber optic system
provides for detecting a temperature transient in an array of
superconducting magnets. A first signal optical fiber is provided in
functional proximity with a first plurality of the superconducting
magnets. A second reference optical fiber is provided in functional
proximity with a second plurality of the superconducting magnets distinct
from the first plurality of magnets. A coherent light outputs light for
input to the signal and reference optical fibers. A phase modulator is
functionally connected to the signal optical fiber or the reference
optical fiber to provide a phase shift of about 90.degree. to the coherent
light in the signal or reference optical fiber. An interferometer then
detects a relative phase difference in coherent light output from the
signal and reference optical fibers. In one embodiment, the output signal
is input to a low pass filter to provide modulator stabilization for noise
reduction and a high pass filter for temperature transient signal
detection.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and form a part of the
specification, illustrate embodiments of the present invention and,
together with the description, serve to explain the principles of the
invention. In the drawings:
FIG. 1 is a schematic diagram in block diagram form of a fiber-optic
temperature transient detector according to one embodiment of the present
invention.
FIG. 2 is a graphical representation of a detector output signal.
FIG. 3 is a schematic diagram in block diagram form of a peak sample and
hold circuit for use in the detector shown in FIG. 1.
FIG. 4 is a schematic diagram in block diagram form of a fiber-optic
temperature transient detector according to another embodiment of the
present invention
FIG. 5 illustrates a fiber-optic arrangement for temperature transient
detection in an array of superconducting magnets.
FIG. 6 is a photographic of an oscilloscope output from a test sample
showing the onset and detection of an actual quench condition.
DETAILED DESCRIPTION OF THE INVENTION
Temperature transient detection according to the present invention uses a
pair of optical fibers (hereinafter "fibers") where one fiber is wound on
a superconducting magnet core with the superconducting wire and forms a
signal arm in a Mach-Zender sensor arrangement. The other fiber is formed
as a reference arm at a location remote from the signal arm, i.e., at a
location that is unlikely to experience a temperature transient
simultaneously with a temperature transient affecting the signal arm. It
will be recognized that the designation of one optical fiber branch as the
signal arm or the reference arm is arbitrary since both arms may be
incorporated in magnets for temperature transient detection. As herein
explained, the present invention incorporates circuitry for stabilizing
the reference arm optical length to minimize or eliminate false quench
signals and also provides for forming a fiber array to detect and locate
quench conditions in an array of superconducting magnets such as may be
found in the superconducting supercollider or in a fusion device.
To illustrate the principles involved, consider that when a magnet
quenches, the quench propagates away from the initial quench point at a
velocity of from 50-100 m/sec and the temperature rises approximately 50
K. Since the change in the index of refraction of fused silica per degree
at 4 K. is approximately 5.times.10.sup.-7 K..sup.-1, the 50 K. rise will
result in a change of the index of refraction of about
2.5.times.10.sup.-5. If coherent light is passed through a fused silica
optical fiber that experiencing the heating transient described above and
is interfered with light passing through an identical length quiescent
reference fiber, a phase shift of .pi. radians will occur between the
light beams in the reference and the detector fibers when a length of
approximately 60 mm has been heated to 50 K. (assuming instantaneous heat
transfer to the core of the fiber). At a temperature transient propagation
velocity of 50 m/sec, the time required for this propagation is
approximately 1.2 msec. The estimated time for thermal diffusion to the
core of a nominal fiber is on the order of 5 msec, so there is a 5 msec
delay before the heating is propagated to the core of the fiber.
This should represent an upper limit since the outer portions of the fiber
are experiencing temperature rises earlier than the core region and
introducing phase shifts in the output signal. Furthermore, phase shifts
of much less than .pi. radians are easily detectable. Thus, the index
change is much faster than required by the application. By the time the
temperature transient has propagated 50 cm (10 msec) with a 50 K.
temperature rise, a phase shift of over 6 radians would have been
generated at the core of the fiber. This is a signal that is several
orders of magnitude greater than the detection threshold.
The basic design of one embodiment of a temperature transient detector
according to the present invention is shown in FIG. 1. Laser source 10,
which may be a 10 mW HeNe laser operating at 632.8 nm, outputs coherent
light to fiber coupler 12 for input to 3 db splitter 14. In another
embodiment, laser source 10 may be a solid state laser, e.g., a diode
laser with a wavelength of about 1.2 .mu.m. Splitter 14 outputs balanced
coherent light to two fibers forming signal arm 16 and reference arm 22.
Single mode fibers are preferred to maximize transmission and to maintain
optical coherence. Thus, signal arm 16 is a length of single mode fiber
that is wound along with superconducting wires to form coils about a
magnet core (not shown). Reference arm 22 is the same length of single
mode fiber as signal arm 16 and is attached to modulator 24, which may be
a piezoelectric modulator crystal, to modulate the detected signal. Signal
arm 16 and reference arm 22 are recombined through 3 db splitter 26 to
output a combined signal for input to detector 28.
The output from detector 28 (FIG. 1) is depicted in FIG. 2 and is a signal
that is modulated by the action of modulator 24 and by the occurrence of a
temperature transient in signal arm 16. Modulator 24 is driven by
oscillator 34 to provide a periodic change in the optical path length of
reference arm 22 and modulates the output from detector 28 from periodic
interference effects as the light in signal arm 16 and reference arm 22
move from being in phase to being out of phase under the action of
modulator 24. The interference pattern that is output from detector 28 is
comprised of two offset peaks having a frequency twice the frequency of
modulator 24. The difference in amplitude of the peaks is a measure of the
difference in optical refraction along the lengths of reference and signal
optical fibers. A temperature transient event along on of the fibers will
introduce an additional phase relationship in light output from fibers 16
and 22 to change the relative amplitudes of the interference peaks as a
function of a temperature transient in one of the fibers.
The output signal from detector 28 shown in FIG. 2 is input to peak sample
and hold circuit 30, which detects the relative intensity of the two peaks
and outputs a signal related to the difference in intensity. The output
signal from peak sample and hold circuit 30 is fed back to oscillator
driver 34 through low pass filter 32. The signal passed by low pass filter
32 will cause the bias level of modulator 24 to compensate for slow path
length variations in either the reference 22 or signal 16 arms due to
noise, e.g., mechanical effects or slow thermal transients. Temperature
transient events are expected to be very fast (>1 KHz) and will be
filtered out by the low pass filter, so that the modulator will not
compensate for phase shift caused by a temperature transient signal.
A temperature transient signal is then passed by high pass filter 36 (FIG.
1) for detection as a change in the amplitude difference between the two
detected signal peaks. This output signal can be used as an alarm signal
and/or a process signal for action that prevents magnet coil damage, e.g.,
heating of the coil to minimize temperature gradients.
A block diagram for peak sample and hold circuit 30 is shown in FIG. 3. The
input signal to circuit 30 is output by detector 28 and shown in FIG. 2,
which shows a sinusoidal fringe, whose peak amplitude is a function of the
optical path length with a length modulation impressed on the optical path
length by phase modulator 24. The optical signal is represented by
positive lobes, the amplitudes of which depend upon the displacement of
the working point. The optical signal contains a component at twice the
modulating frequency and also contains a phase sensitive component at the
modulating frequency. Alternate peaks of the lobes are sampled and held.
Thus, the output from detector 42 is amplified by preamplifier 42 for input
to sample and hold module 44 and to inverter 46. Modulation oscillator 48
provides a sampling signal (on alternate periods) to hold 44 and inverter
46, which also includes a sample and hold function. Thus, alternate
periods of the optical signal are sampled and are summed (with one signal
inverted) by differential amplifier 52, which outputs a positive or
negative signal that is proportional to the amplitude and direction of the
displacement of the working point from the fringe minimum. The output from
amplifier 52 is then integrated by integrator 56 and summed with the
modulation carrier from oscillator 48 at the input to driver 62 for
driving modulator 24. Additionally, compensation network 54 provides a
feed-forward compensation for driver 34 (FIG. 1). In conjunction with the
integrator gain, the signal helps to control the damping for driver 34.
Phase shifter 58 compensates for the time constants of detector 28 and
phase modulator 24. The combined output is the recovered signal from
detector 28 for input to drive circuit 62 for input signal 64 to modulator
driver 34 (FIG. 1). Integrator 56 has a limited dynamic range and is
therefore reset when its output reaches a preset level. Reset unit 66
senses when integrator 56 has saturated and resets it to selected level.
Modulator 24 may be a piezo-electrically driven wafer mounted on a support
form and wound with enough fiber turns to obtain a desired stretch in
length. Typically, the stretch is about 1 micron per 40 volts per 1.5 inch
segment of fiber. A single turn of fiber on the mount requires 20 volts
per micron and a maximum of 25 turns are possible without degradation of
the amplitude response. A suitable piezo driver will accept a maximum
drive voltage of 200 volts and can be controlled with a .+-.5 volt
modulation signal. For the maximum drive voltage, the response is limited
to a maximum of 2 kHz. Faster response (up to 100 kHz) is possible, but
the voltage must be reduced and the amplitude is reduced to a fraction of
a fringe.
Referring now to FIG. 4, there is shown an optical fiber temperature
transient detection system for use with a plurality of magnets. Assuming a
magnet design for use with the superconducting supercollider, each magnet
includes four quarter coils, with inner and outer coils consisting of 16
turns and 20 turns, respectively. Each turn is 15.4 m long and a pair of
fiber optic cables may be wound along with each superconducting cable.
This results in a total of 36 windings for a complete magnet. Since each
fiber optic winding would require about 31 m of length, the total fiber
optic length per magnet assembly would be on the order of 1 km. A
temperature transient would be detected as soon as the temperature-length
product at the temperature transient site exceeds the sensitivity of the
detector (which occurs in about 5 msec).
For the magnet design discussed above, and assuming a reasonable quality
fiber optic cable having an attenuation of 2 db/km, 10 cables can be wound
in series, yielding a total attenuation of 20 db, which will provide more
than adequate signal-to-noise using solid state lasers and detectors. As
noted above, the choice of signal and reference arms is arbitrary so that
10 magnets 78 can be placed in the signal arm 76 and 10 magnets 86 in the
reference arm 82. As discussed above, monitoring light from laser 72 is
split by splitter 74 for balanced input to each arm. Likewise, the return
signals are combined through splitter 88 for detecting the combined signal
on detector 92. The detected signal is input through low pass filter 94 to
phase shifter 84, which includes the components of sample and hold circuit
30, driver 34, and modulator 24 shown in FIG. 1.
The system shown in FIG. 4 can be configured to monitor an array 106 of
magnets, such as shown in FIG. 5. If one fiber of each fiber pair forms a
fiber along x-inputs 102 and the other fiber forms a fiber along y-inputs
104, each fiber has only one magnet in common with any other fiber. Then
an array of 400 magnets may be monitored with only 40 detectors, as shown
in FIG. 5. Various fibers are designated as reference arms for combination
with signal fibers with which the reference fibers do not share a magnet.
X-output fibers 108 and y-output fibers 110 are readily combined to form
appropriate feedback loops for phase shifters 84 (FIG. 4) and for outputs
to a monitoring system that provides a coincidence detection on two
detectors to both detect and localize a temperature transient. It will be
appreciated that the detection array shown in FIG. 5 can significantly
increase the discrimination against false temperature transient alarms
since two fibers must indicate a temperature transient and a single
temperature transient indication in a fiber is ignored.
A demonstration system according to FIGS. 1 and 3 was constructed to verify
system stability and sensitivity. A nichrome wire was wrapped around the
signal arm to simulate a temperature transient condition heating. The
steady state system at room temperature was extremely stable and no
significant output signal was observed even when the signal arm was
subject to mechanical shock.
To test both slow and fast thermal effects, the nichrome coil was heated
with both a dc input and a capacitor discharge. Heating with 100 mA dc
produced no observable signal. However, when the heater was pulsed with a
capacitor discharge on the order of 140 microjoules, an output signal of
about one-tenth maximum was observed. A series of tests was also conducted
with the apparatus immersed in a bath of liquid nitrogen. The liquid
nitrogen had no effect on the fiber transmission or on the interferometer
operation. The nichrome wire was attached to a 5500 microfarad capacitor
that was charged to 20 volts and discharged through the wire. The
sensitivity of the system was calculated to be 0.75 K./fringe shift,
indicating a system sensitivity of about 0.23 mK.
FIG. 6 is a photograph of an oscilloscope trace of the output of detector
28 along with the voltage across a superconducting sample material at a
temperature of about 4.2 K. A heat pulse was applied to the entire length
of the test element (about 3 m). The time of application of the heat pulse
was just before the sample voltage begins to increase. The steady periodic
response from the detector is indicative of a steady change in the optical
length of the signal fiber adjacent the test element. For this case, an
approximate time difference from onset of sample voltage rise to
interferometer detector signal (fringe) registration was about 60 msec. At
the time of quench, the detector fringe frequency was observed to be about
75 Hz, compared with a background frequency of about 2.5 Hz.
The foregoing description of the preferred embodiments of the invention
have been presented for purposes of illustration and description. It is
not intended to be exhaustive or to limit the invention to the precise
form disclosed, and obviously many modifications and variations are
possible in light of the above teaching. The embodiments were chosen and
described in order to best explain the principles of the invention and its
practical application to thereby enable others skilled in the art to best
utilize the invention in various embodiments and with various
modifications as are suited to the particular use contemplated. It is
intended that the scope of the invention be defined by the claims appended
hereto.
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