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Visualization of thermal disturbance and quench propagation in an immersion-cooled superconducting coil in liquid helium
ICEC 15 Proceedings
Visualization of Thermal Disturbance and Quench Propagation in an Immersion-Cooled Superconducting Coil in Liquid Helium Toshio Aihara* and Sadayuki Okada** *Institute of Fluid Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-77, Japan **Hitachi Research Laboratory, Hitachi Ltd., 7-1-1 Ohmika, Hitachi 319-12, Japan Thermal disturbance and quench propagation have been visualized by utilizing attenuation of the luminous energy of reflected light from the interface of conductor surface and liquid helium. The results of optical m e a s u r e m e n t with a high-speed video system were in very close a g r e e m e n t with the m e a s u r e d time history of tap voltages and t e m p e r a t u r e s on the test coil. The visualizability and luminous attenuation characteristics were examined for various conditions.
INTRODUCTION Quench of superconductors has hitherto been detected either by voltage difference in a growing normal zone or by acoustic emission due to sudden thermal expansion, etc. The present authors have succeeded in detecting visually the thermal disturbance and/or quench which may occur unexpectedly in any place at any time in a superconductor. In this study, an artificially induced thermal disturbance and the rapid propagation of normal zone in a pancake-type superconducting coil were visualized by this method and recorded with a high-speed video system. The validity of the optical observation was verified by comparison with the measured time history of tap voltages and temperatures at various locations of the test coil. The visualizability and its causes also were investigated. VISUAL OBSERVATION OF QUENCH PROPAGATION IN A PANCAKE-TYPE COIL Experimental Apparatus and Procedure The test coil of the double-pancake type is schematically shown by parts in Fig. 1. A polyimide-insulated wire of superconducting composite (SC) with the cross section of 1.51mm x 4.98mm and copper/NbTi ratio of 2 was wound spirally around a core cylinder with 32 turns. Thermal disturbance was applied by pulse heating of the 20-~m-thick stainless steel film heater which was sandwiched between the 16th and 17th turns 0fthe SC wire winding. Voltage taps for detecting normal zone growth were soft-soldered onto the coil surface every 3 turns of the winding, and nine 50-~m-diameter chromel-constantan thermocouples were fixed between the SC wires at the locations shown in Fig. 2. The test coil was immersion-cooled in liquid helium at atmospheric pressure, as shown in Fig. 3. Optical phenomena were recorded with a high-speed color video system of 200/ 400 frames per second, by illuminating the coil surface with a 500-watt tungsten lamp. Tsukamoto et al. [1] took high-speed movies of the side view of transient bubble formation around a fine wire stepwise-heated in liquid helium. At the early stage of this study, the present authors also made side-view observations of quench from the direction nearly parallel to the coil surface, but anything new was not found except for bubble formation. Hence, the scheme of downward illumination and top-view observation was adopted. Results and Discussion Figure 4 shows typical still photographs which were reproduced from a video tape for a normal zone propagation induced by pulse input of 18.1 W for 10 ms (equivalent to the energy density 1.5×105 J/m 3) to the film heater under a transport current of 1600 A. Just after power input to the heater, a dark brown zone appears in an arc shape on the coil, at 16th and 17th turns adjacent to the f i l m heater. The dark zone grows primarily in the Cryogenics 1994 Vo134 ICEC Supplement 721
ICEC 15 Proceedings
circumferential direction and draws a concentric circle in about 10 ms. With lapse of time, the dark zone continues to grow inward and outward, rapidly increasing the number of turns and reaches the outermost radius of the coil in about 550 ms. Such rapid propagation of the dark zone closely resembles the results of the authors' numerical analyses [2, 3]. Helium vapor, produced between the coils by quench, bursts out from the outer edge and draft holes of the test coil about 370 ms after quench. Figure 5 shows typical time history of tap voltages and temperatures at the representative locations on the coil. In the figure, I refers to the transport current; I a the pulse current to film heater; t the time after cutoff of the disturbance; Ei~. the voltage difference between ith- andjth-turns; T~.h+t the measured value of a thermocouple inserted between kth- and (k+l)th-turns on line-A in Fig. 2; and C ) the occurrence of quench atjth-turn. Based on the tap-voltage detection, the normal zone reaches the outermost 32nd turn at t -~ 550 ms; at the same time, the temperature of SC wire suddenly increases. On the other hand, the visual observation by video-tape recording shows that the dark zone also reaches the 32nd turn at t -~ 550 ms. During the quench propagation, the expanding of the dark zone always coincides with the time history of the tap voltages and temperatures. It is clear from these facts that the observed dark zone indicates the normal zone. VISUAL OBSERVATION OF THERMAL DISTURBANCE The insulation film was peeled off from the test coil surface for sensitive, visual detection of the faint disturbance, and additional thermocouples were attached on the SC wire surface closest to the film heater and coated with epoxy resin to minimize cooling error. Under no transport current (I = 0), thermal disturbance was applied to the test coil by pulse heating of 1-10 A for 10-50 ms to the heater, equivalent to the energy density 2.6x104-3.5x106 J/m3; then, the transient appearance of a dark zone was recorded with the video system. The experimental results can be summarized as follows: The length and appearance duration of a dark zone are shortened with decreasing pulse input; in the case of a 1.5 A x 50 ms pulse (7.4x104 J/m~), the dark zone appears only in the thermally disturbed area. The dark zone appearance on the video movies is detectable even for the very low disturbance of 5.6x104 J/m s, which induces a slight temperature rise in SC-wire of only 0.3 K at its maximum; however, in the case of 3.3 xl04 J/m s, detection is no longer possible. VISUALIZABILITY, CONDITIONS AND CAUSES Experimental Apparatus and Procedure A mirror-finished strip (10mm × 55mm) of stainless steel film 50~m thick was bonded to an epoxy-resin base plate, and two monitoring thermocouples were attached to the rear surface of strip. This assembly was immersed in liquid helium; the light beam from a 0.95-mW laser was irradiated to the strip. The luminous energy of the reflected light from the pulse-heated strip was converted into an electric signal with the electronic system shown in Fig. 6, of which the overall conversion factor was 4 × 103 V/W. Results and Discussion Transient characteristics of the measured luminous energy are shown in Fig. 7, where @ and @qrefer to the average luminous energy for no heating and pulse heating, respectively, and q0 is the pulse heat flux. As can be seen from the figure, the reflected luminous energy q5 drastically decreases just after pulse heating (shaded part). Based on these measured values, the attenuation rate E~ is calculated by equation (1) and plotted in Fig. 8, and the incipience time of attenuation t~ is plotted in Fig. 9. The linear correlation between Eat and q0 enables us to detect the degree of thermal disturbance only by measuring the e~. Eat
:
1 -(~q/40)
(1)
According to the high-speed video observations, at the moment of attenuation, a dark and bright pattern appearing on the strip surface due to its slight waviness changes to uniform milk-white appearance. Such an unusual change was not observed for a pulse-heated strip in liquid nitrogen until initiation of bubble formation. Additional theoretical analysis shows that refraction of the incident]reflected light in a superheated liquid/vapor layer and at a liquid-solid interface does not induce the luminous energy attenuation. 722 Cryogenics 1994 Vo134 ICEC Supplement
ICEC 15 Proceedings
CONCLUSIONS The thermal disturbance and the rapid propagation of normal zone in a superconducting coil have been visualized as a dark zone on the coil and recorded with a high-speed video system. The appearance of the dark zone can be primarily attributed to attenuation of the reflected luminous energy and light scattering in a very thin thermal layer at the interface of conductor surface and liquid helium. With increasing the wall heat flux, the attenuation rate increases and its incipience time becomes shorter. The attenuation of reflected luminous energy is detectable when the wall heat flux of a conductor is greater than 140W/m = and its wall superheat is greater than 0.3 K. This is of practical importance for detection of thermal disturbance at its early stage. REFERENCES 1 2 3
Tsukamoto, O., Uyemura, T., Ishida, Y., Uyemura, T., and Tsuno, T., Observation of bubble formation mechanism of liquid helium subjected to transient heating Proc. ICEC 8 (1980) 251 Okada, S., Kim, J.-K., Aihara, T., and Kuroda, K., Numerical analysis of thermal stability of an immersion-cooled, pancake-type superconducting coil Cryogenics (1991) 31 547-550 Okada, S., Mhara, T., Kim, J.-K., and Kuroda, K., Numerical analysis of quench propagation of an immersion-cooled, pancake-type superconducting coil Proc. 44th Meeting on Cryogenics and Superconductivity, Japan (1990) 244 END CAPS
~ ~
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HIGH SPEED VIDEO CAMERA
LIGHT SOURCE
DRAFTHOLES .
/
ACRYLIC FLANGES
15ramTHICK
/
ACRYLIC SPACERS
5ramTHICK
GLASS WINDOW oLO ~D 3.4
DOUBLE-PANCAKE
NITROGEN
de~
o. ~0
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c~
dec
-.~LIQUID
HELIUM TEST COIL
TEST PIECE
Figure 3 Outline of experimental apparatus Figure 1 Disassembly view of test coil
Q VOLTAGE TAPS
~
DOUBLE-PANCAKESUPERCONDUCTING COIL
~:~ .GREASE I I / / HEATER FOR
CO~l ~ DISTURBANCE 112~H:21.UI~.
DISTURBANCE THERMOCOUPLES
" THERMOCOUPLES ( LINE B )
50/zm THICK \SUPERCONDUCTOR
Figure 2 Location of thermocouples and voltage taps on test coil Cryogenics 1994 Vo134 ICEC Supplement
723
ICEC 15 Proceedings
0
(a)
E i
1 0 .~/
I
~80~-
(b) I; Z ~
"59-
Figure 5 Time history of voltages and temperatures at representative locations on test coil
(c)
,~o=173 Wlm2
Figure 4 Rapid quench propagation. (a) 10 ms after cutoff of h e a t e r input. (b) 100 ms after cutoff. (c) 200 ms after cutoff. 50
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Figure 7 Attenuation of reflected luminous energy
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Figure 8 Attenuation rate
SILICONPHOTO-DETECTOR \ I ELK-°°°°
f
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Figure 9 Incipience time of attenuation
~ /
'
'
=-N ~
/
" TEST PIECE
Figure 6 Measuring system of luminous energy fi'om a pulse-heated strip 724
100
~, m s
Cryogenics 1994 Vo134 ICEC Supplement
Visualization of Thermal Disturbance and Quench Propagation in an Immersion-Cooled Superconducting Coil in Liquid Helium Toshio Aihara* and Sadayuki Okada** *Institute of Fluid Science, Tohoku University, Katahira, Aoba-ku, Sendai 980-77, Japan **Hitachi Research Laboratory, Hitachi Ltd., 7-1-1 Ohmika, Hitachi 319-12, Japan Thermal disturbance and quench propagation have been visualized by utilizing attenuation of the luminous energy of reflected light from the interface of conductor surface and liquid helium. The results of optical m e a s u r e m e n t with a high-speed video system were in very close a g r e e m e n t with the m e a s u r e d time history of tap voltages and t e m p e r a t u r e s on the test coil. The visualizability and luminous attenuation characteristics were examined for various conditions.
INTRODUCTION Quench of superconductors has hitherto been detected either by voltage difference in a growing normal zone or by acoustic emission due to sudden thermal expansion, etc. The present authors have succeeded in detecting visually the thermal disturbance and/or quench which may occur unexpectedly in any place at any time in a superconductor. In this study, an artificially induced thermal disturbance and the rapid propagation of normal zone in a pancake-type superconducting coil were visualized by this method and recorded with a high-speed video system. The validity of the optical observation was verified by comparison with the measured time history of tap voltages and temperatures at various locations of the test coil. The visualizability and its causes also were investigated. VISUAL OBSERVATION OF QUENCH PROPAGATION IN A PANCAKE-TYPE COIL Experimental Apparatus and Procedure The test coil of the double-pancake type is schematically shown by parts in Fig. 1. A polyimide-insulated wire of superconducting composite (SC) with the cross section of 1.51mm x 4.98mm and copper/NbTi ratio of 2 was wound spirally around a core cylinder with 32 turns. Thermal disturbance was applied by pulse heating of the 20-~m-thick stainless steel film heater which was sandwiched between the 16th and 17th turns 0fthe SC wire winding. Voltage taps for detecting normal zone growth were soft-soldered onto the coil surface every 3 turns of the winding, and nine 50-~m-diameter chromel-constantan thermocouples were fixed between the SC wires at the locations shown in Fig. 2. The test coil was immersion-cooled in liquid helium at atmospheric pressure, as shown in Fig. 3. Optical phenomena were recorded with a high-speed color video system of 200/ 400 frames per second, by illuminating the coil surface with a 500-watt tungsten lamp. Tsukamoto et al. [1] took high-speed movies of the side view of transient bubble formation around a fine wire stepwise-heated in liquid helium. At the early stage of this study, the present authors also made side-view observations of quench from the direction nearly parallel to the coil surface, but anything new was not found except for bubble formation. Hence, the scheme of downward illumination and top-view observation was adopted. Results and Discussion Figure 4 shows typical still photographs which were reproduced from a video tape for a normal zone propagation induced by pulse input of 18.1 W for 10 ms (equivalent to the energy density 1.5×105 J/m 3) to the film heater under a transport current of 1600 A. Just after power input to the heater, a dark brown zone appears in an arc shape on the coil, at 16th and 17th turns adjacent to the f i l m heater. The dark zone grows primarily in the Cryogenics 1994 Vo134 ICEC Supplement 721
ICEC 15 Proceedings
circumferential direction and draws a concentric circle in about 10 ms. With lapse of time, the dark zone continues to grow inward and outward, rapidly increasing the number of turns and reaches the outermost radius of the coil in about 550 ms. Such rapid propagation of the dark zone closely resembles the results of the authors' numerical analyses [2, 3]. Helium vapor, produced between the coils by quench, bursts out from the outer edge and draft holes of the test coil about 370 ms after quench. Figure 5 shows typical time history of tap voltages and temperatures at the representative locations on the coil. In the figure, I refers to the transport current; I a the pulse current to film heater; t the time after cutoff of the disturbance; Ei~. the voltage difference between ith- andjth-turns; T~.h+t the measured value of a thermocouple inserted between kth- and (k+l)th-turns on line-A in Fig. 2; and C ) the occurrence of quench atjth-turn. Based on the tap-voltage detection, the normal zone reaches the outermost 32nd turn at t -~ 550 ms; at the same time, the temperature of SC wire suddenly increases. On the other hand, the visual observation by video-tape recording shows that the dark zone also reaches the 32nd turn at t -~ 550 ms. During the quench propagation, the expanding of the dark zone always coincides with the time history of the tap voltages and temperatures. It is clear from these facts that the observed dark zone indicates the normal zone. VISUAL OBSERVATION OF THERMAL DISTURBANCE The insulation film was peeled off from the test coil surface for sensitive, visual detection of the faint disturbance, and additional thermocouples were attached on the SC wire surface closest to the film heater and coated with epoxy resin to minimize cooling error. Under no transport current (I = 0), thermal disturbance was applied to the test coil by pulse heating of 1-10 A for 10-50 ms to the heater, equivalent to the energy density 2.6x104-3.5x106 J/m3; then, the transient appearance of a dark zone was recorded with the video system. The experimental results can be summarized as follows: The length and appearance duration of a dark zone are shortened with decreasing pulse input; in the case of a 1.5 A x 50 ms pulse (7.4x104 J/m~), the dark zone appears only in the thermally disturbed area. The dark zone appearance on the video movies is detectable even for the very low disturbance of 5.6x104 J/m s, which induces a slight temperature rise in SC-wire of only 0.3 K at its maximum; however, in the case of 3.3 xl04 J/m s, detection is no longer possible. VISUALIZABILITY, CONDITIONS AND CAUSES Experimental Apparatus and Procedure A mirror-finished strip (10mm × 55mm) of stainless steel film 50~m thick was bonded to an epoxy-resin base plate, and two monitoring thermocouples were attached to the rear surface of strip. This assembly was immersed in liquid helium; the light beam from a 0.95-mW laser was irradiated to the strip. The luminous energy of the reflected light from the pulse-heated strip was converted into an electric signal with the electronic system shown in Fig. 6, of which the overall conversion factor was 4 × 103 V/W. Results and Discussion Transient characteristics of the measured luminous energy are shown in Fig. 7, where @ and @qrefer to the average luminous energy for no heating and pulse heating, respectively, and q0 is the pulse heat flux. As can be seen from the figure, the reflected luminous energy q5 drastically decreases just after pulse heating (shaded part). Based on these measured values, the attenuation rate E~ is calculated by equation (1) and plotted in Fig. 8, and the incipience time of attenuation t~ is plotted in Fig. 9. The linear correlation between Eat and q0 enables us to detect the degree of thermal disturbance only by measuring the e~. Eat
:
1 -(~q/40)
(1)
According to the high-speed video observations, at the moment of attenuation, a dark and bright pattern appearing on the strip surface due to its slight waviness changes to uniform milk-white appearance. Such an unusual change was not observed for a pulse-heated strip in liquid nitrogen until initiation of bubble formation. Additional theoretical analysis shows that refraction of the incident]reflected light in a superheated liquid/vapor layer and at a liquid-solid interface does not induce the luminous energy attenuation. 722 Cryogenics 1994 Vo134 ICEC Supplement
ICEC 15 Proceedings
CONCLUSIONS The thermal disturbance and the rapid propagation of normal zone in a superconducting coil have been visualized as a dark zone on the coil and recorded with a high-speed video system. The appearance of the dark zone can be primarily attributed to attenuation of the reflected luminous energy and light scattering in a very thin thermal layer at the interface of conductor surface and liquid helium. With increasing the wall heat flux, the attenuation rate increases and its incipience time becomes shorter. The attenuation of reflected luminous energy is detectable when the wall heat flux of a conductor is greater than 140W/m = and its wall superheat is greater than 0.3 K. This is of practical importance for detection of thermal disturbance at its early stage. REFERENCES 1 2 3
Tsukamoto, O., Uyemura, T., Ishida, Y., Uyemura, T., and Tsuno, T., Observation of bubble formation mechanism of liquid helium subjected to transient heating Proc. ICEC 8 (1980) 251 Okada, S., Kim, J.-K., Aihara, T., and Kuroda, K., Numerical analysis of thermal stability of an immersion-cooled, pancake-type superconducting coil Cryogenics (1991) 31 547-550 Okada, S., Mhara, T., Kim, J.-K., and Kuroda, K., Numerical analysis of quench propagation of an immersion-cooled, pancake-type superconducting coil Proc. 44th Meeting on Cryogenics and Superconductivity, Japan (1990) 244 END CAPS
~ ~
~
.
HIGH SPEED VIDEO CAMERA
LIGHT SOURCE
DRAFTHOLES .
/
ACRYLIC FLANGES
15ramTHICK
/
ACRYLIC SPACERS
5ramTHICK
GLASS WINDOW oLO ~D 3.4
DOUBLE-PANCAKE
NITROGEN
de~
o. ~0
,.3 . ~
c~
dec
-.~LIQUID
HELIUM TEST COIL
TEST PIECE
Figure 3 Outline of experimental apparatus Figure 1 Disassembly view of test coil
Q VOLTAGE TAPS
~
DOUBLE-PANCAKESUPERCONDUCTING COIL
~:~ .GREASE I I / / HEATER FOR
CO~l ~ DISTURBANCE 112~H:21.UI~.
DISTURBANCE THERMOCOUPLES
" THERMOCOUPLES ( LINE B )
50/zm THICK \SUPERCONDUCTOR
Figure 2 Location of thermocouples and voltage taps on test coil Cryogenics 1994 Vo134 ICEC Supplement
723
ICEC 15 Proceedings
0
(a)
E i
1 0 .~/
I
~80~-
(b) I; Z ~
"59-
Figure 5 Time history of voltages and temperatures at representative locations on test coil
(c)
,~o=173 Wlm2
Figure 4 Rapid quench propagation. (a) 10 ms after cutoff of h e a t e r input. (b) 100 ms after cutoff. (c) 200 ms after cutoff. 50
.SW
50 i - -
,.
378
1540
700
.
.--I
~-¢
~
L. Q~
- - ~ -
to
E
-J
0
40
~
•~ t
100
0 ,t,
ms
100 ms
0 100 ,t, ms
0
Figure 7 Attenuation of reflected luminous energy
30
-t+-
1
~s 2o
.<)l=.~,iO~ i i i t
4--
~ 1o --
0
~
10 ~
I
i
103 Pulse input power ~o,
t
0
LA_
103
Purse input power ]o,
W/m2
F1
I
,
f
tit
104
P-e2A
V'I
AMP.
~_J
MIRRORS v
/ "
r HIGHSPEEDVIDEOCAMERA
W/m2
Figure 8 Attenuation rate
SILICONPHOTO-DETECTOR \ I ELK-°°°°
f
10 4
Figure 9 Incipience time of attenuation
~ /
'
'
=-N ~
/
" TEST PIECE
Figure 6 Measuring system of luminous energy fi'om a pulse-heated strip 724
100
~, m s
Cryogenics 1994 Vo134 ICEC Supplement