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A new method for measuring the speed of flux jumps in type II superconductors
A N E W M E T H O D FOR M E A S U R I N G THE SPEED OF FLUX J U M P S IN TYPE II S U P E R C O N D U C T O R S J. R . K E Y S T O N
and M. R. WERTHEIMER
Centre de Recherches sur les Tr~s Basses Temperatures, Grenoble, France Received 9 July 1966
1 N the past several workers ~-4 have used the Faraday rotation of plane polarized light for studying various aspects of intermediate state or mixed state structures in superconductors by looking at one side of the sample only. Recently we have described a cryostat 5 with two sets of windows, in which it has been possible to film simultaneously both faces of a type I I superconducting disc placed perpendicularly to a variable magnetic field. Figures l and 2 show the flux distributions in two niobium specimens, 0.03 and 1 mm in thickness, respectively, after the field had been reduced to zero from a value above Hez. Due to the optical system employed for transferringthe image of the specimen's reverse face (the upper images) into the object plane of the 16 mm camera, the discs are symmetrical about a horizontal axis.
We observed, as in Figures 1 and 2, the same features on both sample faces, even on specimens several millimeters thick, which could be expected from div B = 0. Differences in light and dark contrast, seen particularly in Figure 1, are due to small errors in adjustment of the analysing Polaroid. In order to study flux jumping we have, in the past, used a Fastax WF15 high-speed camera to film one side of the sample only. Flux jumps are rendered visible through crossed Polaroids as bright regions suddenly appearing against the dark background of the virgin superconductor. Replacing this camera by an electronic device employing a photomultiplier, which is described below, and by simultaneously filming the opposite sample face we have, as before, been able to measure the speed of the flux jumps and at the same time obtain information about the spatial distribution of flux immediately before and after flux
I
Figure 1. Frozen-in flux pattern observed in a 0.03 mm thick commercial niobium foil after the field had been cut abruptly from He=. The two images are symmetrical about a horizontal axis (T = 1.83° K) CRYOGENICS.
DECEMBER
1966
Figure 2. Frozen-in pattern observed in a 1 mm thick niobium disc produced in a similar fashion to that in Figure 1. The lower images correspond to the sample face nearer to the camera. Both discs are 13 mm in diameter
341
12 V, 100 W tungstenfilament tamp I~I '
Cerous
I • ~
16 m m
Polarizers
I ~
I
I -----~-
I
I
I
I
Superconducting Amplifier
cam
'
Photo-
multiplier
]
"----.. 50 % Beam splitters"J ~Anatysers~ ~ Figure 3. Schematic diagram of apparatus
jumps. In addition, this new method offers three main advantages over the use of a high speed camera. (1) Because of the relatively short effective running time (about 2 s at 12 000 images/s) of the Fastax WF15, we were limited to rapid sweep rates of applied field; with the electronic detector this is no longer the case. (2) The electronic detector is far more convenient and economical for use in the wide exploratorystudy of flux jumps which we have undertaken. (3) An electronic device is inherently superior to a mechanical one in its time-resolving capabilities. A straightforward photomultiplier as the detecting circuit would be susceptible to saturation and possible damage as the total light reflected from the specimen surface increases with the applied field. We therefore
used the negative feedback circuit shown schematically in Figure 3, which automatically regulates the light source's brightness so as to keep the photomultiplier in its operating range. The actual electronic circuit incorporating the Type 51 AVP photomultiplier tube is shown in Figure 4. For optimum signal-to-noise conditions, it was advantageous to operate the photomultiplier at low accelerating potentials (700-800 V) and high cathode currents. With no photomultiplier current flowing, the light source, a 12 V, I00 W tungsten filament lamp, has a maximum brightness. As the photomultiplier voltage, and thus anode 6urrent, is increased, the base voltage of the first-stage silicon transistor increases up to about 0.6 V, at which point the first transistor starts to conduct. Any further increase in the anode current drastically reduces the current through the lamp, thus keeping the base
Light circuit
i i
i i
-12v,
,
-H.T.
photomultiptier
Oscitloscc =e
Lamp filament
Figure 4. Electronic circuit employed 342
CRYOGENICS"
DECEMBER
1966
voltage essentially constant at 0.6 V. The speed of this feedback mechanism is determined primarily by the thermal response of the lamp filament, which is appreciably longer than the events to be observed. The rapid changes in the total light falling on to the photomultiplier cathode, corresponding to flux jumps, produce transient changes in the anode current. The resulting voltage signal, taken off the amplifier circuit's first stage, is fed into the oscilloscope, where it is amplified, triggers the display, and is recorded by a screen camera. The use of a type E Tektronix low level differential amplifier unit allows the reduction of high and low frequency noise by adjustment of the pass-band. Adjusted for minimum rise time (position 20 = 60 kc/s for the high frequency cut-off, sensitivity 0-5 mV/cm, rise time 5 las) the fastest events observed, of about 100 ~ts duration, are not appreciably influenced by the amplifier characteristics. Summarizing, the feedback keeps the photomulitiplier from saturating, protects it from inadvertent overloading, automatically diminishes the effect of low frequency variations in the light intensity (due, for example, to vibrations in the apparatus), and by keeping the cathode current constant maintains the same low noise level; on the other hand, due to its relatively low speed, it does not affect the recording of the rapid events to be observed. The rise time of the whole circuit under typical operating conditions was measured by placing the photomultiplier in front of an oscilloscope screen displaying a fixed spot, intensity-modulated by a 2 las rise-time square-wave pulse, and detecting the resulting signal on another oscilloscope. Figure 5 shows the response signal's leading edge on a 10 las/cm scale, indicating that the rise time (10 to 90 per cent) is about 15 las. The signal's sign is seen to be negative since, with the chosen circuit configuration, increases in light intensity result in negative voltage changes, and vice-versa. Figure 6 shows two typical traces. Trace (a) is of large amplitude, and trace (b) of small, corresponding to a large and a small flux jump in a 0.06 mm thick cold-rolled niobium foil. The wide straight-line trace (c) corresponds to noise which was of large enough amplitude to trigger the oscilloscope. The time scale is 100 las/cm. Like the flux jump corresponding to trace (a), other jumps in this specimen were found to be of 200 + 50 las duration, in agreement with results obtained using the Fastax camera. The speed was found to be independent of the bath temperature, between 2.09 and 1.3 ° K, of the field sweep rate, between 172 and 2 200 Oe/s, and of the magnetic history of the specimen. Further detailed results will be published elsewhere. The apparatus as described allows convenient simultaneous measurement of the speed and spatial configuration of flux jumps of about 50 las minimum duration. If required, slight modifications could C R Y O G E N I C S . DECEMBER 1966
Figure 5 (below). Response of the detecting circuit to a 2 I~s rise-time light pulse. The response signal's risetime is seen to be about 15 IJs (Time scale 10 I~s/cm)
Figure 6 (above). Traces (a) and (b) correspond to flux jumps in a0-06 mm thick cold-rolled niobium foil at 2.08° K. Field sweep rate: 172 Oe/s (Time scale: 100 IJs/cm)
increase the speed of the detecting circuit to a few microseconds, but at the cost of a reduced signal-tonoise ratio. The authors wish to thank Dr. B. B. Goodman for many helpful suggestions. This work was supported in part by the Direction des Recherches et Moyens d'Essais. REFERENCES ALERS, P. B. Phys. Rev. 105, 104 (1957) DESORBO,W., and HEALY, W. A. Cryogenics 4, 257 (1964) BAIRD, D. C. Canad. J. Phys. 42, 1682 (1964) GOODMAN,B. B., and WERTHEIMER,M. R. Physics Lett. 18, 236 (1965) 5. GOODMAN,B. B., LACAZE,A., and WERTHEIMER,M. R. C. R. Acad. Sci., Paris 262, 12 (1966)
1. 2. 3. 4.
343
and M. R. WERTHEIMER
Centre de Recherches sur les Tr~s Basses Temperatures, Grenoble, France Received 9 July 1966
1 N the past several workers ~-4 have used the Faraday rotation of plane polarized light for studying various aspects of intermediate state or mixed state structures in superconductors by looking at one side of the sample only. Recently we have described a cryostat 5 with two sets of windows, in which it has been possible to film simultaneously both faces of a type I I superconducting disc placed perpendicularly to a variable magnetic field. Figures l and 2 show the flux distributions in two niobium specimens, 0.03 and 1 mm in thickness, respectively, after the field had been reduced to zero from a value above Hez. Due to the optical system employed for transferringthe image of the specimen's reverse face (the upper images) into the object plane of the 16 mm camera, the discs are symmetrical about a horizontal axis.
We observed, as in Figures 1 and 2, the same features on both sample faces, even on specimens several millimeters thick, which could be expected from div B = 0. Differences in light and dark contrast, seen particularly in Figure 1, are due to small errors in adjustment of the analysing Polaroid. In order to study flux jumping we have, in the past, used a Fastax WF15 high-speed camera to film one side of the sample only. Flux jumps are rendered visible through crossed Polaroids as bright regions suddenly appearing against the dark background of the virgin superconductor. Replacing this camera by an electronic device employing a photomultiplier, which is described below, and by simultaneously filming the opposite sample face we have, as before, been able to measure the speed of the flux jumps and at the same time obtain information about the spatial distribution of flux immediately before and after flux
I
Figure 1. Frozen-in flux pattern observed in a 0.03 mm thick commercial niobium foil after the field had been cut abruptly from He=. The two images are symmetrical about a horizontal axis (T = 1.83° K) CRYOGENICS.
DECEMBER
1966
Figure 2. Frozen-in pattern observed in a 1 mm thick niobium disc produced in a similar fashion to that in Figure 1. The lower images correspond to the sample face nearer to the camera. Both discs are 13 mm in diameter
341
12 V, 100 W tungstenfilament tamp I~I '
Cerous
I • ~
16 m m
Polarizers
I ~
I
I -----~-
I
I
I
I
Superconducting Amplifier
cam
'
Photo-
multiplier
]
"----.. 50 % Beam splitters"J ~Anatysers~ ~ Figure 3. Schematic diagram of apparatus
jumps. In addition, this new method offers three main advantages over the use of a high speed camera. (1) Because of the relatively short effective running time (about 2 s at 12 000 images/s) of the Fastax WF15, we were limited to rapid sweep rates of applied field; with the electronic detector this is no longer the case. (2) The electronic detector is far more convenient and economical for use in the wide exploratorystudy of flux jumps which we have undertaken. (3) An electronic device is inherently superior to a mechanical one in its time-resolving capabilities. A straightforward photomultiplier as the detecting circuit would be susceptible to saturation and possible damage as the total light reflected from the specimen surface increases with the applied field. We therefore
used the negative feedback circuit shown schematically in Figure 3, which automatically regulates the light source's brightness so as to keep the photomultiplier in its operating range. The actual electronic circuit incorporating the Type 51 AVP photomultiplier tube is shown in Figure 4. For optimum signal-to-noise conditions, it was advantageous to operate the photomultiplier at low accelerating potentials (700-800 V) and high cathode currents. With no photomultiplier current flowing, the light source, a 12 V, I00 W tungsten filament lamp, has a maximum brightness. As the photomultiplier voltage, and thus anode 6urrent, is increased, the base voltage of the first-stage silicon transistor increases up to about 0.6 V, at which point the first transistor starts to conduct. Any further increase in the anode current drastically reduces the current through the lamp, thus keeping the base
Light circuit
i i
i i
-12v,
,
-H.T.
photomultiptier
Oscitloscc =e
Lamp filament
Figure 4. Electronic circuit employed 342
CRYOGENICS"
DECEMBER
1966
voltage essentially constant at 0.6 V. The speed of this feedback mechanism is determined primarily by the thermal response of the lamp filament, which is appreciably longer than the events to be observed. The rapid changes in the total light falling on to the photomultiplier cathode, corresponding to flux jumps, produce transient changes in the anode current. The resulting voltage signal, taken off the amplifier circuit's first stage, is fed into the oscilloscope, where it is amplified, triggers the display, and is recorded by a screen camera. The use of a type E Tektronix low level differential amplifier unit allows the reduction of high and low frequency noise by adjustment of the pass-band. Adjusted for minimum rise time (position 20 = 60 kc/s for the high frequency cut-off, sensitivity 0-5 mV/cm, rise time 5 las) the fastest events observed, of about 100 ~ts duration, are not appreciably influenced by the amplifier characteristics. Summarizing, the feedback keeps the photomulitiplier from saturating, protects it from inadvertent overloading, automatically diminishes the effect of low frequency variations in the light intensity (due, for example, to vibrations in the apparatus), and by keeping the cathode current constant maintains the same low noise level; on the other hand, due to its relatively low speed, it does not affect the recording of the rapid events to be observed. The rise time of the whole circuit under typical operating conditions was measured by placing the photomultiplier in front of an oscilloscope screen displaying a fixed spot, intensity-modulated by a 2 las rise-time square-wave pulse, and detecting the resulting signal on another oscilloscope. Figure 5 shows the response signal's leading edge on a 10 las/cm scale, indicating that the rise time (10 to 90 per cent) is about 15 las. The signal's sign is seen to be negative since, with the chosen circuit configuration, increases in light intensity result in negative voltage changes, and vice-versa. Figure 6 shows two typical traces. Trace (a) is of large amplitude, and trace (b) of small, corresponding to a large and a small flux jump in a 0.06 mm thick cold-rolled niobium foil. The wide straight-line trace (c) corresponds to noise which was of large enough amplitude to trigger the oscilloscope. The time scale is 100 las/cm. Like the flux jump corresponding to trace (a), other jumps in this specimen were found to be of 200 + 50 las duration, in agreement with results obtained using the Fastax camera. The speed was found to be independent of the bath temperature, between 2.09 and 1.3 ° K, of the field sweep rate, between 172 and 2 200 Oe/s, and of the magnetic history of the specimen. Further detailed results will be published elsewhere. The apparatus as described allows convenient simultaneous measurement of the speed and spatial configuration of flux jumps of about 50 las minimum duration. If required, slight modifications could C R Y O G E N I C S . DECEMBER 1966
Figure 5 (below). Response of the detecting circuit to a 2 I~s rise-time light pulse. The response signal's risetime is seen to be about 15 IJs (Time scale 10 I~s/cm)
Figure 6 (above). Traces (a) and (b) correspond to flux jumps in a0-06 mm thick cold-rolled niobium foil at 2.08° K. Field sweep rate: 172 Oe/s (Time scale: 100 IJs/cm)
increase the speed of the detecting circuit to a few microseconds, but at the cost of a reduced signal-tonoise ratio. The authors wish to thank Dr. B. B. Goodman for many helpful suggestions. This work was supported in part by the Direction des Recherches et Moyens d'Essais. REFERENCES ALERS, P. B. Phys. Rev. 105, 104 (1957) DESORBO,W., and HEALY, W. A. Cryogenics 4, 257 (1964) BAIRD, D. C. Canad. J. Phys. 42, 1682 (1964) GOODMAN,B. B., and WERTHEIMER,M. R. Physics Lett. 18, 236 (1965) 5. GOODMAN,B. B., LACAZE,A., and WERTHEIMER,M. R. C. R. Acad. Sci., Paris 262, 12 (1966)
1. 2. 3. 4.
343