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A method for the direct determination of the phase characteristics of cryotrons
A method for the direct determination of the/phase Characteristics of cryotrons I. A. ARTEMENKO, I. D. V O | T O V I C H , and G. A. M I K H A | L O V t IN an experimental study of cryotrons, the boundary between .the superconducting and non-superconducting states of the gate is usually determined arbitrarily by the appearance of some value of the voltage across the gate. ~-a Such a determination of the transition curve (the phase characteristic) leads firstly, to points for different currents in the gate which correspond to states of the gate with different, necessarily non-zero, values of the resistance; secondly, the experimentally determined threshold gate current depends on the sensitivity of the measuring arrangement and always exceeds the true value, with the difference between them increasing with increasing current in the grid. In the authors' opinion this gives rise to the appearance of tails on the experimentally determined transition curves and to considerable error in the determination of the amplification coefficient of the cryotron. It seems that it would be more correct to take the transition curves at constant gate resistance, since in that case the difference between the experimental and true values of the threshold current, although unavoidable, would not depend on the grid current.
13
J, 12
i
superconductivity of the gate of cryotron K~, the calibration curve is taken of the dependence of the resistance of the gate of cryotron K2 on the current 12 for a constant current I~. (2) The current 12 is given such a value that the maximum possible current I should be persistent in circuit a b c a. It is evident that the value of current I has its upper limit as the threshold current in circuit a b c a in the absence of current in the grid. (The value of current I is determined from the calibration curve). (3) After current 12 is switched off, 13 is turned on, increasing from zero. At some value of I3, the combined field H o f c u r r e n t s I and 13exceeds the critical value and current I decreases to the value at which H becomes less than the critical value. Further increase in current 13 again leads to a reduction of current/, etc. In this way the relation 1 = f(la) is determined and this also represents the transition curve for cryotron K~.
500
E Itc 4O0
t
300
1
.-T 200
IOO
I=
0
IOO
200 Ic
'~" - - - - O
]b ~
0
Figure 1. Cryotron memory element: The gate is 2 mm wide and 5800 A thick; The grid is 0.2 mm wide and 12000 A thick. The silicon monoxide insulator is 7800 A t h i c k
A new method is proposed for determining the true transition curve (this means also the amplification coefficient of the cryotron) in which the superconducting memory unit (Figure 1) is used, consisting of two cryotrons--that under investigation, KI and the registering one K2. The essence of the method is as follows. (1) For a certain large current /3, capable of destroying the t Cybernetics Institute, Academy of Sciences, Kiev, Ukranian S.S.R. Pribory i Tekhnika ~ksperimenta No. 3, 227 (1966). Received 1 March 1967.
240
~
300 (mA)
Figure 2. Experimental transition curves of cryotron K~ at T = 3.6 ° K (Te = 3.81 ° K)
Figure 2 shows the.experimental transition curves in two forms: all points of curve l were taken at constant voltage across the gate (l ~tV), and curve 2 was determined by the method described here.~ (Curve l was taken after the control grid of cryotron K2, in parallel with cryotron KI, had been broken mechanically. The amplification coefficient of the cryotron, found from curve l, is 1-6. Its value, obtained more accurately from curve 2, is 2.2. For most cryotron circuits it is sufficient to have an amplification coefficient slightly larger than unity. The possibility of an accurate determination of the amplification coefficient thus makes it possible to choose the minimum ratios of widths of gate and grid, which increases the speed of action of cryotron circuits. As the memory element was prepared with the destruction of the vacuum after deposition of the tin gates, it is possible that the critical current for the tin-lead connection contact is less than the critical current of the valve + All measurements with liquid helium were carried out with Ya. S. Kan in the Kharkov Physico-Technical Institute. C R Y O G E N I C S " A U G U S T 1967
itself. The flat portion of curve 2 can evidently be explained in this way. A memory element of the type shown in Figure 1 cannot only be used to determine the transition curves of film cryotrons, but also in other studies, for example to determine the dependence of the critical current and critical field of superconductors on temperature, and also in investigation of thermal processes in thin films and substrates.
REFERENCES 1. NEWHOUSE, V. L., BREMER, J. W., and EDWARDS, H. H. Proc.
Inst. Refrig. Engrs. 48, 1395 (1960) 2. BREM~R, J. W. Superconducting Instruments (Mir Publishing House, 1964) 3. ARTEMENKO, A. I., VorrowcH, I. D., a n d MXKAILOV,G. A. Ukr. Phys. J. 8, 798 (1963) This paper has b ~ n especially translated for CRYOOENtCSand is included by permtssmn of the Editors of Pribory i Tekhnika I~ksperlmenta.We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover-to-cover translation of ~ by arrangement with the Russian publisher.
A cryotron relaxation oscillator
b = 0"115
f(cls)
as a t h e r m o m e t e r
Texp(°K) Tea,c(°K)
Y. S. KAN and V. A. R A K H U B O V S K I | T
245 3.605 3.603
n -- 3
206 3.581 3.59
136 3.561 3.56
500 : u
200
T IOO
3-4
3-45 T
3'5 ~
3-55
3"6 I'K]
Figure 1. The temperatu/'e dependence of the frequency of oscillation of a c.g.r.o, for constant drive current of 4£0 m A
The method of measurement was as t~ollows: the temperature dependence of the oscillation frequency of the c.g.r.o., placed immediately under the surface of liquid helium boiling under its own vapour pressure, was determined for constant c.g.r.o, drive current (Figure 1), and then the change in frequency of the c.g.r.o, was measured as it was immersed to different depths under the surface. Then, using Figure 1 as the calibration, the temperature of the liquid at various depths could be determined from the Oscillation frequency of the c.g.r.o. The measurements were made in glass dewars with inner diameters 30, 50, and 100 mm. It can be seen from Figure 2 that the measured temperatures deviate insignificantly from the straight line obtained on the assumption that the temperature gradient in a column of boiling liquid is only determined by the hydrostatic pressure. The deviations ( ~ 1 0 -s degK) 1" Physico-Technical Institute, Academy of Sciences, Khar'kov, Ukranian S.S.R. Pribory i Tekhnika ~ksperimenta No 3, p. 228 (1966). Received 1 March 1967. CRYOGENICS D
"
AUGUST
1967
111 3.536 3.54
85 63 3.511 3.497 3.52 3.5
45 3.47 3.5
~ 3.575 I
"-
T H E oscillator frequency of a cryotron generator of relaxation oscillations (c.g.r.o.), such as described by Cohen et al, 1 is determined by the supercriticality of the drive current, and this constant depends on the temperature of the medium surrounding the generator. The c.g.r.o, can thus act as a thermometer. In order to examine this possibility, the temperature distribution with height in a column of liquid helium-1 under such conditions that mixing was only achieved by vapour bubbles, was measured with a c.g.r.o, of lead-tin wire cryotrons.
~-= 4 x 10-*s
x d= 30mm © d = 50mm [ ] d = 100 mm
• 3.570 l ~" 3.565
x Z , S
x-, l 3.560' o
,25 Column
250 height
3rs
Figure 2. The dependenceofthetemperature of liquid helium on the height of the column in dewars of different 500 internal diameter d
Imm)
correspond to the vertical dimension of the c.g.r.o. (5 mm). The same results were also obtained when a heater, placed on the bottom of the dewar, was switched on, dissipating power between 200 mW and 3 W. The natural heat influx did not exceed 200 mW in the widest dewar and was only a few milliwatts in the narrowest. The measurements showed that a c.g.r.o, could be used as a thermometer of fairly high sensitivity (,--3 x 10-4 degK/c/s at temperatures in the region of ,--3-5 ° K). They also indicated clearly that if special steps are not taken for mixing, appreciable temperature differences can arise between the points furthest apart vertically in systems of considerable height working in liquid helium. The following formula was derived from a theoretical analysis, for computing the temperature of liquid helium from the frequency of oscillation of the c.g.r.o:
T = rex/[1 - B + b exp ( - 1 / 4 n r f ) ] where z is the time constant of an elementary cryotron stage, b is the ratio of c.g.r.o, drive current to the critical drive current at 0 ° K and n the number of stages in the oscillator circuit. This formula is in good agreement with the experimental data shown in the Table. In conclusion, it should be pointed out that it follows from what has been said that a c.g.r.o, can also be used as a liquid helium level indicator with an accuracy of a few millimetres. REFERENCE 1. COHEN, M. L., SMALLMAN, C. R., a n d SLADE, A. E. Proc. Inst. Radio Engrs 9, 1575 (1960) This paper has been especially translated for CIRYOgI~ICSand is included by
permtsston of the Editors of Prlbory i Tekhnika Eksperimenta. We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover.to-cover translation of ~ by arrangement with the Kussian publisher. 24t
13
J, 12
i
superconductivity of the gate of cryotron K~, the calibration curve is taken of the dependence of the resistance of the gate of cryotron K2 on the current 12 for a constant current I~. (2) The current 12 is given such a value that the maximum possible current I should be persistent in circuit a b c a. It is evident that the value of current I has its upper limit as the threshold current in circuit a b c a in the absence of current in the grid. (The value of current I is determined from the calibration curve). (3) After current 12 is switched off, 13 is turned on, increasing from zero. At some value of I3, the combined field H o f c u r r e n t s I and 13exceeds the critical value and current I decreases to the value at which H becomes less than the critical value. Further increase in current 13 again leads to a reduction of current/, etc. In this way the relation 1 = f(la) is determined and this also represents the transition curve for cryotron K~.
500
E Itc 4O0
t
300
1
.-T 200
IOO
I=
0
IOO
200 Ic
'~" - - - - O
]b ~
0
Figure 1. Cryotron memory element: The gate is 2 mm wide and 5800 A thick; The grid is 0.2 mm wide and 12000 A thick. The silicon monoxide insulator is 7800 A t h i c k
A new method is proposed for determining the true transition curve (this means also the amplification coefficient of the cryotron) in which the superconducting memory unit (Figure 1) is used, consisting of two cryotrons--that under investigation, KI and the registering one K2. The essence of the method is as follows. (1) For a certain large current /3, capable of destroying the t Cybernetics Institute, Academy of Sciences, Kiev, Ukranian S.S.R. Pribory i Tekhnika ~ksperimenta No. 3, 227 (1966). Received 1 March 1967.
240
~
300 (mA)
Figure 2. Experimental transition curves of cryotron K~ at T = 3.6 ° K (Te = 3.81 ° K)
Figure 2 shows the.experimental transition curves in two forms: all points of curve l were taken at constant voltage across the gate (l ~tV), and curve 2 was determined by the method described here.~ (Curve l was taken after the control grid of cryotron K2, in parallel with cryotron KI, had been broken mechanically. The amplification coefficient of the cryotron, found from curve l, is 1-6. Its value, obtained more accurately from curve 2, is 2.2. For most cryotron circuits it is sufficient to have an amplification coefficient slightly larger than unity. The possibility of an accurate determination of the amplification coefficient thus makes it possible to choose the minimum ratios of widths of gate and grid, which increases the speed of action of cryotron circuits. As the memory element was prepared with the destruction of the vacuum after deposition of the tin gates, it is possible that the critical current for the tin-lead connection contact is less than the critical current of the valve + All measurements with liquid helium were carried out with Ya. S. Kan in the Kharkov Physico-Technical Institute. C R Y O G E N I C S " A U G U S T 1967
itself. The flat portion of curve 2 can evidently be explained in this way. A memory element of the type shown in Figure 1 cannot only be used to determine the transition curves of film cryotrons, but also in other studies, for example to determine the dependence of the critical current and critical field of superconductors on temperature, and also in investigation of thermal processes in thin films and substrates.
REFERENCES 1. NEWHOUSE, V. L., BREMER, J. W., and EDWARDS, H. H. Proc.
Inst. Refrig. Engrs. 48, 1395 (1960) 2. BREM~R, J. W. Superconducting Instruments (Mir Publishing House, 1964) 3. ARTEMENKO, A. I., VorrowcH, I. D., a n d MXKAILOV,G. A. Ukr. Phys. J. 8, 798 (1963) This paper has b ~ n especially translated for CRYOOENtCSand is included by permtssmn of the Editors of Pribory i Tekhnika I~ksperlmenta.We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover-to-cover translation of ~ by arrangement with the Russian publisher.
A cryotron relaxation oscillator
b = 0"115
f(cls)
as a t h e r m o m e t e r
Texp(°K) Tea,c(°K)
Y. S. KAN and V. A. R A K H U B O V S K I | T
245 3.605 3.603
n -- 3
206 3.581 3.59
136 3.561 3.56
500 : u
200
T IOO
3-4
3-45 T
3'5 ~
3-55
3"6 I'K]
Figure 1. The temperatu/'e dependence of the frequency of oscillation of a c.g.r.o, for constant drive current of 4£0 m A
The method of measurement was as t~ollows: the temperature dependence of the oscillation frequency of the c.g.r.o., placed immediately under the surface of liquid helium boiling under its own vapour pressure, was determined for constant c.g.r.o, drive current (Figure 1), and then the change in frequency of the c.g.r.o, was measured as it was immersed to different depths under the surface. Then, using Figure 1 as the calibration, the temperature of the liquid at various depths could be determined from the Oscillation frequency of the c.g.r.o. The measurements were made in glass dewars with inner diameters 30, 50, and 100 mm. It can be seen from Figure 2 that the measured temperatures deviate insignificantly from the straight line obtained on the assumption that the temperature gradient in a column of boiling liquid is only determined by the hydrostatic pressure. The deviations ( ~ 1 0 -s degK) 1" Physico-Technical Institute, Academy of Sciences, Khar'kov, Ukranian S.S.R. Pribory i Tekhnika ~ksperimenta No 3, p. 228 (1966). Received 1 March 1967. CRYOGENICS D
"
AUGUST
1967
111 3.536 3.54
85 63 3.511 3.497 3.52 3.5
45 3.47 3.5
~ 3.575 I
"-
T H E oscillator frequency of a cryotron generator of relaxation oscillations (c.g.r.o.), such as described by Cohen et al, 1 is determined by the supercriticality of the drive current, and this constant depends on the temperature of the medium surrounding the generator. The c.g.r.o, can thus act as a thermometer. In order to examine this possibility, the temperature distribution with height in a column of liquid helium-1 under such conditions that mixing was only achieved by vapour bubbles, was measured with a c.g.r.o, of lead-tin wire cryotrons.
~-= 4 x 10-*s
x d= 30mm © d = 50mm [ ] d = 100 mm
• 3.570 l ~" 3.565
x Z , S
x-, l 3.560' o
,25 Column
250 height
3rs
Figure 2. The dependenceofthetemperature of liquid helium on the height of the column in dewars of different 500 internal diameter d
Imm)
correspond to the vertical dimension of the c.g.r.o. (5 mm). The same results were also obtained when a heater, placed on the bottom of the dewar, was switched on, dissipating power between 200 mW and 3 W. The natural heat influx did not exceed 200 mW in the widest dewar and was only a few milliwatts in the narrowest. The measurements showed that a c.g.r.o, could be used as a thermometer of fairly high sensitivity (,--3 x 10-4 degK/c/s at temperatures in the region of ,--3-5 ° K). They also indicated clearly that if special steps are not taken for mixing, appreciable temperature differences can arise between the points furthest apart vertically in systems of considerable height working in liquid helium. The following formula was derived from a theoretical analysis, for computing the temperature of liquid helium from the frequency of oscillation of the c.g.r.o:
T = rex/[1 - B + b exp ( - 1 / 4 n r f ) ] where z is the time constant of an elementary cryotron stage, b is the ratio of c.g.r.o, drive current to the critical drive current at 0 ° K and n the number of stages in the oscillator circuit. This formula is in good agreement with the experimental data shown in the Table. In conclusion, it should be pointed out that it follows from what has been said that a c.g.r.o, can also be used as a liquid helium level indicator with an accuracy of a few millimetres. REFERENCE 1. COHEN, M. L., SMALLMAN, C. R., a n d SLADE, A. E. Proc. Inst. Radio Engrs 9, 1575 (1960) This paper has been especially translated for CIRYOgI~ICSand is included by
permtsston of the Editors of Prlbory i Tekhnika Eksperimenta. We are also indebted to the Instrument Society of America and Consultants Bureau Enterprises Inc., who publish their own cover.to-cover translation of ~ by arrangement with the Kussian publisher. 24t