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Some experiments relating to degradation of large superconducting coils
Volume 16, number
PHYSICS
1
LETTERS
1 May 1965
We thus
can write X in terms of v(g and v. When where a0 is the length of the crystal a(?3 -ao, at room temperature, we find from eq. (1)
B
dd4??_ 2aov
(2)
AT
Fig. 1 shows two oscillating curves in which the period AT obeys eq. (1). The upper curve represents a case where the opposite faces of the crystal are flat and parallel better than one minute of arc. The lower curve illustrates a case where the parallelism is poor, with a deviation of approximately 5 min. This results in an interference effect, and the resolution is more limited. In fig. 2 the values of p, obtained from (2), are plotted for different temperatures. These agree fairly well with the values measured by Owen and Williams [3]. The values of the velocity of the ultrasonic waves in lithium were obtained from papers [2,4]. The accuracy of the thermal expansion coefficient is greatly limited by the errors involved in the measurement of v. It is about f 4%. The determination of the thermal expansion coefficient in the present paper should be considered as only preliminary. The accuracy of p determined by this method will probably be increased to lo-8 l/OK in the near future by using an improved technique. I wish to express my deep gratitude to Professor V. Hovi, Head of the Wihuri Physical Laboratory, who has arranged facilities for the performance of the measurements and followed this
Fig. 2. Thermal expansion coefficient of lithium as a function of temperature (a, = 1.80 cm). EImeasured by Owen and Williams [ 31, 0 measured by the writer. work with great interest giving most valuable advice. I am indebted to Drs. E. LXhteenkorva, K. Mansikka, and J. PByhiinen, and to Mr. L. Niemel% for many helpful discussions, and to Mr. A. Korpela for his assistance during the measurements. Referewes
E. MZlntysalo, to be published in Ann.Acad.Sci. Fenn. (Finland) A VI (1965). H. C. Nash and C. S.Smith, J. Phys. Chem.Solids 9 (1959) 113, E.A.OwenandG.I.Williams, Proc.Phys.Soc. A67 (1954) 895. J. Trivisonno and C. S.Smith, Acta Met. 9 (1961) 1064.
*****
SOME
EXPERIMENTS RELATING TO OF LARGE SUPERCONDUCTING
DEGRADATION COILS
D. N. CORNISH and J. E. C. WILLIAMS The Culham Laboratory,
U.K. A.
A number of coils [ 11, 8” inside and 12” outside diameter, have been wound which exhibited the usual degradation exhibited by coils of this size: some experiments have been carried out using these coils to investigate the causes of this behaviour . A number of disc coils were made, clock spring 18
E. A., Culham,
Berkshire,
England
fashion, with only one turn axially and 100 in the radial direction, having the same inside and outside dimensions as the main coils. The wire used was copper plated 0.010” Nb75Zr similar to that used in the main coils. To obtain a coil occupying the minimum axial space, the winding and a $‘I thick backing washer were impreg-
PHYSICS
Volume 16, number 1
x
.
I
. CENTRP x
\
a0
-
END
MAIN
FIELD
(kq)
II
ll”0.D.
DISC
.
SPIRAL
DISC
DISC
-
-
f I-
640$
s
COIL
TURN
o&#,z , , , SCREENED
20-
TWIN
NON-INDUCTIVE
0
IO
20 MAIN
20
TURN
40
so
PI ELD(kq)
Fig. 1. Top. Disc coils at centre and end of a four coil assembly. Bottom. 11” dia disc coil, spiral turn, noninductivespiral turn and a screened turn inserted in a two coil assembly. nated together with epoxy resin. The total coil width was therefore 0.05”., Four main coils were assembled together and
LETTERS
1 May 1965
connected in series. Separately energised disc coils were placed at the centre and one end of the assembly. Quenching curves for the discs were found by traversing different load lines, which were selected by choosing various ratios of disc to main coil current, thus carrying out a “coil simulation” test [2]. The results are shown in the accompanying figure on which the load line for four main coils has been drawn. Similar curves were then found using two main coils only for the following inserts: (1) A disc coil of 11” outside diameter. This diameter is estimated to be inside the zero field region in the main coils. (2) A single turn of wire spiralling from the inner to outer radius of the main coils. (3) as (2) but wound non-inductively. (4) as (2) but screened by threading it through a 0.053”/0.125” dia copper tube. The reference curve was obtained from ‘coil simulation’ tests on short samples of wire using a small 50 kg solenoid. Sandwiching the wire between adjacent coils, however, gives a degraded performance. Moreover, the amount of degradation tends towards that obtaining in the main coils which is more severe when four coils are used than when there are only two. If the spiral turn is screened with 0.02” thick copper normal short sample performance is restored to it. These results tend to suggest that degradation is caused by a combination of the quasisteady field pattern and a flux transient which triggers the quench. So far it has not been found possible to identify the actual flux transient responsible for the quench but it is hoped to do so in the future. References 1. D. N. Cornish and J. E .C. Williams, Cryogenics, to be published. 2. C. H. Rosner and H. W.SchacUer,J. Appl.Phys. 34 (1963)2107.
*****
19
PHYSICS
1
LETTERS
1 May 1965
We thus
can write X in terms of v(g and v. When where a0 is the length of the crystal a(?3 -ao, at room temperature, we find from eq. (1)
B
dd4??_ 2aov
(2)
AT
Fig. 1 shows two oscillating curves in which the period AT obeys eq. (1). The upper curve represents a case where the opposite faces of the crystal are flat and parallel better than one minute of arc. The lower curve illustrates a case where the parallelism is poor, with a deviation of approximately 5 min. This results in an interference effect, and the resolution is more limited. In fig. 2 the values of p, obtained from (2), are plotted for different temperatures. These agree fairly well with the values measured by Owen and Williams [3]. The values of the velocity of the ultrasonic waves in lithium were obtained from papers [2,4]. The accuracy of the thermal expansion coefficient is greatly limited by the errors involved in the measurement of v. It is about f 4%. The determination of the thermal expansion coefficient in the present paper should be considered as only preliminary. The accuracy of p determined by this method will probably be increased to lo-8 l/OK in the near future by using an improved technique. I wish to express my deep gratitude to Professor V. Hovi, Head of the Wihuri Physical Laboratory, who has arranged facilities for the performance of the measurements and followed this
Fig. 2. Thermal expansion coefficient of lithium as a function of temperature (a, = 1.80 cm). EImeasured by Owen and Williams [ 31, 0 measured by the writer. work with great interest giving most valuable advice. I am indebted to Drs. E. LXhteenkorva, K. Mansikka, and J. PByhiinen, and to Mr. L. Niemel% for many helpful discussions, and to Mr. A. Korpela for his assistance during the measurements. Referewes
E. MZlntysalo, to be published in Ann.Acad.Sci. Fenn. (Finland) A VI (1965). H. C. Nash and C. S.Smith, J. Phys. Chem.Solids 9 (1959) 113, E.A.OwenandG.I.Williams, Proc.Phys.Soc. A67 (1954) 895. J. Trivisonno and C. S.Smith, Acta Met. 9 (1961) 1064.
*****
SOME
EXPERIMENTS RELATING TO OF LARGE SUPERCONDUCTING
DEGRADATION COILS
D. N. CORNISH and J. E. C. WILLIAMS The Culham Laboratory,
U.K. A.
A number of coils [ 11, 8” inside and 12” outside diameter, have been wound which exhibited the usual degradation exhibited by coils of this size: some experiments have been carried out using these coils to investigate the causes of this behaviour . A number of disc coils were made, clock spring 18
E. A., Culham,
Berkshire,
England
fashion, with only one turn axially and 100 in the radial direction, having the same inside and outside dimensions as the main coils. The wire used was copper plated 0.010” Nb75Zr similar to that used in the main coils. To obtain a coil occupying the minimum axial space, the winding and a $‘I thick backing washer were impreg-
PHYSICS
Volume 16, number 1
x
.
I
. CENTRP x
\
a0
-
END
MAIN
FIELD
(kq)
II
ll”0.D.
DISC
.
SPIRAL
DISC
DISC
-
-
f I-
640$
s
COIL
TURN
o&#,z , , , SCREENED
20-
TWIN
NON-INDUCTIVE
0
IO
20 MAIN
20
TURN
40
so
PI ELD(kq)
Fig. 1. Top. Disc coils at centre and end of a four coil assembly. Bottom. 11” dia disc coil, spiral turn, noninductivespiral turn and a screened turn inserted in a two coil assembly. nated together with epoxy resin. The total coil width was therefore 0.05”., Four main coils were assembled together and
LETTERS
1 May 1965
connected in series. Separately energised disc coils were placed at the centre and one end of the assembly. Quenching curves for the discs were found by traversing different load lines, which were selected by choosing various ratios of disc to main coil current, thus carrying out a “coil simulation” test [2]. The results are shown in the accompanying figure on which the load line for four main coils has been drawn. Similar curves were then found using two main coils only for the following inserts: (1) A disc coil of 11” outside diameter. This diameter is estimated to be inside the zero field region in the main coils. (2) A single turn of wire spiralling from the inner to outer radius of the main coils. (3) as (2) but wound non-inductively. (4) as (2) but screened by threading it through a 0.053”/0.125” dia copper tube. The reference curve was obtained from ‘coil simulation’ tests on short samples of wire using a small 50 kg solenoid. Sandwiching the wire between adjacent coils, however, gives a degraded performance. Moreover, the amount of degradation tends towards that obtaining in the main coils which is more severe when four coils are used than when there are only two. If the spiral turn is screened with 0.02” thick copper normal short sample performance is restored to it. These results tend to suggest that degradation is caused by a combination of the quasisteady field pattern and a flux transient which triggers the quench. So far it has not been found possible to identify the actual flux transient responsible for the quench but it is hoped to do so in the future. References 1. D. N. Cornish and J. E .C. Williams, Cryogenics, to be published. 2. C. H. Rosner and H. W.SchacUer,J. Appl.Phys. 34 (1963)2107.
*****
19