The effects of magnetic field oscillations on losses in superconducting composites

In the design o f an ac generator which uses a superconducting field winding, it is essential to know the power losses likely to occur as the result of the time-varying fields produced by the loading and fault conditions during operation. The ac losses of a variety of multifilamentary wire samples have been measured under self-field conditions with no external field and under the conditions o f a small alternating field superimposed on a large dc bias field. Both the isothermal calorimeter or helium boil-off method, 1"2 at Warwick University and the magnetization technique at the Rutherford Laboratory were used for the measurements. A comparison o f the experimental results with the known theories will be presented.

The effects of magnetic field oscillations on losses in superconducting composites M.E. Hunt, J.H. Rakels and R.G. Rhodes

Nomenclature B d

alternating magnetic induction amplitude filament diameter

[ Ic It L Pe Ph Y o~

frequency critical current of the conductor transport current twist pitch eddy current loss hysteresis loss volume transverse conductivity

In the application of superconducting magnets, either for tile rotating field of an ac generator or for tile propulsion and levitation of high-speed trains, the magnet winding in both cases is exposed to a large static de bias field on which a small alternating field is superimposed due to the environment in which these magnets are operating. The resulting ac losses can lead to tile generation of sufficient heat to cause premature quenching of the windings and will need to be included in the evaluation of specific designs of magnets for these applications. In the case of the ac generator it has been reported that the cross-sectional area ratio of matrix to superconducting windings over conventional ones is lost. This implies that one should use adiabatic stabilized wire, whereby the superconductor is fabricated as a multiflamentary composite, thus avoiding flux jumps. However, for applications to high-speed trains very high current densities are not so important, so that cryostatic-stabilized wire may possibly be used.

Experimental techniques Ilelium boil-off method at the University of IVanvick. ~,2 The alternating transport current losses were measured in a number of multifilamentary wire samples using the isoM.E.H. is at the Mining and Chemical Company Ltd., London, UK. J.H.R. and R.G.R. are at the Department of Engineering, University of Warwick, Coventry CV4 7AL, UK. Paper received 26 September 1979.

Om ;k Po /at w

matrix conductivity fraction of superconductor in tile conductor permeability of vacuum relative permeability angular frequency

Pm matrix resistivity Phm measured hysteresisloss /)he calculated hysteresis loss eem measured eddy current loss

Ptm totalmeasured loss Ptc

totalcalculated loss

thermal calorimeter method. The samples of typical length 1.5 in were bifilar-wound on to a tufnol former, which was fixed inside a tufnol calorimeter immersed in a helium bath. The power dissipation generated in the sample in tile calorimeter produced a helium gas, flow rate which was monitored by means of a manometer flowmeter as described by Furtado 4. Tile alternating current supply was varied between 10 and 70 A with a frequency range between 16 and 200 tlz. Figs 1,2 and 3 illustrate the system schematical~

Magnetization technique at the Rutherford Laboratory. In this method the loss arising in a bifilar-wound sample when exposed to a small ripple field, superimposed on a dc bias field, is measured by a magnetization technique. The sample is positioned in one of two identical search coils and its magnetization is derived from the out-of-balance signal produced by these coils. Fig. 4 shows a general arrangement of the magnetization apparatus. De levels up to 7 T and ac levels up to 0.8 T at 1 llz were used in the experiments. The resulting field produced inside the bore is measured by a singleturn search coil and the hysteresis loop magnetization versus ac field sweep is displayed on an oscilloscope, the area of this loop being proportional to the loss per cycle in tile sample. Loss measurements are also obtained from electronic analysis of the magnetization and power supply signals.

List of sample specifications Tile samples of NbTi superconductors were supplied by IMI, with a twist pitch, L = 25mm, and other details as follows:

0011-2275180/007390-05 $02.00 9 1980 IPC BusinessPress P ~ v N ~ I : N I C . ~ . J U L Y 1980_

Table 1.

Specifications of the NbTi samples

Sample

Wire No sc, diameter Filaments mm

Niomax $20/40

0.40

Cu to NbTi, ratio

single core 3.00

C361/100 1.00

361

2.00

C361/75

0,75

361

2.00

C361/50

0.50

361

2.00

A61/75

0.75

61

1.35

A61/60

0.60

61

1.35

A61/50

0,50

61

1.35

A61/40

0.40

61

1.35

C40/50

0.50

48

2.00

Tc(oT)*, P(oT) A

Needlevalve

f'/, cm

I

•

Gas flow

125 1050

8X10 ~

Cathedometer 730

8x10 -9

Fig. 2 Schematic diagram of manometer flowmeter 9 -- zero Tesla Experimental results

Figs'5, 6 and 7 show the loss per cycle per volume of tile nine different samples measured. Each of these was bifilar-wound and subjected tode field levels of 1,3 and 5 Tesla. Figs 8, 9 and 10 show self-field losses ofbifilar-wound samples carrying currents at different frequencies without an external applied magnetic field. The results are shown for Niomax $20/40, C361/100, A61/75 and A61/60. In Figs 9 and 10, 1.00

q I S;g~olgeneroto~

J

0.50 I MotchingtronsfoemersI I J Stondocdresistor

I - ~

I ac voltmeter

I

Helium qos

0.10 h 0

I

I

I

I

I

2

3

4

Dissipation ,mW

Fig. 3

Calibration graph of manometer flowmeter

Tufnol colceimetee Former

~-Somple

Bifil~" woundwire I Integr~1769 I

II

IX'r

Fig. 1 Schematic diagram of isothermal calorimeter

Fig. 4

l~

J

- -

M0gnet

General arrangement of the magnetization apparatus

.q.ql

A61/60

where P = It[le (the ratio of transport to critical current)

de field I T Frequency I Hz

Ro = wire radius

150

According to Carr 6 this formula should provide a good approxhnation for the hysteresis loss in a muitifilamentary wire when 2nRo/L is small compared with unity where L = twist pitch.

Eddy current loss. The eddy current loss Pe per unit of volume is, according to CarrT: (emu units) cse/so

(3)

Pc~V= o, =~w2ffRo2 (1 - r)2/L ~

TN I00 -r

150

A61/50

E

dc field 3 T

Frequency I Hz A61/~

0

NSI40 50

• E uI00 T

g,

I 0

0.2

0.4

I 0.6

I O.B

0 -J

50

NS/40

oc ripple field ,T Fig. 5 Transverse field lossesversus ae field superimposed on a IT dc field

the ratio Q/f, (dissipation divided by the frequency) is plotted versus frequency for different currents. Straight lines are obtained, which leads to the experimental relation:

Q/f = a + bf, where a and b are constants. By extrapolating these graphs towards f = o, one can obtain values for a and the slopes give the values for b. It is now possible to distinguish the losses in the superconductor and the normal conductor by rewriting tile above formula as:

~

I 02

0.4

I 0.6

I 0.8

oc ripple field, T Fig. 6 Transverse field losses versus ac field superimposed on a 33dc field

Q = af+ bf 2, where afrepresents the hysteresis loss in the superconductor and b f 2 the eddy current losses in the normal conductor.

I00 dc field 5 T Frequency I Hz

A61/75 L~I/60

/ A 6,,7

Comparison

between

m e a s u r e m e n t s a n d theory

Self-field losses. Tile self-field losses in superconductors, due to the transport current carried by tile conductor, can be divided into hysteresis and eddy current losses given by the following expressions:

llysteresis loss. According to London s, tile average hysteresis loss (Pit) per unit of volume (I 0 of a solid superconducting wire is given by: (emu units) wle2

p,,/v =

I r(2-r) + 2 ( l - F ) . In (1-1-') I

/ / ~48/5o

ME T N -l-

9 E ~ 5o

o

NS/4(j

(1)

which simplifies, for small values of 1-', to (emu units)

o

-

-

-

3rr2Rc]

0.4

0.6

0.8

oc ripple field,T

wlc2r 3

Phlv

0.2

(2)

Fig. 7

Transverse field losses versus ac field superimposed on a 5T

dc field

C R Y O G E N I C S . J U L Y 1980

SULZER

During the last year, ten Turbocryo fridge cold boxes, type TCF, have been shipped from our workshops (four of them to Japan). Tile number of persons employed in our department and also the department's turnover have increased. With seven units successfully commissioned, tile TCF 100 new standard helium plant has definitely established itself in the forefront of the cryogenic world.

[1

-.,.r]..-c,,,.

L J/].-:I[7" [J %JLff We are pleased to announce the receipt of an order from IRD (International Research & Development Co. Ltd.), Newcastle-upon-Tyne, acting on behalf of the United Kingdom Ministry of Defence. It calls for a TCF 100 helium plant to serve as a refrigerator for a superconducting propulsion motor, which is under development at IRD. The project leaders at IRD, highly experienced and renowned for their pioneer achievements in superconduct-

!

,-2~ :7- '

ing rotating machinery development, attached top priority to the selection of a refrigerator with components of proven reliability in long-term operation. Therefore, it is not too surprising that they have finally chosen a completely oil-free Turbocryofridge helium refrigerator with the best components money can buy, namely the non-contaminating labyrinth-piston compressor and turboexpanders with dynamic gas bearings.

Fig. l: Cold boxes of Sulzer Turbocryofridge TCF 100 (right) and TCF 200 (left) standard helium plants providing refrigeration for CERN's new rapid cycling bubble chamber and its superconducting magnet.

,.,.L -GE u _

Fn

,-

o(-~')

HGUULU

_

.

In 1964, the Kemforschungsanlage Ji.ilich ordered two multi-purpose helium refrigerators/liquefiers from Sulzer. Both plants were equipped with oil-bearing turboexpanders and non-contaminating Sulzer labyrinth-piston compressors. In the meantime they have completed more than 80000 and 60000 hours of reliable operation respectively. However, in addition to providing reliable service, these plants have also proved to be highly adaptable to new requirements of their operators and the new technology of their manufacturers. Among several modifications made, two are particularly worthy of note. In one plant, a cold ejector was installed. This almost doubled the helium liquefaction rate. More refrigeration at 10-12 K was required in the throttle circuit of the other plant. This requirement has been met successfully and at relatively low costs by adding a selfacting gas-bearing turboexpander to the circuit. Through this extension, KFA Jiilich became the first to operate oiland gas-bearing turboexpanders combined in one plant.

/---~I

0

0

.z.,.-.]

co

+ +.<:" I,..,l+ ---"11

(u

CERN, ISR Div. (CH): The low-beta insertion of the intersecting storage ring, which consists essentially of eight superconducting quadrupole magnets, was constructed to obtain a high luminosity intersection. To supply the magnets with liquid helium continuously and trouble-free over periods of up to six months - CERN has chosen a TCF 200 Turbocryofridge liquefier. Its guaranteed capacity figures of 2701/h without and 350 I/h with liquid nitrogen precooling - which were exceeded by more than 10 per cent at the acceptance test - puts it at the top of the TCF 200 capacity range and, at the same time, makes it the capacity record holder of all Turbocryofridges supplied up to now (Fig. 3). Messer Griesheim (FRG) and Spectrospin (CH): The 56-1/h TCF 100 helium liquefier for Messer Griesheim and the smaller 17-1/h version for Spectrospin are both serving industrial firms. The outstanding reliability and high availability provided by TCF 100 plants were decisive factors for these procurements. The plant for Messer Griesheim installed at Gellep near Dtisseldorf is used as a reliquefier in the European distribution system for liquid helium. That at Spectrospin in F~illanden near Zurich provides liquid helium for the testing of superconducting magnets used in nuclear magnetic resonance spectrometers. It has been virtually in continuous operation since its commissioning last year.

.'

~..., ,." .;.-

;~ _ ....

Fig. 2: Skid-mounted labyrinth-piston compressor group, type K I05-2D, belonging to the TCFI00 refrigerator at Daresbury Laboratory.

0

Among the numerous Sulzer helium plants commissioned recently, there are f o u r - namely one refrigerator and two open-cycle and one closed-cycle liquefiers - which are especially worthy of mention.

9 o

.....

(;u

(..

Fig. 3: Viewof the TCF 200cold box installed at the ISR Division of CERN.

IKO, Amsterdanu A Turbocryofridge 100 helium refrigerator at the Instituut voor Kernphysisch Onderzoek (IKO) in Amsterdam has been successfully acceptance-tested by the end of 1979. The refrigerator provides refrigeration for the superconducting solenoid (generating an axial field of 5 Tesla) of a muon channel. Guaranteed reliability during continuous running cycles of 2500 hours is required. A noncontaminating Sulzer labyrinth-piston compressor, type K 140-2A, and dynamic gas-bearing turboexpanders make sure that this requirement ist met. Plant capacity: 50W at 4 . 4 K + 6 l/h at 4 . 4 K + 600W at 90K.

o, r s

.,, ,.

,

IJ

For the provision of refrigeration at 3.4 K to be used for superconducting alternator studies by Ansaldo S.p.A. in Genoa, the renowned Italian firm has appointed Sulzer to supply a Turbocryofridge TCF 100 helium refrigerator/ liquefier. Working in a closed circuit as refrigerator/ liquefier, and without liquid nitrogen precooling, it is capable of generating 20 W between 3.4 K and 3.9 K (14g/s mass flow) plus 431/h liquid helium for the cooling of the current leads. The plant can also be operated as a liquefier. It has separate transfer line connections for this purpose and the liquefaction rate is 501/h without and 851/h with liquid nitrogen precooling.

E

" -~

7

/77-

-J

.

.

.

.

.

_

"

One of the remarkable features of this plant is that the subatmospheric helium bath used in the final heat transfer stage to cool the helium stream to the experiment is effected by means of a patented warm ejector arrangement. The driving stream of this ejector is taken at the discharge side of the main compressor, which is a non-contaminating, double-acting, two-stage Sulzer labyrinth-piston machine, type K 160-2F. The warm ejector takes the place of a room temperature vacuum pump, but has distinct advantages in comparison to it: there are no moving parts subject to wear, and the plant remains oil-free. And finally, the hazard of air leaking into the cycle is practically eliminated.

f'l

o

_

---7

--

-" ,r'~ ~ ' 4 ~

The Turbocryofridges TCF 100 and TCF 200 have proved to be the most reliable helium refrigerators and liquefiers. So it is only reasonable to extend the programme to the liquefaction of hydrogen. The principle is that one retains helium as the refrigerant, and uses all the normal components in a standard cold box. Cold helium gas is transferred to a small, simple auxiliary cold box, where the hydrogen is liquefied. All the safety features which have to be followed when working with hydrogen are only necessary for this auxiliary cold box which has no moving elements. Fig. 4 shows the flow diagram of the process. The helium is compressed in a two-stage labyrinth-piston compressor and then cooled in three heat exchangers and expanded in three gas-bearing turboexpanders. The full helium flow is used for the condensation of the hydrogen. The major part of the helium is returned in a cold state to the helium refrigerator. Only a side stream is needed to remove the sensible heat of the hydrogen. The hydrogen stream passes through three stages of ortho-para conversion to secure a para-hydrogen concentration of over 98 per cent in the liquefied hydrogen. The liquid hydrogen is subcooled below 20 K by the cold helium gas, so there are no flash losses when the hydrogen is throttled into the storage dewar. The system is characterized by its excellent part-load efficiency when loading and unloading the helium cycle.

The helium cycle is tight to the outside and virtually does not require any make-up gas even during long periods of operation. For the time being, we have provided for three standard sizes which are defined by the frame size of the Sulzer labyrinth-piston compressors.

~

m Buffer

7tlelium Comprcssorx

llydrogcnfeed

ItXI

'q

llX2

llX4

[

I xl

I

o-p

Fh tlX3

]

/\

[

I

llX6 o-p

Compressor

K I05 K 140 K 160

Liquefaction rate l/h liquid hydrogen (>98% para) 55 105 165

Power requirement kW 150 235 350

'

TCF cold box

'

l

,

llX7

II

li:,'drogen cold _box _ _ _

SUI-ZER

Liquid hydrogen to dewar 9

Fig. 4: Flow diagram ofa Turbocryofridge operating as hydrogen liquefier.

D

d~

The last cold box out of a total of four Turbocryofridge systems ordered by Japanese customers last year left the Sulzer works in March 1980. The TCF 200 cold box (Fig. 5) is one of three Turbocryofridges which have been installed at the new site belonging to Electrotechnical Laboratory of the Agency of Industrial Science and Technology (AIST) in the Tsukuba district. It has a nominal capacity of 405 W at 4.5 K when operated with liquid nitrogen precooling. Unlike the two other TCF 200 systems supplied to AIST, which are helium liquefiers each of 1001/h capacity and equipped with an oillubricated screw compressor, this system will operate with a non-contaminating labyrinth-piston compressor. The plant will provide a superconducting magnet with refrigeration. Fig. 5: Turbocryofridge TCF 200 cold box leaving the Sulzer works to be shipped by air to Tokyo.

Cities Service h e l i m n l i q u e f i e r e x t e n d e d to 12001/h The Helix/Sulzer helium liquefaction plant at Ulysses, Kansas, was built for an initial liquid helium output ofabout 8001/h. However, its cold box - with the exception of the expander equipment - was built from the beginning for an ultimate liquid helium production of well over 20001/h. Six years after initial plant start-up in 1972, a capacity increase to 12001/h was undertaken, namely adding a rotary screw compressor system and adapting the two Sulzer oil-bearing turboexpanders to the higher turbine flow. The extended plant became operational at the beginning of 1980. Up to that time, the plant had been operating for more than 50 000 hours. New U n i o n C a r b i d e ( L i n d e Division) b u l k h e l i u m l i q u e f i e r on s t r e a m The new Helix/Sulzer 1400-1/h bulk helium liquefier shown here (Fig. 6) is now on stream. The three turboexpanders employed in the plant's cold-producing process are of the Sulzer oil-bearing type. The installation is located at Bushton in Kansas. It has been delivered to Linde Division of Union Carbide Corporation. In Kansas, there are now three industrial bulk helium liquefiers operating with Sulzer turboexpanders. Another industrial l Ielix/Sulzer liquefier of 500 l/h capacity, which is also equipped with Sulzer turboexpanders, will soon be operational at the United States Bureau of Mine's Exell plant in Texas. The aggregated present liquefaction capacity of these four plants is in excess of 40001/h, and it is therefore certainly no exaggeration to state that the greatest portion of all liquid helium sold worldwide is liquefied with the help of Sulzer turboexpander technology.

Fig. 6: Hclix/Sulzer 14001/h large-scale helium liquefier nearing completion in the assembly shop. At the top end of the cold box (from right to left): the three Sulzer oil-bearing turboexpanders (with turbine inlet filters above) and the "wet" piston expander of llelix. e./27.07.06/4-V.80-50-Printedin Switzerland

Eddy current loss. For low frequencies, if f < < (2rroxPo#tR~)q and f < < (4rr[L2tzo#tol) with #r = -X/+~., the eddy current

IOOHz

loss is given by: (mks units) 10

Pe/V-rr2B2R~176 (l-X) 2 1 1 + ( _ o

Niomax S/40

2

/ 2

\,,,-o }

I

(5)

50 Hz Table 2. A comparison between measured and calculated values (self field losses)

!

Sample

,J

It(A)

f(Hz)Phm PhcPem

Per

Ptm

Ptr

Ni0max 5

9

~

/"

_

~

-

~

$20/40

20

50 -

23

-

-

C381/100

40

50 -

0.26

-

0.03

0.25

0.29

C361/100 40

100 -

0.52 -

0.12

0.75

0.64

19

-

A61175

30

20 0.16 0.11 0.024 0.017 0.184

0.13

A61/75

30

50 0A2 0.280.142 0.1i

0.56

0.39

I00 HZ

A61/75

30

100 0.83 0.5{; 0.735 0A4

1.57

1.00

361/J00

A61/75

30

200 1.68 1.122.75

4.41

2.86

so Hz

1.76

A61/75

50

20 0.59 0.570.047 0.05

0.64

0.62

A61/75

50

50 1.48 1.3 0.28 0.3

1.76

1.6

A61/75

50

100 2.96 2.6 1.11 1.3

4.07

3.9

Fig.8 Self-field losses versus ac current for the sample Niomax S 20/40 and C361/100

A61/75

50

200 5.92 5.3 4.5

~here

The subscripts m and c refer to measured and calculated values respectively

9

I I0

0

20

I 40

30

I 50

ne current, A

l-k

O1 = ~

5.2

10.42 10.4

Transversefield losses (m Wcm"3) at 1 Hz.

OlTI

A61/75 Bde = 1T, lc = 650A P*m = 7 x 10 q~ 12m

~, = superconductor-to-copper ratio

Bde = 3 T , I c = 4 2 5 A Pm = 11 x 10 "1~ I2m

o= = transverse conductivity

Bale = 5 T , / c = 310A P m = 15 x 10 "1~ I2m

Om = matrix conductivity

* - values taken from Wilson et al 9

Transverse field losses. Murphy et al give the following expressions for the losses:

Conclusions

Hysteresisloss. The full penetration hysteresis loss is, per

From the table of self.field losses it is evident that the agreement between the calculated values and the measured losses is quite good for I t = 50 A but less so for I t = 30 A. No explanation can be given for this other than the insensitivity of the measurements at very low power levels.

unit of volume: (mks units)

SBd/ d"

Phlv- 3rr2Ro 2

(4)

r~le 3.

Bde=

Bdc = 1T tac (T)

Poe

Phc

Ptc

Ptm

Poe

3T

Bdc = 5 T Phr

Pte

Ptm

Pee

Phc

)2

15

7.4

22.4

15

9

4.8

13.8

15

7

).4

60

14.8

74.8

50

37

9.7

36.7

40

28

).6

133

22.2

155.2

90

95

14.6

99.6

75

62

)8

240

29.6

270

150

150

19.4

169.4

130

110

Ptc

Ptm

3.5

10.5

15

7

35

30

72.5

60

i0.5 14

124

100

2o 50A

50

~

15

50A

40

g

30 35A

~

=--i 0

20

.

-

"

.

-

"

[

i

i

i

40

60

80

I00

Frequency,

Fig'. 9

i

~ 2 o A

IO 1

I

120 140 160

,

,

180 200

Hz 0

Self-field loss per cycle versus frequency for A61/75

I 20

I 40

I 60

I 80

I I00

I I I 120 140 160

I I 180 200

Frequency, Hz

In the case o f tile transverse field losses, the agreement between theory and experiment is reasonable but the deviation appears to increase where the field sweep is large in relation to the dc bias field. Tiffs may possibly be due to the fact that the critical current 1c and tile matrix resistivity Pm cannot be assumed to be constants as used in the calculations. Because o f tile limitation o f the apparatus for measuring transverse field losses which was restricted to a sweep frequency of 1 Hz, sufficient data could not be obtained to enable the eddy current and hysteresis losses to be analysed separately. As seen from calculated values, the eddy current losses are very much larger than the hysteresis losses. A possible explanation could be that, at these very low frequencies, only an approximate analytical expression is possible and this, together with the experimental errors at low frequencies, may account for the poor agreement.

Fig. 10

research to be undertaken and also to tile Rutherford Labora tory for their assistance and the use of their facilities.

References 1 2 3 4 5 6 7 8 9

The authors are grateful to tile SRC for a grant enabling this

Self-field loss per cycle versus frequency for A61/60

Rhodes, R.G., Rogers, E.C., Seebold, R.J.A. Cryogenics, 4 (1964) 204 Eastham, A.R., Rhodes, R.G., Supplement to the Bulletin of the International Institute of Refrigeration, Tokyo (1970) 147 Deis,D.W., Reynolds, W.T. Advances in Cryogenic Engineerial 18 (1973) 4OO Furtado, C.S. Cryogenics 12 (1972) 230 London, ll.,Physics Letters 6 2 (1963) 162 Caxr Jr., WJ.,Journal o f Applied Physics 45 2 (1974) 935 Ibid. 929 Murphy, JJl., Deis, D.W., Shaw, BJ., Walker, M.S. lEE Trans. actions on Magnetics MAG-I 1 2 (1979) 317 Wilson,M.N., Waiters, C.R., Lewin, J.P., Smith, P.F. Journal o f Physics 3 (1970) 1517

C R Y O G E N I C S . J U L Y 198flf