THE KARLSRUHE PULSED SUPERCONDUCTING MAGNET PROGRAMME

Status Report+

The Karlsruhe IEKP Magnet Group; compiled by W. SchauerInstitut fur Experimentelle Kernphysik, Gesellschaft fur Kernforschung, Karlsruhe, Germany

Abstract

This paper reviews the progress of the Karls­ruhe superconducting pulsed magnet work. The ex­perimental results of the first two magnets arediscussed and the present status of the subsequentmagnet family U2a, D2b and D3 is described. Resultsof superconductor and material properties measure­ments are presented.

I. Introduction

The high magnetic field capabilities of super­conducting materials make possible the construc­tion of pulsed accelerator magnets which are es­pecially suitable for large synchrotrons with lowrepetition rates. With the construction of the300 GeV proton accelerator now under way at CERN,the application of high field pulsed superconduc­ting magnets to increase the present machine en­ergy to about 1000 GeV has been taken into con­sideration. Convincing demonstrations must showtheir suitability and reliability before a decisioncan be made whether superconducting pulsed magnetsshould be adopted to uprate the machine energy.Each group of the GESSS Collaboration, comprisingthe development teams at Rutherford Laboratory,CEN/Saclay and IEKP Karlsruhe, has built and isgoing to build and test several different prototypemagnets which will be in line with the requirementsof a superconducting accelerator concept. - Thispaper gives the Karlsruhe laboratory status reportwith respect to pulsed superconducting synchrotronmagnets.

II. Magnet Programme

The superconducting pulsed magnets of rel­evance to a synchrotron programme, which have beencompleted and tested, which are being built andwhich are under design in the Karlsruhe IEKP Mag­net Group are listed in Table I. After the twofirst stage magnets ru and DT have been testedproviding a lot of engineering and experimentalexperience, the prototype magnets De and D3 arefixed to produce a 4.5 T central field in an 8 cmaperture by a modified cose current distribution,the coils being surrounded by a cold laminatediron yoke. The variability of the concept is dem­onstrated by the fact, that two different ironyokes (close saturated (a) and far unsaturatediron (b)) and two different conductor configur­ations (five double layers of a nearly squarecable, two or three double layers of flat cable)

+work performed with the financial support of the"Bundesministerium fur Bildung und Wissenschaft",

Bonn.

will be used for dipoles D2a and D2b.Adopting the version which proves to be best

with respect to engineering considerations, meet­ing the design specifications and reliability,dipole D3 will be built with the only essentialchange of doubling the length, its geometrical di­mensions being close to the requirements of asuperconducting synchrotron concept.

The programme summarized in Table I reflectsevidently the conductor development. Dipole Dl waswound from a multifilament superconducting wire,before the cabling and braiding technique necess­ary for high current conductors had been developedfor commercial use. Dipole DT and D2a are woundfrom a InSn-soldered cable to avoid the undesirabletraining effect caused by wire movement. However,one has to put up with a severe disadvantage: thesolder short circuits the individual strands andthereby increases the losses due to matrix currents.Thus, soldered cable magnets, from the point ofview of tolerable ac losses, are only acceptablefor field rise times of about 5 seconds or more. ­Increasing the interwire resistivity by poisoningthe solder and thereby reducinf the losses hasbeen studied by the BNL-Group.

In the next stage dipoles D2b and D3, a fullyinsulated cable or braid will be used. This conduc­tor concept completely removes the disadvantage ofenlarged losses and thus enables shorter cycletimes. To reduce - or even to avoid - training anddegradationJthe multistrand conductor should beimpregnated with a filled epoxy resin system. Indipol~D2 and D3 the spacers, made from fibreglassreinforced epoxy, serve as compact filler, therebyonly a small amount of resin will be needed.

Dipole Dl. In a first stage a single multi­filamentary superconducting wire was used to winda dipole magnet of 0.5 m overall length (Fig. 1).The concept of intersecting ellipses was approxi­mated by a computer programme ~o give a total cal­culated field error of 2 x 10- •

The magnet consists of 7 potted pancakes perpole with geometrical tolerances of ~ 20 ~. Themagnet exhibited training during the first 10quenches to reach nearly its final dc field of 3 Tat 45 A, which is_f3out 30% of the short samplecurrent at ~ = 10 Jlcm wire resistivity. Furthercooldowns brought no significant improvement; prob­ably coil movement has caused the degradation. Inquench case the stored energy was nearly completelyreleased in the magnet due to its high resistancein the normal state. The quench voltages per polewere about 500 V arising in 0.6 s, the currentbeing reduced to 55% of the quench current.

Dipole DT. This magnet shown in Fig. 2 hasbeen wound from a 10 strand cable with a central

306

TABLE 1. IEKP Magnet Parameters

Magnet D1 DT D3

Magnet Parameters

? x2.8

802.8

3 - 10

4.5

801.4

5 - 10

(a)O.3xO.38xl.4;(b)0.3xO.46xl.4

channelsbath cooling, forced cop.vection

103515

channels, bathcooling

10 - 201500

(a)128; (b)142laminated iron yoke at helium temperature

250 x 40 (a)172; (b)216window frameo.37xO. 58xo .40

110 x 800·533

30

4527 (at 3T)

no

channels, bathcooling

Coil aperture diameter ordimensions, inside ofwindings (rom)Length (iron) (m)dc field achieved (T)Pulse field achieved (T)

atfield rise time (s)Specified pulse field (T)

atfield rise time (s)Operating current (A)Stored energy at 4.5T (kJ)Iron sh~eld inner diam-eter or dimensions (rom)Iron shield outerdimensions, crosssection x length (m)Coil cooling

Coil and conductor

Coil design mod. intersect flat race trackellipse, ends unpotted/raceat 900 , potted track ends at

450 , potted

modified case, potted

Number of layers/poleNumber of turns /poleInductivity at 4.5T (mHy)

560027000 at 3T

34} 5 double 1. 2-3 double 1.306 total coil 210

50 (a)130; (b )125

2-3 double L

1M!, Birmingham, EnglandConductor manufacturer

Conductor type

VAC, lIanau,Germany

multifilamentwire

cable with central coppercore, InSn-soldered

flat cable or braid withstrands fully insulated

Dimensions (rom)Cable insulation

Max. current dens. at 4.5Tsuperconductorstrand (kA/crr?-)cable

Number of strandsStrand diameterTransposition pitchFilaments per strandFilament diameterTwist pitchMatrixMatrix: NbTi

(rom)(rom)

( tlIIl)(rom)

10.4

13025

4Cu

1.2 : 1

79} at 3T36

1.9 x 2.,64 2.1 x 2.6glass braid terylene glass

br.10 12

0.5 0.5430 30.4

1045 100010 12 8 - 10 8 - 1012.5 4 ~ 3 ~ 3CuNi Cu

1.5 : 1 1 : 1

( a) (b) I Conductor not yet1130 117 129 finally fixed

53 59 6521 25 27

307

nor training was observed.

The current field characteristics of the mag­net are shown in Fig. 3. The calculated curves liesomewhat higher, which is due to an incorrect setof iron-r-values in the computer programme inputdata, as has been shown by recent low temperaturemagnetization measurements of the iron used in DT;a recalculation will be done. In Fig. 3, two fieldcurves of the coil without the iron yoke are in­cluded. Field measurements were taken with Hallprobes and during field ~ise with an integratingfluxmeter.

kG

For the case of quenching the coils were pro­tected by a comparator device, which inverts thepower supply mode to an energy feed-back mode, ifthere is an ohmic resistance in the coil corre­sponding to a voltage drop of ~1 V. The compara­tor consisted of a copper coil, which was adjustedto give the same induction voltage as did thesuperconducting coil. The maximum current couldbe decoupled from the coil in about 1 second. Evenin the worst case of the coil being totally in thenormal state,its time constant LjR (L = 150 mHyfor unsaturated iron, L ~ 50 mHy at 4.5 T withsaturated iron) is at least one second, thus near­ly the total stored energy could be removed fromcoil. The decoupling device worked perfectly, thecoil could be quenched without observing an in­crease in the boil-off rate.

Losses were determined by measuring the Heboil-off rate and later by a Hall multiplicatordevice. Results are shown in Fig. 4 and Table II.

/~/. nol,on ./

/ )h...,,~/ .. ///

.'if,-/

JO

20

00 200 'CIO 600 800 11D1 lZ00 A. 1400magMt Curmlt I

Field vs current for dipole DT at thecentre (x = y = s = 0), near the coilsurface (x = 1.75 cm, y = s = 0) andat the end (x = y = 0, s = 20 em).

Fig. 3

50

'0

Fig. 1 Dipole ro.

copper core, the cable being InSn-soldered to en­sure mechanical rigidity. The cold laminated ironyoke is of the window frame type with the rectangu­lar window widened in a hyperbolic shape towardsthe ends providing for a linear field decrease.The 1 mm iron sheets are insulated by a 7 pm phos.phate coating and screwed together by means of 4epoxy insulated stainless steel pins passingthrough the outer parts of the yoke. The coilswere fixed within the window by means of wedges.The cable was wound to an unpotted flat race trackcoil in the first test stage. Cooling channelswere spaced 4.5 rom from each other (every two lay­ers). The magnet was mounted vertically and hasbeen operated under bath cooling conditions.After precooling to lN2 temperature,0.15 liter he­lium per kg ire., were necessary for the cool down.

During the first period of operation,the mag­net has been tested for almost 10000 cycles withtriangular pulses of 2, 5 and 10 seconds pulseduration. At a slow field rise of ~O.l Tjs,a cen­tral field of 4.5 T and a peak field of 5.2 T onthe Winding surface were obtained with a magnetcurrent of 1035 A which is in accordance with theguaranteed short sample values. Neither degradation

Fig. 2 Dipole DT.

308

Rise Time Peak Central Field Losses(" Permanent Pulse Mode")

( s) (T) (J!cycle)

45 4·5 <831 3·5 14

5 3·2 1302·5 3·0 1071 2.4 90

TABLE II. Losses in Dipole DT at Peak Field

40

20

with potted coils, whether shorts have made a con­siderable loss contribution. On the other hand,the severe influence of a low resistivity soldirwith respect to coupling losses is well known.The high level of heat production in the coil isthought to be the reason for the lowering of theattainable field with increasing pulse frequency.­The iron hysteretic losses are about 8 J!cycleat maximum peak field, the superconductor hyster­etic losses about 4 to 5 J!cycle, both being smallcompared to the frequency dependent losses men­tioned before.

In a second test stage,the DT-cable is goingto be rewound in two race track coils with endsbent at 450 , now being resin impregnated. Lookingfor possible training and measurements of attain­able field and pulse losses are projected.

The same iron yoke has been used to build amagnet with coils wound from a high p~ity alu­minum tape (8 x 0.3 mm cross section). The tapeis insulated by a 7 pm anodized coating which canstand annealing at 5000 C. The eoils were wound ina flat race track 60 cm long. the ends then beingbent at 45 0 by press-working. The zero field shortsample resistivity ratio of 9200+ could be recover­ed in the bent coils after annealing and subsequentepoxy impregnating to achieve mechanical rigidity.The magnet consists of 2 x 2 coils; one iron yokehalf with coils and cooling channels is shown inFig. 5.

It will be shown from the actual DT test runs

Fig. 4 Losses for dipole magnet DT vs.pulse dur­ation for different central fields (tri­angular pulses).

The boil-off rate method is correct onlywi thin 'V 2CJ1, and rather time expensive to reacha stationary state. The peak field values givenin Table II represent the maximum field attainableduring half an hour pulsing ("permanent pulsemOde"). Going to higher fields no stable permanentpulse operation could be achieved, although somefew pulses could be run at higher fields withoutquenChing the magnet. This peak field is reducedas the rise time decreases. The losses per cycleare approximately proportional to the frequencyand the square of the field. This can be explained

Fig. 5 Iron yoke of dipole DT with Al coils.

+The conductor was produced and delivered by Ver­einigte Aluminium-Werke (VAW), Bonn, Germany.3In the meantime) preparation has been improved toa 0.3 mm tape resistivity ratio of 15000 (allow­ing for size-effect correction).

The total coil resistivity at 4.2 K vs.themagnet central field is given in Fig. 6 showingthe magnetoresistfnce increase, which has beenreported earlier. The upper curve in Fig. 6 wastaken in a later test run, Where probably the mag-

1 •0 0 I 2 ) " S 6

~l

- by the contribution of eddy current lossesin the end region of the iron yoke (ironresistivity ratio = 8), where field compo­nents perpendicular to the laminates exist,

- by coupling losses, due to currents cross­ing the interwire solder resistivity layersand copper core (copper resistivity ratio= 140),

- possibly by shorts between adjacent layers.When the cable was wound off, the fibre­glass tape insulation wrapped around thecable was found damaged at those points,where the coil was wedged in the magnetiron window.

309

Fig. 6 Aluminium magnet coil resistance at 4.2 K'IS magnet central field 1. at the firsttest run; 2. at a later test the resist­ance being increased probably due to coildeformation.

..~.

~ ~ r=9200

Ci

10 20 30

Central Al - magnet fi~d

kG 40

is again due to the incorrect p-values as mention­ed before. Losses have been determined electricallymultiplying current and voltage by a Hall-gener­ator device. In the triangular pulse mode oper­ation they are about one third of those in dc op­eration as is to be expected; they are in factsomewhat lower, as when pulsing the mean resistiv­ity is lower due to the magnetoresistance than thepeak field resistivity responsible for the dc los­ses. The splitting of the loss curves for differentpulse durations is caused by eddy current lossesin the aluminium tape and iron and hysteretic los­ses in the iron.

Though losses in the range of 3 T and 5 sec­onds pulses are comparable for the superconductingand aluminium version of DT, probably potting thesuperconducting coils and especially using a fullyinsulated cable as in dipole D2b and D3 will showthe superiority of the superconducting concept asto pulsed accelerator magnets with about 4 to 5 Tpeak field and pulse durations of 3 seconds ormore.

netic forces ('V 20000 kg over the coil length)during the preceding pulse tests have deformed thecoils slightly and thereby induced lattice defectsresulting in a resistance increase. - Field andloss measurements at 4.2 K are summarized inFig. 7. The discrepancy between calculated andmeasured field levels for a given magnet current

40

30

FiV. 7 Fiel'l B ~ml losses P (tri:mgular pUlses)'IS ~IJrr~nt f0r the 8llJmin 1lm mal~net:

1. 'l~ r j perati0n; 2. ~ sec,~. 6 sec,t. 10 ~;ec plJlse duration.

Dipole D2a, Deb and D3. Dipole D2a, which isactually being wound, and dipole D2b and D3, whichare under design, present various stages in thedevelopment of a superconducting synchrotron di­pole. All three magnets will have coils which arebased on the modified cose current distributionwith an inner and outer diameter of 80 rom and152 rom, respectively. The design induction for allis 4.5 T with a uniformity of 0.1% within a boreof 60 rom diameter; the multipole structure is de­scribed elsewhere.5 The cosS geometry permits oneto build coils which are c,')pnble of generatinguniform fields whether or not the iron shield ispresent.

Two iron yokes are being built presently.+One, which has an inner diameter of 172 rom, be­comes saturated as the magnet goes to high induc­tion; the other, which has an inner diameter of216 rom, remains unsaturated throughout the cycle.The coils of Dea and D2b may be used in eitheriron shell (or with no iron at all). The effectsof iron saturation on ac losses, field and fielduniformity can be measured directly. The expectedfield as a function of current for both shells andno iron shell is shown in Fig. 9, the superconduc­tor critical current as function of Band T beingincluded. The expected iron enhancement of thefield is given in Fig. 10.

The length of the iron shell for D2a and D2bis 1.4 m; the iron sheets are 1 mm thick with a10 pm phosphate coating on each surface. The out­side of the iron is shaped to counteract the effectsof saturation. The ends of the magnet coil lieabout 3 cm inside the laminations (Fig. 8) and areshaped to produce an integrated field which isgood to 0.1%. The conductor positions shown inFig. 8 are calculated by a computer programme inorder to get the desired field homogeneity.

+Brown, Boveri & Cie, Mannheim, Germany.

310

II

~

/

'. //. /~ / ,/

laminatediron yoke

3

9

distances in mm

Fig. 8 Dipole D2a crosssection with con­ductor configur­ation (left) andlongitudinal sec­tion with conduc­tor end configur­ation (below):1. Cooling chan­nels, 1.5 mm inwidth; 2. spacers,glass fibre re­inforced epoxy;3. support rings,stainL steel;4. tube, stainl.steel; 5. yoke topand end plate,stainL steel;6. sleeve, glassfibre reinforcedepoxy; 7. guidepin; 8. Bellevillesprings; 9. Cool­ing hole; 10. frontpanel, stainLsteel.

8

10

2 6 7 4r =35.5

L. .-----L- . I . --------!.. Imagnet

500 600 700 Q~

distance along the bore from the magnet centre

in mm

311

Fig. 10 Calculated iron enhancement factor fordipole J2a central induction; saturatedand unsaturated version with 86 rom and108 rom iron inner radius resp.

Fig. 11 Cryostat for :f'a, if'b (outer vessel).

+ IMI, Birmi np,ham, Jo:nglanll.

++Leybol-l- Her1J:U8, Ki;l n, Germany.

is greatly simplified by reducing the number ofcable layers to 2 or 3 depending on the flat cableor braid superconductor, which is available at thattime. Thereby dipole D2b will be easier to wind,pot and assemble than D2a. Dipole D3 will incor­porate the same coil design as D2b except that thecoil and iron length will be extended to 2.8 m.

The superconductor for dipole D2a is a solder­ed cable with 2.1 x 2.6 mm cross section;+ it iswrapped with a terylene-glass braid insulation0.2 rom thick. The 2.5 x 3 rom insulated conductorcross sections are shown in Fig. 8. A current of2000 A at 5 T and 4.2 K has been guaranteed. It isexpected that the maximum field in the conductorof D2a will be about 4% above the central field.Dipole D2b and 03 will use a flat cable or braidwith fully insulated strands. Our preference istowards the cable, because crossover points whichare potential shorts are eliminated. The cable hasa packing factor of about 70% compared to the braidwith about 50%. It is expected that dipole D3 willhave a conductor similar to that of efb.

Magnetic forces will be supported by stain­less steel rings, the space left between the coiland the iron being filled with helium. Holes inthe iron permit helium to flow from the bottominto the magnet, through the cooling channels ofthe coils and out of the magnet through the holesin the top of the iron. The cooling channels willalso make possible forced cooling lengthwise throughthe coils. The largest dimension of both iron shellspermits them to fit into a cylinder with a diameterof 590 rom. Two cryostats++ have an inner vesselwith a free space of 600 rom diameter by 1530 rom inlength (see Fig. 11). The cryostats have a warmbore which is slightly larger than the 60 rom usefulaperture. They are equipped with two 100 mm diam­eter necks, one for the refrigeration, the otherfor electrical leads and service valves. An outervacuum jacket and a support system is being built,which can be modified for use with dipole D3. \';eintend to make the inner cryostat vessel of dipoleD~ an integral part of the magnet itself .

3000 A

6

2500

5

20001500

Magnet curr~nt

2 3 4MagMt central induction

1000500

Dipole a2a operating current vs centralinduction, calculated for the differentiron versions. Guaranteed short samplevalues of superconducting cable.

160

Fig. 9

15086mm IRON

2u~ 1.40c: 108 mm IRON..E.. 130uc0L:C..c 1.20~

1.10

A

3000

,,$~.q/"

2500 ~~.~

D~sign induction .. _. ~2000

4.5 T

"~~2"c:~ ~ ~<t:;

1500u ,,)/" "c: "~e·cOl ~0~ "1000 "No IRON "

108 mm IRON

500 86 mm IRON

a

Since the radial dimensions of the coil pack­a/.(e in all three 'lip0 les are the sa.me, many 0f the

aceess0ry pa.rts s'lch as bore t'Jbes, force restra.in­in~ fixtllres a.nel hol'linp fixtlJres are i'lentical.The c',il 'l<>sii-,;n, on the other han,J, is q'lite dif­ferent for the three mB.r;nets. .Ji.p0 Ie 'ks. has a 5'10'Jble layer c0il which is fabricate') from almosts,<"are COTI'Jllctor (Fil'. f;). 1t req'Ji res lr) sepa.ratewin1in~ and pottin~ operations and 10 pairs 0fe lectrica.1 1ea'Js to be interc0nnecte'l. 'Jipole 'J2b

312

A selection of tested wires is given inTable III. All samples have a twist pitch of afew millimeters. Conductor (c), (d) and (e) weretaken from braids or cables.

to a well cooled single wire loop sample ('" 15 cm)tested earlier, a considerable reduction in take­off current has been observed. Therefore we had touse single loop samples with the bette~lcooling

performance for resistivity above rv 10 R cm.

• "'fem'10'Cur....t denllity J

,

I • conductor Ib)

..,,, /.I ld I. I.) -

a.I.IT

i·1

I-/

II i --/

!I; l

11 i!I';/ ./It l

10-1 Qem

TABLE III. Parameters of some Tested Wires

Conductor Wire Filament Number of Cu:ScDiameter Filaments Ratio

(rom) ( pm)

( a) 0.4 10 300 3.8 :1(b) 0.4 24 130 1.2 :1(c) 0.28 10 361 1.25:1(d) 0.38 8.5 1000 1 :1(e) 0.2 7.5 400 1 :1

Conductor (a) was known to be mechanically de­fective and showed many broken filaments after thecopper matrix had been carefully etched away. Thecharacteristics for (a) have a small slope ofabout 1.5 = log~/log J nearly constant for thefield range from 1 to 4.4 T. The steepest slope ofabout 50 has been observed for the two conductors(c) and (d). Their characteristics are so closetogether, that only the curve for (d), togetherwith (b) and (e), is shown in Fig. 12. In this dia­gram the upper points give the take-off current.The lowest measured resistivity was 3 x 10-1 n cmat low fields. Microscope photographs of compositecross sections show that the filament diameter forconductor (c) is nearly constant, whereas filamentsin (d) and even more in (e) vary considerably insize; in (b) several filaments are connected to­gether.

One of the central problems of pulsed super­conducting magnet work is the choice of a suitablecomposite superconductor. The many parameters whichcharacterize a multifilament composite and a cableor braid made of them, should be optimized withrespect to mechanical, thermal and electrical per­formance according to technical feasibility. Oneimportant feature to be optimized is the ac lossbehaviour of superconducting composites,8,9 whichis reasonably well understood. On the other hand,the relation between short sample characteristicsof multifilament composites and their behaviour aswire, braid or cable in an unpotted or potted coilor test magnet, i.e. degradation and training ef­fects, still has to be studied more thoroughly.

III. Conductor and Materials

Conductor Studies

Short Sample Measurements. A lot of multi­filament NbTi wires with different filament diam­eters down to 4 ~ has been examined. Their shortsample characteristics were plotted as log~

= f(log J), where the overall wire cross sectionis taken to determine the wire resistivity ~ andthe current density J. The steeper the slope ofthe characteristic, the more the multifilament wireapproaches the behaviour of a single core conduc­tor.10

When wound to a solenoid or magnet~the resis­tivity of the wire might cause degradation depend­ing on cooling conditions. In order to have com·parable cooling conditions in the short sampletests and in magnet operation, the short samples( '" ""i. 5 m long) were wound into a small bifi tarmultilayer coil potted with epoxy resin. Compared

Refrigeration for the dipoles6 will be sup­plied by the Karlsruhe 300 W Linde refrigeratorand later by the 300 W refrigerator built by MesserGriesheim. Cold helium gas is delivered from therefrigerator to the magnet cryostat by a concentrictransfer line system consisting of a rigid transferline from the refrigerator to the magnet test areaand three flexible transfer lines to the magnetcryostats. 7 The J-T-expansion takes place in themagnet cryostat.+

The pulsed dipole magnets are charged with athyristor controlled power supply (BBC) capable ofdelivering ~500 A at 70 V in a ramp function. Thevoltage to do this is generated by a rectifiedthree phase circuit which is thyristor controlled.The remaining 600 Hz ripple as reduced by differentfilters to a few parts in 10 • The power supplycan be used to extract the magnet stored energyduring a quench. A superconducting solenoid isbeing built (4 T, 116 kJ; 8 Tis; split coil length:(2 x 15) + 1 em, inner diameter: 15 cm; 24 strandIMI braid) for testing the power supply and thequench protection device. Field measurements ofdipoles D2 and D3 will be taken by a moving buckingcoi1 system with computer drive and data acqui si tion.,

+Manufacturers involved: rigid transfer line:Leybold-Heraus; flexible transfer line: VacutunBarrier; J-T-expansion including controls: Linde.

Fig. 12 Resistivity if vs current density J forconductors (b), (d) and (e) wound inbifilar coils.

313

A1000 ,....----,,..-----.-.........,.----,-----,-,

Systematically more samples have to be stud­ied, before a satisfactory explanation can be giv­en, how the wire parameters will affect the shortsample characteristic.

Test coils. Different NbTi multifilament wireswere wound to 35 cm long test coils cast in epoxyresin with ends bent at 900 • These coils are simi­lar to the race track coils in dipole Dl.

Looking for training effects, it could beshown that thermal cycling before the electricaltests reduced the training. Once the coil hadbeen trained, the quench currents were constantwithin 0.1~ for some coils and no more affected bythermal cycles. The dc quench currents had to berelated to resistivity values less than 10-12 51 cm,neglecting the other effects which could causedegradation as one has to assume in the dipole mag­net.

The development of conductors for pulsedsuperconducting synchrotron magnets tends to usehigh current ac cables or braids to restrict charg­ing voltages to reasonable levels. Two insulatedbraids of rectangular shape, the first consistingof 48 strands of conductor (c), the second of 24strands of conductor (d) were wound to small un­potted test solenoids of 6 cm length and 4 em innerdiameter with cooling channels between every twolayers. The load lines of these solenoids are shownin Fig. 13. To find the short sample curves, theaveraged single strand values of the (c)- and (d)­wire at a resistivity of 10-13 n cm have beenmultiplied by the number of strands. Testing thesolenoid wound from braided conductor (d),one endof the braid outside the magnet burned out duringpulsing the solenoid with a frequency of 2 Hz. Thebraid end was mechanically unsufficient1y fixed,which might have caused strand breakage duringpulse operation. After repairing the braid end,the

o .20-- Temperature T[c]

-20-'0-60-80

Poisson ratio for different epoxy resinsystems vs temperature.

0 . • a 6

0.3 0.6 0.96 9.6 2'VELOCITY OF DEFORMATION

("10 I min]

LMB 23'..,< ~ Hy 917

/.-' -Zi/4 Cy 221Hy976

~ D \,....' SO"loAlz03

3'~by weight

riaidLy 556

D , .... Hy 91766.,. A11O]by weight

0.30-100

0.40:1

o'aa::

~III

~f 0.35

Low Temperature Material Studies

Fig. 14

solenoid went to a 1~ higher quench current as isshown in Fig. 13 on the load line.

The solenoids which will be tested next willbe potted, including also soldered braids andcables.

Thermal conductivity measurements of differentepoxy resins and superconducting coil windings be­tween 4 K and 10 K have been performed.13 By ad­mixture of A~03-powder the thermal conductivityof the resin could be considerably improved (ClBACY 221/HY 956). Thermal conductivity of coil wind­ings in the coil axis direction could be enlargedby a factor of ~4 by choosing a superconductingwire with a copper matrix and, at the same time,potting the coil with A~03-fi11ed resin, comparedto a CuNi-matrix wire potted with an unfilledresin. - Further measurements of filled and un­filled resins are being performed.

To investigate the mechanical behaviour ofmaterials from 4.2 K to 300 K,a servo-hydraulictesting machine'wi11 be ready for operation by theend of 1972. A similar apparatus which operatesdown to lN2 temperature has been installed in the"lnstitut fiir Kunststoffpriifung" of the UniversityStuttgart. Measurements of the Young and shearmodulus and the Poisson ratio of rigid and flexibleresins, unfilled and filled, with variable defor-

To determine the low temperature mechanical,thermal and electrical properties of materialsused in magnet engineering,an extensive test pro­gramme has been started. Work was concentratedfirst on studying organic structural materialssince generally no comprehensive data are avail­able in this field.

Thermal contraction has been measured for dif­ferent types of epoxy resins.11 Inorganic fillers(quartz, A~O~) of various grain sizes have beenused. The contraction can be reduced considerablyby loading the resin to match the contraction co­efficients of metals. With fibre glass as fillercontraction of resins can in addition be variedby the angle of the filament structure to the di­rection of contraction considered, as has beenshown ear1ier.12

5 T 12 3

~

1000

Short sample curves and load line forunpotted test solenoids of 48 strandbraid (conductor (c)) and 24 strand braid(conductor (d)).

4000

Fig. 13

5000

2000

314

¥5.---------~--------___,

losses of about 50 liters per pAh have reached thecapacity limit of the recently installed heliumgas recovery system. The temperature rise of theprobe under investigation in the beam spot centercan be measured by the calibrated resistivity-tem­perature dependence beyond the transition temper­ature.

Room temperature studies on stabilized NbTimulticore wires with 51.5 MeV deuterons up to dosesof ...,10 1 rads have been performed.19,20 The criti­cal current decreased by I'V 25i at loll rads (Withno recovery) and by ~10% for lower dose rates of1010 rads (With complete recovery). In the resis­tive state the power dissipated below take off isdecreased by- 20% after 4 x 1010 rads irradiation.

Referencesr.7800

Irradiation Tests

mation velocity have been performed. The Poissonratio vs temperature for three systems is shownin Fig. 14. A decrease as the temperature is low­ered has only been observed for the system, whichis flexible at room temperature. - To study crackformation in the resins, a corona test apparar­atus14 ,15 is being built.

Resistivity and magnetoresistance behaviou5of aluminium tape conductor has been examined.1Short sample transverse magnetoresistance with thefield parallel to the long side of the tape crosssection has been measured in a sc solenoid in de­pendence of purity (given by the residual resis­tivity ratio r) and temperature with regard to mag­net application. - As an example,results of A~/~= (9(B,T) - 9(0,T))/5'(0,T) vs temperature for 1 Tand 4 T field and r = 1800 are given in Fig. 15.

A. van Steenbergen, editor, "Progress Report onMagnet Development, Brookhaven Nat. Laboratory:'Proc. 8th Int. Coni. on High Energy Accelera­tors. CERN. Geneva, p. 199.B. Lott, W. Schauer, W. Specking, S. Stumpf,P. TurOWSki, Technical Report Karlsruhe KFK(1912, to be published).W.O. Hannibal, H. Pfundt, G. Winkhaus, Proc.3rd Int. Conf. Magn. Techn., Hamburg, 1~p. 1234.W. Schauer, W. Specking, P. TurOWSki, Proc.3rd Int. Conf. Magn. Techn., Hamburg,~p. 606.H. Brechna, M.A. Green, 1912 Applied Super­conductivity Conf. Annapolis, Maryland-USA,1912 ~ p. 22 6.H. Brechna, M.A. Green, W. Heinz, Proc. 8thInt. Conf. on High-Energy Accelerators, CERN­Geneva, 1911, p. 183.W. Barth et al., 4th Int. Cryog. Eng. Conf.(ICEC 4), Eindhoven-The Netherlands, 1912.K.P. Jlingst, G. Krafft, G. Ries, TechnicalReport Karlsruhe KFK 1211 (1910).G. Ries, H. Brechna, Technical Report Karls­ruhe KFK 1312 (1912).F. Voelker, Part. Ace. 1, 205 (1910).W. Weiss, Oiplomarbeit Karlsruhe (1912).H. Brechna, G. Hartwig, W. Schauer, Proc. 8thInt. Conf. on High-~nergy Accelerators, CERN­Geneva, 1911, p. 218, and: Technical ReportKarlsruhe KFK 1410 (1912).G. Krafft, Technical Report Karlsruhe KFK1341 (1911).H. Brechna, E. Oster, Proc. Int. Sym. onMagnet Techn., SLAC-Stanford

i1965, p.313.

E. SchUhlein, ETZ-A 80, 111 1959).B. Krevet, Diplomarbeit Karlsruhe (1912).H. Becker, H.K. Katheder, E. Seibt, S. Stein­acker, Technical Report Karlsruhe KFK 1684(1972), and: S. Steinacker, DiplomarbeitKarlsruhe (1973).K.R. Krebs, J. Pytlik, E. Seibt, TechnicalReport Karlsruhe KFK 1683 (1912), and:K.R. Krebs, Diplomarbeit Karlsruhe (1972).H. Brechna, W. Maurer, Proc. 8th Int. Conf.on High-Energy Accelerators, CERN-Geneva,1972, p. 224, and: Technical Report KarlsruheKFK 1468 (1971).W. Maurer, H. Brechna, Technical ReportKarlsruhe KFK 1469 (1912).

5·

2.

4.

1.

7·

8.

6.

9·

10.11.12.

18.

15·16.17.

13·

14.

ca.

12 16 20 24 28 32--,,-_____ K

Temperature

Transverse magnetoresistance of aluminiumtape (8 x 0.3 rom cross section) vs temper­ature. B = 1 T, 4 T parallel to the 8 romside; residual resistivity ratio r = 1800.

Fig. 15

To study the nuclear irradiation influence onthe properties of superconductor and structuralmaterials)an experimental setup for low temperatureirradiation measurements has been built.11 It usesthe Karlsruhe isochronous cyclotron as radiationsource with 25 MeV protons, 50 MeV deuterons and100 MeVoc-particles. The probes are irradiated atIHe temperature and measured at 4.2 K in the samebath. The cryostat is equipped with a sc solenoid(6 T; 95 mm i.d.) for magnetic measurements.

An apparatus for magnetization measurementsof superconductors has been built for the irradia­tion setup. Magnetization and hysteretic loss meas­urements for unirradiated samples of NbTi, NbZr,V~Ga, Nb~8n have been performed.18

The irradiation apparatus did run severalcalibration tests for making the possible range ofexperimental conditions with a maximum of 2 ~ deu­teron and 1.5 vA ~-particle current. Irradiationdose and dynamic He-loss measurements with a NbTi1~O fj lI:~ment conductor have been performed. For100 MeV 0(- particle irradiation liquid helium

315