ieee transactions on magnetics, vol. mag-23, no. 2 ... - UT RIS webpage

IEEE TRANSACTIONS ON MAGNETICS, VOL. MAG-23, NO. 2, MARCH 1987

595

5 HZ

A HIGH-POWER MAGNETICALLY SWITCHED SUPEKCONDUCTLNG RECTTPZRR OPERATING AT

*

G . B . J . Mulder, H.J.G. Krooshoop, A. N i j h u i s , H.H..T. t e n Kate and L.J.M. vandeKlundert. University Twente, of Department Applied of Physics, Low Temperature Laboratory, P.O.B. 217, 7500 AE Enschede, The N e t h e r l a n d s .

*

Supported by F.O.M.,

on Matter.

t h eN e t h e r l a n d sF o u n d a t i o nf o rR e s e a r c h

Abstract Above a c e r t a i n c u r r e n t l e v e l , t h e u s e o f a superconductiw rectifier as a c r y o g e n ci cu r r e ns ot u r c e o f f e r s a d v a n t a g e s compared t o t h e u s e o f a power s u p p l y a t room t e m p e r a t u r ew h i c hr e q u i r e sl a r g ec u r r e n tf e e d In some c a s e s , t h e power oE t h r o u g h si n t ot h ec r y o s t a t . such a r e c t i f i e r is immaterial, €orexampleif i t is to beused as a c u r r e n t s u p p l y f o r s h o r t t e s t sampleswith low inductances.Usually,however, a r e c t i f i e r is intended t oe n e r g i z el a r g es u p e r c o n d u c t i n gm a g n e t s , so t h e maximum power a v a i l a b l e becomesanimportantparameter s i n c e i t d e t e r m i n etsh leo a d i n g time. One method of i n c r e a s i n gt h e power of a r e c t i f i e r is t o raise t h e o p e r a t i nf rge q u e n ctIyhnr.iess p e cmt ,a g n e t i c a l l y c o n t r o l l e ds w i t c h e sw i t hv e r yf a s ts w i t c h i n g times are p r e f e r a b l et ot h e r m a l l yc o n t r o l l e do n e s . T h i sp a p e r e p o r t s on the des.ign,as well as t h e e x p e r i m e n t arl e s u l t s , o E a m a g n e t i c a l l ys w i t c h e df u l l wave s u p e r c o n d u c t i n gr e c t i f i e r . Once t h i s r e c t i f i e r i s brought t o i t s d e s i g nf r e q u e n c yo f 5 Hz, t h ea v e r a g e power d e l i v e r e dt ot h ec r y o g e n i cl o a d w i l l be 500 W.

By r e v e r s i nt ghse q u e n cot ehfceo n t r os il g n a l s , pumping down, i.e. d e c r e a s i n gt h el o a dc u r r e n t is also possible. The magnitude the commutation of step on t h em o ~ n e n t a r ys e c o n d a r yc u r r e n t obviouslydepends whichmust t h e r e f o r eb em e a s u r e di no r d e rt og e n e r a t e t h ec o r r e c tp r i m a r yc u r r e n t .F o r a detalledtheoretical is t r e a t m e n t oE superconducting rectiEiers t h er e a d e r r e f e r r e dt o Kef. 1. Here we c o n f i n eo u r s e l v e st ot h e mean power of a r e c t i f i e r w i t h c o n s t a n t f r e q u e n c y :

primary and s e c o n d a r yt r a n s f o r m e ri n d u c t a n c e . c o u p l i n gc o n s t a n to ft h et r a n s f o r m e r . operatingfrequencyoftherectifier. a c o a p l . e t e dl o a d i n gc y c l e . loadcurrentafter

Introduction

In t hpea s t few y e a r s ,e v e r as lu p e r c o n d u c t i n g r e c t i f i e r s were b u i l t and t e s t e d a t t h eU n i v e r s i t yo f performed very Twente ( r e f e r e n c e s I, 2 and 3 ) . They well and d e m o n s t r a t e tdh astu p e r c o n d u c t i n cgo i lfso r 9 and 25 kA c abenen e r g i z ewdi atehnf f i c i e n c y However, t h eo p e r a t i n gf r e q u e n c y was exceeding 36 l i m i t e tdo below 0.1 Hz d utetohaep p l i c a t i oonf t h e r m a l l yc o n t r o l l e ds w i t c h e sw i t hl a r g ea c t i v a t i o n and 4, r e c o v e r y times. An a l t e r n a t i v e was p r e s e n t e di nR e f . a m a g n e t i c a l l yc o n t r o l l e ds w i t c ht h a t was s u c c e s s f u l l y t e s t e d up t o 25 Rz. S i m i l a rm a g n e t i cs w i t c h e s were used i nt h e 500 W, 1 kA r e c t i f i e rd e s c r i b e dh e r e .F o rt h i s r e c t i f i tehor ep e r a t i nf gr e q u e n c y is s u b s t a n t i a l l y h i g h e rt h a n 0.1 Hz and i t is i nf a c tn o td e t e r m i n e d by t hcer y o g e n ipc a rottfhsey s t e m b um t erely by t h e power of the room-temperature supplies used to drive theprimary of t h et r a n s f o r m e r and t h e c o n t r o l c o i l s o f t h es w i t c h e s .

%.

The p r i n c i p l e of a Pull-wave superconducting r e c t i f i e r wl.thinductivecommutationofthesecondary c u r r e n t i s e x p l a i n e d i? f i g u r e 1. A p r i m a r cyu r r e n t with an amplitude of Ip w i l l generate a secondary w i l l i n c r e a s e step-wise c u r r e n t h r o u g ht h el o a dt h a t t o a maximum Inax Each half period of the primary waveform is made upof f o u r parts: pumping p a r t , where a p r i m a r y c u r r e n t s t e p c a u s e s a n i n c r e a s eo ft h es e c o n d a r yc u r r e n t . d e l a y time, w h e r et h ep r i m a r yc u r r e n t is c o n s t a n t , a l l o w i n go n eo ft h es w i t c h e st ob ec l o s e d . a p r i m a cr yu r r esntte p commutation part, where c a u s etshceu r r e niton nhe a loftfhsee c o n d a r y c i r c u i t t o b et r a n s f e r r e dt ot h eo t h e rh a l f . d e l a y time, a l l o w i nognotehfsew i t c h etbsoe closed.

control electronics

I trans

I

’

I

former

switches

f protection diodes

.

ManuscriptreceivedSeptember

Fig. 1

a )C i r c u i to ft h er e c t i f i e rs y s t e m . b)Transformerprimarycurrent. 1. c )C u r r e n ti nc o n t r o lc o i l d )C u r r e n ti nc o n t r o lc o i l 2. 30, 1986. e) C u r r e n tt h r o u g ht h el o a d . 0018-9464/87/0300-0595$01.0001987 IEEE

f

load coil

596

The s w i t c h e s The magnetic switches consist of a swLtching elementplacedbetweenaninner and o u t e rc o n t r o lc o i l having opposite windings. With t h i s geometry acombinat i o n of a homogeneous f i e l d a t t h e s w i t c h e l e m e n t anda c o n t r o lc o i lw i t h low s e l f - i n d u c t a n c e is o b t a i n e d . The sw€tch elements are made of 24 p a r a l l eslt r a n d s of m u l t i f i l a m e n t a r yw i r ew i t h a second c r i t i c a lf i e l d of 0.8 T (MCA, Nbl%Zr/CuNi, 0.3 mm d i a . , 574 f i l a m e n t s ) . is t o compareswitch 1 and 2 P a r t o fo u ri n v e s t i g a t i o n s which d i f f eirnt h ae r r a n g e m e n t of t h e s e 24 s t r a n d s 3 ) . In t h e f i r s t switch 48 s t r a n d s were ( s e ef i g u r e wound a s a f l a t c a b l e i n s e v e r a l l a y e r s . F o r t h e s e c o n l andthen two c a b l e s switch 24 s t r a n d s w e r e f i r s t c a b l e d were simultaneously wound.The d i r e c t i o n s of t r a n s p o r t 3 show how t h e s t r a n d s were c u r r e n t. i n d i c a t e di nf i g u r e connectedafterwards i n o r d e r t o o b t a i n a non-inductive A non-inductlve arrangement is switch elelnent. n e c e s s a r yi no r d e tr om i n i m i z et h es e l f - f i e l d of t h e switchelement and so avoiding a s e r i o u s d e g r a d a t i o n of t h e maximum c u r r e n t . An obvious advantage of s w i t c h 1 i s t h he i g h e r f i l l i n fga c t oorsfu p e r c o n d u c t o r which r e s u l t s in a l a r g e r e s i s t a n c ei nt h en o r m a sl t a t ew i t h a smaller s w i t c h i n g volume ( s e e t a b l e 1).

The f a c t o r d reaches a maximum of 1 . 6 when t h e c u r r e n t h r o u g ht h e magnet is i n c r e a s e dt o 7 l . 5 % of I,,,at. For a givenprimaryenergyofthetransformerthe b e i n c r e a s e d by improving the mean power can only of t h et r a n s E o r m e ro r by r a i s i n gt h e c o u p l i n gc o n s t a n t frequency. An e x p e r i m e n t a l r e c t i f i e r

I n o r d e rt od e m o n s t r a t et h e feasibility ofhighf r e q u e n c ym a g n e t l c a l l ys w i t c h e dr e c t i f i e r s , w ed e s i g n e d anexperimentalmodel,ofwhichtheconstruction was completed afewmonthsago.Designparametersforthis r e c t i f i e r wereaprimaryandsecondarycurrent of 30 A and 1000 A r e s p e c t i v e l y , an o p e r a t i nfgr e q u e n c y of power of 500 W. The c r y o g e n i c about 5 Hz, anaverage p a r t of t h er e c t i f i e r , mounted i n a 0.27 m diameter 2. Subsequently, some c r y o s t a t , Cs shown ifni g u r e w i l l be d i s c u s s e d . aspectsofthisparticularrectifier

Table 1

Switch parameters. switch 1

switch volume open-stateresistance .

switch 2

[litres] ~

f

o p e n - s t a t ec o n t r o lc u r r e n t DIODES ,SWITCH

1

,SWITCH

2

/SWITCH ELEMENT

Bothswitches and t h e i r c o n t r o l c o i l s a r e p r o v i d e d i n order minimize to temperature wih t he da rt a i n s rises. P a r t i c u l a r l yd u r i n gt h e pumping s t a g e , when t h e c o n t r o l c o i l i s energized and t h e momentary d i s s i p a t i o n i nt h es w i t c he l e m e nct a n be a sh i g ha s 50 W, i t is Lmportant that the control coil remains superconducmust not t i n gF. u r t h e r m o r et, h es w i t c he l e m e n it t s e l f heat up o t h e r w i s e a combinationofthermal and magnetic switching would occur.

-CONTROL FIELD COIL

rTRANSFORMER

1

-......-

2L strands + 2L strands

I

r L O A D COIL

2L strands 24 strands

+

-

2 dummy cores

L

+'--~--------t

-HALL

I I

I

PROBE

LOAD COIL

I

6 strands + 1 dummycore

I

I

I

bFig. 2

270 m m

C r o s s - s e c t i o n a lv i e w of t h e r e c t i f i e r mounted i n a 0.27 m d i a m e t e r c r y o s t a t .

+

2L strands 1 dummy core

Fig. 3

Conductorgeometries in t h es w i t c h e s , l o a d c o i l and transformer.

597 The l o a d c o i l

Table

The l o a d c o i l i s wet-wound w i t h a s i x - s t r a n d c a b l e on a s t a i n l e s s steel c o i lf o r m e r using.STYCAST 2850 FT r e s i n . A maximum c o i cl u r r e n o tf 1200 A is expected frompreviouscrtticalcurrentmeasurements on a s i n g l e s t r a n d (MCA, NbTi/Cu, 0.686 mm d i a . , 2070 f i l a m e n t s ) . A t t h ed e s i g nc u r r e n t of 1 kA t h es o l e n o i dg e n e r a t e s a f i e l d of4.5 t e s l a c o r r e s p o n d i n gt o a storedenergyof 6.75 k J . In t h e 5 cm bore and over a lengthof 6 cm we o b t a i n a test-volume o l 1 X homogeneity. The p o s i t i o n s of t h ree c t i f i e r components a r e oftheload chosen in such a way t h a t t h e s t r a y f i e l d c o i l a t t hsew i t c h e s i s less t h a n 20 mT so t h a t is notnecessary., m a g n e t i cs h i e l d i n go ft h es w i t c h e s

two s w i t c h e l e m e n t s two c o n t r o l f i e l d c o i l s transformer primary windings

The t r a n s f o r m e r The t r a n s f o r m e r i s o ft h et o r o i d a la i r - c o r et y p e . T h it so r o i d as lh a paev o i d s a s t r a fyi e l d from t h e t r a n s f o r m e ra n dm a g n e t i cc o u p l i n gw i t ha l lo t h e rc o i l s intherectifier. A s a consequence, a compact construct i o n was p o s s i b l e . The t r a n s f o r m ecro u l d be mounted its f i e l d c l o stetohleo acdo iwl i t h o ustp o i l i n g h o m o g e n e i tolyer a d i ntegox c e s s i vme u t u af ol r c e s . Furthermore,theoperationoftheswitcheswhich are n o ts h i e l d e df o re x t e r n a lf i e l d s i s n o it n f l u e n c e db y t h ep r e s e n c eo ft h et r a n s f o r m e r . S e v edr ai sl a d v a n t at ghoerf so i ds ha al p e compared t o a s o l e n o i d were t a k e nf o rg r a n t e d :a )t h e f a b r i c a t t o n is more d i f f i c u l t ; b) i t t a k e s 2 t o 3 ttmes more superconducting wire t o o b t a i n a comparableselfi n d u c t a n ctcehe);x e tra amount of superconductor i m p lliaersgale.ocrs.csdoe )so;l i n g is bad, The l a t t e r e s p e c i a l l y a t t h ei n n e rs i d eo ft h et o r o i d . two problems were partly overcome by realizing the H w i t h a r e l a t i v e l yl a r g e primary inductance of 0.2 t r a n s f o r m evr o l u maen fde w w i n d i n g sr,e s u l t i n ign a l a r gceo o l i nsgu r f a c e and moderate magnetic field. show C a l c u l a t i o n sf o rt h i s 0.25 m diametertransformer a temperature rise l e s s t h a n 1 K i f t h e d i s s i p a t e d h e a t l o s s e s a t 5 Hz is c o n d u c t e dt ot h eh e l i u m d u et o a.c. b at h r o u gt h e STYCAST i m p r e g n aannt hcdo e il windingswithoutusingheat-drains. The secondary windings are located between two sectiono s pf r i m a r yw i n d i n g isno r d e trog e t good a c o u p l i n gc o n s t a n t ( >97.5 X ) . Theprimary and secondary H r e s p e c t i v e l y . In both i n d u c t a n c e sa r e 0.2 H and280 p same conductor was t r a n s f o r m e ar n dc o n t r ocl o i l st h e a p p l i e d ( MCA, NbTi/CuNi,0.3 mm d i a . , 575 f i l a m e n t s ) .

2

I = 30 A.

H y s t e r e s i sl o s sc o n t r t b u t i o nf o r

1.9 0.9 0.35 0.35

J/period J/pertod J/period J/period transformer sec

The s e c o n cd o n t r t b u t i o n i s ohmic d i s s i p a t i o ni n t hsew i t cehl e m e n tdsu r i ntgh e pumping s t a g eT. h i s c o n t r f b u t i o ni n c r e a s e sl i n e a r l yw i t ht h ef r e q u e n c yo f it dependsonthefract h er e c t i f i e r andfurthermore i s a c t u a l l yu s e df o r t i o n of time o fe a c hp e r i o dt h a t is a l s o t h e pumping s t e pT. h e r e f o r et,h e f f t c t e n c y c l o s e l yr e l a t e dt ot h e mean power of t h er e c t i f i e r . Suppose f o r example t h a t h e magnet is loaded up to 1000 A w i t h a constantfrequencyof 5 Hz and t h a t 35 X of each period i s u s e fdo r pumping. Then, t h e mean power exceeds500 W w h i l et h e ohmic l o s s is 4 % o ft h e magnet energy. Some a d d i t i o n al o l s s essu c h as eddy c u r r e n td i s s i p a t i o ni ns u p e r c o n d u c t o r so rc o n s t r u c t i o n m a t e r i a l s are n e g l i g i bslm y all. So, depending on frequency and p r i m a r y s i g n a l , t h e o v e r a l l e f f i c i e n c y o f therectiftervaries from93.5 t o 97.5 X C o n t r o le l e c t r o n i c s A correo c tp e r a t i ootnhfr e c t i f i ec rabne In o b t a i n e dw i t hv a r i o u ss h a p e so ft h ep r i m a r yc u r r e n t . t h ep r e l i m i n a r y tests u n t i l now a p r i m a r ys i g n a l was used with constant time i n t e r v a lfsot rh e pumping s t a g e ,t h ec o m m u t a t i o ns t a g e and t h ea c t i v a t i o n / r e c o v e r yo ft h es w t t c h e s . A t t h i s moment a r e c t i f i e r c o n t r o l u n i t is u n d e rc o n s t r u c t i o nw h i c hg e n e r a t e s a constant p r i m a r yv o l t a g e and t h e r e f o r e a c o n s t a n tc u r r e n tr a t e during.thecommutationand pumping s t a g e . Thi.6 c h o i c e a l l o w sf o r a combination of h i g he f f i c i e n c ya n dh i g h average power. It a l s o means t h aignte n e r at hl e frequency is n oct o n s t a nbt e c a u s teh e time i n t e r v a l s needed f o r pumping andconmutatLondepend on t h e a c t u a l loadcurrent. is t h e power supplyneeded t o d r i v e Anotherpoint t hc eo n t r co ol i l s and t h t rea n s f o r m eSri.n tchee c o n t r oc lo i al srnee v eorp esni m u l t a n e o u s l y i t is p o s s i butlosee a s i n g l e power s u p pflboyor t h s w i t c h e s . T h i s i s a 40 A, 250 V a m p l i f i e r w i t h u n i p o l a r c u r r e n t and b i p o l avro l t a gees p e c i a l lsyu i t efdo r A similar a m p l i f i w e ri bt hi p o l a r i n d u c t i vl oea d s . c u r r e n t , w h i c h is now beingdeveloped, w i l l beusedfor t h ep r i m a r y of t h et r a n s f o r m e r . Completionoftheabove-mentionedelectronics will enable a rectifier frequency of 5 Hz. Protection

Efficiency The o v e r ae lf lf i c i e ntroh cefyec t i f t e r d e f i n e d as

is

WL

WL

+

WL,

,

where WL is t h e e n e r g y d e l t v e r e d t o t h e l o a d magnetand W L i ~s t~h e e~ n e r g y d i s s i p a t e d in thecryogenicenvironment w i t he x c e p t i o no ft h ed i s s i p a t i o n in t h e magnet is less t h a n 0.2 % i t s e l (f i no u cr a s et h el a t t e r anyway). The f r a c t i o n W L O ~ S t h a t is d i s s i p a t e d in t h e c r y o s t a ct a n b ed i v i d e di n t o two main c o n t r i b u t i o n s . loss in a l l superconF i r stth fei l a m e nht y s t e r e s i s is d u c t i n gp a r t so ft h er e c t i f i e r T . h i sc o n t r i b u t i o n i n d e p e n d a notfhree c t i f i efrr e q u e n c y when h e a t i n g effectsinthesuperconductorandtheirinfluence on Jc a r e d t s r e g a r d e d . Based on t h e r e s u l t s o fp r e v i o u ss h o r t s a m pml e a s u r e m e nththsye,s t e r etlsh oiines s With a primary r e c t i f t e r was e s t i m a t e(dt a b l2e) . A i t t a k e s5 3p e r i o d st oo b t a i n 1000 A amplitudeof30 i nt h e magnet. In t h a t case t h eh y s t e r e s i ls o s s is about2.5 X oftheenergyinthemagnet.

The conductors must b ep r o t e c t e da g a i n s t damage duetoexcessiveheatingafter a quenchanywhere i n the superconductingsystem. F obr o t cho n t r oclo i l s and t hper i m a royt fh e i s achieved by means of t r a n s f o r m etrh i ps r o t e c t i o n q u e n c hd e t e c t o r sw h i c hm e a s u r et h er e s i s t i v ec o m p o n e n t i t exceeds a c e r t a t n l e v e l , a o ft h es u p p l yv o l t a g e .I f quench is assumed and t hceu r r e notth fceo i l in q u e s t i o n is s w i t c h e do f f as q u i c k l y as p o s s i b l e . T h es e c o n d a r yc i r c u i t is p r o t e c t e da g a i n s t a dump of t h el o a dc o i le n e r g y bymeansofdiodesconnected X of a c r o s tsh leo a d c o itle r m i n a l s . ’ More t h a 9n 8 t h iesn e r g y is d i s s i p a t e d in t hdei o d easb o vteh e occurs in t h se e c o n d a r y helium level when a quench c i r c u iw t h i l et h e magnet remains superconducting. On the o t h e r h a n d , . i f t h e magnetquenches, alltheenergy is d i s s i p a t e d in t h e magnet i t s e l f . In t h e l a t t e r case a c o i lc u r r e n t of 1 kA will d e c a yw i t h i n 1 s corresponding t o a maximum temperature rise less than100 K. Both t y p e so f a secondaryquenchwereforcedseveral times a t a magnet c u r r e notf 1 kA and were found completelysafetotherectifier.

598

E x p e r i m e n t a lR e s u l t s Both switches were t e s t e d i n st he ec o n d a r y a s h o r t - c i r c u i t as a load. By r e c t i f i e rc i r c u i tw i t h 2 canbe openingswitch 1 t h e maximam c u r r e n t o f s w i t c h measured and vice versa. These experiments are summarized E i ni g u r e 4 f o r a s i n u s o i dsael c o n d a r y o€ the c u r r e n t . I t s h o u l db em e n t i o n e dt h a tt h ec o o l i n g t r a n s f o r m e r is bad compared ttohceo o l i n g oE t h e S O above a c e r t a i nf r e q u e n c y we expect s w i t c he l e m e n t s t h e maximum s e c o n d a rcyu r r e ntbtolei m i t ebdtyh e t r a n s f o r m e r .F i g u r e 4 and previousmeasurements on t h e a t 5 Hz. t r a n s f o r m e ri n d i c a t et h a tt h i so c c u r s Depending on t h es u p p l yv o l t a g ef o tr h ec o n t r o l we measured switch-on and switch-off times coils, between 25 and 100 m s . T h ec o m p l e t e dr e c t i f i e r was t e s t e d f o r s e v e r a l val u e s of p r i m a r ya m p l i t u d e ,l o a dc u r r e n t and frequency. T h er e c o r d e dl o a dc u r r e n t sf o r pumping up and down f i t t h et h e o r e t i c a lc u r v e sw i t h i n 2 F i g u r e 5 f o r example shows t h e p r i m a r y c u r r e n t and l o a d c u r r e n t f o r a run a t 1.1 Hz and 60 W mean power. A t 700 A a commutation e r r oorc c u rcsa u s i n g a secundary quench. Within 3 seconds98 % o ft h el o a de n e r g y i s d i s s i p a t e di nt h e p r o t e c t i odni o d e sF.or re a s o nms e n t i o n ebde f o rteh e maximum €requency i s l i m i t e d t o 2 Hz f o r t h e p r e s e n t . D u r i nt hge tests, two t y p eofsaf i l u r e s were e n c o u n t e r e d .F i r s t , some q u e n c h e so ft h ec o n t r o lc o i l s a low helium level. A s e c o n df a i l u r e o c c u r e d u teo o c c u r si ft h em a g n i t u d e of t h e commutation s t e p i s n o t c o r r e c t .T h e n ,t h er e m a i n i n gc u r r e n ti no n eh a l fo ft h e s e c o n d a r y c i r c u i t w i l l becommutated r e s i s t i v e l y t o t h e o t h e rh a l fa f t e rt h es w i t c h i s opened. The c u r r e n t r a t e i n v o l v e dw i t h a t o ol a r g ec o m m u t a t i o ne r r o rc a u s e s a quench i n t h e s e c o n d a r y c i r c u i t .

%.

Fig. 4

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5

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10 3 0 5 0 FREQUENCY [Hz I

Maximum s i n u s o i d acl u r r e nitnb o t h a l v e s t hsee c o n d a rcyi r c u i t as a f u n c t i o n frequency.

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Conclusions P r e l i m i n a r y tests otfh e new m a g n e t i c a l l y conwere successEu1. t r o l lseudp e r c o n d u c t i rnegc t i F i e r AEter s e p a r a t e tests of t h ec o m p o n e n t s t, h er e c t i f i e r was assembled and used to charge a I. 'k4, 6.75 kJ magnet. The r e c t i f i e r r e l i a b l y e n e r g i z e s t h i s magnet up t o 1 kA and is f u r t h e r m o r€eu l lpyr o t e c t eadg a i n s t i nt h es y s t e m . The next damage a f t e r a quenchanywhere s t e pi no u re x p e r i m e n t sc o n c e r n sr a i s i n gt h e mean power of t h er e c t i € i e rt o 500 W by i n c r e a s i n gt h eo p e r a t i n g frequencyfrom 2 Hz t o 5 Hz.

I 0

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References 1. H.H.J. t e n Kate, "Superconducting R e c t i f i e r s " , ThesisIJniversityofTwente, The Netherlands,1984. 2. H.H..J t e n Kate, P.B. Bunk, H.A. S t e f f e n s , and L.J.M. Klundert, van de "A t h e r m asl w l yi t c h e d 9 kA s u p e r c o n d u c t i n gr e c t i f i e rf l u x p u m p " , IEEE Trans. on Magn., Vol MAG-17, 2067-2070, 1981. 3 . H.H.J t e n Kate, J.A. Knoben, H.A. S t e f f e n s , and L.J.M. vandeKlundert, "A 25 kA, 0.5 kW t h e r m a l l y switched sc. r e c t i f i eP r "r ,o c . MT-8 Grenoble, J o u r n a ld eP h y s i q u e ,C o l l o q u e C1 supplementau no. 1 tome 45, C1-659-662, 1984.

4. 6.13.5. Mulder, H.H.J. t e n Kate, A. N i j h u i as ,n d L.3.M K lduvenadne r t , "A o f apsetr a t i n g 1 kA", IEEE m a g n e t i c a lcl yo n t r o l l es w d i t cf oh r Trans. on Magn., Vol MAG-21, 686-689,1984.

Fig. 5

TIME [ S I

P r i m a r yc u r r e n t and l o a dc u r r e n tr e c o r d e di n a r u n at 1.1 Hz and a mean power of 60 W.

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