Design and testing of a 5GHz TE10–TE30 mode converter mock-up for the lower hybrid antenna proposed for ITER

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Design and testing of a 5GHz TE10–TE30 mode

converter mock-up for the lower hybrid antenna

proposed for ITER

Julien Hillairet, Joëlle Achard, Stéphane Rasio, Bernard Soler, Cyril Brun

To cite this version:

Julien Hillairet, Joëlle Achard, Stéphane Rasio, Bernard Soler, Cyril Brun. Design and testing of a

5GHz TE10–TE30 mode converter mock-up for the lower hybrid antenna proposed for ITER. Fusion

Engineering and Design, Elsevier, 2012, 87 (3), pp.275-280. �10.1016/j.fusengdes.2012.01.007�.

�cea-02459704�

Design and Testing of a 5 GHz

TE

10

− TE

30

Mode Converter Mo k-Up for the Lower Hybrid Antenna

proposed for ITER

J.Hillairet a,

∗

,J.A hard a ,C.Brun a ,S.Rasio a ,B.Soler a a

CEA,IRFM,F-13108SaintPaul-lez-Duran e, Fran e

Abstra t

The design and overall dimensions of a 5GHz

TE

10

− TE

30

mode onverter are presented. This mode onverter is a RF element of a 20MWCW Lower HybridsystemproposedforITER.Alowpowermo k-upofthisdevi ehasbeen manufa turedat CEA/IRFM and measuredat lowpower. RFmeasurements indi ateareturnlossof40dBandatransmissionlossof4.78dB

±0.03

dBforthe threeoutputs. Theforward onversione ien yfromTE

10

mode to

TE

30

has beenmeasuredfromele tri eldprobingto99.9%. ThegoodRFperforman es obtainedvalidatetheRFdesignofthis element.

Keywords: LowerHybrid,CurrentDrive,LHCD,ITER,ModeConverter

1. Introdu tion

FollowingtheITERS ien eandTe hni alAdivisoryCommittee re ommen-dation, the on eptual design, the R&D program, the pro urement and the installation of a Lower Hybrid Current Drive (LHCD) system on ITER had been reviewed[1℄. A revised5GHzLHCD systemable to deliver20MW/CW onITER andtosustaintheexpe tedhighheatuxes omingfrom theplasma radiation,parti lesuxesandRFlosseshasbeenreported[2, 3℄. Inthisframe, aR&D eortis urrentlyon-goingat CEA/IRFMonthe dierentelementsof theantenna. Inthispaper,wereporttheworkmade on erningthedesignand lowpowerRFtestofa

TE

10

− TE

30

mode onverter.

2. GeneralDes riptionof the ITERLH Antenna

TheLHlaun herdesignpresentlyforeseenforITERisaPassive-A tive Mul-tijun tion (PAM), whi h has been su essfully validated on FTU[4℄at 8 GHz andon Tore Supra[5, 6℄at 3.7GHz. Thenominal refra tiveparallel index

n

k

∗

Correspondingauthor

tionsofpropagationandabsorptionofLHwavesinITERforseverals enarios. Thesesimulationsshowedthatanoptimum



n

k





,denedasatrade-obetween maximizing the urrent drive e ien y and minimizing the powerdeposition in the H-mode pedestal, is foundto be



n

k





= [1.9 − 2.0]

with aexibility of

[1.8 − 2.2]

[7,8℄.

In the present design, the laun her is made of 48 identi al modules, ea h one independently fed by one klystron: twelve in the toroidal dire tion and four in the poloidal dire tion. A module onsists of four a tive waveguides in the toroidal dire tion and six lines of waveguides in the poloidal dire tion (Figure1)[2℄. TheRFpoweris arriedthroughatransmissionlineuptoaRF windowlo atedinsidetheframeand onne tedtoapoloidal3dBsplitterwhi h feedstwo

TE

10

−TE

30

mode onverters. Ea hofthesemode onverters onverts the in ident power from the re tangular

TE

10

mode to the re tangular

TE

30

modeinordertofurtherdividethepowerintothreepoloidalrows, orresponding totheinputofa4-a tivewaveguidesPassiveA tiveMultijun tion. Inthenext se tion,wedes ribethedesign,themo k-upmanufa turingandthelowpower RFtestsofthemode onverter.

Figure1: Left: GeneralCADviewofthe ITERLH antenna. Right: RFCAD modelingof amodule. Theantennaismadeof48identi almodules. TheRFpoweris omingfroma 500kWRF sour e lo ated at the rightof the gure, through the RF window, the hybrid jun tion,the twomode onvertersand the six PAM multijun tions. Allthe elementsof a modulelo atedbehindtheRFwindowareunderthema hineva uum.

3.

TE

10

− TE

30

mode onverter 3.1. Introdu tion

Theaimofthe

TE

10

-

TE

30

mode onverteristo onvertaninput fundamen-talTransverseEle tri (TE)re tangularmode(

TE

10

)intoanotherre tangular mode(the

TE

30

mode). This onversionisthenusedinordertoequallysplitthe powerinthreeinanadaptedsplitter. Su hsplittings heme,usedinbothTore SupraLHantennas sin e1999,isa hievedbyaperturbation ofthewaveguide geometryleadingtomode oupling. Fundamentalinput

TE

10

mode anthusbe

almosttotally onvertedtothe

TE

30

mode,whi hideallydistributesthepower into three in the H-plane[9, 10℄. This wall perturbation mode onversion has beenusedoriginallyonECRH,wherehigh-ordermodesarenotsuitableforlong distan etransmissionandplasmaheating[11, 12℄. Propagationofguided eld into small perturbated wall waveguides an be modeled with the Generalized Telegraphist's equation [1317℄. The resulting set of equations an be solved numeri ally [18, 19℄ oralmost analyti ally with some approximations[15, 16℄. Werefertotheprevious itationsformoredetailsonthe al ulationprin iples. 3.2. RFmodeling

3.2.1. Mode Converter

Inthisse tionwedes ribethemode onverterdesign. Thes hemati draw-ing of a H-plane sinusoidal perturbated wall mode onverter is illustrated in Figure 2. In Figure 2, the input TE

10

mode is oming from the left. Input width

a

0

must be set su iently large in order to permit the

TE

30

mode to propagate, leading to

a

0

>

90

mm at 5GHz. Output width

a

1

is set to in-suretheTE

50

mode to ut-o,i.e.

a

1

6

150

mm. The TE

40

modesis ableto propagateattheoutputwidth

a

1

,however,anin identoddTEmodeuponan H-planedis ontinuitywithalongitudinalsymmetryonlyex itesoddTEmodes inre tangularwaveguides. Asimilar on lusionis obtainedin the aseofeven modes(seeRef.[20℄foranelegantderivationofthisproperty).

Figure2:S hemati drawing(H-plane ut)ofthe

TE

10

− TE

30

mode onverter. The mode evolution along the mode onverter is obtained by solving the generalized telegraphist's equations for a 3.5 periods deformed waveguide of wavelength

λ

w

dened bythefollowingsinusoidalperturbation[9,17,21℄:

a(z) = a

0

+ ε



1 − cos

 2π

λ

w

z



(1) In order to rea h the best mode onversionto

TE

30

mode, a numeri al opti-mizationof parameters

a

0

,

ε

and

λ

w

hasbeenmadein Matlabusinga Gener-alizedTelegraphist'sequationsolver. Thisrstoptimization ledto dimensions

a

0

= 98

mm,

ε = 22.4

mmand

k

w

= 2π/λ

w

= 36.3059

m

−1

.

FurtheroptimizationoftheseparametershavebeenmadeintoHFSS,taking into a ount RF ondu tion losses on walls. In order to avoid spurious mode generationornumeri alinstabilities,someextrastraightre tangularwaveguides

optimizationledtothedimensionsreportedinTable1,whi harevery loseto theones foundinMatlab.

Figure3: HFSSRFmodelofthemode onverter(withouttaperattheinput).

a

0

[mm℄ 98

ε

[mm℄ 22.37

λ

w

=

_k

2π

w

[mm℄ 173.06

k

w

[m

−1

℄ 36.3059

L (= 3.5λ

w

)

[mm℄ 605.71

a

1

= a

0

+ 2ε

[mm℄ 142.8

Table1: Dimensionsofa5GHz3.5periodsTE

10

-TE

30

mode onverter.

Thereturnlossforthefundamentalmodeis20.5dB.Propagationlossesfor opperplatingwalls, al ulatedas

1 −

P

j

|S

j1

|

2

,is 1.36%. Thebandwidthof thedevi e,denedastherangeoffrequen iesforwhi hatleast95%ofthe

TE

10

modeis onvertedto

TE

30

,is120MHz. Ele tri eldintothemode onverteris illustratedinFigure4. Thelengthofthedevi eis,withouttakingintoa ount inputandoutput straightre tangularwaveguides,606mm.

Figure4:Normoftheele tri eld[V/m℄inthemode onverter(

P

in

=1W). In order to feed the mode onverter with a standard WR-229 waveguide (

a = 58.17

mm,

b = 29.08

mm),an inputtaper hasto be addedto mat h the WR-229width. This taperisdes ribedin thenextse tion.

In order to make the transition between the input width

a

0

of the mode onverter and the onventional waveguideWR-229 width whi h is 58mm, an extra taper must be added at the input (instead of the straight re tangular waveguideused before). A osine taperhas been modeled with the following widthequation:

a(z) =

1

2

a

WR229

+ a

0

+

1

2

a

WR229

+ a

0

cos



_z

L

taper

− 1



π



(2) where

L

taper

is the total length of the taper,

a

WR229

is the WR-229 width (58.17mm) and

a

0

the input width of the mode onverter. The model has beendrawnin HFSS and thenoptimizedin order to minimizereturnlossand mode onversionto TE

30

whi h is unwanted at this stage. With a length of

L

taper

= 120

mm,S-parametersarereportedintheTable2below.

|S

11

|

withTE

10

input -59.7dB(0.00104)

|S

21

|

withTE

10

inputandTE

10

output -0.014dB(0.99839)

|S

21

|

withTE

10

inputandTE

20

output -80dB(0.00010)

|S

21

|

withTE

10

inputandTE

30

output -25.7dB(0.05208)

Table2:CosineTapers atteringparametersat5GHz.

Figure5:HFSSRFmodelofthe5GHz osinetaper,ele tri eldnorm[V/m℄for

P

in

= 1

W.

3.4. Mode onverter andinputtaper

The ombinationofthetaperandthemode onverterhasbeenmodeledin HFSS.Themodel onsistsinthetaperandthemode onverterplustwostraight

parametersarereportedinTable3. Letusdenetheforwardmode onversion e ien y as the ratio between the power arried by the

TE

30

over the total forwardpower. A ordingtotheS-parameters,thetheoreti almode onversion e ien yis loseof99.5%.

The ele tri eld inside the devi e is illustrated in Figure 6. Considering aninputnominalpowerof250kW,themaximumele tri eldrea hedintothe mode onverteris6.9kV/ m(Figure6). Thebandwidth,aspreviouslydened, is120MHz(Figure7). Thetotallengthofthedevi e is

725.7

mm.

Parameter 5GHzHFSS 3.7GHz(ToreSupra C4)

S

11

TE

10

-20.5dB -17.7dB

S

21

TE

10

-23dB -33.6dB

S

21

TE

20

-69.1dB -81.9dB

S

21

TE

30

-0.064dB -0.0935dB Length[mm℄ 725.7 944 Max. E-eld[kV/ m℄ 6.9 5.8

Table3: S atteringparametersoftheCosineTaper+ModeConverter(HFSS).For ompar-isonpurpose,ToreSupramode onverterresults arereported. Maximumele tri eld has been al ulatedfor

P

in

= 250

kW.

Figure6:Illustrationofthenormoftheele tri eld[V/m℄intothemode onverterwithits inputtaper(

P

in

= 250

kW).

3.5. Poloidal splitter

Thepoloidalsplitteraimstosplitthepower omingfromthemode onverter, whi hismainlyapureTE

30

mode,intothreedistin twaveguidese tions. This poloidal divider is illustratedin Figure 8. It hasbeenoptimized rstalone in orderto maximizethe powersplitting when ex itingwith aTE

30

mode, then withthemode onverterfastened.

The al ulateds atteringparametersofthisdevi earereportedin Table4. 3.6. Complete assembly

The ompleteassembly,i.e. themode onverterasso iatedtoitsinputtaper andthepoloidalsplitterhasbeenmodeledinHFSS.The al ulatedreturnloss

Figure7:S-parameter onversione ien y,denedas

|S

21

|

2

forTE

30

modeoveroverthetotal forwardpowervsfrequen y. Thetypi aloperationalbandwidthofaklystronis

±10

MHz,as illustratedonthegure.

Parameter 5GHzHFSS

S

11

forTE

30

-46.3dB

S

i1

forTE

30

-4.81dB

Table4: S atteringparametersofthepoloidalsplitter.

is45.5dB.Thetransmissionlosses are4.79dB

±

0.01dBfor thethreeports, loseto theidealvalue 4.77dBwhi h orrespondingto thethird of theinput power. The s atteringparametersevolutionona4.9-5.1GHzfrequen y band arereportedinthenalse tion.

4. Mo k-up manufa turing

A lowpowermo k-upofthis 5GHzmode onverter(withits inputtaper) hasbeenmanufa turedatCEA/IRFMusinga2-axisnumeri aldrillingma hine in Aluminum dire tly from the CATIA CAD model. The poloidal splitter as well as some other waveguide elements used only for measurement purposes, su h as the pull-over des ribedin thenextse tion, have been manufa tured bytheSUMIX ompanyfrom theCAD models. Thetopofea helementshas beens rewtightinorderto insureaverygood RF onta t(Figure9).

5. Low Power RF Measurements

Thetestbedusedtomeasurethemode onverter onsistedin theassembly ofthemode onverter,thepoloidalsplitterandthepull-over.Theaimofthis lastelementisonlytofa ilitatethemeasurementsbyputtingtheoutputports aside. SomeWR187-WR229tapershavealsobeenaddedatea hportsinorder to mat h the ommer ially available WR187 waveguide-to- oaxialtransitions (Figure10).

RF measurements indi ate a return lossof 40 dB and a transmission loss of 4.78 dB

±

0.03 dB for the three ports, very lose from the ideal 4.77 dB orrespondingto thethirdoftheinputpower. Amplitudemeasurementresults are illustrated in Figures 11 and 12; results are in good agreement with the numeri almodelingofthe ompleteassembly. Thephaseshiftbetweentheside portsandthe enter portis

180 deg ±5 deg

asexpe tedbythemodeling.

In order to measure the forward onversione ien y from

TE

10

mode to

TE

30

, a dedi ated waveguide element has been manufa tured. This element allowsdire tele tri eldprobingofthemode onverteroutputse tion,inwhi h analmostpure

TE

30

modeisexpe ted. ThiselementisillustratedinFigures9 and 10. It onsists in a straight waveguideof se tion

142.8 mm × 29.08mm ×

308 mm

, whi hmat hestheoutput se tionofthemode onverter. Three rows ofholes(2mmdiameter,10mmspa ed)aredrilledintheH-plane. Theseholes aremadetosupportandblo ktheprobeinordertomakethemeasurementsat theexa tlysamespatiallo ationswhileholdingtheprobeinaverti alposition. Theprobeisinsertedinsideea hholesequentiallyandmeasurestheamplitude andthephaseoftheele tri eldinsidethewaveguide. The ouplingelementis athin 1mmdiameter opperwire. Theprobegroundpotentialis onne tedto thewaveguide. The ouplinghasbeenmeasuredto be-49dB.On etheprobe is inserted into a probe hole, the oupling element supersedes the waveguide wallbylessthan1mm. Itissupposed fromthesmalldimensionsoftheprobe thattheperturbationsdue toitsusearenegligible.

mode onverter,pull-overandE-eldprobewaveguidesupport.

onverter anthenbededu edwiththefollowingmethod. Werstassumethat theele tromagneti eldinside thewaveguideisalinear ombinationof

TE

m0

modes,ie. atapoint

(x, z)

ofthewaveguide:

E

th

(x, z) =

N

modes

X

m=1

A

m

e

+

m

(x, z) + B

m

e

−

m

(x, z)

(3)

where

x

isthelargestsidedire tionand

z

isthepropagationdire tion. The

e

±

m

termsarethe analyti modaleigunfun tions des ribingforwardandba kward

TE

m0

modesshapes.Analyti alexpressionsofthemodalfun tions anbefound in[22℄forexample.

A

m

and

B

m

aretheasso iatedforwardandba kwardweight oe ients. Theaim ofthemethod is to ndthe

2 × N

modes

best oe ients

A

m

,

B

m

inordertominimizethesquaresoftheerror

χ

2

betweenthetheoreti al eldandthemeasuredeld:

χ

2

(A

m

, B

m

) =

N

mes

X

i=1

(E

mes,i

− E

th,i

)

2

(4)

Solving this problem leads to the forward

A

m

and ba kward

B

m

oe ients, and thus to the forward mode onversion e ien y whi h an be dened as theratiobetweenthepower arriedbythe

TE

30

overthepower arriedbyall forwardmodes,i.e:

η =

|A

3

|

2

P

n

|A

n

|

supportareonlyusedforlow-powerRFmeasurements.

This e ien y hasbeen al ulatedfrom theele tri eld probing after the mode onvertertobe

η =

99.9%.

6. Con lusion

In the frame of the Lower Hybrid R&D for ITER a tivities ondu ted at CEA/IRFM,alowpowermo k-upofa

TE

10

− TE

30

mode onverterat5GHz hasbeendesigned,manufa turedandsu essfullyvalidatedwithlowpowerRF measurements. Further work will on entrate on thermo-me hani al aspe ts relevanttoITERoperational onditions(highpoweraspe ts, ooling,pumping) ofthis mode onverter. Other RFelements ofthe ITER LHantenna, su h as theRFwindows,areaswellunder studyandmanufa turingforquali ation. A knowledgments. Theauthorsa knowledgethesupportoftheSUMIX ompany.Thiswork, supported by the European Communitiesunder the ontra t of Asso iation between EU-RATOMandCEA,was arriedoutwithintheframeworkoftheEuropeanFusion Develop-mentAgreement. Theviewsandopinionsexpressedhereindonotne essarilyree tthoseof theEuropeanCommission.

[1℄ G.T.Hoang, etal.,ALowerHybridCurrent DriveSystemforITER,Nu lear Fusion 49(7),doi:10.1088/0029-5515/49/7/075001 .

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[3℄ L.Marsi, et al., Thermal and me hani al analysisof ITER-relevant LHCD antenna elements,FusionEngineeringandDesignInPress,Corre tedProof,ISSN0920-3796.

-60

-55

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

4,9

4,92

4,94

4,96

4,98

5

5,02

5,04

5,06

5,08

5,1

|S1

1

|

[d

B

]

Frequency [GHz]

|S11| VS frequency

Experimental S11 - #2

Experimental S11 - #3

Experimental S11 - #4

HFSS - S11

Figure11: Reexions atteringparameter

|S

11

|

ofthemode onverteroverthe 4.9-5.1GHz frequen yband. HFSSmodelingresultsareillustratedfor omparison. Threemeasurements havebeenmade, orresponding tothenetworkanalyzerse ondportlo ationonport2,3or 4.

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-8

-7

-6

-5

-4

-3

-2

-1

0

4,9

4,92

4,94

4,96

4,98

5

5,02

5,04

5,06

5,08

5,1

|S

i1

|

[d

B

]

Frequency [GHz]

|Si1| VS frequency

HFSS S21

Experimental S21

HFSS S31

Experimental S31

HFSS S41

Experimental S41

Figure 12: Transmission s attering parameters

|S

i1

|

of the mode onverter over the 4.9-5.1GHzfrequen yband(

i

∈ {2, 3, 4}

). HFSSmodelingresultsareillustratedfor omparison.

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