ScienceDirect. Suppression of higher order modes in an array of cavities using waveguides

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1 Aville online t SieneDiret Physis Proedi 74 (2015 ) Conferene of Fundmentl Reserh nd Prtile Physis, Ferury 2015, Mosow, Russin Federtion Suppression of higher order modes in n rry of vities using wveguides Y.V. Shshkov,*, N.P. Soenin, D.S. Bzyl, V.I. Kminskiy, A.A. Mitrofnov, M.M. Zoov Ntionl Reserh Nuler University MEPhI (Mosow Engineering Physis Institute), Kshirskoye shosse 31, Mosow,115409, Russi LNF-INFN, Vi Enrio Fermi, 40, Frsti (Rome), Itly Astrt An pplition of dditionl hrmoni vities operting t multiplies of the min RF system frequeny of 400 MHz is urrently under disussionin the frmework of the High Luminosity LHC upgrde progrm [1,2]. A struture onsisting of two 800 MHz single ell superonduting vities with grooved em pipes oupled y drift tues hs een suggested for implementtion. However, it is desirle to inrese the numer of single ells instlled in one ryomodule in order to derese the numer of trnsitions etween wrm nd old prts of the ollider vuum hmer. Unfortuntely, it n led to the pperne of higher order modes (HOM) trpped etween the vities. In order to solve this prolem the methods of HOM dmping with retngulr wveguides onneted to the drift tues were investigted nd ompred. We desrie the results otined for rrys of 2, 4 nd 8 vitiesin this pper The Authors. Pulished y Elsevier y Elsevier B.V. This B.V. is n open ess rtile under the CC BY-NC-ND liense ( Peer-review under responsiility ofthe Ntionl Reserh Nuler University MEPhI (Mosow Engineering Physis Institute). Peer-review under responsiility of the Ntionl Reserh Nuler University MEPhI (Mosow Engineering Physis Institute) Keywords:higher order modes; hrmoni vities; lrge hdron ollider; superonduting vities * Corresponding uthor. Tel.: E-mil ddress:shshkovyv@mil.ru The Authors. Pulished y Elsevier B.V. This is n open ess rtile under the CC BY-NC-ND liense ( Peer-review under responsiility of the Ntionl Reserh Nuler University MEPhI (Mosow Engineering Physis Institute) doi: /j.phpro

Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 117 1. Single ell vities 1.1. Struture An initil design of the hrmoni vity ws otined y sling (reduing) the size of the LHC elerting vity operting t 400 MHz y ftor of 2 (Fig.

Unfortuntely, these ouplers hve some drwks, inluding violtion of the ylindril symmetry of the eletromgneti field in the struture, whih gives rise to the trnsverse omponent of the eletri field

2 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) Single ell vities 1.1. Struture An initil design of the hrmoni vity ws otined y sling (reduing) the size of the LHC elerting vity operting t 400 MHz y ftor of 2 (Fig. 1) [3]. HOM dmping is rried out with four ouplers: two dipole nd two rodnd ouplers. Unfortuntely, these ouplers hve some drwks, inluding violtion of the ylindril symmetry of the eletromgneti field in the struture, whih gives rise to the trnsverse omponent of the eletri field (kik-ftor) nd the omplexity of the instlltion of roust power oupler on the sme pipe with HOM ouplers nd possiility of multipting dishrges. Tht is why severl lterntive HOM dmping tehniques hve een studied (Figs. 2-4) [4] Grooved em pipe Fig. 1.Aelerting vity with HOM ouplers. The frequeny of dipole HOMs lies elow the ut-off frequeny of the TE11 wve nd therefore nnot propgte long the drift tue in the struture shown in Fig. 1. The min feture of the struture from Fig. 2 is tht the HOMs frequenies derese to the em pipe ut-off frequeny level nd eyond, whih llows to provide Fig. 2. ()Struture with grooved em pipe; ()its trnsverse wke potentil nd () longitudinl impedne for vity.

118 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 dmping with lod pled in the drift tue outside the ryomodule.

We should not expet multiunh instilities sine the wke field deys ompletely t the distne of 15-25 m, whih orresponds to the tul LHCunh seprtion of 50 ns.

3 118 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) dmping with lod pled in the drift tue outside the ryomodule. The results of wke field simultions onduted with ABCI ode [5] nd CST [6] hs lerly demonstrted tht we mnged to otin truly single mode vityin these strutures. We should not expet multiunh instilities sine the wke field deys ompletely t the distne of m, whih orresponds to the tul LHCunh seprtion of 50 ns. The results otined for single ell strutures with grooved, fluted nd ridged em pipes re similr. In ll the strutures externl Q vlues (Q ext ) reelow 100 for ll HOMs (exept for few modes with low shunt impedne to Q (R/Q) rtio) resulting in very fst wke field dey. This is why we hve lso proposed to omine two suh vities onneted y smller rdius em pipe in single ryostt. d Fig. 3.()Cvity with fluted em pipe; ()its trnsverse wke potentil;() longitudinl impedne nd (d) trnsverse impedne for vity with fluted drift tue Fluted em pipe Another type of vity, whih provides onditions for HOM dmping, is the struture with fluted em pipe [7] (Fig. 3). The dependene of the utoff frequeny on E01, H11 nd H21 modes on the size of the drift tue ws investigted in CST Mirowve Studio. It ws found tht the outer rdius of irregulrities hs strong influene on the utoff frequeny of the wves, nd TE11 nd TE21, respetively, dmping of TE111, TE211 nd TM110, TM210. Also, onfigurtion with three flutes rrnged t 120 degrees insted of four s in [7] signifintly inreses the level of dmping for the qudrupole HOM.

Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 119 1.4. Ridged em pipe The fluted em pipe dmping effiieny will improve if we reverse flutes inwrd the em pipe (Fig. 4).

As it n e seen, the wke potentil deys very fst in this se, too.

4 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) Ridged em pipe The fluted em pipe dmping effiieny will improve if we reverse flutes inwrd the em pipe (Fig. 4). In this se, the ut off frequenies for qudrupole nd dipole modes derese, llowing to redue the em pipe rdius. The lulted wke field deys for this struture is shown in Fig. 4. As it n e seen, the wke potentil deys very fst in this se, too. On the other hnd, the ut-off frequeny of E010 mode is lso lower for this onfigurtion, whih n led to deeper penetrtion of the working mode towrds the lod. To prevent deresing Q 0 of the working mode the em pipe length n e hosen to e longer thn tht in the fluted m pipe se. Shrp peks on the HOM monopole (Fig. 4) nd dipole (Fig. 4d) impedne plotsre not oserved. 2. An rry of two ells Sine the options with single-ell vities hve shown good results for HOM dmping in order to redue the numer of trnsitions etween old nd wrm prt of the ryomodule, we n omine severl of these resontors in the hin of not interting with eh other resontors. Previously, suh lultions were performed for hin of two resontors with grooves [4]. The dey rte of the indued potentil in this struture proved to e very high nd therefore the indued potentils for struture with non-ylindril drift tue nd drift tue with hnnels omined into hin of resontors (Fig. 5, 6)were lulted. Fig. 5 shows tht the indued potentil flls to ner zero when the distne etween the ems is 15 m. There is no shrp pek s shown in the plots of trnsverse (Fig. 5) nd longitudinl (Fig. 5d) impednes, whih indites the effetiveness of this dmpingmethod. The results for struture showed in Fig. 6 re similr to those otined for struture with ridged em pipe. d Fig. 4.() Cvity with ridged em pipe;()trnsverse wke potentil;()longitudinl impedne nd (d) trnsverse impedne for vity with ridged drift tue.

120 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 3. HOM dmping with wveguides We n onlude tht in n rry of two vities HOM dmping is not prolem.

Extrtion of the trpped modes power ould e otined with wveguides tthed to the onneting drift tues (Fig. 8).

At ertin length of the onneting drift tue nd the wveguide, the fst dey of the wke potentil is provided nd, respetively, the sene of shrp peks in the grph of the

These peks orrespond to the monopole HOMs.

5 120 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) HOM dmping with wveguides We n onlude tht in n rry of two vities HOM dmping is not prolem. But if we wnt to inrese the numer of ells, we will fe prolem of prsiti modes dmping trpped in drift tue etween the ells (see Fig. 7 for exmple). Extrtion of the trpped modes power ould e otined with wveguides tthed to the onneting drift tues (Fig. 8). A solution with wings hs proven to e effetive in the extrtion of fields propgting long the em pipes [8]. At ertin length of the onneting drift tue nd the wveguide, the fst dey of the wke potentil is provided nd, respetively, the sene of shrp peks in the grph of the trnsverse impedne is oserved. However, the longitudinl impedne exhiits severl peks tht n e potentilly dngerous (Fig. 8). These peks orrespond to the monopole HOMs. The extrtion of this HOMs is omplited y the ft tht the wve,whih they exite in the wveguide, does not propgte through sine the ut-off frequeny of this wve is muh higher thn the frequeny of these HOM. d Fig. 5.() Arry of vities with ridged em pipes.()trnsverse wke potentil;() trnsverse impedne nd (d) longitudinl impedne for rry of vities with ridged em pipes. Fig 6. Arry of vities with fluted em pipes. Fig. 7. Distriution of the eletri field of trpped modes.

Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 121 Fig. 8.() Arry of two ells with wing type wveguides nd () longitudinl impedne.

Unfortuntely, the use of suh lrge wveguides n lso rek the ylindril symmetry of the fundmentl wve, giving rise to trnsverse impedne, the sme devie n e rther omplited to mnufture.

6 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) Fig. 8.() Arry of two ells with wing type wveguides nd () longitudinl impedne. The wveguide width hs een gretly inresedin order to provide the onditions for monopole modes dmping (Fig. 9). The numer nd the mplitude of the peks is signifintly deresed therefter (Fig. 9). Fig. 9. () Arry of vities with wide wveguides nd () longitudinl impedne. Unfortuntely, the use of suh lrge wveguides n lso rek the ylindril symmetry of the fundmentl wve, giving rise to trnsverse impedne, the sme devie n e rther omplited to mnufture. It ws therefore deided to dd teeth in these wveguides (Fig. 10), whih redue the utoff for monopoly nd other HOMs. This will signifintly redue the size of these wveguides. In suh struture, the fst wke dey ws hieved (Fig. 10), the sene of shrp peks ws otined for oth trnsverse nd longitudinl (Fig. 10) impedne.

122 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) 116 123 Fig. 10.

() Wke potentil nd () longitudinl impednes for rry of vities with ridged wveguides. 4.

10 were good enough for the struture onsisting of four vities (Fig.

As expeted, the dey speed of the wke potentil ws suffiiently high (Fig.

7 122 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) Fig. 10. () Arry of vities with ridged wveguides. () Wke potentil nd () longitudinl impednes for rry of vities with ridged wveguides. 4. Arry of 4 vities Sine the results otined for the strutures in Fig. 10 were good enough for the struture onsisting of four vities (Fig. 11), the wke potentil nd impednes were lso lulted. As expeted, the dey speed of the wke potentil ws suffiiently high (Fig. 11) nd grphis of longitudinl nd trnsverse impedne lso did not revel ny presene of dngerous trnsverse nd longitudinl HOMs. The results were similrfor rrys of6 nd 8 ells. d Fig. 11. () Arry of 4 vities with ridged wveguides. () Trnsverse wke potentil, () longitudinl nd (d) impedne for n rry of 4 vities with ridged wveguides.

8 Y.V. Shshkov et l. / Physis Proedi 74 ( 2015 ) Conlusion The simultion results nd the following nlyses show tht ll options with two vities onneted y the drift tue hve no prolems with HOM dmping. In turn, the HOM dmping in rrys of four vities ould lso e effiient with the help of dditionl wveguides. Arry of two ells with grooved em pipes onneted y the drift tues in our opinion urrently is the most suitle option s ndidte for hrmoni vity design due to: n sene ofhom; no need to use for the HOM ouplers; nxil symmetry of the vity llows the sme level of dmping of HOM nd gretly simplify mnufturing; the detuning of the operting frequeny due to the influene of the Lorentz fore n e redued y the ddition of speil ris. Aknowledgements Work supported y Ministry of Edution nd Siene grnt /r nd y the EU FP7 HiLumi LHC - Grnt Agreement Referenes [1]Rossi L. LHC upgrde plns: options nd strtegy. Proeedings of the IPAC 2011:p.908. [2]Bruning O., Zimmermnn F. Prmeter spe for the LHC luminosity upgrde. Proeedings of the IPAC 2012:p [3]Fidenti F., Tukmntel J., Clg R. LHC Lndu Cvity Design, BR setion meeting, Mrh 29, [4]ShshkovY.V., Soenin N.P., Zoov M.M. et. l. Comprison of higher order modes dmping tehniques for 800 MHz single ell superonduting vities. NIM A. 2014;767: [5] [6] [7]Pdmsee H. Aelerting Cvity Development for the Cornell B-Ftory, CESR-B. Aelertor Siene nd Tehnology. 1991;2: [8]Suetsugu Y. Development of wing-type HOM dmper for KEKB. NIM A. 2004;533:

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