Benvenuti, Cristoforo

Bloess, D

Boussard, Daniel

Geschonke, Günther

Hilleret, Noël

Häbel, E

Juras, S

Kindermann, H P

Stirbet, M

Tückmantel, Joachim

Uythoven, J

Wyss, C

CERN Document Server

IMPROVEMENTS TO POWER COUPLERS FOR THE LEP2SUPERCONDUCTING CAVITIESJ. Tückmantel, C. Benvenuti, D. Bloess, D. Boussard, G. Geschonke, E. Haebel, N. Hilleret,S. Juras , H.P. Kindermann, J. Uythoven, C. Wyss, CERN, Geneva, SwitzerlandM. Stirbet, Institute of Atomic Physics Bucharest, RomaniaAbstract  Power couplers for the 352 MHz LEP2superconducting RF cavities have been plagued byvacuum and electron outbursts which are attributed tomultipacting. Processing of these couplers has been alengthy operation which was often needed again afterhigh power running even if only for a relatively shorttime. We report here on recent progress made inimproved production methods of coupler parts andspecial treatment of surfaces, as well as practical testsand simulations of geometrical coupler modifications.I. INTRODUCTIONAll couplers of the LEP 352 MHz superconductingcavities are located on the cut-off tubes to avoid portson the cavity cells themselves. The first power couplerdesign used a homogeneous 50 Ω antenna line [1].Based on this coupler an adjustable version wasdesigned containing a λ/4 section of 25 Ω impedancewhich houses a choke construction [2].Fig.1 : The adjustable couplerKeeping the RF-line outer diameter of 103 mm withthe proven cylindrical window design as used for theLEP copper cavities (operated at up to 120 kW andbeing tested up to 180 kW [3]) allowed a sufficientsafety margin at the original design value of 60 kW.However, specifications for field level and beam currentwere increased, swallowing all of this safety margin. Italso has to be considered that due to the RFtransformation via the 25 Ω choke the RF currents nearthe ceramic window are strongly increased.During a surface test of a cavity (standing wavecase) a leak developed linked with a burn-mark on theouter coaxial line. Subsequent tests with improvedinstrumentation (local vacuum gauges on the couplers,electron pick-ups, infrared camera) showed that in factlevels with vacuum and electron bursts could beobserved when ramping the power. Static magneticfields could influence the levels — but not suppressthem — thus confirming that multipacting (MP) was theculprit.One-side MP trajectories in coaxial lines hadalready been simulated [4], neglecting the magneticfield. In the power range used by us the influence of themagnetic field is small and with a simple BASICprogram similar tracks with impacts only on the outercoaxial conductor in the 50 Ω part of the line werefound at the E-field maxima of a standing wave wherethe magnetic field is exactly zero [5]. Note the scalinglaw: the RF power where the MP occurs is proportionalto f4. Furthermore two-side MP was detected on the25 Ω line section of the adjustable coupler . Table 1shows, in the range used, the most dangerous predictedand measured levels for the 50 Ω couplers. Psw is thenecessary incident power on the coupler in fullreflection (sc. cavity without beam) producing the peakvoltage VRF in a 50 Ω line section. Use of a systematicscan procedure including the magnetic field [6]identified the same levels.order VRF[kV]Psw(th)[kW]Psw(ex)[kW]3 4.1 42 38-434 3.2 26 26-285 2.6 17 15-18Table 1. One-side MP levels theoretical (th) [5] andexperimental [ex] in 50 Ω lineII. COUPLER PROCESSING ANDDECONDITIONINGProcessing of the couplers (with the local couplervacuum used to control the input power) proceeds intwo stages  In the first stage a pair of couplers ismounted on a resonant copper cavity with strong over-coupling. One coupler is used as input coupler. Thesecond one as output coupler can be connected either toa matched load or an adjustable short circuit.Conditioning takes in general 4-5 days, startingmanually to process the low levels, then using pulsedmode with pulse lengths between 10 µs and 50 ms and aduty cycle around 20 %. Finally ramping up and downunder computer control in cycles of 2-5 minutesprocesses up to 200 kW.A second processing is necessary on the coldcavities and generally done in parallel on the couplers ofa whole module (four cavities in a common cryostat).Initial conditioning has to be repeated and again takesseveral days including pulsed power processing withpeak fields in the cavities reaching 8 MV/m.For couplers from the first production lot, runningat nominal power on the cavities often for only a fewminutes was sufficient, to erase the conditioning donebefore. A reconditioning period of typically severalminutes was necessary to regain nominal field. Also, fora perfectly processed coupler, the well known levelsreappeared after switching off the RF for several hours.These effects, called deconditioning, were attributed togas getting trapped on the cold outer conductor.III. CERAMIC WINDOW IMPROVEMENTOn some of the damaged couplers traces of arcinghave been found on the brazing between ceramiccylinder and Kovar ferrule, on the RF contact betweenKovar ferrule and copper part, or on the relatively thicktitanium coating of the ceramic near to the RF contact(compared to the thin coating on the cylindrical part). Itwas also observed that during high power operation thewindows were strongly heated near the Kovar ferrulesTo avoid these problems the followingmodifications have been made: i) brazing completelypenetrating through the Kovar-ceramic connection, ii)Kovar ferrules machined at the contact surface afterbrazing and welded to the coupler under pressure toimprove the RF contact, iii) more homogeneoustitanium coating of the whole ceramic cylinder with aDC resistance decreased from 500 MΩ to 10-20 MΩ, iv)copper plating of the Kovar ferrules to reduce RF losses,v) Improved air cooling around the ceramic cylinder andthe ferrule. In fact, couplers equipped with the newwindows no longer showed sensitive heating up to 200kW travelling wave.In order to permit even higher RF currents thesimple pressed contact has been replaced by a spiralcontact and tested successfully. But the assembly of thisversion is much more complicated and delicate.However, even though the improved windowsoperated safely under these increased power levels, theiruse did not eliminate the deconditioning phenomenonand further steps to fight MP itself had to be undertaken.IV. MEASURES AGAINSTMULTIPACTINGWork was done in parallel in several areas andalthough tests have been done in the warm and coldstate of the coupler, a full beam test is still pending.A  Choice of fixed coupler versionMovable couplers are attractive because i) theyallow a measurement of the Q versus E curve of a cavitywithout having to break the cavity vacuum and ii) theycan precisely match to a wide range of cavity voltages,beam intensities and stable phases. Unfortunately, dueto the restricted space in the LEP tunnel, the chokenecessary in the movable coupler had to be folded backinto the coupler (25 Ω line). With this geometry twoadditional MP levels appear. The extra risk associatedwith these levels was felt to be unacceptable and themovable coupler version was abandoned for the seriesproduction. Note that coarse estimates of the cavity Qcan be obtained by cryogenic methods and thatimperfect matching essentially results in higher fields inthe coupler. But even at 200 kW both types of couplersare operated far below any electric field breakdownlimit [7].B Improvement of the surfaceAs shown by the simulations, the MP takes place onthe outer conductor. The corresponding line piece isdouble walled to allow He-gas cooling. The originalwork-piece, called ‘extension’, was produced fromrolled  and welded sheet-metal, the welding making itdifficult to obtain a smooth surface. Therefore the newextensions are now produced — including vacuumflanges — from a single stainless steel forging thusavoiding welding or brazing in critical areas.The inner surface of the extension has to be copperplated to reduce RF heating. The original fabricationused ‘standard’ galvanic deposition. To improve thequality of the surface avoiding possible traps for gas,two methods were tried in parallel. One uses a galvanicdeposition with pulsed current showing a smoothersurface; tests allowed faster MP processing in the warmstate. The second deposits copper by sputtering [8]; testshowed faster MP processing in the warm but a tediousone on the cold cavities. A version with improved gascooling also showed good results on a cold cavity. Testsof a sputtered extension submitted to water rinsing areunder way. Titanium coating of the extensions has alsobeen tried, but was not successful.Furthermore, an apparatus to measure the secondaryemission coefficient between 1/10 and 2000 eV wasconstructed and used for measurements on copper andtitanium layer specimen.C. Change of antenna diameterA geometrical modification [7] was done inchanging the antenna line impedance. For impedancesof 50 Ω and higher, the multipacting is 'running' only onthe outer conductor. By reducing the inner conductordiameter and scaling the RF voltage proportionally tothe line impedance one leaves the fieldmap in the outercoaxial region unchanged. This fact was used to transferthe MP levels to higher powers by reducing the standardantenna diameter of 44.8 mm (50 Ω) to 30 mm (75 Ω),thus shifting the levels up by 50% in power [7]. Afurther decrease of the diameter is not considered due toincreased mechanical and thermal problems.Measurements confirmed this approach: the sameMP levels appeared, but at correspondingly higherpower, thus giving more margin for operation. The RFcurrents near the ceramic window are also reduced,compared to the  50 Ω case, thus permitting the use ofthe simple pressed contact. All future couplers will be ofthe 75 Ω type.D. Eccentric CouplerSimulations [9] showed that with an eccentricantenna the MP tracks run around the outer conductortowards the wider gap. Tracks coming from the smallergap are stopped after only a few impacts due to thechanging conditions. It was hoped that tracks startingclose to the wide gap would be concentrated on a smallstrip which would clean more easily.A pair of such  75 Ω couplers with 4 mmeccentricity were built and tested on the warm stand.Their behaviour was not better than the averagestandard coupler. Due to the more complicatedfabrication, the idea was abandoned.E. Injection of Perturbing RFTo break the resonant kinematic conditions of MP,a perturbing RF voltage can be used. This was realisedby adding power at an incorrelated frequency (noiseband or single frequency). Since MP can rise in a shortlap of time, the injected frequency has to be 'far' awayfrom the fundamental one. An injection of about 2% RFpower on a sideband at 200 kHz to 1 MHz frequencydistance (klystron bandwidth limitation) was a sufficientperturbation to make (on the warm test stand) MPdisappear and significantly reduce it on the coldcavities. Since the cavity bandwidth is below 200 Hz,the second signal does not perturb the cavity voltageF.  Electric DC biasPerturbation with static magnetic fields had alreadybeen tested at an early stage. Influence on the levels hadbeen observed but suppression could not be achieved.The coaxial antenna geometry offers a relativelysimple possibility to apply a DC bias voltage. It is trivialthat a DC bias higher than the RF voltage will inhibit allreturn paths for electrons. Simulations [10] showed thatdepending on polarity about -1.5 kV or +2.5 kV withrespect to ground would already be sufficient to obtaincomplete suppression up to 200 kW of travelling wavepower (5.5 kV RF voltage). Experimentally it was foundthat with +2.5 kV MP is completely suppressed.The main problem of this approach has been thecapacitors which are needed to separate RF and DCpaths. For a first proof of principle with a minimum ofmodifications, Mylar or Kapton foils were insertedbetween the doorknob and the waveguide. During thepractical tests on the cold cavities these capacitorsshowed breakdowns with a rate unacceptable formachine operation. The suspicion is that the breakdownsare caused by sharp edges in the waveguide or byparticles introduced in the capacitor during mounting inthe open air as it is actually done. As an intermediatesolution copper plated Kapton foils will be used. Abetter version is under development using a coaxialcapacitor similar to those of the SPS tetrodeamplifiers [11] and mounted in a clean room.To permit DC bias the He gas cooling of theantenna has been replaced by forced air cooling.V. CONCLUSIONThe RF window has been improved to decrease thewindow temperature and to reduce the risk ofbreakdowns. Several approaches to suppress or reduceMP in the LEP2 power coupler have been examined.The choice of a higher antenna line impedance shiftedMP levels up by 50% in power. Two perturbationmethods have been worked out: Injection of about 2%power on a sideband several 100 kHz above the normalRF frequency and application of a DC bias voltage,achieving complete suppression. For the proof ofprinciple a simplified construction has been used, thesafe design for machine application is under way, to befollowed by a large scale beam line test.VI. REFERENCES[1] E. Haebel et al., Proc. 3rd Workshop on RFSuperconductivity, Ed. K. Sheppard, ANL-PHY-88-1.[2] E. Haebel, Part. Accel., Vol. 40, P 155[3] J.P. Boiteux, G. Geschonke, LEP note 570[4] J.P. Budlinger et A. Laisne, NIM 61 (1968),253-259 (in French).[5] J. Tückmantel, Proc. CERN Main CouplerWorkshop, Oct 1992, Editor C. Wyss.[6] E. Somersalo et al, TESLA report 94-14.[7] E. Haebel, S. Juras, CERN LEP2 Note 95-31[8] C. Benvenuti et al, CERN LEP2 Note 94-21.[9] J. Tückmantel, CERN LEP2 Note 94-25.[10] idem, Note 94-26.[11] H.P. Kindermann, R. Maleyran, CERNSPS/88-1 (ARF).ACKNOWLEDGEMENTSWe would like to thank all technicians for theirenthusiastic help, especially J-P. Boiteux andG. Rochepeau, as well as the workshops MT-MF andMT-SM. We also thank M. Karliner, V. Petrov,V. Veshcherevich and V. Yakovlev from INP,Novosibirsk, for suggesting the DC bias of the antennaand G. Bachy for proposing the monolithic extension.

Improvements to power couplers for the LEP2 superconducting cavities

Improvements to power couplers for the LEP2 superconducting cavities

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