D'Elia, Alessandro

D'Elia, A

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EuroTeV High Bandwidth Wall Current Monitor

EU contract number RII3-CT-2003-506395 CARE/ELAN Document-2007-012  - 1 -                                              EuroTeV High Bandwidth Wall Current Monitor                                                       Alessandro D’Elia AB-BI-PI 1    EU contract number RII3-CT-2003-506395 CARE/ELAN Document-2007-012  - 2 -  Acknowledgements We acknowledge the support of the European Community-Research Infrastructure Activity under the FP6 “Structuring the European Research Area” programme (CARE, contract number RII3-CT-2003-506395) EuroTeV High Bandwidth Wall    Current MonitorAlessandro D’EliaAB-BI-PI1Wall Current MonitorsWall Current Monitors (WCM) are commonly used to observe the time profile and spectra of a particle beam by d t ti it i te ec ng s mage curren . 2The “initial” aimThe 3rd generation of CLIC Test Facility (CTF3) foresees a beam formed by bunches separated of       Δb = 67 ps               WCM h. f. cut-off = 20 GHzf t t l l d ti for a o a  pu se ura on oτr = 1.54 μs              WCM l. f. cut-off = 100 kHzBake out temperature: 150 COperating temperature: 20 CVacuum: 10-9 TorrFurthermore 100kHz-20GHz WB signal transmission 3over 10-20m. The gap resonances4With the courtesy of Tom Kroyer (“A Structure for a Wide Band Wall Current Monitor”, AB-Note-2006-040 RF)A more accurate study of the gap resonancesThe resonances due to the cross section changing are “structural”!!!! You cannot delete them you can only to try to    ,       reduce them!  The TM01, with TM01 cut-off ≅ 24GHz}the frequency of about 6.9GHz will b t dTM01 cut-off ≅ 6.9GHz e rappe  between the two coaxial5Feedthrough resonancesWhen the distance between two feedthroughs becomes equal to the free space wavelength, the first azimuthal resonance i th t tappears n e s ruc ure hfeedthrougofnumber == ncF)/(2 nrπ With n =4, one has F =8.3 GHzmmr 23=6With the courtesy of Tom Kroyer (“A Structure for a Wide Band Wall Current Monitor”, AB-Note-2006-040 RF)The whole structureTherefore to have 16 feedthroughs means to push the previous resonance to ≅ 33 GHz7With the courtesy of Tom Kroyer (“A Structure for a Wide Band Wall Current Monitor”, AB-Note-2006-040 RF)The effect of feedthrough’s on the TM01 resonanceIn the transversal plane you have either for vertical or horizontal directions that ⎨⎧>> hdλ ⎩ v01TM d8The effect of feedthrough’sIn order to reduce this enhancement would have,   to happen that the distance between two feedthrough’s   should be at least equal to one half of the resonant mode wavelength2n2d TM01hλπ== rIndeed for a structure having   22mmr =and 43mmc/6.9GHzλTM01 == The optimum9   is for n=6   Some considerationThe two requirements concerning the feedthrough resonances and the effect of the feedthrough enhancement on the gap resonances are in conflict:F dth h G h tee roug  resonances ap resonance en ancemencF = 2d TM01λπ == rhd 2nhdh  has to be, on the one hand, as small as possible, on the other hand, at least equal to one half of the TM01 wavelength10           Three possible solutions found1.2.3.The last two structures present an aperture reduction of 15% and11           30%, respectively. For that reason the first one has been chosen.The chosen structureSilicon Carbide (tang δ = 0.3, εr = 30 )cmr 6.2=12Some geometrical details13S-parameters1415Longitudinal coupling impedance: Real partLongitudinal coupling impedance: Imaginary part16The real structure (1)17The real structure (2)18With the courtesy of Vincent MaireTransmission at the feedthrough19Longitudinal coupling i d R l tmpe ance: ea  parLongitudinal coupling impedance: Imaginary part  20Feedthrough positioning (1)21With the courtesy of Vincent MaireFeedthrough positioning (2)22With the courtesy of Vincent MaireMisalignement problemsWith th t f Vi t M i e cour esy o  ncen  a redB27*20)dB(0010 −≅+−=ΓLLZZZZLog23Really do we need 100kHz low freq cut-off?Let’s make some numerical experimentBunch separation = 83ps   RMS bunch length = 13.3psTrain duration = 8.3nsNb of bunches = 10083 psPeak current = 293A24Let’s apply a perfect low pass filter  25The result in time domain    83 ps 26Let’s apply, to the same signal as before a filter having ,   L f ff 5GHow req cut-o = zHigh freq cut-off= 20GHz27It seems that nothing changes!!!!83 ps 28An interesting exerciseSame condition of before, but some bunches are missed (about 50%)29Perfect 100 bunches spectrumBecause of the diff t leren , arger, bunch spacing, in the spectrum  some new peaks appear at lower frequencies 30Original signalCorrect signal recovering!!!Wrong signal recovering!!!  31Some considerationIf some bunches are missed we need a proper low frequency    ,       cut-off in order to solve the larger bunch spacing appearing in th t lik k t l f i Th f the spec rum e new pea s a  ower requenc es. ere ore elow frequency cut-off should be settled up in relation to the maximum expected missed bunch ratio. 32Applying the WCM “real” signalLow freq cut-off ≅ 2GHz500 ps ≅ 6 bunches missed33(Or 30 by compensating down to 400 MHz)The result in time domain    83 ps 34A last academic exercise108 psRMS bunch length = 13.3psT i d ti 2 583 ps 457 psra n ura on = . nsNb of bunches = 23Peak current = 293A35Just to have more fun it has been added also a random noise level of about 10% with respect to the signal amplitude36108 ps83 ps457 ps 37Measurements on the existing design8 feedthroughs BeamThe existing design is based on a previous design for the CTF2 (63 MHz ≤ bandwidth ≤ 10 GHz )but• Bigger volume of ferrite in order to lower the l. f. cut-off to 100 kHz• The miniature feedthrough modified in order to extend their bandwidth38          beyond 20 GHz  Experimental setup and testbench39Old measurements (March 2006)≅ 7GHzFrightening results!!!!!40With the courtesy of Lars Soby and Ivan PodaderaNew measurements (November 2006)≅ 9 GHz Quite better measurements!!!!!41What was wrong?Th i t l t h d b d RF t tBad RF contacts!!!e exper men a  se up s owe  very a   con ac s between WCM and the two external straight tubes. In order to improve the contacts some pasty stripes of conducting        material has been used…. Unfortunately it cannot be used in vacuum….42For frequencies higher than 12 GHz strong reflections occur because of the adapting cone are not enough smoothed.13.13GHZ43With the courtesy of Raquel FandosBy making the transitions longer the resonances get less dramaticL=200mmWith the courtesy of Raquel Fandos44Conclusions and outlooks• WCM specifications has been reviewed in a more iti l h i l t i t t i tcr ca  way, s ow ng ess s r ngen  cons ra n s• The e-m design is accomplished, giving pretty good results • At the end of the next week the mechanical designs will be sent to the mechanical workshop to start the machining and the assembling• The testbench has been improved• On December the first measurements and the       characterization are foreseen 45

EuroTeV High Bandwidth Wall Current Monitor

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