Boine-Frankenheim, O.

Eidam, L.

Niedermayer, U.

GSI Repository

Bench measurements of beam coupling impedances for SIS100 componentsL. Eidam ∗1, U. Niedermayer1,2, and O. Boine-Frankenheim1,21GSI, Darmstadt, Germany; 2TEMF, TU-Darmstadt, GermanyIntroductionCoupling impedance describes the force acting back onthe beam due to the electromagnetic properties of the accel-erator environment. The development of the high intensitySynchrotron SIS100 requires a detailed knowledge of theimpedance contribution of individual components to pre-vent beam instabilities and additional heat load. Besidesanalytic calculations, which are only possible for simplifiedstructures, numerical calculations are in progress. Benchmeasurements are required to cross-check simulations andtheir input parameters.Measurement systemMeasurements are performed by adding a central con-ductor on the beam axis to a DUT (Device Under Test) andconnecting this formed transmission line to a VNA (Vec-tor Network Analyzer). Describing this transmission lineby an equivalent circuit the DUT adds a serial impedanceto the equivalent line of a reference beam pipe. This ad-ditional impedance causes a variation of the wave number,which can be determined by changes of the scattering ma-trix. Assuming a successful matching of the transmissionline to the VNA, the reflections vanish and the correlationfor the longitudinal coupling impedance is [1]:Z‖ = Z0 · ln(SR21SD21)·(1 +ln(SD21)ln(SR21))(1)where SR21 / SD21 describes the transmission component ofthe scattering matrix for the reference/DUT. To determinethe transverse coupling impedance two anti-parallel drivenwires are needed [2]:Z⊥ =cZωΔ2(2)whereΔ is the displacement of the wires andZ comes fromEq. 1.For low frequencies the accuracy can be increased by us-ing a multiturn coil instead of two anti-parallel drivenwires and measuring directly impedance variations by aLCR-meter. The sensitivity increases by number of turnssquared:Z⊥ =c · (ZDUT − ZRef )ωΔ2N2(3)As example the low frequency transverse impedance of acircular pipe will be discussed. The coupling impedancehas been determined by several numerical and analyticalcalculations [3, 4]. A very good agreement between mea-surements and calculations has been achieved as seen in∗ l.eidam@gsi.deFig. (1). The drawback of using coils with many turns isthe lower resonance frequency. Nearby the resonance mea-surements are not possible, so this method is limited up toseveral MHz. To achieve good results in a broad frequencyrange two different coils were produced. One for high ac-curacy at low frequencies and one for measurements up to2 MHz.1001000100001000 10000 100000 1e+06Re(Z trans) [Ω/m]f [Hz]ReWallConstantan; N=53Copper; N=51000100000.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55 0.6Z [Ω]f [MHz]Z [Ω]Figure 1: Measurement of the real part of the couplingimpedance for a circular beam pipe compared to analyti-cal results made by Rewall [4], the small picture shows thefirst resonance of the coil with 53 turns.Conclusion and OutlookThe experimental setup for measuring couplingimpedances in the low frequency regime has been buildsucessfully. The measurement results for a reference beampipe structure were compared to analytic calculations. Theexperimental setup for measurements above 2 MHz ispresently under construction. The first planed device undertest will be a kicker magnet module, where the ferrite andalso the supply network cause to large impedances. Theresults of both measurement methods will be comparedwith numerical calculations of the measurement setupusing CST MWS [5].References[1] V.G. Vaccaro, Coupling Impedance Measurements: An Im-proved Wire Method, INFN/TC-94/023[2] G. Nassibian, F. Sacherer, NIM 159 (1979)[3] U. Niedermayer, NIM A687, 2012[4] N. Mounet, B. Salvant, E. Me˙tral, ”ReWall”,www.cern.ch/imp, 2010[5] CST Studio Suite 2012 R©, www.cst.comGSI SCIENTIFIC REPORT 2012 PHN-FAIR-11235

Bench measurements of beam coupling impedances for SIS100 components

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Bench measurements of beam coupling impedances for SIS100 components

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