Achard, Claude

Adolphsen, C E

Dehler, M

Jensen, E

Luong, M

McCormick, D

Ross, M

Slaton, T

Wilson, Ian H

Wuensch, Walter

Achard, C

Wilson, I

Wuensch, W

Adolphsen, C

CERN Document Server

An asset test of the CLIC Accelerating Structure

EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN – PS DIVISIONCERN/PS-2000-044 (RF)CLIC Note 443AN ASSET TEST OF THE CLIC ACCELERATING STRUCTUREC. Achard, M. Dehler*, E. Jensen, M. Luong**, I. Wilson, W. Wuensch,CERN, Geneva, SwitzerlandC. Adolphsen, M. Ross, T. Slaton, D. McCormick, SLAC, California, USAAbstractTransverse wakefield suppression in the CLIC (Compact Linear Collider) multibunch accelerating structure,called the TDS (Tapered Damped Structure), is achieved primarily through heavy damping. In order toverify the performance of the TDS design and the validity of the theoretical tools used to model it, a 15 GHzversion of the TDS has been constructed and tested in the ASSET facility at SLAC. The test has directlydemonstrated transverse wakefield suppression of over a factor 100, with an excellent agreement betweenthe measured and the calculated wakefield.*) Paul Scherrer Institute, Villigen, Switzerland**) CEN-Saclay, Gif-sur-Yvette, France7th European Particle Accelerator Conference, 26-30 June 2000, Vienna AustriaGeneva, Switzerland15 August 2000AN ASSET TEST OF THE CLIC ACCELERATING STRUCTUREI. Wilson, W. Wuensch, C. Achard,  M. Dehler *, E. Jensen, M. Luong **, CERN, Geneva,SwitzerlandC. Adolphsen, M. Ross, T. Slaton, D. McCormickSLAC, California, USA Present Addresses: * Paul Scherrer Institute, Villigen, Switzerland,** CEN-Saclay, Gif-sur-Yvette, FranceAbstractTransverse wakefield suppression in the CLIC(Compact Linear Collider) multibunch acceleratingstructure, called the TDS (Tapered Damped Structure), isachieved primarily through heavy damping. In order toverify the performance of the TDS design and the validityof the theoretical tools used to model it, a 15 GHz versionof the TDS has been constructed and tested in the ASSETfacility at SLAC. The test has directly demonstratedtransverse wakefield suppression of over a factor 100,with an excellent agreement between the measured andthe calculated wakefield.1  INTRODUCTIONA common feature for most linear collider designs isthat multiple-bunch trains are accelerated during each RFpulse (called multibunching) in order to reach designluminosities (1035 cm-2 sec-1 at 2 TeV for CLIC [1]) in apower efficient way. As a result of multibunching, amajor concern for the linear collider designs is bunchtrain emittance growth. Long-range transverse wakefieldsinduce a bunch-to-bunch coupling of transverse motions.Major efforts in the various linear collider studies havegone into developing accelerating structures in whichlong range transverse wakefields are suppressed, so as tolimit this contribution to emittance growth.For CLIC, beam dynamics simulations indicate that thesingle bunch transverse wakefield must be suppressed bya factor 100 in the interval between bunches in the train -0.67 ns, which corresponds to 20 (30 GHz) acceleratingmode cycles - and that the wakefield must continue todecrease for longer times [2].A structure, called the TDS (Tapered DampedStructure), capable of such a performance has beendeveloped in the context of the CLIC study [3].Wakefield suppression is achieved primarily throughdamping. Each cell of the 150-cell TDS is damped by itsown set of four individually terminated waveguides. Thisproduces a Q of below 20 for the lowest dipole modeband. The waveguides have a cutoff frequency of 33 GHzwhich is above the 30 GHz fundamental but below allother higher order modes. Cell and iris diameters aretapered along the length of the structure in order toinduce a frequency spread in the dipole bands which iscalled detuning. Iris dimensions range from 4.5 to3.5 mm in the TDS. Detuning causes a decoherence ofwakefield kicks, further suppressing the transversewakefield. A cut-away view of the TDS cell and dampingwaveguide layout is shown in Figure 1.Figure 1: A cross-sectional view of the TDS cell anddamping waveguide layout. A number of theoretical techniques have beendeveloped to compute the time dependent transversewake of  the TDS [3,4,5]. The techniques have beenbenchmarked with cold model RF measurements thatincluded cell dipole mode quality factors and thereflection coefficient spectrum of the loads [6]. In orderto make a definitive verification of the TDS performanceand of the theoretical tools used to model it, a teststructure has been constructed at CERN and tested in theASSET facility at SLAC [7].2 ASSET TDS MODELThe TDS test model prepared for the ASSETmeasurement is a somewhat simplified version of the30 GHz CLIC structure. The most importantsimplification has been to scale the structure up by afactor of two, to 15 GHz in order to increase theminimum iris. This is done in order to avoid creatingbeam transmission difficulties during testing andparticularly during SLAC B-factory operation. The B-factory beam passes through structures installed in theASSET facility and since opportunities to install andremove the test TDS were limited, the structure had to bemade so it could be left installed for an extended period. The scaling was limited to a factor of two in order tomaintain a sufficiently strong transverse wakefield kickfrom the structure, which decreases with the cube of thefrequency. The smallest kick which can be measured inASSET is about 0.1 V/(pCmm), although the exact valueis influenced by the available driving positron bunchcharge. The factor two scaling results in an initialwakefield kick of 120 V/(pCmm), and thus makespossible direct measurement of suppression factors of upto 1000.The second simplification was to assemble thestructure by bolting rather than by brazing. Thissimplification has a negligible influence on the heavilydamped dipole modes which have with loaded qualityfactors less than 20. Quality factors achieved in un-damped bolted cells are lower than in un-damped areeasily over 200 to 300. The disks were made fromaluminium in order to ease machining, reduce cost andreduce weight. The bolted stack of plates was mounted ina vacuum tank and the cells were pumped via thedamping waveguides.A close-up view of the ASSET TDS cell and dampingwaveguides is shown in Figure 2. Figure 3 shows hestructure being assembled.Figure 2: Close-up view of the TDS cell, with the fourdamping waveguides and silicon carbide loads.Another difference between the 30 GHz TDS and the15 GHz ASSET test structure is in the load. Since thecomplex permittivity of silicon carbide is a function offrequency [6], a load had to be specially designed for thetest structure. The reflection coefficient spectrum of theload is shown in figure 4 and does not differ significantlyfrom the load designed for 30 GHz structure.Figure 3: The structure being assembled.Figure 4: Measured load reflection coefficientmagnitude spectra. These values were used to computethe wake.The resulting  transverse wakefield computed using thedouble-band circuit model [4] appears in Figures 5 and 6.Total wakefields from the structure are presented in thispaper although per meter values are the same since the150 cell, 15 GHz structure is exactly 1 m long. Forreference, the CLIC bunch spacing scaled to 15 GHz is1.33 ns. 3 MEASUREMENTS AND RESULTSThe structure was measured in ASSET during onenight-time break in B-factory operation. The principles0.00.10.20.30.40.516 18 20 22 24 26 28 30and procedures for the measurement of the transversewakefield were the same as for previous measurements ofSLAC X-band structures [7]. The now well establishedmeasurement process proceeded without incident.Plots on two different time-scales of the measuredtransverse wakefields in the vertical plane are shown inFigures 5 and 6.Figure 5: Measured (dark) and computed (light)transverse wakefield in units of V/(pCmm) against timein units of ns.Figure 6: Log plot of the measured (dark) and computed(light) transverse wakefields in units of V/(pCmm)against time in units of ns.The agreement between the measured and computedwake is excellent and represents a direct verification ofthe performance of the design and of the accuracy oftechniques used to model it.The only unexpected feature of the data was a 7.6 GHzcomponent of the wake that emerged after the wake hadfallen below about 0.2 V/(pCmm). The close-up on datawhere the signal appears is shown in Figure 7. Since thecomponent occurred at a frequency well below thefundamental mode of the structure, it was clear the signalhad its origin in a feature of the experimental set-uprather than in the structure itself.  A quick a posteriorianalysis of the set-up and a MAFIA 2-D run identified thevacuum chamber to beam pipe transitions as theoffending components. The transitions had a narrowcircular gap that behaved as a high-Q coaxial resonator.The computation matched the observed signal in bothfrequency and in kick amplitude. A new sliding-fittransition has been manufactured and will be installed ifscheduling allows. Figure 7: Close-up view of the measured wakefieldaround 2.1 ns in units of V/(pCmm) against time in unitsof ns. A 2.9 V/(pCmm) 7.6 GHz wake is superimposedon the data.REFERENCES[1] J.P.Delahaye, I. Wilson, “CLIC, a Multi TeV erLinear Collider”, CERN/PS 99-062 (LP), November1999.[2] D.Schulte, “Emittance Preservation in the MainLinac of CLIC”, EPAC’98, Stockholm, June 1996.[3] M.Dehler, I.Wilson, W.Wuensch, “A TaperedDamped Accelerating Structure for CLIC”,LINAC'98, Chicago, August 1998.[4] M.Dehler, “Modeling a 30GHz Waveguide LoadedDetuned Structure for the Compact Linear Collider(CLIC)”, CERN/PS RF 98-09.[5] W.Wuensch, “Calculation of the Transverse WakeFunction of a Highly Damped Periodic Structure”,PAC’99, New York, March 1999.[6] M.Luong, I.Wilson, W.Wuensch, “RF Loads for theCLIC Multibunch Structure”, PAC’99, New York,March 1999. [7] C. Adolphsen et. al. “Wakefield and Beam CenteringMeasurements of a Damped and Detuned X-BandAccelerator Structure”, PAC’99, New York, March1999-1-0.8-0.6-0.4-0.200.20.40.60.81.9 2 2.1 2.2 2.3

An Asset Test of the CLIC Accelerating Structure

An Asset Test of the CLIC Accelerating Structure

Abstract

Similar works

Full text

Available Versions

CERN Document Server

CERN Document Server