This paper outlines the RF design of the CLIC (Compact Linear Collider) 30
GHz main linac accelerating structure and gives the resulting longitudinal and
transverse mode properties. The critical requirement for multibunch operation,
that transverse wakefields be suppressed by two orders of magnitude within 0.7
ns (twenty fundamental mode cycles), has been demonstrated in a recent ASSET
experiment. The feasibility of operating the structure at an accelerating
gradient of 150 MV/m for 130 ns has yet to be demonstrated. Damage of the
internal copper surfaces due to high electric fields or resulting from metal
fatigue induced by cyclic surface heating effects are a major concern requiring
further study.Comment: Paper to be submitted to LINAC2000 - paper ID TUA1

Wilson, I.

Wuensch, W.

English

arXiv

This paper outlines the RF design of the CLIC (Compact Linear Collider) 30 GHz main linac accelerating structure and gives the resulting longitudinal and transverse mode properties. The critical requirement for multibunch operation, that transverse wakefields be suppressed by two orders of magnitude within 0.7 ns (twenty fundamental mode cycles), has been demonstrated in a recent ASSET experiment. The feasibility of operating the structure at an accelerating gradient of 150 MV/m for 130 ns has yet to be demonstrated. Damage of the internal copper surfaces due to high electric fields or resulting from metal fatigue induced by cyclic surface heating effects are a major concern requiring further studyThis paper outlines the RF design of the CLIC (Compact Linear Collider) 30 GHz main linac accelerating structure and gives the resulting longitudinal and transverse mode properties. The critical requirement for multibunch operation, that transverse wakefields be suppressed by two orders of magnitude within 0.7 ns (twenty fundamental mode cycles), has been demonstrated in a recent ASSET experiment. The feasibility of operating the structure at an accelerating gradient of 150 MV/m for 130 ns has yet to be demonstrated. Damage of the internal copper surfaces due to high electric fields or resulting from metal fatigue induced by cyclic surface heating effects are a major concern requiring further study.This paper outlines the RF design of the CLIC (Compact Linear Collider) 30 GHz main linac accelerating structure and gives the resulting longitudinal and transverse mode properties. The critical requirement for multibunch operation, that transverse wakefields be suppressed by two orders of magnitude within 0.7 ns (twenty fundamental mode cycles), has been demonstrated in a recent ASSET experiment. The feasibility of operating the structure at an accelerating gradient of 150 MV/m for 130 ns has yet to be demonstrated. Damage of the internal copper surfaces due to high electric fields or resulting from metal fatigue induced by cyclic surface heating effects are a major concern requiring further study

Wuensch, Walter

CERN Document Server

arXiv:physics/0008101   17 Aug 2000THE CLIC MAIN LINAC ACCELERATING STRUCTUREI. Wilson, W. Wuensch, CERN, Geneva, SwitzerlandAbstractThis paper outlines the RF design of the CLIC(Compact Linear Collider) 30 GHz main linacaccelerating structure and gives the resultinglongitudinal and transverse mode properties. The criticalrequirement for multibunch operation, that transversewakefields be suppressed by two orders of magnitudewithin 0.7 ns (twenty fundamental mode cycles), hasbeen demonstrated in a recent ASSET experiment. Thefeasibility of operating the structure at an acceleratinggradient of 150 MV/m for 130 ns has yet to bedemonstrated. Damage of the internal copper surfacesdue to high electric fields or resulting from metal fatigueinduced by cyclic surface heating effects are a majorconcern requiring further study.1  INTRODUCTIONA major effort has gone into developing anaccelerating structure for the CLIC main linac that canoperate with an average gradient of 150 MV/m, thatmaintains an acceptable short-range wakefield, and thathas highly suppressed long-range transverse wake fields.The latter design criterion is a result of multibunching -a common feature among linear collider designs inwhich multiple-bunch trains are accelerated during eachRF pulse in order to reach design luminosities (1035 cm-2sec-1 at 3 TeV for CLIC [1]) in a power-efficient way.For CLIC, beam dynamics simulations indicate thatthe amplitude of the single-bunch transverse wakefieldmust decrease from its short-range value by a factor of100 during the first 0.67 ns (which is the time betweenbunches in the train and which corresponds to twenty30 GHz accelerating mode cycles) and that it mustcontinue to decrease at least linearly for longer times [2]. A structure, called the TDS (Tapered DampedStructure), capable of such transverse wakefieldsuppression, has been developed [3]. The suppression isachieved primarily through damping. Each cell of the150-cell TDS is damped by its own set of fourindividually terminated waveguides. This produces a Qof below 20 for the lowest dipole band. The waveguideshave a cutoff frequency of 33 GHz which is above the30 GHz fundamental but below all other higher-ordermodes. Cell and iris diameters are tapered along thelength of the structure in order to induce a frequencyspread in the dipole bands which is called detuning. Irisdimensions range from 4.5 to 3.5 mm. Detuning de-coheres the wakefield kicks further suppressing thetransverse wakefield.A first pass through the RF design of the TDS, forboth fundamental mode performance and transversewakefield suppression, has now been made.  Certainweaknesses in the design, with regard to operation atvery high gradients, have become apparent and are beingaddressed in a complete re-optimisation of the structure.2. GEOMETRY AND FUNDAMENTALMODE CHARACTERISTICSThe geometry of a TDS cell is shown in Figure 1. Afull structure consists of 150 cells. Iris diameters (2a)vary linearly from 4.5 mm at the head of the structure to3.5 mm at the tail. The relationship between irisdiameter (2a) and cell diameter (2b) necessary tomaintain a fundamental mode frequency of 29.985 GHzwas obtained using HFSS. Q, R'/Q, and group velocity(vg) as a function of b were then calculated. Therelationship between a and b is plotted in Figure 2 andQ, R'/Q, and vg are plotted in figure 3.Figure1: Geometry of TDS cell and dampingwaveguides.The nominal CLIC beam consists of a train of 0.6 nCbunches spaced by 20 cm. Operating the structure at anaverage accelerating gradient of 150 MV/m results in theloaded and unloaded accelerating gradient profilesplotted in Figure 4. The corresponding power flows areplotted in Figure 5.Figure 2: Relationship between 2b (y axis) and 2a (xaxis) for correct fundamental mode frequency. Units aremm.Figure 3: Fundamental mode characteristics as afunction of cell number. R'/Q is in k:/m, vg/c is apercentage. Q is nearly constant and has a value of 3600.Figure 4: Loaded and unloaded accelerating gradient[MV/m] as function of cell number.Figure 5: Loaded and unloaded power flows in units ofMW as a function of cell number.The highest surface field occurs in the first cell of thestructure and has a value of  420 MV/m. For a 150 nslong pulse, a pulsed temperature rise of about 250 qCoccurs in the cell wall between the waveguides.3. DIPOLE MODE CHARACTERISTICS.The damping waveguide terminating load is madefrom silicon carbide. The geometry of the load is shownin Figure 6. The principle behind this compact load, andthe method by which the complex permittivity wasobtained  is described in reference [5]. The amplitude ofthe reflection coefficient, ~S11~, as a function offrequency was calculated using HFSS - see Figure 7. Thetransverse wakefield was then calculated using thedouble-band circuit model described in reference [4].The result is plotted in Figure 8. The transversewakefield suppression of the TDS and the validity of thetools used to model it was verified in an experiment inthe ASSET facility [6] at SLAC. The calculated andmeasured wakefields are plotted in Figure 9.Figure 6: Geometry of the 30 GHz load.Figure 7: Amplitude of the reflection coefficient, ~S11~,as a function of frequency in units of GHz.Figure 8: Computed transverse wakefield.Figure 9: Results from the ASSET test.  The amplitudeof the transverse wake in units of V/(pCmmm) isplotted against time in ns. The test was made using ascaled 15 GHz structure.4. CONCLUSIONSA complete first pass has been made through thedesign of the TDS, for both the accelerating and thetransverse modes. The transverse mode suppression iswell within the specifications for CLIC and has beendemonstrated experimentally.The situation for the fundamental mode for anaccelerating gradient of 150 MV/m is less favourable.The 420 MV/m peak surface electric field at the input ofthe structure is quite high, the feasibility of which mustclearly be demonstrated.The 250q pulsed surface heating of the cell walls is ofeven greater concern, since it is probably well above anacceptable value. The temperature rise may be reducedby optimising the cell geometry, in particular bydecreasing both the thickness and width of the couplingiris. In this way the coupling of the damping waveguidesto the dipole mode can be maintained while decreasingthe perturbation of the fundamental mode currents.An overall shift to smaller irises (dimension 2a) andfewer cells - thus maintaining the same RF to beamefficiency - would reduce both the peak surface electricfield and the pulsed surface heating in the first cell.Beam dynamics simulations are needed to determinehow much increase in the transverse wakefield due to thesmaller irises can be tolerated.ACKNOWLEDGEMENTSThe authors would like to acknowledge their sincereand deep appreciation to Micha Dehler and MichelLuong, who have both made substantial contributions tothe development of this structure. Special thanks are alsoextended to Erk Jensen for maintaining the circuitmodel.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] M.Luong, I.Wilson, W.Wuensch, “RF Loads for theCLIC Multibunch Structure”, PAC’99, New York,March 1999. [6] C. Adolphsen et. al. “Wakefield and BeamCentering Measurements of a Damped and DetunedX-Band Accelerator Structure”, PAC’99, New York,March 1999.

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