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damping

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MOP020 CLIC Two-beam Module Design and Integration vacuum, alignment, quadrupole, linac 91
 
  • A. Samoshkin, D. Gudkov, G. Riddone
    CERN, Geneva
 
 

The CLIC (Compact LInear Collider) design is based on two-beam acceleration concept developed at CERN, where the RF power is generated by a high current electron-beam (Drive Beam) running parallel to the Main Beam. The Drive Beam is decelerated in special power extraction structures (PETS) and the generated RF power is transferred via waveguides to the accelerating structures (AS). The accelerating gradient must be very high (100 MV/m) to reach the high energy for the electron-positron collisions. To facilitate the matching of the beams, components are assembled in 2-m long modules, of few different types. In some of them the AS are replaced by quadrupoles used for the beam focusing. Their alignment and positioning is made by using the signals from the beam-position monitors (BPM). Special modules are needed in damping region or to carry out dedicated instrumentation and vacuum equipment. The module design and integration has to cope with challenging requirements from the different technical systems. This paper reports the status of the engineering design and reports on the main technical issues.

 
MOP023 The Accelerating Structure for a 500 GeV CLIC linac, luminosity, wakefield, accelerating-gradient 100
 
  • A. Grudiev, D. Schulte
    CERN, Geneva
 
 

The rf design of an accelerating structure for the 500 GeV CLIC main linac is presented. The design takes into account both aperture and HOM damping requirements coming from beam dynamics as well as the limitations related to rf breakdown and pulsed surface heating. In addition, the constraints related to the compatibility with 3 TeV CLIC have been taken into account. The structure is designed to provide 80 MV/m averaged accelerating gradient at 12 GHz with an rf-to-beam efficiency as high as 39.8 %.

 
MOP025 ACE3P Computations of Wakefield Coupling in the CLIC Two-beam Accelerator wakefield, simulation, coupling, linear-collider 106
 
  • A.E. Candel, K. Ko, Z. Li, C.-K. Ng, V. Rawat, G.L. Schussman
    SLAC, Menlo Park, California
  • A. Grudiev, I. Syratchev, W. Wuensch
    CERN, Geneva
 
 

The Compact Linear Collider (CLIC) provides a path to a multi-TeV accelerator to explore the energy frontier of High Energy Physics. Its novel two-beam accelerator concept envisions rf power transfer to the accelerating structures from a separate high-current decelerator beam line consisting of power extraction and transfer structures (PETS). It is critical to numerically verify the fundamental and higher-order mode properties in and between the two beam lines with high accuracy and confidence. To solve these large-scale problems, SLAC's parallel finite element electromagnetic code suite ACE3P is employed. Using curvilinear conformal meshes and higher-order finite element vector basis functions, unprecedented accuracy and computational efficiency are achieved, enabling high-fidelity modeling of complex detuned structures such as the CLIC TD24 accelerating structure. In this paper, time-domain simulations of wakefield coupling effects in the combined system of PETS and the TD24 structures are presented. The results will help to identify potential issues and provide new insights on the design, leading to further improvements on the novel CLIC two-beam accelerator scheme.

 
MOP067 First High Power Tests of CLIC Prototype Accelerating Structures with HOM Waveguide Damping linac, higher-order-mode, target, HOM 208
 
  • S. Döbert, A. Grudiev, G. Riddone, W. Wuensch, R. Zennaro
    CERN, Geneva
  • C. Adolphsen, F. Wang, J.W. Wang
    SLAC, Menlo Park, California
  • T. Higo, S. Matsumoto, K. Yokoyama
    KEK, Ibaraki
 
 

Prototype accelerating structures for the Compact Linear Collider (CLIC) are being developed and high-power tested in a collaboration between SLAC, KEK and CERN. Several undamped, low group-velocity and strongly tapered prototypes (of the so-called T18 design) have been operated above 100 MV/m average gradient at a very low breakdown rates. Recently two new structures with the same iris apertures but now including higher order mode damping waveguides in each cell (TD18 design) have been tested at SLAC and KEK. The damped versions could be processed to similar gradients but an increased breakdown rate was observed. The damping waveguides lead to a magnetic field enhancement in the outer diameter of the cells which results in increased pulsed surface heating. The maximum pulsed temperature rise is 80 deg at the design gradient of 100 MV/m compared to only 20 deg for the undamped version. The high-power tests of the two TD18 structures are analyzed with special emphasis on the influence on breakdown rate of the enhanced magnetic field and consequent increased pulsed surface temperature rise.

 
MOP068 Design of the CLIC Main Linac Accelerating Structure for CLIC Conceptual Design Report HOM, linac, wakefield, impedance 211
 
  • A. Grudiev, W. Wuensch
    CERN, Geneva
 
 

The design of the CLIC main linac accelerating structure has been refined based on an improved understanding of the high-gradient limits given by rf breakdown and pulsed surface heating. In addition, compact couplers have been developed and HOM damping loads have been designed. The rf design has also been made consistent with details of the present manufacturing procedure, based on bonded asymmetrical disks, and with requirements coming from integration of the accelerating structure in the two-beam module which includes all subsystems. This completion and refinement of the structure design has been made to produce the self-consistent parameter set required for preparation of the CLIC conceptual design report.

 
MOP069 Thermal Fatigue of Polycrystalline Copper in CLIC Accelerating Structures: Surface Roughening and Hardening as a Function of Grain Orientation laser, electron, radio-frequency, vacuum 214
 
  • M. Aicheler
    CERN, Geneva
 
 

The accelerating structures of CLIC will be submitted to 2 x 1010 thermal-mechanical fatigue cycles, arising from Radio Frequency (RF) induced eddy currents, causing local superficial cyclic heating. In order to assess the effects of superficial fatigue, high temperature annealed OFE Copper samples were thermally fatigued with the help of pulsed laser irradiation. They underwent postmortem Electron Backscattered Diffraction (EBSD) measurements andμhardness observations. Previous work has confirmed that surface roughening depends on the orientation of near-surface grains*,**. It is clearly observed that, through thermal cycling, the increase of hardness of a crystallographic direction is related to the amount of surface roughening induced by fatigue. Near-surface grains, oriented [1 0 0] with respect to the surface, exhibiting very low surface roughening, show limited hardening whereas grains oriented in [1 1 0], exhibiting severe surface roughening, show the most severe hardening. Consistently, surface roughening and hardening measured on [1 1 1] direction lie between the values measured for the other directions mentioned.


* Aicheler M et al.; 2010; Submitted to Int. Journal of Fatigue
** Aicheler M; 2009, Journal of Physics: Conference Series, Proceedings of ICSMA15

 
MOP073 Numerical Validation of the CLIC/SwissFEL/FERMI Multi Purpose X Band Structure wakefield, impedance, dipole, FEL 223
 
  • M.M. Dehler
    PSI, Villigen
  • A.E. Candel, L. Lee
    SLAC, Menlo Park, California
 
 

Currently an X-band traveling wave accelerator structure is fabricated in a collaboration between CERN, PSI and Sincrotrone Trieste (ST). PSI and ST will use it in their respective FEL projects, CERN will test break down limits and rates for high gradients. A special feature of this structure are two integrated wake field monitors monitoring the beam to structure alignment. The design used an uncoupled model for the fundamental mode, assuming the overall behavior to be the superposition of the individual components. For the wake field monitors, an equivalent circuit was used. This approach has been proven to produce valid structure designs. None the less it cannot approach the quality of a numerical electromagnetic simulation of the full structure, which is ideal for a validation capturing the differences between design models and the real cavity as e.g. internal reflections inside the structure or higher order dispersive terms altering the response of the wake field monitor. Using SLAC's family of massive parallel codes ACE-3P, first results are presented for the fundamental mode and the first transverse mode. They are compared with earlier simulations using simplified models.

 
MOP076 An Experimental Investigation on Cavity Pulsed Heating cavity, site, feedback, vacuum 232
 
  • L. Laurent, V.A. Dolgashev, C.D. Nantista, S.G. Tantawi
    SLAC, Menlo Park, California
  • M. Aicheler, S.T. Heikkinen, W. Wuensch
    CERN, Geneva
  • Y. Higashi
    KEK, Ibaraki
 
 

Cavity pulsed heating experiments have been conducted at SLAC National Accelerator Laboratory in collaboration with CERN and KEK. These experiments were designed to gain a better understanding on the impact of high power pulsed magnetic fields on copper and copper alloys. The cavity is a one port hemispherical cavity that operates in the TE013-like mode at 11.424 GHz. The test samples are mounted onto the endcap of the cavity. By using the TE013 mode, pulsed heating information can be analyzed that is based only on the impact of the peak magnetic field which is much bigger in value on the test sample than on any other place in the cavity. This work has shown that pulsed heating surface damage on copper and copper alloys is dependent on processing time, pulsed heating temperature, material hardness, and crystallographic orientation and that initial stresses occur along grain boundaries which can be followed by pitting or by transgranular microfractures that propagate and terminate on grain boundaries. The level of pulsed heating surface damage was found to be less on the smaller grain samples. This is likely due to grain boundaries limiting the propagation of fatigue cracks.

 
MOP103 Studies on High-precision Machining and Assembly of CLIC RF Structures simulation, cavity, alignment, collider 301
 
  • J. Huopana
    HIP, University of Helsinki
  • S. Atieh, G. Riddone
    CERN, Geneva
  • K. Österberg
    Helsinki University, Department of Physics, University of Helsinki
 
 

The Compact Linear Collider (CLIC) is currently under development at CERN as a potential multi-TeV e+e' collider. The manufacturing and assembly tolerances for the required RF components are essential for the final efficiency and for the operation of CLIC. The proper function of an accelerating structure is sensitive to mechanical errors in the shape and the alignment of the accelerating cavity. The current tolerances are in the micron range. This raises challenges in the field of mechanical design and demands special manufacturing technologies and processes. Currently the mechanical design of the accelerating structures is based on a disk design. Alternatively, it is possible to create the accelerating assembly from quadrants, which has the potential to be favoured for the mass production due to simplicity and cost. In this case, the functional shape inside of the accelerating structure remains the same and a single assembly uses less parts. This paper focuses on the development work done in design and simulation for prototype accelerating structures and describes its application to series production.

 
TUP098 Wakefield Monitor Development for CLIC Accelerating Structure wakefield, linac, cavity, alignment 641
 
  • F. Peauger, W. Farabolini, P. Girardot
    CEA, Gif-sur-Yvette
  • A. Andersson, G. Riddone, A. Samoshkin, A. Solodko
    CERN, Geneva
  • R.J.M.Y. Ruber
    Uppsala University, Uppsala
  • R. Zennaro
    PSI, Villigen
 
 

To achieve high luminosity in CLIC, the accelerating structures must be aligned to an RMS accuracy of 5 μm with respect to the beam trajectory. Position detectors called Wakefield Monitors (WFM) are integrated to the structure for a beam based alignment. This paper describes the requirements of such monitors. The development plan and basic feature of the WFM as well as the accelerating structure working at 12 GHz and 100 MV/m are shortly described. Then we focus on detailed electromagnetic simulations and design of the WFM itself. In particular, time domain computations are performed and an evaluation of the intrinsic resolution is done for two higher order modes at 17 and 24 GHz. The mechanical design of the accelerating structure with WFM is also presented. Precise machining with a tolerance of 2.5 μm and a surface roughness of 0.025 μm is demonstrated. The fabrication status of three complete accelerating structures with WFM is finally presented for a feasibility demonstration with beam in CTF3 at CERN.

 
THP009 Critical Dipole Modes in JLAB Upgrade Cavities cavity, HOM, cryomodule, dipole 776
 
  • F. Marhauser, J. Henry, H. Wang
    JLAB, Newport News, Virginia
 
 

The 12GeV upgrade of CEBAF is currently in progress. Ten new cryomodules will be installed at completion of the project to increase the energy from 6GeV to 12GeV. Each cryomodule houses eight seven-cell Low Loss type cavities. The damping of HOMs is crucial to prevent from beam break-up (BBU) instabilities at the desired beam currents as experienced with an upgrade demonstration cryomodule which needed to be de-installed recently. Detailed HOM surveys of a complete string of cavities in a cryomodule as well as individual cavities revealed the existence of critical dipole modes below and above beam tube cutoff that needed extensive experimental and numerical analyses. Results and their consequences for the 12 GeV upgrade cryomodules are detailed.

 
FR104 Progress of X-Band Accelerating Structures collider, linear-collider, vacuum, linac 1038
 
  • T. Higo
    KEK, Ibaraki
 
 

A CERN-SLAC-KEK collaboration on high gradient X-band accelerator structure development for CLIC has been ongoing for the past three years. A major outcome has been the stable 100 MV/m gradient operation of a number of CLIC prototype structures. The design of the structures, which have very strong higher-order-mode damping, is based on newly developed high-power scaling laws. The structures are being fabricated using the technology which was developed in the GLC/NLC projects which is giving excellent reproducibility. The features of this new generation of high-gradient normal conducting structures and their testing results are reviewed.

 

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