IPAC2012 - List of Authors (Grudiev, A.)

Paper	Title	Page
TUPPD069	Schottky-Enabled Photoemission and Dark Current Measurements - Toward an Alternate Approach to Fowler-Nordheim Plot Analysis	1563
	E.E. Wisniewski, W. Gai, J.G. Power ANL, Argonne, USA H. Chen, Y.-C. Du, Hua, J.F. Hua, W.-H. Huang, C.-X. Tang, L.X. Yan, Y. You TUB, Beijing, People's Republic of China A. Grudiev, W. Wuensch CERN, Geneva, Switzerland E.E. Wisniewski Illinois Institute of Technology, Chicago, Illinois, USA
	Field-emitted dark current, a major gradient-limiting factor in RF cavities, is usually analyzed via Fowler-Nordheim (FN) plots. Traditionally, field emission is attributed to geometrical perturbations on the bulk surface whose field enhancement factor (beta) and the emitting area (A) can be extracted from the FN plot. Field enhancement factors extracted in this way are typically much too high (1 to 2 orders of magnitude) to be explainable by either the geometric projection model applied to the measured surface roughness or by field enhancement factors extracted from Schottky-enabled photoemission measurements. We compare traditional analysis of FN plots to an alternate approach employing local work function variation. This is illustrated by comparative analysis of recent dark current and Schottky-enabled photoemission data taken at Tsinghua S-band RF gun. We conclude by describing a possible experimental plan for discrimination of variation of local work function vs. local field enhancement.

TUPPR015	Choke-Mode Damped Structure Design for the CLIC Main Linac	1840
	H. Zha, H. Chen, W.-H. Huang, C.-X. Tang TUB, Beijing, People's Republic of China A. Grudiev, J. Shi, W. Wuensch CERN, Geneva, Switzerland
	Choke-mode damped accelerating structures are being studied as an alternative to the CLIC waveguide damped baseline structure. Choke-mode structures hold the potential for much lower pulsed surface heating and reduced cost since no milling is required. We propose a new choke geometry which has significant suppression of higher order dipoles. By impedance matching and detuning of the first dipole pass-band, the wakefield suppression is comparable to the baseline design with waveguide damping. A fully featured choke mode structure with the same accelerating gradient profile and filling time as the nominal CLIC design has been designed. It has the potential to replace the waveguide damped design without changing any of the machine layout or the beam parameters.

TUPPR025	Higher-Order Modes and Beam Loading Compensation in CLIC Main Linac	1867
	O. Kononenko, A. Grudiev CERN, Geneva, Switzerland
	Compensation of transient beam loading is one of the major performance issues of the future compact linear collider (CLIC). Recent calculations, which consider only the most important fundamental mode, have shown that the 0.03% limit on the rms relative bunch-to-bunch energy spread in the main beam can be reached by optimizing the RF power pulse shape for the TD26, the CLIC baseline accelerating structure. Here, using HFSS and massively parallel ACE3P codes developed at SLAC, we perform an additional dedicated study of the influence of higher-order modes on the energy spread compensation scheme. It is shown that taking these modes into account in the accelerating structure does not increase the rms energy spread in the main beam above the CLIC specification level. Results of the HFSS and ACE3P simulations are also in a good agreement.

TUPPR026	Conceptual Design of the CLIC Damping Ring RF System	1870
	A. Grudiev CERN, Geneva, Switzerland
	In order to achieve high luminosity in CLIC, ultra-low emittance bunches have to be generated in both electron and positron damping rings. To achieve this goal, big energy loss per turn in the wigglers has to be compensated by the RF system. This results in very strong beam loading transients affecting the longitudinal bunch position and bunch length. In this paper, conceptual design of the RF system for the CLIC damping ring is presented. Baseline and several alternatives are discussed and the corresponding requirements for the cavities and the RF power sources are presented in order to meet stringent tolerances on the bunch-to-bunch phase and bunch length variations.

TUPPR027	Study of Multipolar RF Kicks from the Main Deflecting Mode in Compact Crab Cavities for LHC	1873
	A. Grudiev, J. Barranco, R. Calaga, R. De Maria, M. Giovannozzi, R. Tomás CERN, Geneva, Switzerland
	A crab cavity system is under design in the frame work of the High Luminosity LHC project. Due to transverse space constraints on one hand and the RF frequency requirements on the other hand, the design of the crab cavities has to be compact. This results in the crab cavity shape being far from axially symmetric and, as a consequence, higher order multipolar components of the main deflecting mode are non-zero. In this paper, multipolar RF kicks from the main deflecting mode have been calculated in the compact crab cavities for LHC. They are compared to the multipolar error in magnetic elements of LHC. The influence of the RF kicks on the beam dynamics has been investigated and possible acceptable tolerances are presented.

TUPPC086	Conceptual Design of the CLIC damping rings	1368
	Y. Papaphilippou, F. Antoniou, M.J. Barnes, S. Calatroni, P. Chiggiato, R. Corsini, A. Grudiev, J. Holma, T. Lefèvre, M. Martini, M. Modena, N. Mounet, A. Perin, Y. Renier, G. Rumolo, S. Russenschuck, H. Schmickler, D. Schoerling, D. Schulte, M. Taborelli, G. Vandoni, F. Zimmermann CERN, Geneva, Switzerland C. Belver-Aguilar, A. Faus-Golfe IFIC, Valencia, Spain A. Bernhard KIT, Karlsruhe, Germany M.J. Boland ASCo, Clayton, Victoria, Australia A.V. Bragin, E.B. Levichev, S.V. Sinyatkin, P. Vobly, K. Zolotarev BINP SB RAS, Novosibirsk, Russia M. Korostelev Cockcroft Institute, Warrington, Cheshire, United Kingdom E. Koukovini EPFL, Lausanne, Switzerland M.A. Palmer CLASSE, Ithaca, New York, USA M.T.F. Pivi, S.R. Smith SLAC, Menlo Park, California, USA R.P. Rassool, K.P. Wootton The University of Melbourne, Melbourne, Australia L. Rinolfi JUAS, Archamps, France A. Vivoli Fermilab, Batavia, USA
	The CLIC damping rings are designed to produce unprecedentedly low-emittances of 500 nm and 5 nm normalized at 2.86 GeV, in all beam dimensions with high bunch charge, necessary for the performance of the collider. The large beam brightness triggers a number of beam dynamics and technical challenges. Ring parameters such as energy, circumference, lattice, momentum compaction, bending and super-conducting wiggler fields are carefully chosen in order to provide the target emittances under the influence of intrabeam scattering but also reduce the impact of collective effects such as space-charge and coherent synchrotron radiation. Mitigation techniques for two stream instabilities have been identified and tested. The low vertical emittance is achieved by modern orbit and coupling correction techniques. Design considerations and plans for technical system, such as damping wigglers, transfer systems, vacuum, RF cavities, instrumentation and feedback are finally reviewed.

WEPPR064	Very Short Range Wake in Strongly Tapered Disk Loaded Waveguide Structures	3072
	A. Grudiev CERN, Geneva, Switzerland
	Electron bunches are very short, both in linear collider and in X-FEL projects. Furthermore, typical disk-loaded waveguide structures used for particle acceleration are tapered. For example in CLIC, in order to achieve high accelerating gradient, the structure is only 26 cells long, which results in strong tapering. In this paper, very short range wake is investigated in the regime where the number of cells needed to arrive at steady state is much larger than the number of cells in a single tapered structure. In this case the very short range wake is dominated by the wake from the smallest aperture. The results of an analytical model and numeric solutions are compared.

WEPPR065	Electromagnetic Simulations of the Impedance of the LHC Injection Protection Collimator	3075
	B. Salvant, V. Baglin, B. Goddard, A. Grudiev, E. Métral, M.A. Timmins CERN, Geneva, Switzerland
	During the 2011 LHC run, significant vacuum and temperature increase were observed at the location of the LHC injection protection collimators (TDI) during the physics fills. Besides, measurements of the LHC transverse tune shift while changing the TDI gap showed that the impedance of the TDI was significantly higher than the LHC impedance model prediction based on multilayer infinite length theory. This contribution details the electromagnetic simulations performed with a full 3D model of the TDI to obtain both longitudinal and transverse impedances and their comparison with measured observables.

WEPPR067	Study of Fundamental Mode Multipolar Kicks in Double- and Single-feed Power Couplers for the CLIC Main Linac Accelerating Structure	3081
	A. Latina, A. Grudiev, D. Schulte CERN, Geneva, Switzerland
	Multipolar kicks from the fundamental mode have been calculated in the CLIC baseline accelerating structure with double–feed input and output power couplers. The influence of such multipolar kicks on the main linac beam dynamics has been investigated. Furthermore, an alternative design of the couplers with single-feed has been studied and compared with the double-feed. Such an alternative would significantly simplify the waveguide system of the main linac but potentially introduce an harmful dipolar kick from the fundamental mode. The geometry of the coupler has been optimized in order to minimize such a dipolar kick and keep it below threshold levels determined with beam dynamics simulations. Influence of the higher order multipoles has been investigated as well and acceptable levels have been determined.

WEPPR070	Beam Coupling Impedance Simulations of the LHC TCTP Collimators	3090
	H.A. Day, R.M. Jones UMAN, Manchester, United Kingdom F. Caspers, A. Dallocchio, L. Gentini, A. Grudiev, E. Métral, B. Salvant CERN, Geneva, Switzerland
	As part of an upgrade to the LHC collimation system, 8 TCTP and 1 TCSG collimators are proposed to replace existing collimators in the collimation system. In an effort to review all equipment placed in the accelerator complex for potential side effects due to collective effects and beam-equipment interactions, beam coupling impedance simulations are carried out in both the time-domain and frequency-domain of the full TCTP design. Particular attention is paid to trapped modes that may induce beam instabilities and beam-induced heating due to cavity modes of the device.

WEPPR077	Analysis of Long-range Wakefields in CLIC Main Linac Accelerating Structures with Damping Loads	3111
	G. De Michele Paul Scherrer Institut, Villigen, Switzerland G. De Michele EPFL, Lausanne, Switzerland A. Grudiev CERN, Geneva, Switzerland
	The baseline design of the CLIC accelerating structure foresees a moderate detuning and heavy damping of high order modes (HOMs), which are the source of long-range transverse wakefields. Such unwanted fields produce bunch-to-bunch instabilities so the HOMs must be suppressed. In order to damp these modes, the CLIC RF structure is equipped with lossy material inserted into four rectangular waveguides coupled to each accelerating cell. The lossy material absorbs EM (electromagnetic) wave energy with little reflection back to the accelerating cells. In the past, computations of the long-range wake of CLIC accelerating modes have been done using perfectly absorbing boundaries to terminate the damping waveguides. In this paper, 3D EM simulations of CLIC baseline accelerating structure with HOMs damping loads will be presented. A comparison between different EM codes (GdfidL, CST PARTICLE STUDIO®) will be discussed as well as the analysis of different types of absorbing materials with respect to the wakefields damping.

THPPC011	Design of an Accelerating Structure for a 500 GeV CLIC using Ace3P	3296
	K.N. Sjobak, E. Adli University of Oslo, Oslo, Norway A. Grudiev, W. Wuensch CERN, Geneva, Switzerland
	Funding: Research Council of Norway An optimized design of the main linac accelerating structure for a 500 GeV first stage of CLIC is presented. A similar long-range wakefield suppression scheme as for 3 TeV CLIC based on heavy waveguide damping is adopted. The accelerating gradient for the lower energy machine is 80 MV/m. The 500 GeV design has larger aperture radius in order to increase the maximum bunch charge and length which is limited by the short-range wakefields. The cell geometries have been optimized using a new parametric optimizer for Ace3P and details of the RF cell design are described. Parameters of the full structure are calculated and optimized using a power flow equation.

THPPC022	Enhanced Coupling Design of a Detuned Damped Structure for CLIC	3323
	A. D'Elia, A. Grudiev, V.F. Khan, W. Wuensch CERN, Geneva, Switzerland T. Higo KEK, Ibaraki, Japan R.M. Jones UMAN, Manchester, United Kingdom
	The key feature of the improved coupling design in the Damped Detuned Structure (DDS) is focused on the four manifolds. Rectangular geometry slots and rectangular manifolds are used. This results in a significantly stronger coupling to the manifolds compared to the previous design. We describe the new design together with its wakefield damping properties.

Paper

Title

Page

Schottky-Enabled Photoemission and Dark Current Measurements - Toward an Alternate Approach to Fowler-Nordheim Plot Analysis

1563

E.E. Wisniewski, W. Gai, J.G. Power
ANL, Argonne, USA
H. Chen, Y.-C. Du, Hua, J.F. Hua, W.-H. Huang, C.-X. Tang, L.X. Yan, Y. You
TUB, Beijing, People's Republic of China
A. Grudiev, W. Wuensch
CERN, Geneva, Switzerland
E.E. Wisniewski
Illinois Institute of Technology, Chicago, Illinois, USA

Field-emitted dark current, a major gradient-limiting factor in RF cavities, is usually analyzed via Fowler-Nordheim (FN) plots. Traditionally, field emission is attributed to geometrical perturbations on the bulk surface whose field enhancement factor (beta) and the emitting area (A) can be extracted from the FN plot. Field enhancement factors extracted in this way are typically much too high (1 to 2 orders of magnitude) to be explainable by either the geometric projection model applied to the measured surface roughness or by field enhancement factors extracted from Schottky-enabled photoemission measurements. We compare traditional analysis of FN plots to an alternate approach employing local work function variation. This is illustrated by comparative analysis of recent dark current and Schottky-enabled photoemission data taken at Tsinghua S-band RF gun. We conclude by describing a possible experimental plan for discrimination of variation of local work function vs. local field enhancement.

TUPPR015

Choke-Mode Damped Structure Design for the CLIC Main Linac

1840

H. Zha, H. Chen, W.-H. Huang, C.-X. Tang
TUB, Beijing, People's Republic of China
A. Grudiev, J. Shi, W. Wuensch
CERN, Geneva, Switzerland

Choke-mode damped accelerating structures are being studied as an alternative to the CLIC waveguide damped baseline structure. Choke-mode structures hold the potential for much lower pulsed surface heating and reduced cost since no milling is required. We propose a new choke geometry which has significant suppression of higher order dipoles. By impedance matching and detuning of the first dipole pass-band, the wakefield suppression is comparable to the baseline design with waveguide damping. A fully featured choke mode structure with the same accelerating gradient profile and filling time as the nominal CLIC design has been designed. It has the potential to replace the waveguide damped design without changing any of the machine layout or the beam parameters.

TUPPR025

Higher-Order Modes and Beam Loading Compensation in CLIC Main Linac

1867

O. Kononenko, A. Grudiev
CERN, Geneva, Switzerland

Compensation of transient beam loading is one of the major performance issues of the future compact linear collider (CLIC). Recent calculations, which consider only the most important fundamental mode, have shown that the 0.03% limit on the rms relative bunch-to-bunch energy spread in the main beam can be reached by optimizing the RF power pulse shape for the TD26, the CLIC baseline accelerating structure. Here, using HFSS and massively parallel ACE3P codes developed at SLAC, we perform an additional dedicated study of the influence of higher-order modes on the energy spread compensation scheme. It is shown that taking these modes into account in the accelerating structure does not increase the rms energy spread in the main beam above the CLIC specification level. Results of the HFSS and ACE3P simulations are also in a good agreement.

TUPPR026

Conceptual Design of the CLIC Damping Ring RF System

1870

A. Grudiev
CERN, Geneva, Switzerland

In order to achieve high luminosity in CLIC, ultra-low emittance bunches have to be generated in both electron and positron damping rings. To achieve this goal, big energy loss per turn in the wigglers has to be compensated by the RF system. This results in very strong beam loading transients affecting the longitudinal bunch position and bunch length. In this paper, conceptual design of the RF system for the CLIC damping ring is presented. Baseline and several alternatives are discussed and the corresponding requirements for the cavities and the RF power sources are presented in order to meet stringent tolerances on the bunch-to-bunch phase and bunch length variations.

TUPPR027

Study of Multipolar RF Kicks from the Main Deflecting Mode in Compact Crab Cavities for LHC

1873

A. Grudiev, J. Barranco, R. Calaga, R. De Maria, M. Giovannozzi, R. Tomás
CERN, Geneva, Switzerland

A crab cavity system is under design in the frame work of the High Luminosity LHC project. Due to transverse space constraints on one hand and the RF frequency requirements on the other hand, the design of the crab cavities has to be compact. This results in the crab cavity shape being far from axially symmetric and, as a consequence, higher order multipolar components of the main deflecting mode are non-zero. In this paper, multipolar RF kicks from the main deflecting mode have been calculated in the compact crab cavities for LHC. They are compared to the multipolar error in magnetic elements of LHC. The influence of the RF kicks on the beam dynamics has been investigated and possible acceptable tolerances are presented.

TUPPC086

Conceptual Design of the CLIC damping rings

1368

Y. Papaphilippou, F. Antoniou, M.J. Barnes, S. Calatroni, P. Chiggiato, R. Corsini, A. Grudiev, J. Holma, T. Lefèvre, M. Martini, M. Modena, N. Mounet, A. Perin, Y. Renier, G. Rumolo, S. Russenschuck, H. Schmickler, D. Schoerling, D. Schulte, M. Taborelli, G. Vandoni, F. Zimmermann
CERN, Geneva, Switzerland
C. Belver-Aguilar, A. Faus-Golfe
IFIC, Valencia, Spain
A. Bernhard
KIT, Karlsruhe, Germany
M.J. Boland
ASCo, Clayton, Victoria, Australia
A.V. Bragin, E.B. Levichev, S.V. Sinyatkin, P. Vobly, K. Zolotarev
BINP SB RAS, Novosibirsk, Russia
M. Korostelev
Cockcroft Institute, Warrington, Cheshire, United Kingdom
E. Koukovini
EPFL, Lausanne, Switzerland
M.A. Palmer
CLASSE, Ithaca, New York, USA
M.T.F. Pivi, S.R. Smith
SLAC, Menlo Park, California, USA
R.P. Rassool, K.P. Wootton
The University of Melbourne, Melbourne, Australia
L. Rinolfi
JUAS, Archamps, France
A. Vivoli
Fermilab, Batavia, USA

The CLIC damping rings are designed to produce unprecedentedly low-emittances of 500 nm and 5 nm normalized at 2.86 GeV, in all beam dimensions with high bunch charge, necessary for the performance of the collider. The large beam brightness triggers a number of beam dynamics and technical challenges. Ring parameters such as energy, circumference, lattice, momentum compaction, bending and super-conducting wiggler fields are carefully chosen in order to provide the target emittances under the influence of intrabeam scattering but also reduce the impact of collective effects such as space-charge and coherent synchrotron radiation. Mitigation techniques for two stream instabilities have been identified and tested. The low vertical emittance is achieved by modern orbit and coupling correction techniques. Design considerations and plans for technical system, such as damping wigglers, transfer systems, vacuum, RF cavities, instrumentation and feedback are finally reviewed.

WEPPR064

Very Short Range Wake in Strongly Tapered Disk Loaded Waveguide Structures

3072

A. Grudiev
CERN, Geneva, Switzerland

Electron bunches are very short, both in linear collider and in X-FEL projects. Furthermore, typical disk-loaded waveguide structures used for particle acceleration are tapered. For example in CLIC, in order to achieve high accelerating gradient, the structure is only 26 cells long, which results in strong tapering. In this paper, very short range wake is investigated in the regime where the number of cells needed to arrive at steady state is much larger than the number of cells in a single tapered structure. In this case the very short range wake is dominated by the wake from the smallest aperture. The results of an analytical model and numeric solutions are compared.

WEPPR065

Electromagnetic Simulations of the Impedance of the LHC Injection Protection Collimator

3075

B. Salvant, V. Baglin, B. Goddard, A. Grudiev, E. Métral, M.A. Timmins
CERN, Geneva, Switzerland

During the 2011 LHC run, significant vacuum and temperature increase were observed at the location of the LHC injection protection collimators (TDI) during the physics fills. Besides, measurements of the LHC transverse tune shift while changing the TDI gap showed that the impedance of the TDI was significantly higher than the LHC impedance model prediction based on multilayer infinite length theory. This contribution details the electromagnetic simulations performed with a full 3D model of the TDI to obtain both longitudinal and transverse impedances and their comparison with measured observables.

WEPPR067

Study of Fundamental Mode Multipolar Kicks in Double- and Single-feed Power Couplers for the CLIC Main Linac Accelerating Structure

3081

A. Latina, A. Grudiev, D. Schulte
CERN, Geneva, Switzerland

Multipolar kicks from the fundamental mode have been calculated in the CLIC baseline accelerating structure with double–feed input and output power couplers. The influence of such multipolar kicks on the main linac beam dynamics has been investigated. Furthermore, an alternative design of the couplers with single-feed has been studied and compared with the double-feed. Such an alternative would significantly simplify the waveguide system of the main linac but potentially introduce an harmful dipolar kick from the fundamental mode. The geometry of the coupler has been optimized in order to minimize such a dipolar kick and keep it below threshold levels determined with beam dynamics simulations. Influence of the higher order multipoles has been investigated as well and acceptable levels have been determined.

WEPPR070

Beam Coupling Impedance Simulations of the LHC TCTP Collimators

3090

H.A. Day, R.M. Jones
UMAN, Manchester, United Kingdom
F. Caspers, A. Dallocchio, L. Gentini, A. Grudiev, E. Métral, B. Salvant
CERN, Geneva, Switzerland

As part of an upgrade to the LHC collimation system, 8 TCTP and 1 TCSG collimators are proposed to replace existing collimators in the collimation system. In an effort to review all equipment placed in the accelerator complex for potential side effects due to collective effects and beam-equipment interactions, beam coupling impedance simulations are carried out in both the time-domain and frequency-domain of the full TCTP design. Particular attention is paid to trapped modes that may induce beam instabilities and beam-induced heating due to cavity modes of the device.

WEPPR077

Analysis of Long-range Wakefields in CLIC Main Linac Accelerating Structures with Damping Loads

3111

G. De Michele
Paul Scherrer Institut, Villigen, Switzerland
G. De Michele
EPFL, Lausanne, Switzerland
A. Grudiev
CERN, Geneva, Switzerland

The baseline design of the CLIC accelerating structure foresees a moderate detuning and heavy damping of high order modes (HOMs), which are the source of long-range transverse wakefields. Such unwanted fields produce bunch-to-bunch instabilities so the HOMs must be suppressed. In order to damp these modes, the CLIC RF structure is equipped with lossy material inserted into four rectangular waveguides coupled to each accelerating cell. The lossy material absorbs EM (electromagnetic) wave energy with little reflection back to the accelerating cells. In the past, computations of the long-range wake of CLIC accelerating modes have been done using perfectly absorbing boundaries to terminate the damping waveguides. In this paper, 3D EM simulations of CLIC baseline accelerating structure with HOMs damping loads will be presented. A comparison between different EM codes (GdfidL, CST PARTICLE STUDIO®) will be discussed as well as the analysis of different types of absorbing materials with respect to the wakefields damping.

THPPC011

Design of an Accelerating Structure for a 500 GeV CLIC using Ace3P

3296

K.N. Sjobak, E. Adli
University of Oslo, Oslo, Norway
A. Grudiev, W. Wuensch
CERN, Geneva, Switzerland

Funding: Research Council of Norway
An optimized design of the main linac accelerating structure for a 500 GeV first stage of CLIC is presented. A similar long-range wakefield suppression scheme as for 3 TeV CLIC based on heavy waveguide damping is adopted. The accelerating gradient for the lower energy machine is 80 MV/m. The 500 GeV design has larger aperture radius in order to increase the maximum bunch charge and length which is limited by the short-range wakefields. The cell geometries have been optimized using a new parametric optimizer for Ace3P and details of the RF cell design are described. Parameters of the full structure are calculated and optimized using a power flow equation.

THPPC022

Enhanced Coupling Design of a Detuned Damped Structure for CLIC

3323

A. D'Elia, A. Grudiev, V.F. Khan, W. Wuensch
CERN, Geneva, Switzerland
T. Higo
KEK, Ibaraki, Japan
R.M. Jones
UMAN, Manchester, United Kingdom

The key feature of the improved coupling design in the Damped Detuned Structure (DDS) is focused on the four manifolds. Rectangular geometry slots and rectangular manifolds are used. This results in a significantly stronger coupling to the manifolds compared to the previous design. We describe the new design together with its wakefield damping properties.