LINAC2014 - List of Authors (Yakovlev, V.P.)

Paper

Title

Page

On the Design of Higher Order Mode Antenna for LCLS II

161

M.H. Awida, I.V. Gonin, T.N. Khabiboulline, K.S. Premo, O.V. Pronitchev, N. Solyak, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Funding: Operated by Fermi Research Alliance, LLC, under Contract DE-AC02-07CH11359 with the U.S. DOE
The upgrade of the Linac Coherent Light Source (LCLS-II) necessitates a major modification to the higher order mode (HOM) antenna of the conventional ILC elliptical 9-cell cavity. Due to the continuous wave nature of the proposed LCLS II Linac, the HOM antenna is required to bare higher RF losses. A modified design of the HOM antenna is presented in this paper ahead with a thorough thermal quench study in comparison with the conventional ILC design.

MOPP049

Dipole Kick due to Geometry Asymmetries in HWR for PXIE

165

P. Berrutti, T.N. Khabiboulline, V.A. Lebedev, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Funding: Work supported by D.O.E. Contract No. DE-AC02-07CH11359
Project X Injector Experiment (PXIE) will have a family of half wave resonators having frequency=162.5 MHz and beta optimal=0.11. During cavity production, when the niobium parts are assembled and welded together, it is fundamental to control the frequency of the accelerating mode in order to meet the specified operating value. For the HWR of PXIE the tuning will be achieved by trimming one end of the resonator only, this will introduce unwanted asymmetry in the cavity geometry leading to a dipole kick for the particles traveling through the cavity. The cavity geometry will be different from the ideal, once the cavity is assembled, because of small misalignment of the niobium parts and because of the welding shrinkage. Misalignments of the inner conductor and the beam pipes can be expected. The asymmetry due to tuning process along with production misalignments, have been simulated and the equivalent dipole kick has been calculated.

Poster MOPP049 [1.441 MB]

MOPP052

Development of 5-Cell β=0.9 650 MHz Elliptical Cavities for Project X

171

I.V. Gonin, M.H. Awida, M.H. Foley, A. Grassellino, C.J. Grimm, T.N. Khabiboulline, A. Lunin, A.M. Rowe, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Several 5-cell 650 MHz elliptical cavities have been fabricated for the PIP-II Project. Two versions of the cavities have been designed to accelerate protons of relative group velocity of β=0.9 and β=0.92 in the high energy region of the linac. In this paper, we report the development status of these cavities, summarize the results of the quality control measurements performed on five initial prototypes, and outline the VTS test results.

MOPP053

TTF-III Coupler Modification for CW Operation

174

I.V. Gonin, T.N. Khabiboulline, A. Lunin, O.V. Prokofiev, N. Solyak, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

LCLS-II linac is based on XFEL/ILC superconducting technology, but CW regime of operation requires the modification of components to satisfy LCLS-II requirements. TTF-III coupler is considered as a candidate for a fundamental power coupler for the 1.3 GHz 9-cell accelerating structure at the LCLS-II project. In this paper we discuss the results of multiphysics analysis of the coupler working at various operating regimes. Two major modifications are proposed in order to meet the LCLS-II requirements and eliminate possible overheating: reducing the length of antenna (cold part) and increasing the thickness of a cooper plating on the inner conductor of the warm part of the coupler.

MOPP126

Untrapped HOM Radiation Absorption in the LCLS-II Cryomodules

351

K.L.F. Bane, C. Adolphsen, C.D. Nantista, T.O. Raubenheimer
SLAC, Menlo Park, California, USA
A. Saini, N. Solyak, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Funding: Work supported by Department of Energy contract DE–AC02–76SF00515.
The superconducting cavities in the continuous wave (CW) linacs of LCLS-II are designed to operate at 2 K, where cooling costs are very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length-especially in L3 the final linac-the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K and located between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the amount of beam power that needs to be removed at 2 K.

TUPP047

PXIE RFQ Bead Pull Measurements

535

P. Berrutti, T.N. Khabiboulline, V. Poloubotko, G.V. Romanov, J. Steimel, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA
D. Li, J.W. Staples
LBNL, Berkeley, California, USA

Funding: Work supported by D.O.E. Contract No. DE-AC02-07CH11359
Project X Injector Experiment radio frequency quadrupole has recently been built for Fermilab by Berkley laboratory. This RFQ will be placed after the low energy beam transport (LEBT) and before the medium energy beam transport (MEBT). The RFQ will operate at 162.5 MHz in CW regime; its function is to accelerate and focus particles coming from the LEBT at 30 keV, and to deliver a beam at 2.1 MeV to the MEBT. In order to make sure that the RFQ meets the specifications of field flatness and frequency the field in the vanes should be measured using bead pull technique. FNAL created a new single wire bead pull set up for the RFQ of PXIE. The measurements are used to find the electrical center of the structure, then the amplitude of the electromagnetic field in all the sectors of the RFQ; and the tuning will be based on these measurements. This paper describes the bead pull experimental set up, the software developed for this particular application and the measurements taken.

Poster TUPP047 [1.089 MB]

TUPP049

Test Stand for 325 MHz Power Couplers

538

S. Kazakov, B.M. Hanna, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

325 MHz superconducting Single Spoke resonators (SSR1) will be utilized in the Project X Injector Experiment (PXIE). Developed in Fermilab the main power coupler will be supply 2kW CW RF power to each cavity. Fermilab developed and designed the special test stand where the couplers will be tested up to 10 kW and design properties be confirmed. This paper describes the design of the coupler test stand and preliminary results of the tests.

TUPP054

Study of Beam-Based Alignment for the LCLS-II SC Linac

544

A. Saini, N. Solyak, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA
T.O. Raubenheimer
SLAC, Menlo Park, California, USA

The Linac Coherent Light Source (LCLS) is an x-ray free electron laser facility. The proposed upgrade of the LCLS facility is based on construction of 4 GeV superconducting (SC) linac. The achievable performance of linac is determined by beam sensitivity to various component errors. In this paper we review misalignment tolerances of LCLS-II SC linac and discuss possible beam-based alignment algorithm to meet these tolerances.

TUPP055

Progress on Euclid SRF Conical Half-Wave Resonator Project

547

E.N. Zaplatin
FZJ, Jülich, Germany
T.L. Grimm
Niowave, Inc., Lansing, Michigan, USA
A. Kanareykin
Euclid TechLabs, LLC, Solon, Ohio, USA
V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Funding: This Work is supported by the DOE SBIR Program, contract # DE-SC0006302.
Euclid conical Half-Wave Resonator (cHWR) project develops 162.5 MHz β=v/c=0.11 accelerator structure for the high-intensity proton accelerator complex proposed at Fermi National Accelerator Laboratory. The main idea of this project is to provide a self-compensation cavity design together with its helium vessel to minimize the resonant frequency dependence on external loads. A unique cavity side-tuning option is also under development. Niowave, Inc. proposed a complete cavity production procedure including preparation of technical drawings, processing steps and resonator high-gradient tests to demonstrate such possibility for the private company. Here we present the procedure of the cavity and helium vessel fabrication, cavity preparation and initial experimental results.

THPP048

Design of a Compact Lever Slow/Fast Tuner for 650 MHz Cavities for Project X

957

I.V. Gonin, E. Borissov, T.N. Khabiboulline, Y.M. Pischalnikov, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Fermilab is developing 5-cell elliptical 650 MHz β=0.6 and β=0.9 cavities for Project X. A compact fast/slow lever tuner intended for both types of cavities has been developed for final tuning of the resonance frequency of the cavity after cooling down and to compensate the resonance frequency variations of the cavity during operation coming from liquid helium pressure fluctuations. The updated helium vessel (presented at this conference) is equipped with the tuner located at one of the end of the cavity. The tuner design and results of ANSYS analysis of their properties are presented.

THPP049

Design of 162.5 MHz CW Main Coupler for RFQ

960

S. Kazakov, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, J. Steimel, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Project X Injector Experiment (PXIE) at Fermilab will utilize 162.5 MHz CW RFQ accelerating cavity. Design of new main power coupler for PXIE RFQ is reported. Two identical couplers are supposed to deliver approximately 100 kW total CW RF power to RFQ. Unique design of the coupler allows providing DC bias for multipactor suppression. Results of RF and thermal simulations along with mechanical design are presented.

THPP050

Status of 325 MHz Main Couplers for PXIE

963

S. Kazakov, B.M. Hanna, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, L. Ristori, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

The Project X Injector Experiment (PXIE) at Fermilab will include one cryomodule with eight 325 MHz single spoke superconductive cavities (SSR1). Each cavity requires approximately 2 kW CW RF power for 1 mA beam current operation. A future upgrade will require up to 8 kW RF power per cavity. Fermilab has designed, procured and tested two prototype couplers for the SSR type cavities. Status of the 325 MHz main coupler development for PXIE is reported.

THPP051

Design of a Quasi-Waveguide Multicell Deflecting Cavity for the Advanced Photon Source

966

A. Lunin, I.V. Gonin, T.N. Khabiboulline, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA
A. Zholents
ANL, Argonne, Ilinois, USA

This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (called QMiR) being considered as a transverse RF deflecting cavity for the Advanced Photon Source’s (APS) Short Pulse X-ray project. QMiR forms a trapped dipole mode inside a beam vacuum chamber while High Order Modes (HOM) are heavily loaded. It results a sparse HOM spectrum, makes HOM couplers unnecessary and allows to simplify the cavity mechanical design. The form of electrodes is optimized for producing 2 MV of deflecting voltage and keeping low peak surface electric and magnetic fields of 54 MV/m and 75 mT respectively. Results of detailed EM analysis, including HOM damping at the actual geometry of beam vacuum chamber, will be presented.

Poster THPP051 [1.250 MB]

THPP054

Study of Coupler's Effect in Third Harmonic Section of LCLS-II SC Linac

969

A. Saini, A. Lunin, N. Solyak, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA
T.O. Raubenheimer
SLAC, Menlo Park, California, USA

The Linac Coherent Light Source (LCLS) is an x-ray free electron laser facility. The proposed upgrade of the LCLS facility is based on construction of 4 GeV superconducting (SC) linac which will use two stage bunch compression scheme in order to achieve short bunches with high peak current. In order to reduce non-linear effects in first bunch compressor, third harmonic section is utilized to linearize longitudinal phase space of the beam. However, transverse phase space of beam may get distorted due to coupler RF kicks and coupler wake kicks resulting from the asymmetry of input and HOM couplers in 3.9 GHz cavity. In this paper, we discuss coupler's effects and estimate resulting emittance dilution in third harmonic section. Local compensation of coupler kicks using different orientation of cavities are also addressed.

THPP124

Wakefields in the Superconducting RF Cavities of LCLS-II

1147

K.L.F. Bane, T.O. Raubenheimer
SLAC, Menlo Park, California, USA
A. Romanenko, V.P. Yakovlev
Fermilab, Batavia, Illinois, USA

Funding: Work supported by Department of Energy contract DE–AC02–76SF00515.
The superconducting cavities in the linacs of LCLS-II are designed to operate at 2K, where cooling costs are very expensive. In addition to an unavoidable static load and the dynamic load of the fundamental 1.3 GHz accelerating rf, there will be higher order mode (HOM) power deposited by the beam. Due to the very short bunch length the LCLS-II beam spectrum extends into the THz range. Ceramic absorbers, cooled to 70K and located between cryomodules, are meant to absorb much of this power; understanding their effectiveness, however, is a challenging task. In this report we calculate the amount of power radiated by the beam in the different portions of the linac as the bunch length is changed by the bunch compressors. We consider both the steady state radiation as well as transients that arise at the beginning of the linac structures. In addition, transitions due to changes in the vacuum chamber aperture at the ends of the linacs are also considered. Finally, under the assumption that all the wake power ends up in the SRF cavity walls, we estimate the wall heating and the possibility of breaking the Cooper pairs and quenching the cavities.

Paper	Title	Page
MOPP046	On the Design of Higher Order Mode Antenna for LCLS II	161
	M.H. Awida, I.V. Gonin, T.N. Khabiboulline, K.S. Premo, O.V. Pronitchev, N. Solyak, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Funding: Operated by Fermi Research Alliance, LLC, under Contract DE-AC02-07CH11359 with the U.S. DOE The upgrade of the Linac Coherent Light Source (LCLS-II) necessitates a major modification to the higher order mode (HOM) antenna of the conventional ILC elliptical 9-cell cavity. Due to the continuous wave nature of the proposed LCLS II Linac, the HOM antenna is required to bare higher RF losses. A modified design of the HOM antenna is presented in this paper ahead with a thorough thermal quench study in comparison with the conventional ILC design.

MOPP049	Dipole Kick due to Geometry Asymmetries in HWR for PXIE	165
	P. Berrutti, T.N. Khabiboulline, V.A. Lebedev, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Funding: Work supported by D.O.E. Contract No. DE-AC02-07CH11359 Project X Injector Experiment (PXIE) will have a family of half wave resonators having frequency=162.5 MHz and beta optimal=0.11. During cavity production, when the niobium parts are assembled and welded together, it is fundamental to control the frequency of the accelerating mode in order to meet the specified operating value. For the HWR of PXIE the tuning will be achieved by trimming one end of the resonator only, this will introduce unwanted asymmetry in the cavity geometry leading to a dipole kick for the particles traveling through the cavity. The cavity geometry will be different from the ideal, once the cavity is assembled, because of small misalignment of the niobium parts and because of the welding shrinkage. Misalignments of the inner conductor and the beam pipes can be expected. The asymmetry due to tuning process along with production misalignments, have been simulated and the equivalent dipole kick has been calculated.
	Poster MOPP049 [1.441 MB]

MOPP052	Development of 5-Cell β=0.9 650 MHz Elliptical Cavities for Project X	171
	I.V. Gonin, M.H. Awida, M.H. Foley, A. Grassellino, C.J. Grimm, T.N. Khabiboulline, A. Lunin, A.M. Rowe, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Several 5-cell 650 MHz elliptical cavities have been fabricated for the PIP-II Project. Two versions of the cavities have been designed to accelerate protons of relative group velocity of β=0.9 and β=0.92 in the high energy region of the linac. In this paper, we report the development status of these cavities, summarize the results of the quality control measurements performed on five initial prototypes, and outline the VTS test results.

MOPP053	TTF-III Coupler Modification for CW Operation	174
	I.V. Gonin, T.N. Khabiboulline, A. Lunin, O.V. Prokofiev, N. Solyak, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	LCLS-II linac is based on XFEL/ILC superconducting technology, but CW regime of operation requires the modification of components to satisfy LCLS-II requirements. TTF-III coupler is considered as a candidate for a fundamental power coupler for the 1.3 GHz 9-cell accelerating structure at the LCLS-II project. In this paper we discuss the results of multiphysics analysis of the coupler working at various operating regimes. Two major modifications are proposed in order to meet the LCLS-II requirements and eliminate possible overheating: reducing the length of antenna (cold part) and increasing the thickness of a cooper plating on the inner conductor of the warm part of the coupler.

MOPP126	Untrapped HOM Radiation Absorption in the LCLS-II Cryomodules	351
	K.L.F. Bane, C. Adolphsen, C.D. Nantista, T.O. Raubenheimer SLAC, Menlo Park, California, USA A. Saini, N. Solyak, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Funding: Work supported by Department of Energy contract DE–AC02–76SF00515. The superconducting cavities in the continuous wave (CW) linacs of LCLS-II are designed to operate at 2 K, where cooling costs are very expensive. One source of heat is presented by the higher order mode (HOM) power deposited by the beam. Due to the very short bunch length-especially in L3 the final linac-the LCLS-II beam spectrum extends into the terahertz range. Ceramic absorbers, at 70 K and located between cryomodules, are meant to absorb much of this power. In this report we perform two kinds of calculations to estimate the effectiveness of the absorbers and the amount of beam power that needs to be removed at 2 K.

TUPP047	PXIE RFQ Bead Pull Measurements	535
	P. Berrutti, T.N. Khabiboulline, V. Poloubotko, G.V. Romanov, J. Steimel, V.P. Yakovlev Fermilab, Batavia, Illinois, USA D. Li, J.W. Staples LBNL, Berkeley, California, USA
	Funding: Work supported by D.O.E. Contract No. DE-AC02-07CH11359 Project X Injector Experiment radio frequency quadrupole has recently been built for Fermilab by Berkley laboratory. This RFQ will be placed after the low energy beam transport (LEBT) and before the medium energy beam transport (MEBT). The RFQ will operate at 162.5 MHz in CW regime; its function is to accelerate and focus particles coming from the LEBT at 30 keV, and to deliver a beam at 2.1 MeV to the MEBT. In order to make sure that the RFQ meets the specifications of field flatness and frequency the field in the vanes should be measured using bead pull technique. FNAL created a new single wire bead pull set up for the RFQ of PXIE. The measurements are used to find the electrical center of the structure, then the amplitude of the electromagnetic field in all the sectors of the RFQ; and the tuning will be based on these measurements. This paper describes the bead pull experimental set up, the software developed for this particular application and the measurements taken.
	Poster TUPP047 [1.089 MB]

TUPP049	Test Stand for 325 MHz Power Couplers	538
	S. Kazakov, B.M. Hanna, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	325 MHz superconducting Single Spoke resonators (SSR1) will be utilized in the Project X Injector Experiment (PXIE). Developed in Fermilab the main power coupler will be supply 2kW CW RF power to each cavity. Fermilab developed and designed the special test stand where the couplers will be tested up to 10 kW and design properties be confirmed. This paper describes the design of the coupler test stand and preliminary results of the tests.

TUPP054	Study of Beam-Based Alignment for the LCLS-II SC Linac	544
	A. Saini, N. Solyak, V.P. Yakovlev Fermilab, Batavia, Illinois, USA T.O. Raubenheimer SLAC, Menlo Park, California, USA
	The Linac Coherent Light Source (LCLS) is an x-ray free electron laser facility. The proposed upgrade of the LCLS facility is based on construction of 4 GeV superconducting (SC) linac. The achievable performance of linac is determined by beam sensitivity to various component errors. In this paper we review misalignment tolerances of LCLS-II SC linac and discuss possible beam-based alignment algorithm to meet these tolerances.

TUPP055	Progress on Euclid SRF Conical Half-Wave Resonator Project	547
	E.N. Zaplatin FZJ, Jülich, Germany T.L. Grimm Niowave, Inc., Lansing, Michigan, USA A. Kanareykin Euclid TechLabs, LLC, Solon, Ohio, USA V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Funding: This Work is supported by the DOE SBIR Program, contract # DE-SC0006302. Euclid conical Half-Wave Resonator (cHWR) project develops 162.5 MHz β=v/c=0.11 accelerator structure for the high-intensity proton accelerator complex proposed at Fermi National Accelerator Laboratory. The main idea of this project is to provide a self-compensation cavity design together with its helium vessel to minimize the resonant frequency dependence on external loads. A unique cavity side-tuning option is also under development. Niowave, Inc. proposed a complete cavity production procedure including preparation of technical drawings, processing steps and resonator high-gradient tests to demonstrate such possibility for the private company. Here we present the procedure of the cavity and helium vessel fabrication, cavity preparation and initial experimental results.

THPP048	Design of a Compact Lever Slow/Fast Tuner for 650 MHz Cavities for Project X	957
	I.V. Gonin, E. Borissov, T.N. Khabiboulline, Y.M. Pischalnikov, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Fermilab is developing 5-cell elliptical 650 MHz β=0.6 and β=0.9 cavities for Project X. A compact fast/slow lever tuner intended for both types of cavities has been developed for final tuning of the resonance frequency of the cavity after cooling down and to compensate the resonance frequency variations of the cavity during operation coming from liquid helium pressure fluctuations. The updated helium vessel (presented at this conference) is equipped with the tuner located at one of the end of the cavity. The tuner design and results of ANSYS analysis of their properties are presented.

THPP049	Design of 162.5 MHz CW Main Coupler for RFQ	960
	S. Kazakov, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, J. Steimel, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Project X Injector Experiment (PXIE) at Fermilab will utilize 162.5 MHz CW RFQ accelerating cavity. Design of new main power coupler for PXIE RFQ is reported. Two identical couplers are supposed to deliver approximately 100 kW total CW RF power to RFQ. Unique design of the coupler allows providing DC bias for multipactor suppression. Results of RF and thermal simulations along with mechanical design are presented.

THPP050	Status of 325 MHz Main Couplers for PXIE	963
	S. Kazakov, B.M. Hanna, T.N. Khabiboulline, V. Poloubotko, O.V. Pronitchev, L. Ristori, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	The Project X Injector Experiment (PXIE) at Fermilab will include one cryomodule with eight 325 MHz single spoke superconductive cavities (SSR1). Each cavity requires approximately 2 kW CW RF power for 1 mA beam current operation. A future upgrade will require up to 8 kW RF power per cavity. Fermilab has designed, procured and tested two prototype couplers for the SSR type cavities. Status of the 325 MHz main coupler development for PXIE is reported.

THPP051	Design of a Quasi-Waveguide Multicell Deflecting Cavity for the Advanced Photon Source	966
	A. Lunin, I.V. Gonin, T.N. Khabiboulline, V.P. Yakovlev Fermilab, Batavia, Illinois, USA A. Zholents ANL, Argonne, Ilinois, USA
	This paper reports the electromagnetic design of a 2815 MHz Quasi-waveguide Multicell Resonator (called QMiR) being considered as a transverse RF deflecting cavity for the Advanced Photon Source’s (APS) Short Pulse X-ray project. QMiR forms a trapped dipole mode inside a beam vacuum chamber while High Order Modes (HOM) are heavily loaded. It results a sparse HOM spectrum, makes HOM couplers unnecessary and allows to simplify the cavity mechanical design. The form of electrodes is optimized for producing 2 MV of deflecting voltage and keeping low peak surface electric and magnetic fields of 54 MV/m and 75 mT respectively. Results of detailed EM analysis, including HOM damping at the actual geometry of beam vacuum chamber, will be presented.
	Poster THPP051 [1.250 MB]

THPP054	Study of Coupler's Effect in Third Harmonic Section of LCLS-II SC Linac	969
	A. Saini, A. Lunin, N. Solyak, V.P. Yakovlev Fermilab, Batavia, Illinois, USA T.O. Raubenheimer SLAC, Menlo Park, California, USA
	The Linac Coherent Light Source (LCLS) is an x-ray free electron laser facility. The proposed upgrade of the LCLS facility is based on construction of 4 GeV superconducting (SC) linac which will use two stage bunch compression scheme in order to achieve short bunches with high peak current. In order to reduce non-linear effects in first bunch compressor, third harmonic section is utilized to linearize longitudinal phase space of the beam. However, transverse phase space of beam may get distorted due to coupler RF kicks and coupler wake kicks resulting from the asymmetry of input and HOM couplers in 3.9 GHz cavity. In this paper, we discuss coupler's effects and estimate resulting emittance dilution in third harmonic section. Local compensation of coupler kicks using different orientation of cavities are also addressed.

THPP124	Wakefields in the Superconducting RF Cavities of LCLS-II	1147
	K.L.F. Bane, T.O. Raubenheimer SLAC, Menlo Park, California, USA A. Romanenko, V.P. Yakovlev Fermilab, Batavia, Illinois, USA
	Funding: Work supported by Department of Energy contract DE–AC02–76SF00515. The superconducting cavities in the linacs of LCLS-II are designed to operate at 2K, where cooling costs are very expensive. In addition to an unavoidable static load and the dynamic load of the fundamental 1.3 GHz accelerating rf, there will be higher order mode (HOM) power deposited by the beam. Due to the very short bunch length the LCLS-II beam spectrum extends into the THz range. Ceramic absorbers, cooled to 70K and located between cryomodules, are meant to absorb much of this power; understanding their effectiveness, however, is a challenging task. In this report we calculate the amount of power radiated by the beam in the different portions of the linac as the bunch length is changed by the bunch compressors. We consider both the steady state radiation as well as transients that arise at the beginning of the linac structures. In addition, transitions due to changes in the vacuum chamber aperture at the ends of the linacs are also considered. Finally, under the assumption that all the wake power ends up in the SRF cavity walls, we estimate the wall heating and the possibility of breaking the Cooper pairs and quenching the cavities.