IPAC2011 - List of Keywords (accelerating-gradient)

Paper	Title	Other Keywords	Page
MOOCA02	Two Beam Test Stand Experiments in the CTF3 Facility	linac, acceleration, diagnostics, ion	29
	W. Farabolini, F. Peauger CEA/DSM/IRFU, France J. Barranco, S. Bettoni, B. Constance, R. Corsini, M. Csatari, S. Döbert, A. Dubrovskiy, C. Heßler, T. Persson, G. Riddone, P.K. Skowroński, F. Tecker CERN, Geneva, Switzerland D. Gudkov, A. Solodko JINR, Dubna, Moscow Region, Russia M. Jacewicz, T. Muranaka, A. Palaia, R.J.M.Y. Ruber, V.G. Ziemann Uppsala University, Uppsala, Sweden
	The CLEX building in the CTF3 facility is the place where essential experiments are performed to validate the Two-Beam Acceleration scheme upon which the CLIC project relies. The Drive Beam enters the CLEX after being recombined in the Delay loop and the Combiner Ring in intense beam trains of 24 A – 150 MeV lasting 140 ns and bunched at 12 GHz, although other beam parameters are also accessible. This beam is then decelerated in dedicated structures installed in the Test Beam Line (TBL) and in the Two-Beam Test Stand (TBTS) aimed at delivering bursts of 12 GHz RF power. In the TBTS this power is used to generate a high accelerating gradient of 100 MV/m in specially designed accelerating structures. To assess the performances of these structures a probe beam is used, produced by a small Linac. We reported here the various experiences conducted in the TBTS making use of the versatility the probe beam and of dedicated diagnostics.
	Slides MOOCA02 [3.003 MB]

MOPC013	Design, Fabrication and High Power RF Test of a C-band Accelerating Structure for Feasibility Study of the SPARC Photo-injector Energy Upgrade	impedance, klystron, FEL, radiation	89
	D. Alesini, R. Boni, G. Di Pirro, R. D. Di Raddo, M. Ferrario, A. Gallo, V.L. Lollo, F. Marcellini INFN/LNF, Frascati (Roma), Italy G. Campogiani, A. Mostacci, L. Palumbo, S. Persichelli, V. Spizzo Rome University La Sapienza, Roma, Italy T. Higo, K. Kakihara, S. Matsumoto KEK, Ibaraki, Japan S. Verdú-Andrés TERA, Novara, Italy
	The energy upgrade of the SPARC photo-injector from 170 to 250 MeV will be done by replacing a low gradient 3m S-Band structure with two 1.5m high gradient C-band structures. The structures are traveling wave, constant impedance sections, have symmetric waveguide input couplers and have been optimized to work with a SLED RF input pulse. A prototype with a reduced number of cells has been fabricated and tested at high power in KEK (Japan) giving very good performances in terms of breakdown rates at high accelerating gradient (>50 MV/m). The paper illustrates the design criteria of the structures, the fabrication procedure and the high power RF test results.

MOPC018	Operation Status of C-band High Gradient Accelerator for XFEL/SPring-8 (SACLA)	electron, klystron, acceleration, target	104
	T. Inagaki, C. Kondo, T. Ohshima, Y. Otake, T. Sakurai, K. Shirasawa RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan T. Shintake RIKEN Spring-8 Harima, Hyogo, Japan
	XFEL project in SPring-8 have constructed a compact XFEL facility. In order to shorten an accelerator length, a C-band (5712 MHz) accelerator was employed due to a higher accelerating gradient than that of an S-band accelerator. Since a C-band accelerating structure generates a gradient of higher than 35 MV/m, the total length of an 8 GeV accelerator fits within 400 m, including 64 C-band RF units, 4 S-band RF units, an injector and three bunch compressors. The accelerator components were carefully installed by September 2010. Then we have performed high power RF conditioning. After 500 hours of the conditioning, the accelerating gradient of each C-band structure was reached up to 35 MV/m without any particular problem. The RF breakdown rate is low enough for an accelerator operation. Since February 2011, we started the beam commissioning for XFEL. The C-band accelerator has accelerated the electron beam up to 8 GeV, with an accelerating gradient of 33-35 MV/m in average. The energy and the trajectory of the electron beam was stable, thanks to the stabilization of a klystron voltage of 350 kV within 0.01% by a high precision high voltage charger. The facility was recently named SACLA (SPring-8 Angstrom Compact free electron LAser).

MOPC035	Design and Machine Features of 2.2-m C-band Accelerating Structure	cavity, linac, vacuum, electron	148
	C.H. Yi, M.-H. Cho, S.H. Kim, H. Lee POSTECH, Pohang, Kyungbuk, Republic of Korea W. Namkung PAL, Pohang, Kyungbuk, Republic of Korea
	Funding: This work is partly supported by the MEST, Korea and POSTECH BK21 Program. And this work was supported by the Korea Student Aid Foundation (KOSAF) grant funded by the Korea government. A compact linac system is designed using a longer accelerating column in a C-band linac. It reduces the total number of RF units for the given linac beam energy and results in the cost-effective use of RF powers. For the 10 GeV PAL-XFEL project, a C-band accelerating column of 2.2-m long is investigated, which is 22% longer than 1.8-m for the SACLA at SPring-8. The detailed RF and thermal characteristics are presented by an analytic model.

MOPC101	Vertical Test of PEFP Prototype SRF Cavity	cavity, electron, linac, SRF	307
	H.S. Kim, Y.-S. Cho, H.-J. Kwon KAERI, Daejon, Republic of Korea
	Funding: This work was supported by Ministry of Education, Science and Technology of the Korean Government. The PEFP Proton linac is a 100-MeV machine which consists of a proton injector, a 3-MeVRFQ and 100-MeV DTL. For the extension of the machine beyond 100 MeV, SRF technology is under consideration. As a prototyping activity, a superconducting RF cavity with a geometrical beta of 0.42 and a resonant frequency of 700 MHz has been designed, fabricated and tested. The cavity is an elliptical shape with 5 cells stiffened by double-ring structure. A design accelerating gradient is 8.0 MV/m at the operating temperature of 4.2 K and maximum duty factor is 9%. For the vertical test of the cavity, a cryostat with a vacuum jacket and multi-layer insulation was prepared. The RF system for driving the cavity is based on PLL to track the resonant frequency. In case of lack of RF power, a two-way RF power combiner based on splitted coaxial transmission line is considered. The details of the vertical test setup and test results will be presented in this paper.

MOPC111	Progress of ILC High Gradient SRF Cavity R&D at Jefferson Lab	cavity, SRF, niobium, cryogenics	334
	R.L. Geng, J. Dai, G.V. Eremeev, A.D. Palczewski JLAB, Newport News, Virginia, USA
	Funding: US Department of Energy Latest progress of ILC high gradient SRF cavity R&D at Jefferson Lab will be presented. 9 out of 10 real 9-cell cavities reached an accelerating gradient of more than 38 MV/m at a unloaded quality factor of more than 8·10⁹. New understandings of quench limitation in 9-cell cavities are obtained through instrumented studies of cavities at cryogenic temperatures. Our data have shown that present limit reached in 9-cell cavities is predominantly due to localized defects, suggesting that the fundamental material limit of niobium is not yet reached in 9-cell cavities and further gradient improvement is still possible. Some examples of quench-causing defects will be given. Possible solutions to pushing toward the fundamental limit will be described.

WEPC018	Self-focusing Effects in Compact C-band Standing-wave Accelerating Structure for X-ray Imaging Applications	focusing, bunching, electron, linac	2046
	H.R. Yang, M.-H. Cho, S.H. Kim, W. Namkung, S.J. Park POSTECH, Pohang, Kyungbuk, Republic of Korea J.-S. Oh NFRI, Daejon, Republic of Korea
	In electron RF linacs for industrial X-ray imaging applications, compact structures are preferred for mobility. The electron beam spot size of 1 – 2 mm is required for the spatial resolution of images at the X-ray conversion target. Applying self-focusing effects to the accelerating structure, external magnets can be removed and then the accelerator system becomes more compact. We design a C-band electron linac, which is capable of producing 6-MeV, 80-mA pulsed electron beams with an RF power of 1.5 MW. It uses a bi-periodic and on-axis-coupled accelerating structure with a built-in bunching section. It uses the π/2-mode standing-waves. The first bunching cell has an asymmetric geometry which maximizes the RF phase focusing. On the other hand, the normal cells are designed for the electrostatic focusing to be maximized. In this paper, we present design details of the accelerating cells and the beam dynamics simulation by the PARMELA code.

WEPC043	Beam Transport in a Dielectric Wall Accelerator for Intensity Modulated Proton Therapy	proton, focusing, beam-transport, emittance	2106
	Y.-J. Chen, D.T. Blackfield, S.D. Nelson, B. R. Poole LLNL, Livermore, California, USA
	Funding: This work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA2A27344. We are developing a compact dielectric wall accelerator (DWA) for intensity modulated proton therapy (IMPT) with a goal of fitting the compact proton DWA in a single room. To make the accelerator compact, the DWA needs to have a very high accelerating gradient. Also, beam transport in the DWA should be done with as few external lenses as possible. We have developed a transport scheme to transport the proton bunch in the DWA and to focus the charge bunch on the patient without using any external focusing lenses. The transport scheme would allow us change the proton beam spot size on the patient easily and rapidly. Results of simulations using 3-D, EM PIC code, LSP* will be presented. * G. J. Caporaso, Y-J Chen and S. E. Sampayan, Rev. of Accelerator Science and Technology, vol. 2, p. 253 (2009). ** Alliant Techsystems Inc., http://www.lspsuite.com/.

WEPS045	Feasibility Study of a High-gradient Linac for Hadrontherapy	linac, ion, cyclotron, cavity	2589
	S. Verdú-Andrés, U. Amaldi, A. Degiovanni TERA, Novara, Italy A. Faus-Golfe, S. Verdú-Andrés IFIC, Valencia, Spain P.A. Posocco CERN, Geneva, Switzerland
	Funding: The research leading to this results has been funded by the Seventh Framework Program [FP7/2007-2013] under grant agreement number 215840-2. Compact, reliable and little consuming accelerators are needed for tumor treatment with hadrons. As solution, TERA proposes CABOTO (CArbon BOoster for Therapy in Oncology), a linac which boosts the energy of carbon ions and H₂ molecules coming from a cyclotron. The linac, typically a Side-Coupled Linac (SCL), is divided into several modules. The beam energy can be varied in steps of about 15 MeV/u without using absorbers by acting on the power (amplitude and/or phase) that feeds the different modules of the linac. This work presents the structure design of a 5.7 GHz high repetition rate SCL for a cyclinac, that accelerates carbon ions from 150 up to 400 MeV/u in less than 25 meters. The beam dynamics for this linac and its particular energy selection system is also discussed for different beam energy outputs.

WEPS062	Design and Beam Dynamics Study of Hybrid ESS LINAC	linac, cavity, cryomodule, proton	2640
	M. Eshraqi, H. Danared, W. Hees, A. Jansson ESS, Lund, Sweden
	The European Spallation Source, {\sc ESS}, will use a superconducting linear accelerator delivering high current long pulses with an average beam power of 5~MW to the target station at 2.5~GeV. A new cryomodule architecture is proposed which allows for a transition between cryomodules in the sub-100~K region, this region can work even at room temperature. This new hybrid design will generate a lower heat load with respect to a fully segmented design - while still providing easy access to individual cryomodules for maintenance and repair. This paper will present a review of the {\sc linac} design, beam dynamics studies and a preliminary cryogenic analysis of the transition region.

WEPZ007	Multi-mode, Two-beam Accelerator with Feedback	cavity, feedback, impedance, RF-structure	2778
	S.V. Kuzikov, M.E. Plotkin IAP/RAS, Nizhny Novgorod, Russia
	A high-gradient accelerator consisted of the test and the drive beam structures is reported. The accelerating structure can be based on dielectric or corrugated cavities separated each other by irises. Each cavity is operated by several axisymmetric, TM-like eigen-modes with longitudinal indices to be related to frequencies. These modes are excited at Fourier harmonics of the drive current which consists of bunches spaced with the same period as test bunches. The superposition of the excited modes introduces a short RF pulse propagated in-phase with a moving test bunch and after reflection by iris (a feedback) this pulse can accelerate next bunch. Such longitudinally-sweeping RF field promises a reduction of the exposure time and due to compact space shape can help to obtain high shunt impedance.

Paper

Title

Other Keywords

Page

MOOCA02

Two Beam Test Stand Experiments in the CTF3 Facility

linac, acceleration, diagnostics, ion

29

W. Farabolini, F. Peauger
CEA/DSM/IRFU, France
J. Barranco, S. Bettoni, B. Constance, R. Corsini, M. Csatari, S. Döbert, A. Dubrovskiy, C. Heßler, T. Persson, G. Riddone, P.K. Skowroński, F. Tecker
CERN, Geneva, Switzerland
D. Gudkov, A. Solodko
JINR, Dubna, Moscow Region, Russia
M. Jacewicz, T. Muranaka, A. Palaia, R.J.M.Y. Ruber, V.G. Ziemann
Uppsala University, Uppsala, Sweden

The CLEX building in the CTF3 facility is the place where essential experiments are performed to validate the Two-Beam Acceleration scheme upon which the CLIC project relies. The Drive Beam enters the CLEX after being recombined in the Delay loop and the Combiner Ring in intense beam trains of 24 A – 150 MeV lasting 140 ns and bunched at 12 GHz, although other beam parameters are also accessible. This beam is then decelerated in dedicated structures installed in the Test Beam Line (TBL) and in the Two-Beam Test Stand (TBTS) aimed at delivering bursts of 12 GHz RF power. In the TBTS this power is used to generate a high accelerating gradient of 100 MV/m in specially designed accelerating structures. To assess the performances of these structures a probe beam is used, produced by a small Linac. We reported here the various experiences conducted in the TBTS making use of the versatility the probe beam and of dedicated diagnostics.

Slides MOOCA02 [3.003 MB]

MOPC013

Design, Fabrication and High Power RF Test of a C-band Accelerating Structure for Feasibility Study of the SPARC Photo-injector Energy Upgrade

impedance, klystron, FEL, radiation

89

D. Alesini, R. Boni, G. Di Pirro, R. D. Di Raddo, M. Ferrario, A. Gallo, V.L. Lollo, F. Marcellini
INFN/LNF, Frascati (Roma), Italy
G. Campogiani, A. Mostacci, L. Palumbo, S. Persichelli, V. Spizzo
Rome University La Sapienza, Roma, Italy
T. Higo, K. Kakihara, S. Matsumoto
KEK, Ibaraki, Japan
S. Verdú-Andrés
TERA, Novara, Italy

The energy upgrade of the SPARC photo-injector from 170 to 250 MeV will be done by replacing a low gradient 3m S-Band structure with two 1.5m high gradient C-band structures. The structures are traveling wave, constant impedance sections, have symmetric waveguide input couplers and have been optimized to work with a SLED RF input pulse. A prototype with a reduced number of cells has been fabricated and tested at high power in KEK (Japan) giving very good performances in terms of breakdown rates at high accelerating gradient (>50 MV/m). The paper illustrates the design criteria of the structures, the fabrication procedure and the high power RF test results.

MOPC018

Operation Status of C-band High Gradient Accelerator for XFEL/SPring-8 (SACLA)

electron, klystron, acceleration, target

104

T. Inagaki, C. Kondo, T. Ohshima, Y. Otake, T. Sakurai, K. Shirasawa
RIKEN SPring-8 Center, Sayo-cho, Sayo-gun, Hyogo, Japan
T. Shintake
RIKEN Spring-8 Harima, Hyogo, Japan

XFEL project in SPring-8 have constructed a compact XFEL facility*. In order to shorten an accelerator length, a C-band (5712 MHz) accelerator was employed due to a higher accelerating gradient than that of an S-band accelerator. Since a C-band accelerating structure generates a gradient of higher than 35 MV/m, the total length of an 8 GeV accelerator fits within 400 m, including 64 C-band RF units, 4 S-band RF units, an injector and three bunch compressors. The accelerator components were carefully installed by September 2010. Then we have performed high power RF conditioning. After 500 hours of the conditioning, the accelerating gradient of each C-band structure was reached up to 35 MV/m without any particular problem. The RF breakdown rate is low enough for an accelerator operation. Since February 2011, we started the beam commissioning for XFEL. The C-band accelerator has accelerated the electron beam up to 8 GeV, with an accelerating gradient of 33-35 MV/m in average. The energy and the trajectory of the electron beam was stable, thanks to the stabilization of a klystron voltage of 350 kV within 0.01% by a high precision high voltage charger.
*The facility was recently named SACLA (SPring-8 Angstrom Compact free electron LAser).

MOPC035

Design and Machine Features of 2.2-m C-band Accelerating Structure

cavity, linac, vacuum, electron

148

C.H. Yi, M.-H. Cho, S.H. Kim, H. Lee
POSTECH, Pohang, Kyungbuk, Republic of Korea
W. Namkung
PAL, Pohang, Kyungbuk, Republic of Korea

Funding: This work is partly supported by the MEST, Korea and POSTECH BK21 Program. And this work was supported by the Korea Student Aid Foundation (KOSAF) grant funded by the Korea government.
A compact linac system is designed using a longer accelerating column in a C-band linac. It reduces the total number of RF units for the given linac beam energy and results in the cost-effective use of RF powers. For the 10 GeV PAL-XFEL project, a C-band accelerating column of 2.2-m long is investigated, which is 22% longer than 1.8-m for the SACLA at SPring-8. The detailed RF and thermal characteristics are presented by an analytic model.

MOPC101

Vertical Test of PEFP Prototype SRF Cavity

cavity, electron, linac, SRF

307

H.S. Kim, Y.-S. Cho, H.-J. Kwon
KAERI, Daejon, Republic of Korea

Funding: This work was supported by Ministry of Education, Science and Technology of the Korean Government.
The PEFP Proton linac is a 100-MeV machine which consists of a proton injector, a 3-MeVRFQ and 100-MeV DTL. For the extension of the machine beyond 100 MeV, SRF technology is under consideration. As a prototyping activity, a superconducting RF cavity with a geometrical beta of 0.42 and a resonant frequency of 700 MHz has been designed, fabricated and tested. The cavity is an elliptical shape with 5 cells stiffened by double-ring structure. A design accelerating gradient is 8.0 MV/m at the operating temperature of 4.2 K and maximum duty factor is 9%. For the vertical test of the cavity, a cryostat with a vacuum jacket and multi-layer insulation was prepared. The RF system for driving the cavity is based on PLL to track the resonant frequency. In case of lack of RF power, a two-way RF power combiner based on splitted coaxial transmission line is considered. The details of the vertical test setup and test results will be presented in this paper.

MOPC111

Progress of ILC High Gradient SRF Cavity R&D at Jefferson Lab

cavity, SRF, niobium, cryogenics

334

R.L. Geng, J. Dai, G.V. Eremeev, A.D. Palczewski
JLAB, Newport News, Virginia, USA

Funding: US Department of Energy
Latest progress of ILC high gradient SRF cavity R&D at Jefferson Lab will be presented. 9 out of 10 real 9-cell cavities reached an accelerating gradient of more than 38 MV/m at a unloaded quality factor of more than 8·10⁹. New understandings of quench limitation in 9-cell cavities are obtained through instrumented studies of cavities at cryogenic temperatures. Our data have shown that present limit reached in 9-cell cavities is predominantly due to localized defects, suggesting that the fundamental material limit of niobium is not yet reached in 9-cell cavities and further gradient improvement is still possible. Some examples of quench-causing defects will be given. Possible solutions to pushing toward the fundamental limit will be described.

WEPC018

Self-focusing Effects in Compact C-band Standing-wave Accelerating Structure for X-ray Imaging Applications

focusing, bunching, electron, linac

2046

H.R. Yang, M.-H. Cho, S.H. Kim, W. Namkung, S.J. Park
POSTECH, Pohang, Kyungbuk, Republic of Korea
J.-S. Oh
NFRI, Daejon, Republic of Korea

In electron RF linacs for industrial X-ray imaging applications, compact structures are preferred for mobility. The electron beam spot size of 1 – 2 mm is required for the spatial resolution of images at the X-ray conversion target. Applying self-focusing effects to the accelerating structure, external magnets can be removed and then the accelerator system becomes more compact. We design a C-band electron linac, which is capable of producing 6-MeV, 80-mA pulsed electron beams with an RF power of 1.5 MW. It uses a bi-periodic and on-axis-coupled accelerating structure with a built-in bunching section. It uses the π/2-mode standing-waves. The first bunching cell has an asymmetric geometry which maximizes the RF phase focusing. On the other hand, the normal cells are designed for the electrostatic focusing to be maximized. In this paper, we present design details of the accelerating cells and the beam dynamics simulation by the PARMELA code.

WEPC043

Beam Transport in a Dielectric Wall Accelerator for Intensity Modulated Proton Therapy

proton, focusing, beam-transport, emittance

2106

Y.-J. Chen, D.T. Blackfield, S.D. Nelson, B. R. Poole
LLNL, Livermore, California, USA

Funding: This work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA2A27344.
We are developing a compact dielectric wall accelerator (DWA) for intensity modulated proton therapy (IMPT) with a goal of fitting the compact proton DWA in a single room*. To make the accelerator compact, the DWA needs to have a very high accelerating gradient. Also, beam transport in the DWA should be done with as few external lenses as possible. We have developed a transport scheme to transport the proton bunch in the DWA and to focus the charge bunch on the patient without using any external focusing lenses. The transport scheme would allow us change the proton beam spot size on the patient easily and rapidly. Results of simulations using 3-D, EM PIC code, LSP** will be presented.
* G. J. Caporaso, Y-J Chen and S. E. Sampayan, Rev. of Accelerator Science and Technology, vol. 2, p. 253 (2009).
** Alliant Techsystems Inc., http://www.lspsuite.com/.

WEPS045

Feasibility Study of a High-gradient Linac for Hadrontherapy

linac, ion, cyclotron, cavity

2589

S. Verdú-Andrés, U. Amaldi, A. Degiovanni
TERA, Novara, Italy
A. Faus-Golfe, S. Verdú-Andrés
IFIC, Valencia, Spain
P.A. Posocco
CERN, Geneva, Switzerland

Funding: The research leading to this results has been funded by the Seventh Framework Program [FP7/2007-2013] under grant agreement number 215840-2.
Compact, reliable and little consuming accelerators are needed for tumor treatment with hadrons. As solution, TERA proposes CABOTO (CArbon BOoster for Therapy in Oncology), a linac which boosts the energy of carbon ions and H₂ molecules coming from a cyclotron. The linac, typically a Side-Coupled Linac (SCL), is divided into several modules. The beam energy can be varied in steps of about 15 MeV/u without using absorbers by acting on the power (amplitude and/or phase) that feeds the different modules of the linac. This work presents the structure design of a 5.7 GHz high repetition rate SCL for a cyclinac, that accelerates carbon ions from 150 up to 400 MeV/u in less than 25 meters. The beam dynamics for this linac and its particular energy selection system is also discussed for different beam energy outputs.

WEPS062

Design and Beam Dynamics Study of Hybrid ESS LINAC

linac, cavity, cryomodule, proton

2640

M. Eshraqi, H. Danared, W. Hees, A. Jansson
ESS, Lund, Sweden

The European Spallation Source, {\sc ESS}, will use a superconducting linear accelerator delivering high current long pulses with an average beam power of 5~MW to the target station at 2.5~GeV. A new cryomodule architecture is proposed which allows for a transition between cryomodules in the sub-100~K region, this region can work even at room temperature. This new hybrid design will generate a lower heat load with respect to a fully segmented design - while still providing easy access to individual cryomodules for maintenance and repair. This paper will present a review of the {\sc linac} design, beam dynamics studies and a preliminary cryogenic analysis of the transition region.

WEPZ007

Multi-mode, Two-beam Accelerator with Feedback

cavity, feedback, impedance, RF-structure

2778

S.V. Kuzikov, M.E. Plotkin
IAP/RAS, Nizhny Novgorod, Russia

A high-gradient accelerator consisted of the test and the drive beam structures is reported. The accelerating structure can be based on dielectric or corrugated cavities separated each other by irises. Each cavity is operated by several axisymmetric, TM-like eigen-modes with longitudinal indices to be related to frequencies. These modes are excited at Fourier harmonics of the drive current which consists of bunches spaced with the same period as test bunches. The superposition of the excited modes introduces a short RF pulse propagated in-phase with a moving test bunch and after reflection by iris (a feedback) this pulse can accelerate next bunch. Such longitudinally-sweeping RF field promises a reduction of the exposure time and due to compact space shape can help to obtain high shunt impedance.