Summary :
High intensity photon beams have various applications in advanced accelerators, from medical imagery (X-rays) to high energy physics (polarized positron beams, photon colliders) passing by nuclear physics (fundamental and applied). They can be obtained by laser-Compton backscattering off electrons, the main advantage being the possibility to produce high flux monochromatic photon beams. In this context, an optical cavity is a unique system to reach the requested laser beam power at high repetition rates. LAL and KEK are developing such light sources and are trying to push forward the technical limits to increase the maximal power stored in these optical cavities.

French members :
A. Martens, K. Cassou, I. Chaikovska, R. Chiche, K. Dupraz, P. Favier, D. Jehanno, V. Soskov, T. Williams, F. Zomer

Japanese members :
Y. Honda, S. Araki, M. Fukuda, U. Hosaka, T. Omori, K. Sakaue, T. Takahashi, N. Terunuma,

References :
[1] J Bonis et al 2012 JINST 7 P01017, http://dx.doi.org/10.1088/1748-0221/7/01/P01017
[2] T Akagi et al 2012 JINST 7 P01021, http://dx.doi.org/10.1088/1748-0221/7/01/P01021
[3] ThomX Technical Design Report, http://hal.in2p3.fr/in2p3-00971281
[4]Chaikovska et al, High flux circularly polarized gamma beam factory: coupling a Fabry-Perot optical cavity with an electron storage ring, Scientific Reports 6, Article number: 36569 (2016)
[5]Liu et al, Laser frequency stabilization using folded cavity and mirror reflectivity tuning, Optics Communications 369 (2016) 84–88.

Summary :
Magnetic shielding is a key technology for super-conducting RF cavities.It is necessary to optimize the shielding method, including the choice of materials and the shape of the magnetic shield, and to establish a method for quality control suitable for large-scale production.We plan to continue to evaluate the permeability of various shielding materials, and also investigate possible causes for the performance degradation of the shielding material at cryogenic temperature. We would also like to study the dependence of cavity performance on the permeability

French members :
J. Plouin, E. Cenni, O. Napoly

Japanese members :
K. Umemori, T. Kubo, M. Masuzawa, K. Tsuchiya

References :
[1] Mika Masuzawa, et al. “Study of Magnetic Shielding Materials and Fabrication of Magnetic Shield for Superconducting Cavities” IEEE Transactions on Applied Superconductivity, 2014. Volume: 24, Issue: 3
[2] Mika Masuzawa, Juliette Plouin, et al. “MAGNETIC SHIELDING: OUR EXPERIENCE WITH VARIOUS SHIELDING MATERIALS”, Proc. For SRF2013, Paris, 2013.
[3] Juliette Plouin, “Magnetic Shielding Activities for IFMIF/CEA and Study on the Magnetic Shielding in the FJPPL Collaboration”, presented at “MSU FRIB Workshop on Magnetic Shielding for Cryomodules” Feb.4-6, 2013.
[4] Mika Masuzawa, et al. “Magnetic Properties of Shielding Materials for Superconducting Cavities” IEEE Transactions on Applied Superconductivity, 2012. Volume: 22, Issue: 3

Summary :
SuperKEKB e+e- collider is designed to provide a very high luminosity for Belle-II experiment, using nano-beam scheme. In this context, the fast luminosity monitoring collaboration aims to develop a reliable instrumentation to measure the luminosity and provide fast input for luminosity feedback corrections as well as for luminosity optimization phase. The measurements are based on the detection of the electron or positron arising from the Bhabha scattering process at zero degree photon angle, which offers a significant amount of signal. The fast luminosity monitoring system will be set ten to thirty meters downstream of the interaction point, ideally both in the High and Low energy rings, just immediately outside the beam-pipe. Several kind of detectors are under development, combining thus complementary technologies, namely scintillator, Cerenkov counter and diamond sensor, all requiring fast readout electronics enabling both the measurements of the whole-ring averaged luminosity and the bunch by bunch luminosity.

French members :
*C. Rimbault *, Ph. Bambade, D. Jehanno, V. Kubytskyi, Y. Peinaud

Japanese members :
S. Uehara, Y. Funakoshi, T. Kawamoto, M. Masuzawa, T. Oki

References :
[1] D. El Khechen, C. Rimbault, P. Bambade, “Fast Luminosity Monitoring using diamond sensors for the Super Flavour Factories SuperKEKB”, proceeding for the 5th International Particle Accelerator Conference (IPAC2014), Dresden, Germany, 15-20 June 2014.
[2]"First Tests of SuperKEKB Luminosity Monitors during 2016 Single Beam Commissioning”　（Dima El Khechen et al.） Proceedings of IPAC2016, May 8 -13, 2016, Busan, Korea.
[3] "BEAM COMMISSIONING OF SuperKEKB"　（Y. Funakoshi et al.） Proceedings of IPAC2016, May 8 -13, 2016, Busan, Korea.

Website :
https://www.belle2.org/

Summary :
KEK and CEA/Irfu are pursuing their effort to improve technology related to ILC cavity production/surface treatment. Presently, main steps studied are :
- Cavity Fabrication at Cavity Fabrication Facility at KEK
- Vertical Electro-polishing of cavities (VEP set-up at CEA Saclay and at Marui Galavanizing Company ltd.)
Alternative environmental-friendly surface treatments are also investigated (Electro-chemical Buffing).
The proposal mainly consists in the continuous process of cavity from fabrication at KEK through surface treatment and test at Saclay.
The aim is to optimize the process with the view of an industrial fabrication of cavities.

French members :
T. Proslier, C. Antoine, S. Berry, F. Eozenou, A. Four, C. Madec, C. Servouin,

Japanese members :
T. Kubo, H. Hayano, S. Kato, H. Monjushiro, T. Saeki, M. Sawabe

References :
[1] K. Nii et al., “Vertical Electro-polishing of Nb single-cell cavity using cathode with variable-geometry wings and its results of vertical test”, LINAC 2014, MOPP108.

Summary :
As a demonstrator for Future Linear Colliders (FLC) with nanometer (nm) size e+-e- beams, ATF2 (Accelerator Test Facility) has achieved 44nm vertical beam size, very near to its 37nm goal. It is realized under low intensity beam and the studies on beam intensity dependence will continue. Next major step for our French and Japanese Teams is maintaining the beam position stability of 2nm at IP. To achieve this, numerous studies on Beam Position Monitors in a vacuum chamber with internal moving mechanisms were done. Two diamond strip sensor based scanners were installed for horizontal and vertical beam halo studies together with a vertical dedicated collimator. Clear cuts in the halo from upstream apertures were identified. Ground Motion sensors are being used for vibration source identification and for the development of a new Ground Motion feedforward acting on the beam stability. In addition, improvement of the QF1FF Final Focus magnet support has to be done. These studies will be of paramount importance for the preparation of the ILC, in this context the French team was invited to join a new KEK-based study group charged with re-optimizing ILC IP/BDS/DR parameters for a staged ILC project starting with a 250 GeV centre-of-mass energy. This program is the ideal framework for the training of students in the Accelerators Physics field.l

French members :
A. Faus-Golfe, P. Bambade, F. Bogard, L. Brunetti, P. Cornebise, A. Jeremie, S. Wallon, R. Yang

Japanese members :
K. Kubo, S. Araki, S. Kuroda, Y. Morikawa, T. Naito, T. Okugi, T. Tauchi, N. Terunuma

References :
[1] G.White et al., Experimental Validation of a Novel Compact Focusing Scheme for Future Energy-Frontier Linear Lepton Colliders, Phys. Rev. Lett. 112 (2014) 034802
[2] T. Okugi et al., Linear and second order optics corrections for the KEK Accelerator Test Facility final focus beam line, Physical Review Special Topics - Accelerators and Beams 17, 023501 (2014)
[3] E. Marin et al., Design and high order optimization of the Accelerator Test Facility lattices, Physical Review Special Topics - Accelerators and Beams 17, 021002 (2014)
[4] D. Wang et al., Analytical Estimate of ATF Beam Halo Distribution, Chinese Physics C 2014 Vol. 38(12): 127003
[5] Y. Renier et al., Trajectory measurements and correlations in the final focus beam line at the KEK Accelerator Test Facility, Physical Review Special Topics - Accelerators and Beams 16, 062803 (2013)
[6] Bambade P. et al., Present status and first results of the final focus beam line at the KEK Accelerator Test Facility, Phys. Rev. ST Accel. Beams 13, (2010) 042801
[7] D. Bett et al., GROUND MOTION COMPENSATION USING FEED-FORWARD CONTROL AT ATF2, WEPOR005, 7th International Particle Accelerator Conference (IPAC 2016), Busan, Korea, 8-13 May, 2016
[8] Nuria Fuster-Martinez et al., ATF2 Beam Halo Collimation Background and Wakefield Measurements in 2016 Runs, IPAC 2017
[9] Renjun Yang et al., Measurements of Transverse Beam Tails in the Accelerator Test Facility 2 of KEK, IPAC 2017
[10] Renjun Yang et al., Theoretical Study of Beam Tail Formation in the Accelerator Test Facility of KEK, IPAC2017

Website : http://lcdev.kek.jp/ATF2/

Summary:
Field emission is one of the main issues for quality factor degradation at high gradient operation. We propose to develop a detection system equipped with scintillating fiber in order to precisely detect field emission sources on the cavity surface. Currently, this is achieved by PIN diodes that require long installation time and cannot be mounted on cavities in their final configuration (with helium tank).
The photon energy, produced by electrons impact on the cavity surface, can be estimated once the scintillating fibers are connected to a multi-channel analyzer. This will allow not only to detect the electron impact locations but also their energy, hence determine their origin more precisely respect to other detection systems.
A detection system prototype will be designed and manufactured with different fibers (geometry and composition) and equipped with photomultiplier and multi-channel analyzer

French members :
E. Cenni, J. Plouin

Japanese members :
Y. Yamamoto, H. Sakai, K. Umemori

References:
[1] H. Sakai, T. Furuya, T. Takahashi, S. Sakanaka, and K. Umemori, IPAC 10.
[2] Y. Yamamoto, H. Hayano, E. Kako, S. Noguchi, M. Sato, T. Shishido, K. Umemori, and K. Watanabe, in Proceeding PAC (2009).
[3] Y. Yamamoto, H. Hayano, E. Kako, S. Noguchi, T. Shishido, K. Umemori, K. Watanabe, H. Sakai, K. Shinoe, and S. I. Moon, SRF2007 Peking Univ Beijing China (2007).

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