The present configuration will not generate background from scattering of halo-electrons at beam pipe in the final doublet if the halo is perfectly collimated at the upstream QBPMs with 20mm diameter aperture. However, this collimation may not be perfect. Since the beam halo has been estimated be about 107 electrons/bunch and the signals are order of 103. In general, a single collimation would suppress a beam halo with 10-4. M. Oroku investigated the background acceptance from the scattering by simulation based on GEANT4 with a following condition and geometry.
First, 1.3GeV electrons are hitting the beam pipe with incident angle of 0.3mrad at QF1. The beam pipe has 35mm inner diameter with 1.6mm-SUS thickness all along the beam line up to a conical gamma collimator for a detector. The gamma collimator is located just behind the last dipole magnet, i.e. 3m from IP. This magnet discriminates signals (photons) from the beam and charged background. The detector is located at end of beam dump.
The background acceptance was defined by relative energy deposit to the incident energy in the detector. The acceptance was calculated as a function of (angular) aperture of the gamma collimator, i.e. 5%, 1% and 0.1% with the aperture of 2.2mrad, 1.3mrad and 0.83mrad, respectively. Also, the signal acceptance was estimated to be 95%, 80% and 60%, respectively.
Q : Is the beam pipe diameter 40mm in the final doublet ?
A ; Yes. In this simulation, it was set to be 35mm since the largest beam size, which is collimated by upstream QBPMs with 20mm aperture, is 35.7mm at QF1 in X. The purpose of this study is the estimation of background reduction by the gamma collimator. So, the major conclusion would be similar even in case of 40mm. We have a beam test plan to verify the basic property of the gamma collimator in this fall at ATF.
There were following suggestions. As pointed out, the geometry should be the same as the design which includes beam pipe, IPBPM in a vacuum pipe of Shintake monitor and QBPM in front of the dipole magnet. Since the vertical aperture of the dipole magnet is 32mm, the beam halo could hit in the magnet. Such background must be carefully estimated . Results should be expressed in S/N as well as the background acceptance. In parallel, it should be evaluated a possibility of adding a quadrupole magnet between IP and the dipole magnet in order to focus the beam in vertical direction.
Major motivation of new optics is to improve the current one which uses only 1/3 of laser power in the interference at IP , which can not measure the vertical beam size in 360nm to a few μm, which can not measure the horizontal beam size for smaller dynamic range and which has very limited space for the fringe phase detector. The new optics will be mounted on a new optical table which has more rigidity.
The new optics can change the crossing angle by reflecting the light path rather than splitters and shutters without the laser power loss, i.e. 1/3 reduction. Nominal laser spot size of σ=15μm can be decreased to σ=5μm to increase the signals from 2,900 to 8,700 Compton photons. The nominal horizontal beam size of &sigmax=2.8μm can be measured in the laserwire mode where the single laser is focused to σ=5μm. Following 4 laser crossing angles of 2o, 6o, 30o and 174o are made in order to cover the vertical beam size from a few μm to 35nm . The 4 angles can be selected by a mirror. The interference patterns are stabilized with feedback on phases.
Preliminary optical layout was shown. The table size is larger than the present one, especially in horizontal direction. Reduction of the size was suggested for re-configuring the injection optics.
Although the new Shintake monitor can cover the vertical beam size in all region, Honda monitor would be useful for its simplicity, robustness against background and fast signals , especially early tuning stage.
Finally, the conceptual design will be completed by August 2007, and the detailed component design will be finished by October 2007. The new table will be fabricated in January 2008. The optics will be assembled on the table for February 2008 until end of March 2008.
Q : The schedule seems to be very aggressive. Are you confident ?
A : Yes, since the company promised us that the new table will be fabricated for three months after they get the design which our group prepare by October 2007. The assembling must be straight-forward. This work is part of Suehara's Doctor thesis.
Monalisa system was shown in a 3D image at first time. The basic idea is to use compact straightness monitor (CSM) between tops of QD0 and Shintake monitor and distance meters to monitor pole tips and IP-BPM. The CSM is installed in a vacuum pipe system which consists of two pipes and a double bellow system between them for force-free. The two pipes are rigidly supported. First approximation is that the double bellow system decouples perfectly, which will be verified by an integrated FEA of Shintake-Table+MONALISA-Vacuum-System .
Following fundamental questions were raised and answered;
The Monalisa group will estimate stability of this system consisting of Shintake monitor, QD0 and Monalisa by FEA. Therefore D.Urner would like to know detailed structure of Shintake monitor system as well as QD0 (QC3) . Tatsuya Kume (KEK) is a contact person of Shintake monitor support system.
C. Spencer commented that there is the third QC3 at SLAC which can be be sent to LAPP for real measurement and an inspection for Monalisa. She will arrange such possibility within 2 weeks.
Q : Is it OK (for D.Urner) ?
A : Two weeks are fine, while we would be happy to know as soon as possible.
Especially, Shintake monitor group is urgently requested to pass the detailed information of signal-hole in a new beam dump to N.Terunuma, since the beam dump must be constructed by this fall for first installation on the new floor.
The next ATF2 meeting will be held on 11th July 2007, 2:30pm -, 3-gokan, 425, KEK, TV-ID#=31110 with the KEK Gatekeeper and ILC Webex.