WG-4: Demonstration of final Focus system and controlling beam orbit at nano meter level

 

1) Goals

　We have two goals. One is to demonstrate maintaining beam spot size obtained by compact final focus system. The other is to demonstrate controlling beam position at focus point (IP) at nano-meter-level.

 

　It is crucially important for ILC to do a good physics soon after its commissioning. Therefore, we have to (i) stably produce intense beam with a small beam-size and (ii) maintain orbits of electron- and positron-beam with a nano-meter precision.

　In 1994, FFTB/SLAC succeeded in producing a small beam spot with 70nm height. However, it will not be a demonstration of ILC final focus system in the strict sense of the word, since ILC will adopt a new compact final focus system proposed by P.Raimondi and A.Seryi in 2000. Furthermore, FFTB/SLAC did not demonstrate to maintain the beam-size at all. Measured beam jitter at IP was about 20nm, which is much larger than the beam-size of 5nm at ILC. The source of and controlling method of the jitter were not studied. Therefore, two goals of ATF2 were not achieved yet. Beam orbit stabilization is crucially important to make stable collision, and is to be demonstrated.

　The 37nm beam obtained at ATF2 is more ideal for nano-meter stabilization test, since it is about a half of that at FFTB/SLAC. Nano-BPM, which is under development, will enable us to make a good test of nano-meter stabilization test, even if the beam size is much bigger than ILC beam.

 

　Our experience at ATF shows that “now-how”, which is hardly evaluated, is crucially important to maintain a high-quality beam. Accumulation of knowledge and experience, as well as educating young people is another goal of ATF2. We should start ATF2 program as soon as possible and keep studying just before conditioning of ILC, so as to ensure quick start of physics program at ILC.

 

 

 

　The goal is summarized as follows; in order to ensure quick start of physics program at ILC, we will study/demonstrate:

 

(A) Achievement of 37nm beam size

　(A1) Demonstration of a new compact final focus system proposed by P.Raimondi

　　　and A.Seryi in 2000,

　(A2) Maintenance of the small beam size

　　　　 (FFTB/SLAC did not maintain the beam size)

 

(B) Control of beam position

　(B1) Demonstration of beam orbit stabilization with nano-meter precision at IP.

　　　　　　 (The beam jitter at FFTB/SLAC was about 20nm.)

　(B2) Establishment of beam jitter controlling technique at nano-meter level

　　　　 with ILC-like beam

 

For the goal of (A), cavity-BPMs with 100nm resolution must be installed at every quadrupole magnets and a beam size monitor (BSM) shall be set at the focal point (IP).　 The BSM is based on laser interferometer with higher modes for smaller beam size than that of FFTB/SLAC.　 The present normalized emittance is estimated to be 4.8 x 10-8 m at the ATF extraction line, as it is three times larger than that of the dumping ring.　 The emittance will be improved to the nominal value of 3 x 10-8m by adjusting x-y coupling.　 Very preliminary study shows that power supplies of bending and some of quadrupole magnets must be stable within 10-5, which may require a development. Tuning methods will be established based on BSMs as well as BPMs.　　 Vertical beam jitter of 30% beam size can be acceptable for this goal like the FFTB/SLAC, which is typically　 2um at the extraction line.　 In order to maintain the small beam size, i.e. goal (A2), a feedback system must be implemented at the upstream of IP.

 

In addition, to achieve the goal of (B1), the nano-BPM must be developed with the resolution of less than 2nm, which is the present research program by Japan-US group at the ATF.　 The beam jitter shall be controlled to be less than the resolution of nano-BPM, 2nm, i.e. about 5% beams size at the ATF extraction line.　　 The IP jitter shall be decomposed into ones scaled with/without beam size by the nano-BPM as well as the upstream BPMs.　　　

 

After installation of kicker with 300 nsec pulse width in October 2005, the ATF can provide a beam-train with 3 bunches separating in 150nsec.　 The third bunch can be further stabilized by the fast feedback system, FEATHER and FONT,　 under developments　 by KEK and UK groups.　 Therefore, the nano-BPM will verify the performance of the fast feedback system, which has to be implemented in ILC, at nanometer level.　

 

For the goal of (B2), the ATF beam parameter can be configured as the ILC like; i.e. 20 bunches with about 300nsec separation at 5Hz, for instance .　 For this purpose, a very fast kicker must be developed with the rise time of less than 1ns and stable pulse height. This goal must be longer term even during the ILC construction.

 

As mentioned above, the most urgent task is the jitter control at the ATF extraction line. The present jitter is 10% beam size at the laser-wire position (dispersion free), the dumping ring, while it is about 100% with a few minutes period, i.e. 5um, whose slow component is considered to come from the extraction kicker system.　 The improvement of the kicker system will be conducted with the double kicker scheme which has been experimentally verified.　 New kicker magnet will be shipped to the ATF from SLAC in next summer.　　 Feed-forward system is expected to reduce the jitter at the same level at the dumping ring; i.e. 10% beam size.

 

 

2) Urgency

 

At ATF, there are active research and developments programs of the Nano-BPM (KEK, SLAC and LLNL groups), which is based on cavity-type beam position monitor (BPM) with a target position resolution of less than 2nm, and the fast intra-bunch feedback system ( FEATHER of KEK, Tokyo Metropolitan univ. and FONT of UK, SLAC ). Especially, the Nano-BPM has a very important role in the ATF2 project for dispensable device to measure the beam position with nanometer level resolution at the focal point, i.e. IP. 　The ATF2 shall be based on success of these R&Ds. The ATF2 is strongly expected to substantially reduce a risk factor for stable collision between the nanometer beams, i.e. high luminosity at ILC.　 The risk reduction must require R&Ds of beam position and profile monitors, active supports of final doublet and the fast feedback system at the final focus system.　 In parallel, establishment of tuning and orbit correction is a task of pressing urgency.　　 These R&Ds can be implemented in a swift and comprehensive manner at the ATF2.　　 Also, the ATF2 shall cultivate young human resources capable of operating the beam delivery system at ILC in future. Without the ATF2, it must be hard to reduce the risk that we may not achieve the design luminosity for many years. We would like to start the construction in JFY2005 and the commissioning will be in April, 2007.

 

 

 

 

3) Participating members and roles

 

The ATF2 shall be planned/designed and constructed with an international collaboration as the FFTB at SLAC.　 Therefore, the participations are expected from all over the world; KEK, SLAC, FNAL, BNL, DESY, CERN, Daresbury lab. for instance.　 Especially, KEK will take care of infrastructure of the facility at least as the host institute.　 Currently, H.Hayano, Y.Higashi, Y.Honda, Y.Iwashita (Kyoto univ.), M.Kuriki, S.Kuroda, K.Kubo, M.Masuzawa, T.Okugi, T.Sanuki (Tokyo univ.), R.Sugahara, T.Takahashi (Hiroshima univ.), T.Tauchi, J.Urakawa V.Vogel, H.Yamaoka and K.Yokoya are contributing in the proposal in Japan.　 Most of the ATF staffs (http://atfweb.kek.jp/atf/people.html) will be available in the construction and operation, too.

 

 

4) Cooperative relationship to domestic and foreign institutes (companies)

 

Each research institute shall carry out research and development regarding the assigning tasks, and construct them in each country. KEK must efficiently receive them.　 In this way, the construction of the ATF2 must be a super miniature version of ILC construction for the international sourcing.　 Also, the operation of the ATF2 must be international, i.e. it could be a super miniature version of the global accelerator network.

 

 

5) Oversea activities and potential collaborators

 

Possibilities of test facility have recently been explored for the compact final focus system at CTF3 (CLIC Test Facility 3), CERN and a new line B/SLAC.　 At CTF3 beam energy ranges from 200 to 500MeV and the focused beam size is 500nm,　 where possibilities of laser-Compton scattering and plasma experiment are also explored like the ATF2.　 At the new line B/SLAC, beam energy can be 30GeV and 14GeV with beam parameters of SPPS (Sub-Picosecond Photon Source) and LCLS (Linac Coherent Light Source), respectively, where the beam sizes are 160nm and 70nm, respectively.　 (Reference is available at a URL of http://www-project.slac.stanford.edu/ilc/meetings/workshops/US-ILCWorkshop/talks/optics_tests.ppt .)　 The ATF2 has the smallest beam size which is only smaller than that achieved at the FFTB/SLAC. Therefore, the ATF2 is the most suitable for verification of nanometer beam control towards ILC.

 

The ILC-WG4 has begun to play a central role in investigating the possibility of the ATF2 and preparation of the proposal. Towards the completion of proposal in next March, the outline was already decided and the authors will be decided soon. The international collaboration will be formed at the ATF2 mini-workshop in 5 January, 2005, SLAC, which is held on the day before the MDI workshop, 6-8 January, 2005, SLAC.　 Potential collaborators are described in the section (3).