Minutes of 46th FFIR/BDSIM meeting on 1/29/2003

The meeting was held in a room of 425 at KEK, 10:00-11:00, 1/29/2003. We had a special talk by Sekazi Mtingwa,MIT, Then, we discussed on implementation of synchrotron radiation in SAD, FEATHER, and an issue of crossing angle.

(1) Special talk on "Heavy flavor production using Compton Backscattered photons on fixed targets at the linear colliders", (S.Mtingwa, MIT)

(transparencies, 25 pages, pdf ,1.2MB ) We had a special talk by Sekazi Mtingwa (MIT) who is visiting at KEK for short period. He has proposed a fixed target experiment by backscattered photons at extraction line of TeV linear colliders, which is called as "BackGammon". Physics targets are B-, Tau-physics (their rare decays, CP violations, mass of tau-neutrino and Lorentz structure in tau-decay). Assuming the CLIC parameters (Ne=109/bunch, 2,000Hz, Ee=1TeV) with a "mini-beam" system, annual production yields of B-pairs and (polarized) tau-pairs were calculated to be 6x109 and 109, respectively. The mini-beam system consists of about 100 lasers with the same number of trim dipoles. The extracted beam collides with the lasers one after another. The backscattered photons are separated from the beam and are displaced vertically by the trim dipoles. The BackGammon could be also used as tune up for photon-photon collider. He would like to study background issues for the BackGamma experiment in future.

(2) SAD-synchrotron radiation(T.Ohgaki, KEK)

(transparencies, 7 pages, pdf ,1.1MB ) Ohgaki compared old and new SAD-calculations on synchrotron radiations. Major difference is the implementation of angular distribution in the new one as explained at the previous meeting. He showed spatial distributions for core and flat beams with both SAD. However, he found a very different average photon energy. The new one generated lower energy photons by an order of magnitude. There should be no difference in the photon energy. So, he was strongly encouraged to debug the new SAD routines.

(3) Fast feedback system "FEATHER"(N.Delerue, KEK)

(transparencies, 4 pages, pdf ,541KB ) Nicolas presented the schedule of beam test and the first results. He obtained machine times of 1/28 9:00~13:00, 1/29 13:00~ and 1/31 17:00~22:00. His plan is (1)BPM calibration, (2) kicker calibration and (3) tests of simple, delayed and improved models. First results are showed as follows. BPM outputs were measured, while BPM outputs differentiation could not be measured (wire badly plugged). Beam was not affected when a signal was sent to the kicker. These 2 problems are to be investigated during today shift. We also briefly discussed on an issue which is how to test a system that has a 40 ns latency with ATF trains (2.8nsec x 20 bunches) . One desired possibility is to make more bunches in a train. In this case, major concern may be the length of flat top at the extraction kicker magnet. He also explained about opto-FEATHER which uses optical components. He expected much shorter latency than usual circuits. Simple model would require optical combiner, optical amplifier and optical splitter. After he discussed with people from Tsukuba university, the combiner and splitter are turned out to be available by photonic crystals. The optical amplifier may be a maser system

(4) Crossing angle issue(K. Yokoya, KEK)

Yokoya briefly explained the related issues on the crossing angle. Major issues of the crossing angles are (a) very tight tolerance of relative phase between two crab cavities, (b) background from disruptive beam-beam interaction and (c) extraction beam line to the beam dump. The phase tolerance can be translated to the relative length (delta_L) of the two cavities separating by about 20m (L), that is, delta_L ~ sigma*_x/crossing angle ~0.2/0.02 ~ 10 micron-m over L-20m. Even the crossing angle of 7mrad, delta_L ~ 30micron-m. Without crab cavities, in the case of small crossing angle the luminosity loss will be about 15%, while the loss will become more than 50% with 20mrad crossing angle. Therefore, the JLC group took the design of small crossing angle whose minimum magnitude is limited by beam-beam blow-up interaction (BBU), that is 6~8mrad for L*=2m at least. Next issue was the background, specially low energy electron-positron pairs largely deflected by colliding beam. The issue has been studied by detailed simulations,and it turned out that it can be controllable with proper masking system and collimation upstream. With larger crossing angle the extraction line could be relatively easily designed including a "large" beam dump with thick shield. Recently, A.Seryi (SLAC) presented a calculation of BBU for 190 bunches and L*=4m, He found very large beam blow-up. However, his formula is very old and it is an approximation assuming small crossing angle, So, more realistic estimation must be needed for large crossing angle. He, who has originally derived the formula, will do that soon.

His recent result (transparency) is available here.

The next meeting will be on 6 February (Thu.), 2003,13:30 - 15:00 am at 3 gokan, 425 .