Minutes of 15th FFIR meeting on 9/13/2000

The meeting was held in a room of 3-425 at KEK, 13:30-14:00, 9/13/2000. We discussed on (1)progresses (2) report from ATF2 meeting and (3) talks at the next meeting.

(1) Progresses We purchased the precision workstation 620 (Pentium III Xenon, DELL) for the ANSYS analysis of the support tube prototype. It works far faster than the old PC which Yamaoka has. The shaking/vibration system of Air Brown Co. will be shipped here at the end of October and the prototype support tube is also ready at that time. First results of the prototype will be obtained in November. Sugimoto also upgraded CPU-chips in his PC. So, he is ready to study a new masking system in heavy simulations with backgrounds.

(2) Report from ATF2 meeting There was the ATF2 meeting in 9/11. The ATF2 meeting has been formed to discuss possibilities of the 2nd phase of ATF international collaboration. S.Kuroda presented the first trial of Pantaleo's FF optics at ATF. He could achieved 30nm vertical beam size with about 30m long optics at Ebeam =1.5GeV. He raised an issues which parameter set should be used for further study, e.g. should we dedicate to the ATF beam ? . We agreed that the parameters shall be "same"(scaled) as those at JLC-1(Ebeam=250GeV). So, the aspect ratio(sigma_x_sigma_y at IP) is 100, the energy spread is +/-0.5% and L* is 4m. In order to verify our aim, optics must be made at three beam energies of 1.3(ATF), 45 (SLAC) and 250(JLC-1)GeV. They shall be compared in terms of tunability, tolerance and any specialty at particular energy.
T.Okugi reported the bunch compressor(BC) system at ATF. The BC consists of L-band cavity and a chicane. First, he showed a possible scheme of longitudinal collimation. The (BC-)longitudinal tail in BC must be generated by (DR-)longitudinal and (DR-)energy tails in beam at the dumping ring. For the collimation, the chicane can be used for the DR-longitudinal tail and a new dispersion section in front of the BC1 for the DR-energy tail. Major issues in the BC-system are (a)multibunch longitudinal position shift due to beam loading in the DR and (b) emittance growth due to coherent radiation at the chicane. For (a), phase-amplitude modulation in the cavity must be implemented and for (b) he would like to have three chicanes for various magnitudes of emittance growth. Until next meeting, he will design a test BC-line including the above considerations.
K.Kubo studied a possible positron beam instability at ATF, which has been observed at KEK-B. The instability has a density threshold of electron cloud at positron-beam orbit. The threshold is proportional to energy and synchrotron frequency and inversely proportional to averaged beta-function. The density is proportional to bunch intensity, energy, revolution-frequency and inversely proportional to bunch spacing. At the KEK-B, the density was measured by tune shift and was also estimated by simulation, although they differed by a factor of 2. Assuming multibunch beams at ATF-DR, the density was calculated at the ATF from scaling of the KEK-B value. Compared to the estimated threshold, the instability may occur at the ATF too. However, the density strongly depends on geometry of beam pipes. For an example, the density must be smaller for beam pipe with ante-chamber which prevent electrons from drifting into beam pipe. KEK-B has no such ante-chamber, while ATF has it. Since PEP2 also has it, he will re-estimate this issue by scaling from the PEP2-case.
T.Tauchi reviewed recent progresses in measurements of ground motions especially at SLAC, which have been reported at EPAC2000 and LINAC2000 in this summer. At SLC-tunnel,i.e. shallow depth, atmospheric pressure changes were major cause of the diffusive motions (ATL rule). So, a deep tunnel is expected to have very small diffusive motion (A) since the atmospheric effect must be very small there. At very low frequency range of day, week and year, systematic motions dominated the diffusive motion at the SLC tunnel, and the same behavior has been observed at LEP tunnel (deep depth). During discussions, it was stressed that we should have a measured "KEK model" for ground motions and tools of estimation for beam stabilities with the model as NLC and TESLA groups.

(3) Talks at the next meeting We will have following talks at next meeting. (a) H.Yamaoka, ANSYS analysis results on the prototype and comparison with actual one, (b) Y.Sugimoto, a new masking system, (c) K.Kubo, neutron-shielding system at dump line. We also expect progress reports from superconducting QC, beam tails in SAD, new FF optics and etc. .

The next meeting will be on 4 October (Wednesday), 2000 13:30 - 15:30 at 3 gokan, 425.