Minutes of 6th FFIR meeting on 3/27/2000

The meeting was held in a room of 3-425 at KEK , 13:30-15:30, 3/27/2000. We discussed on (1) re-population after pre- and post- linac collimations, (2) superconducting QC and others (BDIR2000).

(1) "Re-population in beam-tail" by T.Tauchi. (Transparencies, pdf, 9 pages, 1.57MB He first briefly reviewed the NLC estimations of repopulation as follows. The pre-linac collimation is located after NLC-10GeV pre-linac. The probabilities of repopulation have been estimated to be 10^{-5} and 10^{-9} for pre- and post-linac collimations, respectively, where the post-linac means "after(downstream of) main linac". The first number was informed at the ISG5 meeting and the latter is published in a paper entitled "Thermal-photon and residual-gas scattering in the NLC beam delivery" by I.Reichel, F.Zimmermann, T.O.Raubenheimer and P.Tenenbaum, SLAC-PUB-8012. The paper shows that dominant contribution is beam-gas (Coulomb) scattering and the number was a simulation result saying "the simulations show that the particles scattered at angles below 1 micro-rad are not lost. .... we thus assumed a limit theta_min=2 micro-rad...." .

Then, he introduced TESLA estimations which have been presented at LC99 by R.Brinkmann. The TESLA number was result from an attempt to determine order-of-magnitude fractional part of beam outside BDS acceptance (~30 sigma_y, 3% dE/E) taking account of sources of residual halo from damping ring, beam-gas scattering in the linac, Toushek effect and dark current. The dominant one was the beam-gas scattering. The probability was 5.7x10^{-8} for 60 km total length and 250GeV beam energy.

Finally, he showed his calculations for JLC based on the beam-gas scattering. The probabilities are proportional to (1/theta_min)^2, therefore integral of beta-function along the beam line because of Coulomb scattering. For the post-linac collimation, the probabilities outside 6 sigma_x and 35 sigma_y are 8.9x10^{-8} and 4.2x10^{-6} horizontally and vertically, respectively, for 500GeV beam energy assuming a vacuum pressure of 10^{-8} torr. Of-course, if the theta_min=2 micro-rad, we got the same number as the NLC. For the pre-linac collimation, the probabilities outside 3 sigma_x and 30 sigma_y are calculated to be 2.2x10^{-7} in both directions, assuming beta_x=beta_y=100m and the total length of main linac=11km. There seems to be a 2 order-of-magnitude "discrepancy" comparing to the NLC number of 10^{-5}.

After this FFIR meeting, we asked this question of T.O.Raubenheimer (SLAC) who has actually executed simulations for NLC. He replied that "I had estimated 10**-8 re-population from beam-gas scattering and 10**-5 from a large betatron oscillation (with 10^{-8} vacuum pressure).". So, the NLC number corresponds to the large betatron oscillation.

Anyway, our estimation shows that repopulation probabilities due to beam-gas scattering are very similar both at the pre- and post-collimations. This can emphasize an importance of pre-linac collimation regarding beam tails for background issues. Apparently, more studies must be needed for designing both collimation schemes in terms of backgrounds as well as machine protection.

(2) "Superconducting quadrupole(QC) magnet" by K.Tsuchiya. He showed an idea for "slit" in coils for synchrotron radiations which are generated by disrupted beam after collisions. The vertical width of slit seems to be too small for synchrotron radiations emitted at upstream (opposite ) final doublet while it is enough for radiations generated there. It was suggested that the SQC must have the original large aperture and the slit.

(3) Others. There will be "Beam delivery and Interaction region workshop 2000" in 7/3-5,2000, Daresbury,UK, which has been proposed at LC99. There are three working groups;(a) Luminosity stabilisation and Diagnostics (b) Collimation systems, Background and IR related issues and (c) Novel Concepts for Linear Colliders. We discussed how we can participate in this workshop. At least, Tauchi will go to the workshop. The other participations depend on the financial support. Since there are GG2000 (photon linear collider physics/detector workshop) in 6/14-17 and EPAC in 6/26-30, we are asking BDIR2000 organizers to move the date between these conferences. SLAC people like also to move the date since they have very important town meetings to determine a next US project in future. If the date can be moved, we could have larger possibilities for the participations.

We will have a next FFIR meeting in 4/12 (Wednesday). The place is 3-go-kan 4F meeting room (425) and the time is 13:30-15:30.