Minutes of 40th FFIR meeting on 10/3/2002

The meeting was held in a room of 525 at KEK, 10:00-12:30, 10/3/2002. We discussed on R&D status of support tube prototype, collimation system, final focus system and simulation of BDS etc. . Finally, other future efforts are listed.

(1) R&D status of support tube

(transparencies, 5 pages, pdf ,763KB ) H.Yamaoka shortly introduced major results on the vibration tests of the cantilever (case-1: 20t x 100W x 695L ,where t=thickness, W=width, L=length in a unit of mm) and various types of both-end support systems (case-2: 10t x 50W x 1520L, 2 cantilevers connecting without (case-3) or with (case-4) a thin plate of 2.5t x 100W x 200L), which have been presented at the 5th ACFA workshop on Physics and Detector at Linear Collider. The ends were vibrated by the long stroke shakers with sweeping sine waves of 0.2 x 10-3 gal (uniformly) at 1< f < 1000Hz for spectrum analysis.

Case-1, case-2 and case-4 have good agreement with the ANSYS calculations, while case-3 disagreed with the calculation. Reason of the disagreement was understood by imperfection or uncontrollable errors of two "identical" structure of cantilevers. A thin plate was turned out to be very effective to recover the "predictable or controllable" property(vibration) in the both end support system. It must be important to calculate this recovery quantitatively as a function of stiffness of thin plate since thinner structure is very preferable in the actual detector. A series of measurements can conclude that amplitude caused by ground motion is very small at f > 100Hz with a support tube and cultural noise must be problem because it dominates at f>10Hz.

Future efforts will be (1) measurement of oscillating amplitude with different phases at both ends, (2) manufacturing of 1/10 model with realistic structures and (3) stiffness test of the model and study of a vibration isolation system.

Toward the goal of (1), he recently succeeded to simulate the vibration of two ends with different "magnitudes" (although not phases) in ANSYS.

For (2), he started to design the 1/10 model with and without tapered connections, whose@technical drawing was shown at this meeting . He will order the manufacturing within a month.

(2) Collimation system

(transparencies, 8 pages, pdf ,451KB ) T. Ohgaki studied the collimation system which S.Kuroda has designed by modification of the latest design of NLC-BDS (1400m, L*=3.51m) . The collimation system consists of betatron (0-400m), energy (400-650m) and final focus (850-950m) collimations from the upstream. One typical feature of this BDS(Beam Delivery System) is an existence of octupole doublets after the energy collimation, which are called as tail folding octupoles (TFO).

He found that apertures of spoiler and absorber (SPE and ABE, respectively) must be made wider than originals at the energy collimation as follows because of additional bending magnets for small crossing angle; SPE: Ax=1.6mm to 3.1mm and ABE: Ax=3.4mm to 5.8mm, where the momentum acceptance at the final focus system is assumed to be &&sigmap/p=0.9%.

After this adjustment of apertures, he calculated the acceptance of beam-transverse profile. The results are 16 &sigmax x 51 &sigmay and 11 &sigmax x 46 &sigmay with ON and OFF of TFO, respectively, where the smallest apertures are AB9 (the last betatron collimator, 911m from IP) and SP2 and 4(final focus collimator, 76 and 228m from IP, respectively) with ON and OFF, respectively. The ON-configuration must be safer than the OFF-one in terms of background problem (muons and synchrotron radiations) at IP.

He also tried to calculate synchrotron profile at IP. However, his method was doubtful. He will re-calculate it considering that the largest source is the final doublet.

There were following suggestions; (1) to verify the momentum acceptance of 0.9% at the final focus system, (2) to calculate the synchrotron profile at IP in order to determine the apertures of the collimation system.

Future effort will be (1) estimation of muon background by MUCARLO in cooperation with Namito-san, (2) calculation of wake field at the spoilers and absorbers and (3) understanding of TFO-role quantitatively and optimization of the collimation system so on.

(3) Beam Delivery System simulation

(transparencies, 11 pages, pdf ,3MB ) K. Tanabe (M1 student at University of Tokyo) has developed a simulation program of BDS based on GEANT4 for background studies with detectors. He put all the magnets of the Kuroda-BDS (1400m, crossing angle of +/- 3 mrad ) as well as spoilers and absorbers in the program.

Tracking along the 1.4km beam line, particles horizontally scooted down at 100 micron m from the collision point and focal spot sizes were RMSx=3.0 micron m and RMSy=370nm while the design values are RMSx=211nm and RMSy=2.7 nm, where tracking in magnetic field was used original GEANT4 routine. Beam envelops were very similar to the beta-function. Therefore, he speculated that the reason must be "error" in GEANT4. He will improve the tracking in magnetic field by consulting CLIC study.

He also studied synchrotron radiations emitting from magnets. He found that the average number of photons and energy are 3.4 per electron and 1.9MeV, respectively. Even with this performance, the synchrotron radiations will not hit beam pipes of 10mm radius if beams are Gaussian. Beam with flat fail could be simulated with and without absorbers.

Future effort will be (1) improvement by employing the CLIC method, (2) simulation of muon pairs created at collimators, (3) study on dump line, and (4) estimation of backgrounds at detectors.

(4) Final focus system

S. Kuroda investigated an option of permanent magnets for final doublet. Strength of the permanent magnet must be changed in a wide range ( say from Ebeam=250GeV to 45GeV) without replacement. One method is to use half-turns of longitudinally segmented pieces with K values (field strength of quadrupole magnet), for instance K-KKK instead of KKKK to reduce the total strength by 50 percent, since the permanent magnet consists of many pieces.

In his calculation, the nearest final focus magnet of 1.1m length is assumed to consist of 99 pieces with the same length of about 1cm. Pattern of K-values of pieces is arranged (K-KKKK-KKK......) so that the integrated strength is the same as the usual configuration as kkkkk..... ., where K=2k. Without any other change, beam sizes were calculated to be &sigmax=537nm and &sigmay=34.5nm, while nominal values are &sigmax=211nm and &sigmay=2.7nm, respectively. Q magnets at upstream have been adjusted in order to recover the beam sizes. Resultant sizes were &sigmax=350nm and &sigmay=3.64nm.

He also studied a case of half beam energy by doubling the emittance. At this moment, beam sizes were calculated to be &sigmax=439nm and &sigmay=7.08nm, which would be compared to be &sigmax=305nm and &sigmay=3.99nm, respectively.

(5) Other future effort

The next meeting will be on 17 October (Thu.), 2002 10:00 - 12:00 at 3 gokan, 525.