Minutes of 41st FFIR meeting on 10/17/2002

The meeting was held in a room of 525 at KEK, 10:00-12:00, 10/17/2002. We discussed on fast feedback system, collimation system, final focus system and SLEPT project etc. .

(1) Fast feedback system

(transparencies, 4 pages, pdf ,14KB ) Dr. Nicolas Delerue came from France to KEK as a JSPS foreign fellow in this October. He will stay KEK for two years. Research plan is (a) simulation study on the performance of fast(intra-pulse) feedback system by using CAIN, (b) design and manufacturing of the system especially focused on the electronics and (c) beam test at ATF (multi-bunch beam, 2.8nsec separation, 20 bunches/pulse). The fast feedback system consists of BPM to measure beam positions after collisions, stripline kicker to kick incoming beams for correction, and the electronics of BPM processor and amplifier etc. .

First, he studied luminosities as a function of vertical beam-offset at IP by CAIN, where the beam parameters are those of the roadmap report, i.e. 1.4nsec bunch separation, 192 bunches/pulse, intensity of 0.75 x 10^{10}/bunch, sigma*x/sigma*y=243/3.0 nm, beta*x/beta*y=8/0.11mm and the nominal luminosity of 2.5 x 1034/cm2/s. At the offset of 4 (10) sigma*y the luminosity drops to 50% (17%) of the nominal one. Second, deflection angles were calculated after collisions as a function of the offset, where the angle is the average one. Values of the angle are 40, 120 and 190 micro radian at the offset of 1, 3 and 6 sigma*y, respectively. Then, he made a simple algorithm of the feedback with vertical beam offset only for this moment, so called "linear model", in reference to the Schulte's paper ( LCC-0026 20/09/99, CLIC Note 415). In his simulation, a key parameter is a gain ( g ) which is defined as delta/sigma*y= g def.angle/sigma*y', where delta is the correction factor in unit of sigma*y, and def.angle is the deflection angle measured by BPM, and sigma*y' is a sigma of vertical beam divergence at IP. Trailing bunches will be kicked (or corrected) bunch by bunch with a constant gain of g. Generally, the "corrected" offset would start to oscillate if g is large. He found that such oscillation could disappear with g=0.05 and the luminosity recovered at 40th bunch after the first bunch passed through BPM.

(2) Collimation system

(transparencies, 7 pages, pdf ,446KB ) Momentum acceptance was calculated to be +/-1.5% at the collimation system with apertures of Ax(SPE)=3.1mm, Ax(ABE)=5.8mm, which can be simply estimated by dispersions( etax=0.19917 and 0.36611 at SPE and ABE, respectively, i.e. A-x = etax dE/E ) .

Spread of synchrotron radiations was estimated at the final focus quadrupole magnet(QD0) across the IP, i.e. s=L*=3.5m ( s=0 at IP) . The maximum divergent angles are thetax=112 micro-radian and thetay=25 micro-radian in horizontal and vertical directions, respectively, which are produced at the final doublet(QD0+QF1). The spread can be calculated by linear optics simply as sigma(SR)x=(s+2.9m) x thetax and sigma(SR)y=s x thetay with s=3.5m. Putting s=3.5m, we obtained sigma(SR)x=0.72mm and sigma(SR)y=87.6micro-m. Therefore, the spread corresponding to 11 sigmax x 46 sigmay is calculated to be sigma(SR)x=7.93mm and sigma(SR)y=4.03m. The spreads are small enough for passing through the final magnet without interaction. He also calculated energy loss by radiations along the beam line by using SAD for two types of beam (Gaussian and flat of 20 sigmax x 60 sigmay) with/without octupoles (OCT on/off). The results are summarized in the table, where first, second and third numbers are transmission rate, average energy loss and number of photons per electron emitted.

OCTGaussianflat (20sigmax, 60 sigmay)
on100%,9.25MeV, 44.6photons/e 994%,21.1MeV,51.9photons/e
off100%,9.10MeV, 43.8photons/e 92.2%,17.7MeV,44.7photons/e

There was a suggestion that SAD should be modified in order to calculate energy of photons and emitted angles for comparison with simulation results of GEANT4.

He demonstrated an effect of the tail-folding octupoles which can allow the use of wider spoiler jaw settings, say more than double. He installed MUCARLO-program in jlc.cc.kek,jp in order to estimate muon background in cooperation with Namito-san. In present BDS, there seems to be no space for 120m long muon-attenuator while only 70m long attenuator can be used. Effect of the shorter attenuator shall be studied. If necessary, it might be desirable to modify the BDS for the 120m long attenuator.

(3) Final focus system

(transparencies, 1 page, pdf ,400KB ) He has been studied an effect of error in strength of QD0 (final focus magnet). Range of the error is from -1% to +1%. The error must be corrected by adjusting optics at the upstream assuming that beta-functions are fixed to be beta*x=8mm, beta*y=110micron-m and eta*x=0, deltap=+/- 0.3 (0.1) %. The matching condition is to achieve the nominal beam size at IP, i.e. sigma*x=200nm, sigma*y=2.7nm. Resultant range of error was obtained to be from -0.7% to +0.4%, where he found successful corrections. It may indicate that the permanent quadrupole must be segmented into 300 pieces at least.

(4) SLEPT project

(transparencies, 1 page, pdf ,xxKB ) Beam transportation has been successfully demonstrated from LINAC though BDS. At the entrance of the linac, a Gaussian beam is divided into many slices in phase space for the SLEPT inputs. At the exit of the linac, the slices are converted into particles for tracking by SAD. By using this program, the vertical beam size was calculated to be 4.8nm at IP with misalignment of rms=10 micro-m at the linac-structures. Ground motion effect will be estimated possibly for three models of A,B,C in the TRC-report.

The next meeting will be on 30 October (Wed.), 2002 10:00 - 12:00 at 3 gokan, 425.