Minutes of 63rd FFIR/BDSIM meeting on 11/26/2003

The meeting was held in a room of 425 at KEK, 10:00-10:40, 11/26/2003. We discussed on LINAC simulation, update of the dumpline, final focus system and discussion on the crossing angle.

(1)LINAC simulation (T. Yamamura)

(transparencies, pages, pdf,)

Yamamura has investigated the effect of sextupole fields in an emittance growth at the main LINAC. From B3/B2=0 to B3/B2=1, the emittance growth was calculated to be less than 1%, where Bn is a coefficient of Bn(r/a)n-1, a=20mm. He also calculated the effect of scattering with residual gass molecules. Total 400,000 electrons were injected to the main LINAC with (x, x', y, y')=(0,0,0,0). All of them were scattered with angular distribution of 1/theta2, while the total probability was estimated to be about 10-5. At the LINAC exit, most of them have transverse positions of about um which are well inside original angular distributions.

(2)Update of the dumpline design (K. Kubo)

(transparencies, 5 pages, pdf, 22kB)

Kubo updated the optics of the dumpline with two beam dumps for beamstrahlung photons and beam. The disrupted beam is horizontally bent in opposite side of incoming beam in order to make enough space for the beam dump. He designed the optics both for 7 and 20 m radian. The optics must be implemented in the BDS routine by University of Tokyo group.

(3)Final focus system (S. Kuroda)

(transparencies, 4pages, pdf,234kB)

First, Kuroda introduced the optics from the final doublet to IP; the invariant emittance is 3 x 10-6(8)m, the momentum spread is 0.3% and the beam sizes are &sigma*x=222 (&sigma*y=2.72) nm, where values in parentheses are those of vertical component. The field strength of QF1(1m long x 2) and QD0(1.1m long x 2) is K1=0.0913 and -0.187 in 1/m, respectively. One major characteristics of this optics is non-zero eta' at IP.

Rolling errors of QD0 were calculated for |dtheta(QD0)|< 0.2 m radian by using SAD. Rolling of 1u radian must enlarge the vertical beam size by 10%, while the horizontal beam size would remain within 10% even by rolling of 0.2 m radian. Area of beam IP profile (&sigmax x &sigmay) was also calculated as a function of rolling. The function was linear and symmetric in signs of the rolling. Apparent tolerance of the rolling is 1 u radian, while a present technology can reach 10 u radian.

The calculation can be simply understood by following approximation, which was given by Yokoya at the meeting;
delta_y is a vertical position at IP due to the rolling of theta.
delta_y= y' L, y' = K1 2 theta &sigmax at s=L (QD0), theta is the rolling angle of QD0.
delta_y=4 theta &sigmax with K1 L= 2
&sigmax=&sigma*x L / beta* with beta*=100mm, L=3.5m
delta_y = 4 nm with theta=10-5 (10 u radian), &sigmax=100um.

(4) Discussion

We also discussed on the crossing angle. While the standard crossing angle is 7mrad in the roadmap report 2003, larger crossing angle (20mrad as the NLC) has been motivated for longer L* in order to reduce the beam-beam blowup during crossing between the final quadrupole magnet. The larger angle may be necessary for 3-5TeV collisions. We have discussed on this issues at previous meeting on 4/23/2003 and Yokoya's talk on 7/18/2003. We do not have consensus for adopting the large crossing angle yet. Since choice of the large crossing angle depends on technology of the crab cavity, we decided to invite Akai-san for a talk on status of crab cavity at the KEK-B factory at coming meeting.

The next meeting will be on 10th December, 2003,10:00 -noon at 3 gokan, 425 .