1) Review of head-on optics design - tbd for Lew Keller
2) Discussion of 2mrad and headon extraction - Yuri Nosochkov
3) Feedbacks and tuning in BDS - Glen White
Status of design of head-on interaction region was discussed. Design is studied by task force (J.Payet, O.Napoly, C.Rippon, D.Uriot, D.Angal-Kalinin, F.Jackson, M.Alabau-Pons, P.Bambade, J.Brossard, O.Dadoun, C.Rimbault, L.Keller, Y.Nosochkov, A.Seryi, R.Appleby). In the assumption of 250GeV/beam energy, it was possible to shorten the FD, move electrostatic separator closer to IP, that allowed reducing the voltage of electrostatic separator to the value used in LEP operation. Separation of the beam at first parasitic collision (at 46m from IP) was sufficient to avoid multibunch beam-beam instability for all parameter sets with nominal bunch separation. It is suggested to put an intermediate dump into the extraction beamline to collimate early the energy tail. The tail was collimated below -30% of energy. As was shown in the next presentation, collimation below -10% may actually be required. The bends B1 and B2 were noted to have large gaps, so that the outgoing beam see almost full field of these bends and thus deflected back to the incoming beam by some amount. This would also be needed to be taken into account.
The first version of the complete head-on extraction optics was created by D.Angal-Kalinin.
The second part of this optics is similar to 2mrad collimation and diagnostic
section, but to control the horizontal dispersion and to get R22=+0.5 at the
second focus (R22=-0.5 is not possible in this design due to additional kick
from QD2A), the optics has been modified. This optics
is certainly very new and no optimization was done. Yuri made first studies of
chromatic properties of this new optics and compared with 2mrad extraction
optics. It was found that for the nominal beam energy, the head-on optics has
smaller beta-functions and beam sizes. However, for the beam with energy offset,
the beta-functions, dispersions, or orbit grow significantly. For dE = -30% the
orbit deviation reach 1.5m. This is explained by overfocusing by the final
doublet, while in 2mrad case the sextupole in FD provide smooth focusing in
large energy range.
The group discussed possible means to improve performance of this new optics. Any chromatic correction require further studies. One of the solutions would be to collimate all the tail below -10% of energy.
Glen presented simulations of 5Hz and intratrain feedbacks and tuning in BDS.
The BDS tuning need improvements, the luminosity achieved is yet far from
nominal. The reason for not satisfactory performance is not yet known, a
suspicion was that the strength errors applied to magnets were too large (0.001
for all magnets including FD) and the sextupole knobs were running out of the
linear range. This will be studied further. Glen also assumed both corrector
coils and movers for every magnet. One would need to study what is min needed
set of correctors.
Glen described three intratrain feedbacks included into simulations: the IP, the angle feedback and the feedback after the linac. The latter was included with very low gain and did not affect simulations. One of the present issue with the latter is that BPM resolution requirement is 100nm which require cavity BPMs however the required bunch by bunch mode makes it difficult. The kicker strength needed is about 100 times of the IP kicker. An alternative of increasing beta-functions in the location of the BPMs and kicker could mean several hundred meters of additional length of BDS. This will be studied further.
Andrei Seryi, 06/21/06