Summary of talk

Agenda

1) Summary of crab cavity meeting - A.Seryi, K.Ko, et al
2) Wakes effects and BDS vacuum chamber aperture - K.Bane
3) Effect of beam sweaping on the losses - Y.Nosochkov
4) Low field dipoles - C.Spencer
5) Discussion of IR hall sizes and BDS CF&S layout

Summary

Summary of crab cavity meeting (Fermilab, May 8-9) was presented. See the talks at the link above. In particular, the Fermilab design and manufacturing technology was discussed. Results of optimization by Kwok Ko and his colleagues were discussed. The needed redesign, to reduce Qexternal from the first dipole band was discussed.

Karl presented calculations of the effect of resistive wakes due to BDS vacuum chamber on the beam. Considered errors included injected beam offset or jitter, and vacuum chamber misalignment.
    At first, stainless steel vacuum chamber was assumed. In this case 80% of emittance growth was found for one sigma angular offset at the entry. Most of the growth come from the region of large beta-function in geometrical aberration correction section. About half of that growth is due to decreased aperture near tail folding octupoles. (The latter is easy to fix, since the chamber can be enlarged in this region and have smaller aperture only in the octupoles, which are SC. ) In terms of the tolerance for offset and jitter, for 25% emittance growth, the offset should be less than 0.5sigma and jitter less than 0.2sigma. Thus, the growth was clearly too big, unacceptable.
    Then, simulations were done assuming Cu plated SS chamber in the region 900-1250m (the IP was at 1582m for the partial optics considered). This improved situation several times. The tolerances for 25% emittance growth become less than 1.75sigma for offset and less than 0.5sigma for jitter.
    Tolerances on vacuum chamber misalignment were investigated. If the chamber assumed to be misaligned in 10m chunks, then 100microns rms misalignment result in about 10-20% emittance growth for SS chamber and 1-2% for SS Copper plated in 900-1250m region.
    Next study will include investigation of the chamber with increased aperture near tail folding octupoles, and chambers with the aperture increased in all drifts and bends. The latter will have larger effects of geometric wakes. (For presently studied chamber, the geometric wakes are 30times smaller than resistive wakes). However, the overall effect may be smaller and need to be studied.

Yuri studied effect of beam rastering on the losses in extraction line for 20/14mrad optics. The beam is rastered with 3cm radius to decrease temperature rise in the water. For some parameters, especially high luminosity and 1TeV, rastering can increase losses quite noticeably. It was discussed if it is possible, if we rely on rastering to avoid water boiling and window damage, to shorten the extraction line. This will then decrease losses on collimator and reduce cost as well. These studies will proceed. Single bunch damage for the window need to be investigated in this case, as this is relevant to determine how short the extraction line can be made.

Cherrill presented the idea for low field dipoles for FF. The strength range needed is 5-300Gs. The designed bends are air cooled. The 12m bends (in optics) were suggested to be split to 2m chunks. The bends have small transverse size (~24cm full width) so that there is enough separation from the photon cone in 2mrad IR. The question of the achievable range (at low field in particular) may require further studies.

The old and new GLD collider hall were discussed, as well as the general BDS CF&S layout. The new hall assumes that detector assembled on beamline, which has consequences for beam commissioning. It was concluded that possibility to commission BDS while detector is assembled is important. So, it is suggested to use older sizes for the moment, for both IRs, and have more discussion of the new hall sizes and its concequencies.

For BDS layout, two versions, with alcoves and with elevated service tunnel were discussed. In particular, emergency escape in case of fire, from one of BDS tunnels, when another tunnel just had the beam, was discussed. As a result, the "elevated access and service tunnel" version was suggested for considerations for Vancouver time scale. The following issues affected the choice:
    a) The need for accessible space in laser wire region, kicker PS region and in close proximity of Compton IPs of the polarimeter chicanes, upstream and downstream of IP.
    b) The need for ~3000m^s space for beam dumps water treatment system in proximity of beam dumps.
    c) The need for low rad area for BPM electronics within 100m.
    d) The need to place the power supplies of large power consuming extraction line magnets in separate room and minimize cables length in beam tunnel, for the reasons of temperature stability.
    e) The need to provide access to one BDS arm while another is running, and also provide an emergency escape in case of fire, taking into account radiation conditions in other beamline immediately after beam shutdown.

Andrei Seryi, 05/21/06