Agenda and Summary:
1) Crab cavity aperture evaluation -- approach and preliminary results --
2) Modification of DID for detectors with TPC -- B.Parker, A.Seryi
3) Anti-DID to improve background -- A.Seryi
4) Length of diagnostics section and aperture for betatron errors -- M.Woodley
Crab cavity aperture evaluation :
Sasha started evaluation of crab-cavity apertures and we discussed the assumptions and preliminary results. The issue is that 3.9GHz crab-cavity considered for baseline has 30mm aperture (diameter) and the question is whether this is sufficient. Sasha considered different collimation scenarios and varied the aperture of the crab cavity. The crab-cavity is placed between photon masks MSK1 and MSK2, which have aperture smaller than 10mm to protect Final Doublet from soft photons coming from the bend. The first mask mostly shadows the crab cavity, but not entirely, since it is rather far from the cavity -- there are up to 1E5GeV/bunch of photons (mean energy <E>~30keV) from the beam core, deposited at the cavity, if its aperture is below than 30mm. One need: 1) understand what these photons do for crab-cavity operation; 2) understand if the second mask MSK2 can be relocated so that the crab-cavity would be protected better, or if additional mask could be added.
Modification of DID for detectors with TPC :
Andrei and Brett have shown that DID design can be modified to provide very
small field in the region |z|<0.5m, to ease TPC calibration. As suggested by Dan
Peterson in discussion with Witold Kozanecki, the DID field in this central
region |z|<0.5m is desirable to be reduced ten times, while large DID field near
|z|=2m is acceptable.
Modification of the DID field is done with two DIDs wound on the same radius but which have different length. Adjusting their current, one can flatten the field. The combined DID field in the central region |z|<0.5m is decreased about 65 times with respect to the earlier DID design. The effectiveness of the combined DID for the optics is expected to be the same.
Anti-DID to improve background :
Andrei have shown that DID with reversed polarity, anti-DID, can be used to
zero or reduce the effective crossing angle for outgoing beam (and low energy
pairs). The anti-DID (or "DIDNT", dubbed by Tom Mattison) would be easier to use
if the initial crossing angle is smaller, such as intermediate 14mrad, because
synchrotron radiation effects are significantly reduced.
The use of anti-DID would be easier (and even may be possible at 20mrad, with full or reduced strength), if one would not have to compensate the vertical angle at the IP (~100urad). This would complicate polarimetry, as the IP collision trajectory would be nominally misaligned with the beam axis at the polarimeters. This can be partly mitigated, at least for downstream polarimeter, by using vertical correctors at the polarimeter IP to match the IP angles, according to Mike Woods. Such correction may not be possible in the upstream polarimeter due to synchrotron radiation emittance growth.
Length of diagnostics section and aperture for betatron errors :
Mark have shown that if we need to increase the beam size in the diagnostic section to three microns, to be compatible with present laser wire specifications (1% precision measurement of sigmaY and 3.7% presicion measurement of sigmaX with the same laser) (see talk of Grahame Blair at Snowmass), then the length of diagnostics section grow to 600m per side, adding 1.2km to the site. The accuracy requirements and possibility of future improvement of laser wire performance need to be further discussed.
Andrei Seryi, 09/28/05