Dear Glen,

I think it is possible to reduce the length of energy collimator
for the horizontal emittance growth by the Synchrotron radiation.

The problem is, however, the vertical betatron collimation is still remained.
The gap of the vertical collimator is better to design as large as possible
in order to reduce the wake effect and to reduce the tolerance of orbit adjustment.

In my design, the main vertical betatron collimation 
is also assumed to the energy collimator section (highest betaY),
and the vertical beta function at energy collimator is designed to betaY=4000m 
and the phase advance in between energy colliator to QD0 was adjusted 
in order to increase the collimator gap of the vertical spoiler.

In present design, the vertical full gap hight is about 2mm,
and the gap is comparable to the horizontal one at betatron collimator section.

When we reduce the energy collimator length, the vertical beta function is also reduced (in generally).
Therefore, the vertical beta collimation also should be set at betatron collimation section,
and the gap of the collimator will be reduced to about half to the present one.
 
This is just a comment for this issue.

Sincerely,

Toshiyuki OKUGI, KEK


----- Original Message -----
Dear colleagues,

As reported in the recent global timing working group report (EDMS D1097725), the BDS group was invited to look into ways to cut ~150m from the length of the positron system (optionally also from the electron side) to fix the timing mismatch between e- and e+ beams which exists in the current TDR lattices. There is a change request in process looking at increasing the linac length by ~1.5km (CR4 - EDMS D1092915) for additional reasons which will fix this problem if that is approved, but here Nick has also asked for a quick look into the possibility of shortening the BDS as an input into these discussions.
I think the most obvious candidate is for shortening the energy collimation system, which comes mainly at the cost of an additional 2.1% horizontal emittance growth and 220 MeV energy loss for the 1 TeV machine (negligible impact for 500 GeV).
As a suggested response to these requests, please find attached short note. Let me know if you have any comments or other suggestions on this topic.

Best regards,

- Glen.

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