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[Ext-GDE-28] RTML BC Change Request



Dear Colleagues,

I am announcing that CCB has received a change request for the
RTML section from T.Tenenbaum who is representing the RTML AG.

The change request relates to clarification of specifications for
and possible implementations of the beam diagnostic section located
closest to the RTML entrance.

The requester suggests Class 1 for this change request, and the
CCB chair concurs, so it will be treated as Class 1.
I would like to ask D.Schulte and K.Kubo to act as CCB reviewers.

Now this request is brought to review by CCB, and is also open to 
general discussion. Any members of GDE or GDE-related task groups
with comments or questions on this request, please direct them to 
ml-ccb@xxxxxxxxxxxx and/or ml-ext-gde@xxxxxxxxxxxx by the end of
Friday, March 24, PST.

BCD is available at
   http://www.linearcollider.org/wiki/doku.php?id=bcd:bcd_home
BCD change history is available at
   http://www.linearcollider.org/wiki/doku.php?id=bcd:bcd_history

Sincerely,

- Nobu Toge (KEK, Accelerator Lab)
  email: toge@xxxxxxxxxxxx
  voice: +81-29-864-5224
  fax:   +81-29-864-3182
--- Begin Message --- Attached is my proposed change to the RTML BCD text. The requested change is CLASS 1.

SUMMARY OF THE CHANGE REQUEST: The request eliminates the requirements for intra-bunch emittance measurement at the measurement station closest to the entrance of the RTML, clarifies that that station uses either a metal wire scanner or an OTR screen and that both are provided at the given location, and updates the table of instrumentation with the OTR screen count.

REASON FOR THE CHANGE REQUEST: My previous change request for the baseline configuration left the BCD in a state which is not internally consistent. Specifically, the section on Emittance and Trajectory Measurement still called for "fast" intra-train measurement, which implies multiple laser wire scanners which operate intra-train, whereas the table of instruments currently only calls out 1 wire scanner at that location, which implies (slow) quad scans for emittance measurement. The changed text clarifies the issue in favor of the latter interpretation.

Over the month of March there have been many discussions about the emittance measurement station in the skew correction section of the RTML: what sorts of diagnostics should be present, and how they would be used. At the same time, the lattice design of the RTML has proceeded and clarified a number of issues. Out of this process, we have converged on the choice of 1 OTR screen and 1 wire scanner for the skew correction section emittance measurement for the following reasons:

1. The beam energy at this location is 5 GeV, and the layout of the accelerator calls for weak focusing and no bending through the emittance diagnostic and collimation sections, with the first bend in the entrance to the RTML turnaround. The combination of low beam energy, weak lattices, no bending, and collimation meant that there was considerable concern that a laser wire signal would not be detectable, especially against the anticipated backgrounds generated by beam halo hitting the collimators. Furthermore, if we chose to perform emittance measurements with the RTML's insertable tune-up stopper in place (so that a potentially untuned beam does not try to go around the turnaround and into the bunch compressor), the laser wire signal would be even harder to detect.

2. In the lattice for the upstream RTML regions, it has been possible to identify a candidate location for the skew correction section monitor where the beam size is 180 um RMS (x) by 13 um RMS (y). At such a location a metal wire is a candidate measurement device, but probably only for relatively low beam power (ie, single bunch or a few bunches per train).

3. Such a large spot would also be consistent with an OTR screen, which would have a significant speed advantage over a wire scanner since a digitized OTR image produces a full beam spot reconstruction on a single pulse, as opposed to the 100-200 pulses needed for a wire scanner.

4. On the other hand, experience with profile monitors of all types under these sorts of circumstances is mixed, whereas wire scanners are extremely well understood, and the relevant experts agree that a metal wire can be made to work here. Therefore, we went ahead and provided both 1 OTR and 1 metal wire in the skew correction section.

IMPACT OF THE CHANGE REQUEST: The requested changes would restore internal consistency to the RTML BCD section, and close out a portion of our original change request (from 27 Jan 2006) which has been in limbo since that time. More substantively, the requested changes would put into the skew correction section an emittance diagnostic which is more robust given its operating environment than what was originally envisioned. The diagnostic would no longer provide non-invasive measurement of the extracted emittances, but it is our judgement that (1) providing such a capability at this location would be difficult given all of the obstacles to extracting laser wire signals, and (2) this capability is probably not needed since we can perform invasive measurements of the beam at this location relatively quickly, and non-invasive measurements can be made on the diagnostic section downstream of the turnaround, which is relatively nearby (furthermore, the beamline between the skew correction and the end of the turnaround is relatively simple FODO lattices and we are relatively unconcerned about complicated and subtle effects in this region destroying the beam; so the downstream monitors should be more than adequate for most needs).

As always, I can be contacted by e-mail at quarkpt@xxxxxxxxxxxxxxxxxx I am looking forward to working with you on this request.

-PT

Attachment: rtml.20mar2006.proposed.doc
Description: MS-Word document


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