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[Ext-GDE-91] ML Change Request - Nov.16, 2006 (CCR#21)
- Subject: [Ext-GDE-91] ML Change Request - Nov.16, 2006 (CCR#21)
- From: N.Toge <toge@xxxxxxxxxxxx>
- Date: Thu, 16 Nov 2006 21:35:32 +0900 (JST)
I am announcing that CCB has received a change request proposal for
the ML section of GDE/ILC BCD from C.Adolphsen, who is representing
the ML AG.
The change request seeks to apply two revisions to the BCD -
1. Revise the relative spacing of neighboring cavities in the cryomodules
to be 5+4/3 lambda (lambda = free-space wavelength of L-band):
This results in a small increase (+7.7mm) in the cryomodule spacing,
and thus results in a small increase of the ML CF/S cost. However,
this introduces a potential of using 3dB hybrids to feed a pair
of neighboring cavities, thereby removing the need for expensive
circulators, which in turn is expected to result in substantial
cost reduction. It is also expected to suppress acceleration of
dark current from upstream cavities, and to make the CM assembly
easier, according to the proponents.
However, introduction of 3dB hybrids in place of circulars is
not part of this CCR.
2. Make the 8-cavity cryomodule with quad equal in length to
the 9-cavity cryomodule:
The length of a 9-cavity cryomodule, as discussed in CCR#20
(currently under review) is 12.590m. The length of an 8-cavity
cryomodule with a quad in it presently is 12.543m. This CCR proposes
to have the 8-cav (with quad) CM to have the same length, 12.590m.
While this also is expected to result in a small increase of
the ML CF/S cost, it is expected to allow a better flexibility in
configuring the ML optics lattice, according to the proponents.
The proponents suggest Class-0 for this change request.
While the cost impact, as stated by the proponents, is not exactly zero,
I would tentatively assign Class-0 to this change request, hereafter
call it CCR#21, and assign W.Funk as the reviewer, pending confirmation.
I would like to ask the understanding by the proponents that since part 2
of this CCR#21 is contingent on CCR#20, depending on the progress of
CCR#20 review, the CCB conclusion on part 1 might come out sooner as a
Now this request is brought to review by the 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,
Nov.17, 2006, PST.
My thanks in advance for your cooperation.
BCD is available at
BCD change history is available at
- Nobu Toge (KEK, Accelerator Lab)
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- Subject: [CCB-897] Linac Change Request: Change in Cavity Spacing
- From: "Adolphsen, Chris" <star@xxxxxxxxxxxxxxxxx>
- Date: Thu, 16 Nov 2006 00:51:40 -0800
The Main Linac Group proposes
1) Changing the relative spacing of neighboring cavities in the cryomodules to be 5.75 * lambda = 1326.9 mm (lambda is the L-band free-space wavelength). This is 7.7 mm longer than the spacing in the current BCD, which is based on the TDR design (for reference, the TTF cryomodules have a 6 * lambda = 1384.6 mm spacing - no cryomodule has yet been built with the TDR spacing).
The motivation for this change is
A) With the 5.75 lambda spacing, cavities could be fed in pairs using 3dB hybrids, thus eliminating the expensive circulators.
B) This spacing will suppress upstream dark current acceleration (electrons accelerated on-crest upstream will be 180 degrees off-crest in the next cavity).
C) The increased spacing makes the cavity string easier to assemble.
2) Making the 8-cavity cryomodule with quad equal in length to the 9-cavity cryomodule.
The motivation is to allow more flexibility to configure the linac lattice - that is, all cryomodules would be interchangeable.
With these changes, each linac would be about 30 m longer, which would increase the machine cost by less than 1 M$. Thus we believe these changes to be Class 0.
Change to BCD text on page 36.
The length of the cryomodule with the quad is 12.543 m, close to the 12.590 m length of the 9 cavity cryomodule (the rf unit length is thus 37.723 m).
should be changed to
The length of the 8-cavity cryomodule with quad will be made equal to that of the 9 cavity cryomodule to allow flexibility to configure the linac lattice.
Also the paragraph
The TDR cavity spacing of 283 mm was optimized based on the flange connection and bellow scheme used at TTF. Shorter spacing is possible (see ACD below), but the TDR spacing is assumed until the engineering implications are better understood. With this choice, the linac packing fraction (ratio of active to actual length) would be about 70%.
should be changed to
The 283 mm spacing specified in the TDR between the 1036.2 mm long cavity 'bodies' was optimized based on the flange connection and bellows scheme used at TTF. Shorter spacing is possible (see ACD below), but a somewhat longer spacing (290.7 mm) is chosen to allow cavities to be fed in pairs using 3dB hybrids (R&D is underway to verify the feasibility of this scheme). With this choice, the linac packing fraction (ratio of active to actual length) is about 70%.
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