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[Ext-GDE-91] ML Change Request - Nov.16, 2006 (CCR#21)

Dear Colleagues,

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 
separate statement.

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
   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
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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.

Cost Impact

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 sentence

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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