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ILC “Minimum Machine” specifications proposal

Posted by Walker_Nicholas

  • 11:23:30 am on July 9, 2008 | # | 9
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    The Technical Design Phase R&D Plan rel. 2 includes plans for the so-called 200-500 GeV centre-of-mass Minimum Machine studies. Specific milestones include a top-level specification for the minimum machine by the end of 2008, which will include a plan for related studies to be concluded in 2009, leading to a re-baseline of the machine design in early 2010 (end of TD Phase 1).

    To begin discussions on the initial specifications and scope of the subsequent design studies, we have produced a memo called Towards a Minimum 500 GeV Machine Definition outlining the approach we intend to take, and some initial top-level (conceptual) ideas for reducing the cost of the ILC, primarily via further integration of the central injector systems and the Beam Delivery System.

    The memo has been distributed to the GDE mailing list for comment. We invite the GDE to read it and comment using this web.log.

    Nick Walker & Ewan Paterson.

    Accelerator Design


  • Nobu Toge
    4:57 am on 10 July 2008 | # |

    I agree with the general need for some kind of a minimum machine to consider for sake of design refinement, cost refinement and other exercises. So I am not questioning that part. But I have a few technical questions, primarily for my better understanding:

    1. The DR circumference is noted as 6.4km. RDR used to have 6.7km. Are you intending to state 6.4km, in which case I would like to understand its rationale (For instance, now we know how to compatify the RF sections, etc — but I kinda doubt), or is this a typo?

    2. Fig 1 shows that the DRs now have their straight sections aligned with respect to canonical BDS lines. The offset between the DR straights and the BDS is noted as 200m. Why 200m? 100m seems to suffice, although I may be forgetting something (may be the rooms to make for injectors and boosters etc?)

    3. In relation to 2, I guess the beam dumps are to be placed on the side opposite to the DRs… correct?

    4. Undulator is now located at the e- linac end (max beam energy there is therefore 250GeV) and its length is 100m (RDR used to have 150m). Are you intending to shorten the undulator by 30% and thus accepting the reduction of positron population by 30%, or is this a typo? Also, at the Ecm lower than 300GeV (E_elec lower than 150GeV) the positron yield will be rapidly dying out.  Are you saying we more or less give up on running at ECM ~ 250GeV or below? (This is just a question. If the physics says that it is OK, I am OK, but I do not know if the physics is actually OK.)

    I might come up with some additional questions/comments later, but this is it for now.

  • Walker_Nicholas
    5:00 am on 10 July 2008 | # |

    Dear Nobu,
    Many thanks for your prompt reply. The document we sent out was intended to stimulate these types of comments, so I guess we can claim success.  Many of your comments and questions pertain to details which are still to be worked out, but clearly they are appropriate and we will add them to the list.  Ewan should also reply, but here are some quick responses to your points (my point of view).

    1. DR Circumference: As you point out I expect this is a typo. (Should be checked).

    2. I think 200m is nothing more than a 'conservative guess' on how far you would have to be offset to clear the IR region. I tend to agree with you that this could be smaller (as small as possible? Are there other constraints?). These types of details need to be sketched out. Off the top of my head I think it could be less than 100m.

    3. Dumps: the dumps are only offset by ~3m from, so I doubt there is any interference if they are on the same side as the DRs. Ewan's proposal for the connection tunnels/beamlines to the DRs are closer to the IR than the current dump locations. Andrei has a proposal for re-using the primary beam dump also as the commissioning/emergency dump which would require an additional tunnel (see his talks from Dubna); in this case clearly they would have to be on the other side to avoid interference (unless there is a clever way to reuse these tunnels in someway.) Again, these are things to be sketched out to some scale to see if they work.

    4. From all your points, I suspect this one to be the topic of most discussion. The undulator length issue needs to be reviewed (I don't think Ewan was that exact in his distances). The energy range between 200-300 GeV Ecm is clearly impacted; in that sense, this scenario does affect the physics scope in the WWS document which prescribes only 1/gamma luminosity scaling over the full range. Should we immediately reject this proposal based on that? Or should we evaluate what the physics impact might be and see what the WWS think about it? If I remember correctly, when Tom Himel and I worked on the original post-Snowmass White Paper on this subject, the final decision to put the source at 150GeV was primarily taken on physics-scope grounds. All of the argument for and against are quire well documented, and it certainly warrants having that info reviewed. Something to do before the next WebEx meeting on the 18th July.

  • Nobu Toge
    5:02 am on 10 July 2008 | # |

    In my opinion we do not have to categorically reject this shortened wigglers at the linac end, on a simple ground of being incompatible with the ILCSC physics param document.   In fact today I looked for a ref describing how the intensity / luminosity would scale if the beam energy gets below 150GeV, and I could not find a systematic survey report on that topic. Maybe I did not look carefully enough, but this perhaps would prompt us (i.e. some experts within ourselves) to re-examine the numerics, and I think that IS good. It is good since it helps us advance our collective understanding of our machine design.  However, whether being dogmatic or not, there still is a real non-zero possibility of Higgs to be really rather light, and if so we will have to be receptive to such a possibility. This perhaps would dictate how we may have to supplement this linac-end-wigglers with possible bypass lines for e+ production for low energy operation and so on.  And, when describing this to the physicists we (mostly you the PMs and EC) will have to be extra careful since we could be easily interpreted as starting to touch these "project scoping" issues.

    The physics folks are hypersensitive to this type of stuff as you are aware.  Or maybe you consider this as being the first immunizing shot for them which they need anyways at some point <g>.

  • Andy Wolski
    5:09 am on 10 July 2008 | # |

    Dear Nobu,

    I see that Ewan already answered your question about the damping rings circumference; but just for completeness (and general information) see the spreadsheet posted on the Cornell Damping Rings Wiki site:

    This shows the possible fill patterns (and timing scheme) for the "EDR" (should be "TDP") baseline lattice with circumference 6476.440 m. 

    You can also see the corresponding data for the RDR lattice, which had circumference 6695 m. 
    Tor spotted that the RDR circumference failed to satisfy a constraint on the bunch spacing in the main linac, arising from the subharmonic buncher in the electron source; but unfortunately this was noticed too late to make a correction for the RDR. 
    This issue prompted re-examination of the circumference, to find a better solution.

  • Mark Woodley
    9:00 pm on 11 July 2008 | # |

    In Figure 1 (B) (p. 7) of the memo you show the main linacs as anti-parallel (actually, colinear). The net bend angle for each BDS system is zero (required for the upstream polarimetry). This implies head-on collisions. Is that the intent? Or are some new, yet to be designed, bending systems for generating the 14 mrad crossing angle envisioned?
    Also, in your discussion of the Minimum Machine you state that “A minimum 500 GeV BDS will be initially considered”. I assume that this means a minimum length BDS system capable of operating at a maximum energy of 250 GeV (for a maximum center-of-mass collision energy of 500 GeV).
    (excuse me if I’m picking at nits … )

  • Ewan Paterson
    11:03 am on 12 July 2008 | # |

    Regarding your comments on Fig 1 (B), the fact that the tunnels are shown co-linear is purely the ‘artists’ rendering and they should be assumed to be intersecting at the approptiate non zero crossing angle. We should have noted this in the memo.
    You are correct that for the first step of the study of a ‘mimimum’ design, we assume the shorter 500 GeV BDS, as shown in Fig 3, not the RDR version as in Fig 2. Once this configuration is understood, we should consider whether the longer BDS might also be possible.

  • Tom Himel
    3:42 am on 14 July 2008 | # |

    The memo in many places refers to making top level “specifications” for the minimum configuration. I believe what is really meant is a top level “design.” “Specification” means a description of what it will do. An example would be reaching an energy of 500 GeV. “Design” is a description of how the goals in the specification will be accomplished. An example would be the location and circumference of the damping rings. To avoid mis-interpretation (and possible panic by physicists who think we are considering changing the physics specifications for the ILC), I’d suggest we call what we are doing a top level design.

  • Ewan Paterson
    12:27 pm on 14 July 2008 | # |

    With reference to the document on a Minimal Machine Design” it is clear that everyone has trouble in using Fig 1 to picture possible linac tunnel layouts. To help ‘I hope’ I have produced a very schematic plan view of a possible tunnel layout, which represents a case where we combine injector systems,E+ and E-, with a short 500 GeV version of the BDS. The horizontal and vertical scales are extremely assymetric, 200:1, and the tunnels are co-linear, not at the real beam crossing angle. These are to simplify the sketch which can be found in the EDMS file
    I would appear that it is in principle possible to fit everthing together in a 4.5 m tunnel and in one plane. The exception being an electron injector linac for an auxilliary positron or keep alive source. There are also many questions, for example :- what are the real sizes of equipment like collimators etc/ We have a feel for the sizes of cryostats and magnets but not for all the miscellaneous equipment.
    An obvious question is where the primary beams cross from one side of the tunnel to the other, as they always did in the RDR, what is the CF&S preferred solution for the passage of people and equipment?
    This scale does not allow showing the DR’s, the IR or the shafts/vaults which I propose combine the function of end of linac and e+ vault. There location is shown.
    There are obviously many details missing as the schematic soon becomes too dense but I hope it is helpfull for our continuing discussions.

  • Gudrid Moortgat-Pick
    1:13 am on 16 July 2008 | # |

    Dear Nobu, dear Nick, dear Ewan, I would like to make a comment concerning the new undulator position at the end of the linac.
    I agree with Nobu that the consideration of a bypass line would be important to be considered. One expects a luminosity reduction of a factor 3-4 in the range of sqrt{s}=200-300 GeV. Having a bypass one could guarantee to keep the reduction factor to a factor 2 for the whole range (referring, for instance, to the studies done for Snowmass).
    Concerning the physics scenarios we definitely could live with the factor 2, but even a factor 4 may be ok depending on the physics scenario and the outcome of precision measurements at sqrt{s}=500 and at 350 GeV, the top threshold.

    Let me shortly summarize the important physics topics for the sqrt{s}=500 GeV machine: a) measurements of the properties of the top quark in the continuum and at top threshold; b) measurements of the Higgs properties; c) measurements of possible new particles, for instance light supersymmetric particles and d) hints for new physics discovered via detection of deviations from Standard Model predictions, the so called ‘indirect searches’.
    Starting with the easy cases:
    Concerning item a) we should be still on the luminosity-safe side, so no problem;
    concerning item d) high energy is favoured, so no lumi problems with the new undulator position is expected.
    More complicated cases:
    Concerning item b): the upper mass limit for a Standard Model Higgs is <207 GeV at 95% c.l., derived from the electroweak precision measurements and the LEP2 exclusion, so we would be just in the critical energy range. Most physics studies for the Higgs, however, will first be done at sqrt{s}=500 and at 350 GeV. Using the results of these studies at high energy would enable to optimize the needed energy steps for a threshold scan a priori. In other words, even for Higgs studies where we really need to go to the threshold, probably only a few energy steps are needed (Studies have been made showing that 3 steps are sufficient to reach the precision goal). So, suffering under luminosity reduction for maybe 3 energy steps is not nice but maybe bearable and from results at sqrt{s}=500 GeV and 350 GeV, we could already conclude whether we really need the bypass or not — even for such cases.
    Concerning the last outstanding item c): we do not really expect many new particles in the energy range just between sqrt{s}=200-300 GeV, otherwise we would not always cry for higher energy and more luminosity. But even for maybe a few light supersymmetric particles and only in case we really need threshold scans, one would use the same strategy as for the Higgs: results at high precision measurements at high energy to outline if and how many energy steps might be required for the threshold scans. So, my guess, also no problems with the lumi reduction in the range of sqrt{s}=200-300 GeV concerning item c).
    Last remark: It has been shown that it will be important to have high luminosity and an easy path to polarized beams at 500 GeV, to have optimal tools for the physics studies, to get systematics under control and to verify new physics properties etc. Also, concerning these aspects I believe the new undulator position providing a higher yield may be advantageous.

    I wrote a 10 pages physics summary concerning the undulator position in times of the post-Snowmass ‘White Paper’, providing also physics references. In case there is any interest, I could easily forward it to you. The physics has not much changed since that time, so, the arguments and references should still be appropriate.
    Best wishes, Gudi