There has been a lot of BDS-related work presented recently in the run up to the ALCW conference, and also during the conference itself. I thought it would be instructive to try and summarize where we are currently and try to list the work required going forward. Especially in light of quite a few suggestions during presentations that would require change requests to the TDR baseline. Nick and Beno: please view items in this list so tagged as additional entries into the change request register. Whilst some of the change requests are probably fairly simple and can be expected to be put through with minimum discussion, there are others which will require proper review. I foresee dedicated BDS fuze meetings in the coming months to address these issues.

There was a “close-out” session during the conference to finalize discussions related to CR2 (single L*), where the decision to adopt a single L* of 4.1m was presented, with the additional request made of maintaining QF1 L* at 9.5m. Please see the talks in this and the previous BDS session for details (https://agenda.linearcollider.org/event/6557/timetable/#20150421). A final report is also available on EDMS (D1100205). I expect the decision to be finally ratified by the CMB in the coming days. We now also have a complete set of matched lattice configuration files (decks) which correspond to the official TDR configuration from Mark Woodley. These decks will be available shortly from EDMS, but for now (and for future developments) can be downloaded from the lattice development repository site (https://bitbucket.org/whitegr/ilc-lattices/downloads). It is intended to use this repository for collaborative development work related to the ILC lattices in the future. Currently XSIF format is supported (as historically has been the case for LC deck descriptions) but we are looking to find a way to support multiple formats in the future.

As a result of CR2, we need to update the BDS decks with new matched lattices for the various TDR parameter sets. But in addition, as a result of the optics development and simulation work which has been undertaken over the past several months, there are many foreseen changes in addition to just the FFS optics which need to be incorporated. In the post-TDR era, we are bound by change-control management to carefully document any changes made to the TDR baseline design. There will then be, by necessity, a staged approach to updating the official decks and BDS design via a series of carefully considered further change requests.

Below is a list of further work required and envisioned in the coming months, where I have indicated in each case whether formal change request documentation needs to be provided. My criteria for listing a CR required is that the item is not strictly cost-neutral or significantly changes TDR design assumptions. I have compiled this list from presentations and conversations at ALCW and during recent BDS fuze meetings.

Note that there is an unavoidable co-dependency between some of these items. e.g. many items would require re-matching of the FFS. Ideally we would address items in dependency-order, but due to resource limitations I suspect we will have to re-do some things as certain items are addressed at later times.

- Re-matched decks for TDR parameter sets to be provided as part of CR2 implementation plan with L*=4.1m. There has been, however, more than one suggested approach for the matching procedure with different calculated IP distributions. The most notable difference being the philosophy of using FFS octupole magnets as part of the baseline matching solution. Use of octupoles yields smaller rms spot sizes but with small differences in the core sizes which most relate to luminosity. Questions have been raised about the impact of the use of the FFS octupoles on the collimation performance. We need to properly expound these differences and converge on a common solution before updating the lattices.

- The FD coil packages include many different field types (dipole, sextupole, octupole, solenoid + compensation coils). The exact distribution inside the FD cryostats is being re-evaluated for the new L* configuration. Some possible changes include: co-winding SD0 onto QD0B, separation of octupole coil placed either d/s of QD0B or in between QD0A and QD0B. Use of “sweet-spot” field cancelation technique which impacts field seen by extracted beam. Assumption is 2 quad packages each 1.1m long for QD0 and 2 quad packages for QF1. QF1 requested to drop in total length 2 -> 1m. Anti-solenoid field configuration is also being re-assesed based on IP aberration cancellation. All these options need to be engineered into new FD magnet design and impact on FFS matching/tuning and extraction line optics assessed.

- Some matching quads introduced for the u/s polarimeter chicane exceed the requested 9.6kG max pole-tip field level for 1 TeV machine. Need to split the quads (introduce extra quads) which also will require re-matching. May impact extraction system.

- This suggestion widens the absolute collimator apertures, but it is a departure from the primary design of the collimation system which was set down by a collimation task force many years ago (for NLC) and inherited for all subsequent ILC studies. We need to understand this change request in the context of these studies which were done previously. For example, it was assumed that the energy collimator provides the service of removing particles whose energy has been degraded by the betatron collimation system but not completely absorbed. Hence, this proposed change will alter the collimation efficiency and needs to be studied as part of the wider collimation study program to properly assess its impact. Also new full calculations of wakefield effects need to be made.

- It was reminded during recent tuning studies that the required tolerances for sextupole moves for multi-knob application is ~50nm. This agrees with previous assumptions from the RDR-era. This is a highly non-trivial mover accuracy and an engineering solution needs to be looked at to ensure this is possible. Maybe a dual solution is best- usual O(1um) resolution mover for initial alignment plus trim coils for fine-adjustment? This would require a more complex magnet design, hence the potential CR.

- Baseline lattice exists for 1 TeV optics that delivers expected luminosity, difference from 500GeV lattice just filling in bend sections. Several changes to this baseline suggested: Additional magnets other than just bends; lengthen collimation section; lengthen and re-design emergency extraction system. These CR’s have an impact on the baseline design lattices in addition to 1 TeV config and need to be studied. 1 TeV lattice sets the total BDS length. Needs to be evaluated in conjunction with CR4 (1.5km linac tunnel extension) and timing task force.

- Many engineering issues raised as part of central-region integration into common tunnel raised at ALCW (some related aspects integrated into above) - see talks on thursday pm (https://agenda.linearcollider.org/event/6557/timetable/#20150421). Resources to study many of these issues are currently not identified. Efforts in this study will continue.

After the assumed completion of the CR2 process in the coming days, the most immediate next step will be to roll out the new FFS decks in the shortest timescale possible. To this end, I will be sending out a meeting agenda shortly after the CR2 announcement to begin discussions of this process.