ILC Beam Delivery System lattice design changes since the RDR

D. Angal-Kalinin

ASTeC, STFC, Daresbury Laboratory

This note summarises the status of changes made to ILC BDS decks after ILC RDR was published and comments on the remaining lattice design work which needs to be done in the future.

Summary of changes required/requested/studied after the RDR is listed below. These changes were made separately and were not released as a final BDS version, which was foreseen to be done during the EDR phase. The decks for the corresponding changes are available at (http://projects.astec.ac.uk/ilcdecks/), with the corresponding directories highlighted below in red. Archives of these directories (in .zip format) are available at http://projects.astec.ac.uk/ilcdecks/Archives.

1.       In the RDR design (ILC2006e_RDR_decks), the functionalities of machine protection, detection of laser wire photons were combined in the upstream polarimeter chicane in both e+ and e- BDS. The implications of these combined functionalities were discussed in the "Workshop on polarization and Energy measurements at ILC" held in Zeuthen in April 2008 :

http://indico.desy.de/contributionDisplay.py?sessionId=1&contribId=10&confId=585

The BDS team has agreed to separate these functionalities by adding another chicane for upstream polarimetry. This change was implemented in October 2010 and is mentioned in point 5 below.

 

2.       RDR BDS decks (ILC2006e_RDR_decks) were not modified for the push-pull operation; i.e. the inner length (D1B in the decks) between final doublet (QD0 and QF1 in the decks) was not increased which will allow operation with different L*(D0 in the decks) whilst maintaining the location of QF1 fixed in the tunnel. A. Seryi has prepared such decks (Andrei_Different_Lstar_not_tuned) for different L* for the purposes to study collimation depth and muons for different L* (D0) and inner-FD drift (D1B). The optics was not tuned in these decks for beam size studies and the dipoles were not decimated. Betax at the IP is 0.015m in these decks as compared to 0.021m in the RDR.

 

In (Andrei_Different_Lstar_not_tuned) directory, there is one "standard" optics (with L*=3.51m) and three modified versions. For all modified versions location of QF1 is fixed, and moved out, with respect to standard version, by 1.69m.

 

L*=3.51m - 351LD0_135D1B -- standard (RDR ILC 2006e), not push-pull

L*=3.51m push pull - 351LD0_304D1B

L*=4.0m - 400LD0_255D1B,

L*=4.5m - 450LD0_205D1B

These modifications increase the Length of FFS to 1587.212 m from 1582.319m in the RDR.

 

The dipoles in these decks are not decimated (so as to install only few at 250 GeV beam energy reserving a space for installing additional dipoles for 500 GeV beam without changing the beam line geometry).

Another change incorporated in these decks was to place the energy spectrometer upstream of the matching quadrupoles QM11-16, compared to the RDR where the energy spectrometer was embedded in the matching section. This provides better matching for different IP parameters without affecting the optics in energy spectrometer.

 

3.       The phase advances of the spoilers in the RDR deck were optimised by F. Jackson to improve the collimation efficiencies (Optimised_Collimation_decks). This work was reported in the paper (F. Jackson et al, PAC07, THPMN073). The modifications were done to ILC2006e RDR deck and thus were not compatible for the push-pull operation.

 

4.       SB2009 AD&I Decks used for AD&I CFS drawings (SB2009_decks_AD&I) (J. Jones, D. Angal-Kalinin, N. Collomb)

 

The major modification to electron side BDS was required for SB2009 due to change of location of the undulator based positron source as a part of proposed central integration. These decks are used for AD& I CFS drawings.

 

Modifications to RDR Electron BDS:

In order to protect the small gap undulator, the sacrificial collimators in the beginning of RDR BDS & detection of off-energy error beam for fast abort were moved before the undulator. For detection of off-energy error beam, energy spectrometer chicane (beam line EFFs from ilc612_eff1.xsif) was copied before the undulator and the fast abort line was mirrored from the ILC2006e deck. The fast abort line needs to be modified as the beam dump at the end will not be full power beam dump (11/18 MW) but only a few kW. If this line will be used for commissioning of the linac then the power rating of the dump and the design of fast abort line needs to be revised accordingly.

 

After the fast abort take-off section, matching quads match twiss to the undulator section, which is then followed by the Undulator. The positron target is located ~400m from the undulator and needs a transverse offset of 1.5m at the target location. To provide 400m path for photons, a dogleg has been designed to take the beam to the BDS. The constraints are to provide the required offset of 1.5m in 400m and with a drift of 40m parallel to the target location without any components. In order to keep the emittance growth at 500 GeV beam to a few percent, Theoretical Minimum Emittance lattice is designed for this dogleg. This lattice gives ~2.8% emittance growth at 500 GeV beam energy. The details of the dogleg design and the tolerances are described in paper [J. Jones & D. Angal-Kalinin, IPAC10 WEPE031].

 

A matching section at the end of the dogleg matches twiss to the beginning of the skew correction/emittance measurement section of the RDR. The MPS collimator from the combined chicane (MPS, polarimetry and laser wire photon detection) has been removed (replaced by a drift) as the MPS functionality has been moved before the undulator. Additional polarimeter chicane is envisaged but not included in this version. It was planned to shorten the Final Focus System (FFS) to allow this additional chicane to be included without affecting the overall AD&I BDS length.

 

A fast abort line becomes DC tuning line and thus the kickers can be replaced by DC magnets. This is not done and needs to be designed. Since the fast abort line is kept the same in this version and the distance after kickers "DMBB0" is reduced from 52.8 m to 14.2 m. This leads to clashing of the elements in the AD&I deck. This should not be the case when the DC line will be re-designed with few strong DC dipoles instead of pulsed magnets.


The matching quadrupoles at the end of ebsy2 match the twiss to start of FFS (beginning of betatron collimation section). Rest of the line (betatron collimation, energy collimation, matching section, energy spectrometer, matching section, final focus, IP) remains the same as in the RDR.

 

On the positron side: the line is unchanged and is same as RDR (ILC2006e).

 

5.       Changes to decks above used for AD&I CFS drawings (SB2009_Nov10):

Upstream polarisation chicane was separated in both electron and positron side of the BDS.

First chicane after the skew correction and emittance measurement on the electron side is now only for detecting the laser wire photons whereas on the positron side it is for MPS as well as detection of laser wire photons. Matching quadrupoles at the end of DMBB0 obtain beam waists in the polarimeter chicane and a matching section is added after the polarimeter chicane to match twiss parameters to the Final focus. This line adds ~150m extra length in both the BDS length.

 

In order to avoid clashing of the DC tuning line, the drift space 'DMBB0' was again restored to 52.8m (from 14.2m). This length will definitely reduce after design of the DC tuning line will be done, but at this stage the original length gives better flexibility of re-designing this section.

 

6.       RDR Final focus reduction (PushPull_tuned_LReduction) (R. Versteegen, CEA, Saclay)

 

The RDR deck has been designed to keep the emittance growth due to synchrotron radiation to minimum (DIMAD SYNC option 2, @500 GeV, emit/emit0 = 1.0078 (emit0 = 1e-5 m)). It is possible to allow it to increase to a few percent at 500 GeV beam energy.

 

Attempt was made by R. Versteegen to first tune the push-pull final focus decks (L*=3.51m) and then to shorten it. It should be noted again as mentioned in point 2 above that the dipoles are not decimated in these decks. However these studies are important at 500 GeV beam when all the dipoles are installed and thus need to be optimized at 250 GeV with decimated dipoles.

 

The length reduction is done by multiplying all the dipole lengths and drifts by 0.87 in the energy collimator and the FFS in order to approximately double emittance growth in these sections. The linear optics and sextupoles were re-tuned to optimise the luminosity and the bandwidth.


Final focus length is reduced by ~130m, which will almost compensate for the increase in length (~150m) due to additional of separate polarimeter chicane. The decks are kept separately for future optimization and inclusion of decimated dipoles at 250 GeV. It is easy to include this part in SB2009 decks by simply re-matching the quadrupoles at the end of ebsy2 to start of the final focus.

 

7.       Work needs to be done in the future

a.       Travelling focus will need another deflecting cavity to be introduced at pi/2 phase from the FD (A. Seryi, PAC09, WE6PFP082) in both e- and e+ BDS. Present decks do not include these cavities.

b.      Design of DC tuning line on electron BDS side.

c.       Modifications in the design of fast abort line before the undulator for low power beam dump.

d.      Shortening of different push-pull final focus decks with different L* and decimated dipoles at 250 GeV energy.

e.      Explore possibility to combine DC tuning dump on e- side and post collision beam e+ dump to one full power dump (and similarly e+ fast abort/tuning line dump and e- post collision beam dump in to one). (Some sketches were presented by D. Angal-Kalinin at ALCPG09, Albuquerque :

http://ilcagenda.linearcollider.org/materialDisplay.py?contribId=293&sessionId=28&materialId=slides&confId=3461)