LHC Injection Working Group

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Summary notes of the meeting held on 15 November 2006

1. Injection Aperture (J. Uythoven, round table)

J. Uythoven reported on the n₁calculations he had performed on the recombination chamber (VCTY) close to D2 in IR8 right and IR2 left (slides), after B. Jeanneret had noticed problems with the available aperture. In fact these chambers provide enough aperture for the circulating beam, however not enough for the "to-be-injected" but unkicked beam and the kicked out circulating beam (up to 15 mm are missing in the vertical plane to reach n₁=7). The n₁ calculations for the stored beam assumed 4 mm closed orbit error, 4 mm alignment error and worst-case polarity of the separation-crossing bumps for the circulating beam and for the injected beam 2 mm trajectory error. The aperture problem is located in one of the round (r = 28 mm) "legs" of the 5 m long y-shaped chamber. The problem would be solved with an elliptical chamber (28´40 mm is not sufficient for n₁=7) or preferably a racetrack shaped chamber. The horizontal dimension must not be changed due to the space requirements of the ZDC in IP2. A similar chamber design could be applied to both IPs. According to C. Rathjen and J.M. Jimenez  a racetrack chamber is easier to build. B. Jeanneret stated that an elliptical chamber with a vertical dimension larger than 40 mm might also not be desirable in view of the collective effects (RF losses) and might deform under vacuum. It was also proposed to keep the 28 mm horizontal chamber diameter all the way through to the D2 (presently there is a transition at about 4 m from the D2 towards the IP from a chamber with a slightly larger diameter to 28 mm). The implications on the equipment (ZDC, TCTH, BPMs,...) have to be carefully checked by C. Rathjen who will only be back at the beginning of December. The IR8 installation is nearing completion, and a solution is urgently required. It was hence proposed to prepare drawings with racetrack chamber dimensions prepared by B. Jeanneret and to have them approved by C. Rathjen as soon as he is back. IP2 is somewhat less urgent with installation taking place in March 2007. D. Macina mentioned that the anti-collision system of the ZDC which prevents the detector to collide with the beam pipes will have to be modified according to the new chamber design. So far circular chambers had been assumed. With regard to the latest version of the beam parameters in the injection regions Y. Papaphilippou mentioned no further optics changes are planned for the time being. It was mentioned that future variations in the separation- and crossing schemes in the combined experimental-injection IRs would have to conform with the installed hardware.

J. Uythoven will systematically check all apertures in the injection regions.

2. TDI (Y. Kadi, L. Bruno,  A. Grudiev, round table)

Two issues have come up concerning the TDI built for IR8: the out-gassing time during the 250 ºC bake-out is much longer than estimated from the tests some years ago and the impedance from the TDI with its present design would make up for 15 - 20 % of the total machine impedance budget at 450 GeV, during the filling time.

Out-gassing: The out-gassing could be speeded up by increasing the bake-out temperature, but only if the out-gassing rate is dominated by diffusion. The current TDI design is compatible with a bake-out at 350 ºC. A test of the most sensitive parts of the assembly will be carried out (time in oven about 12 - 20 h). J.M. Jimenez will perform a bake-out test of a BN segment as used in the present TDI. While the TDI is not conforming to the vacuum requirements J.M.Jimenez agreed to install it in the present state to move on with installation. An extended in-situ bake-out must be carried out. During the 2007/2008 shutdown the TDI IR8 will be replaced by an improved version.

Impedance: Recent measurements have shown that the 5 mm gaps between adjacent absorber blocks would create a sizeable contribution to the machine impedance as they have no electrical connection between them.  Reducing them to the bare minimum as demanded by the thermal dilatation during bake-out (order of 2 - 3 mm for 250 ºC) would not much reduce the problem. A sort of RF spring contacts have been proposed to provide the contact. Y. Kadi is checking the mechanical feasibility of these with simulations. Another issue is the coating of the TDI blocks (presently Ti). A. Grudiev mentioned that Cu would be preferable in terms of impedance, but L. Bruno replied that Cu would peal off during the bake-out due to the different thermal coefficients. If the coating material would have to be changed, out-gassing and coating procedures would have to be re-checked. Other absorber materials will also be re-considered. The studies have to be expedited to be able to incorporate the changes into the next TDI for IP2, to be installed in spring 2007. This one will also include the ferrites proposed earlier, to be screwed on the front frame of the tank to reduce trapped modes, which are presently still missing in IR8.

V. Mertens inquired whether similar impedance problems would not occur with the TCDS and TCDQ, which also have slots (2 mm). A. Grudiev replied that the geometry is different.   

3. List of open issues and possible future topics

A collection of items for follow-up and topics for future work and discussions was reviewed. The main remarks were related to the following issues:

TI 2 - TCDI integration: For the time being no further transfer line collimators are produced, but Y.Kadi expects that the hardware should still arrive in good time for the installation (April to August 2007). The energy deposition studies for the foreseen TI 2 locations are still not completely finished. It appears that two out of the seven locations are not perfect in the sense that the downstream masks would not protect the magnets sufficiently. The installation of additional dummy collimators is being investigated. Pending the result of the studies it was decided to freeze the TI 2 layout in the present state and to move ahead with the update of the general layouts and the detailed installation drawings.

TCLIA: The TCLIA drawings (two-in-one design) will be ready for execution by the end of this year.

TI 8 - TCDI integration: The last TCDI in TI 8 will be installed in December 2006.  

Injection permit loop: There will no proper injection permit loop yet in 2007. Direct links between the beam dumping system and the injection kickers are being put in place. The experiments will have a software injection veto.

TCDDM/TCDD: Trapped mode studies have been carried out for both TCDD absorbers in 8 and 2. They will be equipped with ferrites. The fixed TCCDM for IR 8 is brazed and waiting for coating. The design of the tank and jaw of the movable TCDD in IR2 is finished, the design of the supports is starting and will be completed by end of this year.

TCDI in TI 8 and quenches in the LHC: A paper was presented at EPAC'06 by V. Kain. Y. Kadi and S. Beavan. Fluka simulations for the two most critical collimator locations TCDIV.87804 and TCDIH.87904 showed that for ultimate beam intensity and all studied accidental impact scenarios quenches would occur in the LHC. Nominal losses during LHC injection would not lead to quenches.  

Energy interlocking in the SPS: The only energy interlock on the SPS side is provided via the BETS (beam energy tracking system) on the main bends which internally interlocks the extraction kicker MKE. It is now set to become active at 1 % of the nominal energy.

The list of open actions and future topics will be updated accordingly and the items for follow-up in the InjWG be prioritised in the coming days.

V. Kain + V. Mertens, 17/11/06.