Summary notes of the LCU meeting on 07/04/2009

Present: MA, GB, CB, HB, SF, MG, WH, JJ, EL, FS, YS, RT, Daniela Macina, Rob Appleby, DW.

Report from meetings

• SLM -> MG
  Daniel Wollmann was introduced. He just started as fellow in our group and works in the collimation team.
  MG said that there was a follow up on alignment of the vacuum chamber in the LHC. Up to 10 mm horizontal and 17 vertical offsets were measured on the beam pipe. We are asked to give feedback on the threshold value (typically 3 mm) above which alignment is needed.
  The MQY and MQM transfer functions a low current are very non-linear. In the FiDeL model implemented in LSA and used to compute the machine settings for the 2008 beam commissioning, the transfer function was assumed to be constant below 150 A and 200 A, respectively. RT computed the beta beating induced by this choice, which is about 4 %. This feature should be used in any computation involving data taken during the 2008 beam commissioning. 

Status of aperture studies for replacing the y-shaped vacuum chamber in IR2 -> MG (pdf) and Rob Appleby (pdf)

MG introduced the subject. Closing the vertical tertiary collimators TCTVB around ALICE to protect the triplet reduces the acceptance for neutrons in the forward region (ZDC). The collimators could be opened to not cut into the acceptance but this would reduce the triplet protection (even though no failure scenario in the vertical plane is known).

An alternative would be to displace the y-shaped vacuum chamber to install the ZDC upstream of the TCTVB. The issue is whether this displacement will have an impact on the aperture and whether such an impact is acceptable or the vacuum chamber should be modified.
IP2 also has the injection of beam 1. Aperture considerations have to cover also failure scenarios. This is currently followed up by Jan Uythoven (who prepared the slides showed by MG). The main failure scenario at injection is kicker misfiring, i.e., injected beam not kicked, or circulating beam kicked. A similar analysis was already performed as the y-shaped vacuum chamber had to be modified already once to increase its aperture. The aperture bottleneck for the failure scenarios remains D2. The y-shaped vacuum chamber, whenever displaced, becomes a second aperture limit. The maximum displacement is 2.5 m for which the n1 drops to about 5. An optimized integration, resulting in a reduced displacement, would be helpful, as with 1 m displacement, only n1 is 7.6. In all the computations performed by Jan the mechanical tolerances are 1 mm in horizontal and vertical planes. Jan estimated the increase in the chamber size (injection leg) to restore a decent beam aperture: as the distance between the two legs cannot be reduced too much, an increase of the vertical size is needed to gain some aperture in the horizontal one.

Rob Appleby showed studies of the aperture at IP2 in terms of n1 for the nominal and longitudinally displaced y-chamber. Aperture is most critical at injection. The mechanical tolerance is 4 mm (radial). There was some discussion on the definition of n1 and if it would be justified to assume a reduced alignment tolerance of only 1 mm at critical elements at injection.
A preliminary conclusion was that one should really consider a modified y-chamber for the left side of IP2. The right side is not affected by injection failure scenarios and might be left as it is (apart from the displacement). Daniela Macina said she is in contact with the collimation team and the vacuum group to see if the TCTs could be displaced.
SF suggested to re-optimize the crossing scheme to gain in aperture.

The subject will be followed up and more should be known by the time of the next LCU meeting.

AOB

None

Last update: 17-April-09

MG & HB

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