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Update to BBSIM PEPII geometry: gnbpip
Terry Geld
Simulation Review
23 Sept 1998

Contributors: C.Cheng, S.Devmal, G.DeDomenico, S.Emery, T.L.Geld, D.Kirkby, B.Meadows


We are in the process of major updates to the PEPII beamline geometry in BBSIM. These changes have been driven by the background commissioning group's need for a more accurate simulation of the beamline in order to understand the sources and reproduce the level of the high backgrounds seen during the HER and LER commissioning runs. In the original simulation used in the TDR, MDCI and MDCII, the beamline simulation included preliminary versions of B1, Q1, the support tube, the Q2 septum chamber and the iron endplugs. The lost particle files were based on a TURTLE simulation which only used 1/3 of the ring. As discussed at various BaBar collaboration meetings, the background commissioning group has found much higher backgrounds than predicted by the simple geometry and Turtle rays. A significant fraction of the background is due to lost particles originating from the distant arcs. Additionally, the pressure profile is much worse than the nominal 1nTorr. As part of understanding the background sources, a full simulation of the beamline used for the January HER run (including the Q4 & Q5 magnets) was created, along with TURTLE rays which travelled one full turn of the ring. For the HER+LER commissioning in July and the fall, the real beamline is now complete, except for the Q2 septum which is a temporary aluminum version, not the final copper version. Therefore, the background commissioning group has embarked on a major update to the geometry which is needed not only by BkgCom, but by BaBar. After the geometry update is finished, along with new TURTLE rays and a better understanding of the pressure profile, a more accurate study can be made of the effects of realistic background on BaBar, rather than the underestimates used in MDCI & MDCII.


  • Beampipe: update beampipe dimensions
  • B1: include lead segment, update magnet values
  • Q1: update vacuum chambers, magnet values, support structure
  • Masks: update and include all masks
  • Add bellows
  • Add ion pump with lead shielding
  • Support Tube: update including asymmetric stainless pieces with carbon fiber in middle and end support flanges (flanges not quite finished)
  • Remove non-existent tungsten shields at ends of ST
  • Endplugs: update endplugs to final version
  • Q2: create Q2 magnet material and coils
  • Q2 septum: update to 98 aluminum version
  • Q4: create Q4 magnet material and field
  • Q5: create Q5 magnet material and field
All this was included in CVS tagged versions gnbpip V00-04-01 and gnbbg V00-05-18. It has not been released yet.

In Progress / To Do list

  • Q2 field values (in progress)
  • LER beampipes (in progress)
  • New TURTLE decks and events for updated HER/LER (in progress)
  • Fix some non-coplanar mask volumes (in progress)
  • Corrector magnets
  • HER beampipes backwards of Q5 and BV1 (prob necessary to study DIRC backgrounds)
  • 1999 copper Q2 septum (not built yet)

Major overlap problems

There are two overlap problems which need to be solved. The first is an overlap between the SSVD and the beampipe mother volume, BPMO (BPMO includes B1 & Q1). The second is an overlap between the forward IFR, MCPF, and the forward iron endplug. Both of these overlap problems existed in the original (TDR, MDCI, MDCII) geometry but must be fixed for future studies.

Pictures of new geometry and overlap problems

  • Overview of beamline
  • BPMO: beampipe, B1, Q1, bellows, ion pump
  • STMO: support tube and endplugs
  • ACCU: accelerator components: Q2, Q4 & Q5
  • Overlaps in TDR gnbpip:
    • There are some overlaps between BPMO and SSVD in the BBSIM used in MDCI and MDCII (these pictures used release 6.10.6). The small outcropping in the forward SSVD overlaps the BPMO and overlaps part of the B1 magnet. The third picture also shows the BPMO mother (in red) does not encompass all the magnet volumes.
    • There are also a major overlap between the forward endplug and MCPF .
  • Overlaps in V00-04-01 gnbpip:
    • A similar overlap between BPMO and SSVD exists in the updated version of gnbpip. The first plot shows just the outline of BPMO (in red) and SSVD (in blue). Both volumes, with all daughter volumes, are shown in the second plot. The last two plots show close-ups of overlap sections. With some redefinitions of both BPMO and SSVD, these overlaps are probably solvable.
    • Once again, there is a major overlap between the forward endplug and MCPF in the updated version of gnbpip.

Last modified: Mon Sep 21 15:38:29 PDT 1998