Preliminary minutes for 37th TREX - Special VELO topic

Presented by Paula Collins - presentation available on Indico.

• Status of the VELO installation.
• Summary of RF foil limitations.
  • Vacuum pressure - no major concern for the VELO (*1,2).
  • Temperature limits for the foil -30°C / +200°C.
  • Differential pressure 10mbar.
• Available temperature readout.
  • Side C connected and in DIP. Side A not connected (not available until the installation).
  • SMOG2 both halves connected and in DIP.
  • VELO TC readouts (from the vessel) connected and in DIP (*3). These are PT100s (delay and accuracy).
• RF foils temperature
  • Paula has presented studies performed by E. Jans - evolution of T with VELO position, beam energy and intensity. Slow stabilization of the temperatures with variation in range of 1-2 °C. 
  • Most important temperature effect stays powering of the modules (+17°C with effect on RF foil +7°C )
  • Slide 12 shows evolution of the temperatures during warm-up of the new C-side detector and effect on the read-outs on the SMOG storage cell (active cooling of the C-side).
  • In case of temperature increase on the C-side>20°C - alarm + changing VELO position by +/-1mm
  • In case of temperature increase on the C-side> 80°C - beam dump signal.
• Request for the new VELO open position from 29mm to 15mm - per side (Ø58mm to Ø30mm).
  • This is to reduce buckling effect of the downstream wake-field suppressor (TREX30/04/2021).
  • Jorg has pointed that there is no protection against the fault in horizontal plane.
  • Aperture of the Q1 (horizontal Ø51.3mm, vertical Ø41.7mm)
  • Aperture of the beam-pipe (Ø54mm)
  • Benoit has pointed that the new shape of the VELO may actually present closest point for "open position" which does not belong to the aperture channel of the VELO but outer part of the box. Preliminary study attached lower to these minutes.
  • Jorg & Daniel - potential change in the opening must be addressed to MPP and approved by the machine.
  • Daniel proposed to have an informal meeting before the MPP panel discussion.

In case we stay with actual planning:

It is expected that the limited intensity of the machine during first weeks of the commissioning should not bring any important temperature load to the system.

No movements of the VELO are foreseen before the A-side installation (VELO nominal open position 29mm).

Within this scenario there are no major issues foreseen by impedance, machine protection, vacuum and LHC OP.

In case of an additional installation delay:

The beam intensity limits must be re-discussed and agreed by both machine and experiment. Connection of the TC monitoring on the A side will provide additional information however requires neon venting of the IP8.X, opening detector volume and pump-down of the IP8.X (intervention for at least 4 days). Additional neon venting is not preferred option by TE-VSC as it presents a risk and effects the vacuum conditions of the IP8.X. 

(1*) The increase of the temperature will likely impose a limit for the beam vacuum (pressure increase caused by excessive thermal outgassing may trigger sector valves interlock as well as increase saturation of NEG in the IP8.X and connected vacuum sectors). The limit proposed by the LHCb (80°C) measured on the C side (with active cooling) must be set as a function of beam vacuum pressure (agreed by TE-VSC).

Post meeting notes (JS):

• (2*) Are there any increased HV breakdown risk for the modules in case of increased degassing.
• (3*) Cables for the TC readouts on the vessel and head side are not radiation hard (same cables as for the bake-out?). If so is there any concern regarding the reliability of the readouts. Do we know exact position of the thermocouples?

Slide 12 - Temperatures during VELO warm-up (only C side of the VELO, SMOG2 readouts):

• RF foil (C-side) went from 3°C to 16.5°C (T 13.5°C)
• SMOG2 (C-side readouts 3,4,5) went from 14°C to 18.5°C (deltaT 4.5°C)
• SMOG2 (A-side readouts 1,2) went from 18.5°C to 20°C (deltaT 1.5°C)
• T_Offset RF-foil to SMOG2 (C) at cold 11°C
• T_Offset RF-foil to SMOG2 (A) at cold 14.5°C
• T_Offset SMOG2 (C) to SMOG2 (A) at cold 4.5°C
• T_Offset RF-foil to SMOG2 (C) at warm 2°C
• T_Offset RF-foil to SMOG2 (A) at warm 3.5°C
• T_Offset SMOG2 (C) to SMOG2 (A) at warm 1.5°C

Preliminary geometrical study of the RF box aperture VS beam-screen of Q1 and smallest BP diameter attached to these minutes (pictures):

• At 29mm of opening is the closest box point 24mm;
• At 27mm of opening is the closest box point 22.6mm;
• At 15mm of opening is the closest box point 14.1mm;

Preliminary minutes written by J. Sestak 19-20/4/2022.