1. MPP, 18 th of June 2008 1 MPP 18 th of June 2008 Outcome of special CPP meeting of 16.06.2008 on Stand-Alone Cryostats Helium Level Issue R. van Weelderen Participants: Vittorio Parma; Serge Claudet; Juan Casas-Cubillos; Udo Wagner; Francois Millet; Nuria Catalan Lasheras; Herve Prin; Laurent Jean Tavian; Rajvir Singh Doohan; Prabhat Kumar Gupta; Shailesh Govind Gilankar

2. MPP, 18 th of June 2008 2 Observations (sector 2-3, sector 8-1) Level reading, when overfilling, from ~75 % to 15-0 % within 1 s, followed by oscillations (even with filling valve fixed): period 90 s Level reading variation in phase with respect to vapour fraction (which varies mainly due to header D pressure variations): approximately 32% level/ 11% vapour fraction (or 360 mbar header D) 2.8 %/mbar (8 mm/mbar) (sector 7-8) ¾ cryostats level down to zero within 10 seconds every 3-10 days (suspicion: Taconis oscillations). ¼ cryostat ok. (sector 4-5, sector 5-6, sector 6-7) 4-5, 5-6 ok, 6-7 tends to 5-6 behaviour ~ok (General) statistically there seems to be an increased susceptibility to problematic behaviour according tunnel slope and accompanying cryostat installation (left/right, tilt)

3. MPP, 18 th of June 2008 3 Stand alone cryostats orientation 0 slope + slope - slope arrows indicate cryostat inclination towards or outwards from the center

4. MPP, 18 th of June 2008 4 Observations: 2-3 (over-fill, level in blue)

5. MPP, 18 th of June 2008 5 Observations: 8-1 (over-fill, level in blue)

6. MPP, 18 th of June 2008 6 Observations: 7-8 (spontaneous level loss, level in blue)

7. MPP, 18 th of June 2008 7 Observations: 8-1 (level – header D pressure correlation)

8. MPP, 18 th of June 2008 8 Stand alone cryostats helium level measurement  Implementation: –Level probe in pipe separated from the cold-mass. –Level-probe pipe connects to room temperature. –Level-probe pipe to cold mass connections at the bottom for the liquid equilibration and near the top of the cold mass for the cold vapour equilibration. The cold vapour connection is of a relatively small diameter (2 mm ID)

9. MPP, 18 th of June 2008 9 Stand alone cryostats helium level measurement Level probe housing Cold vapour pressure equilibration capillary (620 mm long, 2 mm inner diameter) Exhibits 1) High hydraulic impedance 2) Low points susceptible to liquid helium accumulation) Magnet vapour exhaust to header D (30 mm inner diameter) Note that the stand-alone cryostats are installed in left/right versions, and inclined either towards or outwards with respect to the LHC center. Level control and cool-down inlets (CL1 & CL2)

10. MPP, 18 th of June 2008 10 Numbers to keep in mind Level reading change per mbar of generated pressure difference between LT-housing and cold-mass volume: ~86 mm/mbar (21 %/mbar) vapour flow generated ∆P

11. MPP, 18 th of June 2008 11 Numbers to keep in mind: susceptibility of cold flow capillary to generate pressure drops 86 mm equivalent level difference 172 mm equivalent level difference

12. MPP, 18 th of June 2008 12 Numbers to keep in mind: susceptibility of cold flow capillary to generate pressure drops 86 mm equivalent level difference 172 mm equivalent level difference

13. MPP, 18 th of June 2008 13 Cryostat piping (courtesy V. Parma) LD1: exhaust CL1: filling CL2: level Q5L8 Slope: 0.36%

14. MPP, 18 th of June 2008 14 Conclusions & Recommendations (1/2) LT-implementation 1. due to the narrow cold vapour exhaust the ensemble is extremely sensitive to the consequences of pressure variations of any source which will directly translate into changes in level reading 2. The cold-vapour exhaust implementation exhibits low points and therefore presents a trap for liquid, effectively nullifying the level reading once liquid enters Cryostat piping 1. the magnet vapour exit (LD1) and the LT-cold-vapour exhaust capillary are joined, therefore increasing the exposure of the capillary to liquid droplets of the main helium exhaust stream. 2. the magnet liquid fill line (CL2) is positioned near (about 20cm) and below the magnet vapour exhaust (LD1), thereby increasing the fraction of liquid droplets in the vapour stream near the LD1 and capillary join. Operation 1. The present level control is estimated to be above the magnet liquid inlet (CL2), therefore increasing the fraction of liquid droplets in the vapour stream near the LD1 and capillary join.

15. MPP, 18 th of June 2008 15 Conclusions & Recommendations (2/2) Short term: the maximum level to be controlled shall be below the liquid fill point (CL2 inlet) of the cold mass secundary: In case of spurious excess cold mass heat loads the cold mass cool- down supply (CL1 inlet) could, instead of being closed, be set to a low liquid bleed to suppress possible thermo-acoustic oscillations of the cold-mass helium volume Long term: increase the cold-vapour exhaust capillary size from 2 mm ID to at least 8 mm ID (to desensitize the LT-implementation to the consequences of pressure variations of any source) eliminate low points in the cold-vapour exhaust capillary (to desensitize the LT-implementation to vapour outlet blockage by liquid helium) Increase the liquid fill point (CL2 inlet) of the cold mass