1. Vac meeting 08 Oct 08 Agenda 1. needed operations for towers displacements (Frederic R.) final comments on the 2m bottom not accessible anymore 2. Larger tower links (1m in diameter) without valves between towers (Antonio P. + All) 3. Upgrade of SR tower: (Pierre M.) 4. Improved clean air flow in UHV towers: (Pierre M.) 5. Vacuum system upgrade in order to reduce the residual pressure (Antonio P. + All) _ Sensitivity _ Present status _ Possible solutions _ Considerations from ligo vacuum expert about water _ Air from bench towers (oring permeation, desorption from cables …) 6. Vacuum Control System upgrade: (LAL team )

2. Vacuum Upgrade 1)Actual status 2)Sensitivity 3)Needed improvement and possible solutions 4)Ligo vacuum expertise 5)1m link option

3. 1.10^-6 mbar About 7 months Actual Vacuum in Virgo W arm N arm W arm recently

4. W tube (600m, 1200m, 2400m) pumped during a recent stop reached 5E-8mbar, and went back to around 2E-7 mbar when reconnected to the towers (right pic ) Gas load from towers is considerable Towers are frequently vented and connected to the tubes after a few days of pumping (left pic). Pressure can reach temporarily 1E-6mbar.

5. Noise due to water at different pressures Beam size : 60mm-8.5mm- 60mm (slide by G.Cella) Water 3E-8 mbar

6. The phase noise due to vacuum of tubes has been checked by experts (GCella) and the proposed formula has been experimentally validated (LIGO) It is needed to improve the vacuum by more than one order of magnitude. Two orders of magnitude to get a margin of ~>5, the minimum considering a safety factor on design and to not limit the AdvV progress Ligo limits correspond to our full baked conditions 1E-9mbar H2, 1E- 10mbar H2O,...

7. Considered solutions: virgo vac system has been designed and tested to be fully baked at 150C: beam tubes + towers to reach < 1E-9 mbar, H2. Towers can’t be baked because much time consuming + risky : Baking only tubes: already in use by Ligo using Coldtraps. Increasing pumping speed: it was already discarded in the original design. Not feasible to get a factor 100, while the cost is considerable even to get a factor 10 on pumping speed because the need of doubling the stations Others ?

8. full baked case (total pressure 1E-9, hydrogen with other gases) ‘limit’ Feasible solution

9. Ligo expertize, baking tubes but not towers limit of the water load to be released inside tubes is 400mbar.liter, for all the service life It comes from a model based on water adsorption on tube walls, verified on the real case during the bake (a simulation for our case shall be done) This amount is released from towers to tubes in few months without cryotraps. AdvV towers will have additional internal load (large amount of peek). Cryotraps are recommended to lower the HC pressure in mirror chambers, possibly risky for AdvV mirrors. (RW,Sept’08)

10. Cryotrap principle is already present in Virgo (towers conductance, DT tower) They are traps but in particular also large cryogenic pumps ~10^5 liter/sec, 100 times than actual figure in towers. Different postions can be studied ”Ligo” Small option Additional ‘false’ valve

11. Cryotraps about 4m long, 1m diameter, can be made as an LN2 reservoir made out of aluminum and put inside the first portion of tube (Ligo courtesy)

12. Other advantages of cryotraps The need of prototypes and long design phase is reduced thanks to Ligo experience. Bake of tubes could be postponed. Once separated from towers, the pressure in tubes will improve along time. ITF duty cycle: faster recovery of tower service after venting Needed to allow the ‘1m diameter link’ option. They shall reduce water and HC in central area by a factor 100, (as we got in the tower baking test) helping to connect mirror towers to bench towers.

13. without ‘brewster windows’ we need to consider ‘ air’, in particular from bench towers upper parts Cryotraps don’t pump air, that will be released from polymeric materials after each venting and through orings due to permeation, and small leaks. After water we need to deal with air, exact amount is to be evaluated Could a very simplified IVC adopted on DT/IB/MC?

14. Needed answer ‘today’: Can now decide to have ‘’large flange not accessible’’? (pic by A.Paoli and T.Zelenova

15. Before the AdvV review, Nov 3rd: It is needed an update of the AdV conceptual design, including a revised plan and budget