How to deal with leaks in the QRL and magnet insulation vacuum

1. How to deal with leaks in the QRL and magnet insulation vacuum Paul Cruikshank for AT/VAC Germana Riddone for AT/ACR Chamonix XIV Workshop, 17-21 January 2005 Chamonix XIV Workshop, 17-21 January 2005

2. Contents • Insulation vacuum overview • Heat loads • Helium leaks • Air Leaks • Detection & Localisation • RHIC data • Repairs & downtime • Summary Chamonix XIV Workshop, 17-21 January 2005

3. Insulation vacuum Leaks – why? Origin: In-situ welds ~ 150000 ! Imported leaks Thermal cycles Faulty o-rings Damaged sealing surfaces Etc… Chamonix XIV Workshop, 17-21 January 2005

4. Insulation vacuum- Sectorisation Chamonix XIV Workshop, 17-21 January 2005

5. Insulation vacuum– which gases? Chamonix XIV Workshop, 17-21 January 2005

6. Heat load limitationsMagnet cryostat • Degraded vacuum will increase heat loads to the 50-75 K, 4.6-20 K, 4.5 K and 1.9 K LHe and 1.9 K GHe cooling circuits. • For the 1.9 K LHe level, the maximium available power per sector is 2400 W, and the local limitation per 214 m vacuum sub-sector is 180 W. Under operation with nominal beam, these limitations are reached with a degraded vacuum 1 E-2 Pa (1 E-4 mbar). • Heat inleak at 1.9 K versus residual pressure on the CTM Chamonix XIV Workshop, 17-21 January 2005

7. Heat load limitationsQRL • Degraded vacuum will increase heat loads to the 50-75 K, 4.5-20 K and 4 K VLP cooling circuits. • Under operation with nominal beam, limitations are reached with a degraded vacuum 0.1 Pa (1 E-3 mbar). Chamonix XIV Workshop, 17-21 January 2005

8. Helium leaks- LHC String Test Observations • 24 hours delay before any observation of the He leak or heat load, • Without the cryopumping effect, the vacuum would have degraded in 30 minutes, • System is allowed to degrade to 1 E-4 mbar, • The internal gauge gives only a qualitative measurement, • Auxiliary pumping returns the system to nominal conditions. Chamonix XIV Workshop, 17-21 January 2005

9. Helium Leaks – 200 days LHC operation • Scaling to the cryomagnet & QRL vacuum system: • With no auxiliary pumping, 214 m cryomagnet vacuum sub-sectors, 200 days continuous operation/year and 1 E-4 mbar as threshold, ~ 500 mbar.l of helium can be pumped, • Integral helium leak rate < 3 E-5 mbar.l/s (for 214 m) • or ~ 1 E-7 mbar.l/s/meter of machine • For the QRL, the adsorbtion capacity of the 4 K surface/meter of cryostat is ~ 20 % of magnet cryostat • Integral helium leak rate < 1 E-5 mbar.l/s (for 428 m) Chamonix XIV Workshop, 17-21 January 2005

10. Special measures in case of helium leaks- thermal cycles • In the magnet cryostat, the 1.9 K surfaces will adsorb the majority of the helium • From experimental data, if the system is at 1 E-5 mbar: • A thermal cycle from 1.9 K to 4.2 K will release 60 % of adsorbed He • A thermal cycle to 1.9 K to ~ 25 K will release 99 % of adsorbed He • The released helium will be pumped by fixed pumping station, and at cool-down the helium pressure is given by the reduced surface coverage. • A thermal cycle for 2 cells from 1.9 K to 4.2 K to 1.9 K will take ~ 20 hrs. • Such thermal cycling could be useful if the time to rise to 1 E-4 mbar due to a leak is several weeks. • He adsorption isotherms on stainless steel Chamonix XIV Workshop, 17-21 January 2005

11. Special measures in case of helium leaks- auxiliary pumping 1 • For degraded vacuum, the pressure of the insulation vacuum can be reduced by turbomolecular pumping groups. The turbo MTTF is 7 years. • The baseline is 0.5 l/s per meter of cryostat • ~ 50 CHF/m = 2 MCHF total • Changing the by-pass valve configuration gives 1 l/s/m. Standalones have 1 to 5 l/s/m • Additional pumps could be added to arc sub-sectors, without breaking vacuum, to give 3 l/s per meter. • Turbo group at a vacuum barrier. Chamonix XIV Workshop, 17-21 January 2005

12. Special measures in case of helium leaks- auxiliary pumping 2 • String Test – 400 l/s pumping on a 1 E-3 mbar.l/s leak reduced the heat load to nominal conditions. Tests were not made at higher leak rates, but the theoretical maximum would be ~ 4 E-2 mbar.l/s) • For a local turbo pump to be effective, the axial conductance of the cryostat needs to be bigger than the speed of the pump > 200 l/s. • Since helium leaks will be inside the MLI envelope, the integral transverse conductance must also be of the same order. To achieve < 1 E-4 mbar inside the MLI with a helium leak of 1 E-2 mbarl/s, an aperture equivalent to 100 l/s is sufficient ~ 1 mm2/m cryostat – the value on String 2 was > 400 l/s • A 214 m magnet cryostat (axial conductance ~ 350l/s) with baseline auxiliary pumping speed would allow a theoretical maximum helium leak rate ~ 1 E-2 mbar.l/s. • A 428 m QRL cryostat has an axial conductance of ~ 33 l/s due to the fixed points every 53.5 m. The transverse conductance need to be determined. Chamonix XIV Workshop, 17-21 January 2005

13. Air Leaks • Unlike helium leaks, which may be initiated or amplified under cryogenic conditions, air leaks can be identified & repaired before cool-down. • However, with 5 ppm helium in ambient air, their magnitude needs to be limited. The specified integral air leak rate per 214 m vacuum sub-sector is 1 E-5mbar.l/s • The gas load from thermal outgassing is ~ 1 E-3 mbar.l/s for the same sub-sector. • Air leaks which develop during operation will be observed as a large change in vacuum pressure but minor cryogenic heat load. A 214 m sub-sector could operation for 200 days with a leak of 1 mbar.l/s Chamonix XIV Workshop, 17-21 January 2005

14. Detection & Localisation of leaks • Detection Sensor: Vacuum pressure measurement • Vacuum pumping groups Cryogenic flow control valves • Cryogenic temperature sensors • Mass spec. leak detectors (mobile) • Localisation Feature: • Localisation steps (helium leak): Remote localisation from the control room • Localisation in-tunnel with mobile equipment • Warm-up • Localisation in-tunnel with mobile equipment • Break vacuum & open interconnect(s) • Confirm leak position • During String 2 experiments, simulated helium leaks could be longitudinally located to within one interconnect with the vacuum system closed, but only when the thermal shield and MLI were not installed. Chamonix XIV Workshop, 17-21 January 2005

15. Data from RHIC • Heat load limit for degraded vacuum in mid E-4 mbar range. • Baseline auxiliary pumping is 0.5 l/s/m. • In year 2000, several leaks > 1 E-3 mbar.l/s were present in the vacuum system, requiring additional turbos to be installed. • Degradation has been observed following thermal cycles (2 arcs have been cycled 5 times, and 30% of machine 8 times). • Leak repairs (faulty welds) have all been made during annual shutdowns. • The machine is now running with the baseline auxiliary pumping. • 4 out of 12 valve boxes, located outside the machine tunnel, have suffered from brazing flux corrosion problems, leading to repairs and or replacement – leaks > E-2 mbar.l/s Chamonix XIV Workshop, 17-21 January 2005

16. Repairs & downtime • If a helium leak cannot be contained with a combination of thermal cycles and auxiliary pumping, repairs will be necessary. The cryogenic and vacuum sectorisation allows local warm-up of a sector eg QRL only, standalone only, cryomagnet sub-sector only. Intervention times for the QRL have not been studied yet. • ‘Long Intervention’ • ‘Short Intervention’ (fast warmup 100 g/s) • eg for leak repair at an interconnect • Extract from LHC-PM-ES-0002.00 with warmup & cooldown values from Chamonix XII. Chamonix XIV Workshop, 17-21 January 2005

17. Summary • The cryogenic heat load limit for the 1.9 K LHe circuit occurs with a degraded vacuum of 1 E-4 mbar in the magnet cryostat. The cryogenic heat load limit for the 4.5–20 K or 4 K VLP circuit occurs with a degraded vacuum of 1 E-3 mbar in the QRL cryostat. • For a 214 m magnet sub-sector: • Helium leaks up to 3 E-5 mbar.l/s can be adsorbed by the 1.9 K surfaces without exceeding 1 E-4 mbar for 200 days continuous operation. Thermal cycles up to 4.2 K will partially regenerate the surfaces. • For a 428 m QRL sub-sector: • Helium leaks up to 1 E-5 mbar.l/s can be adsorbed by the 4 K surfaces without exceeding 1 E-3 mbar for 200 days continuous operation. Thermal cycles will regenerate the surfaces. • The installed auxiliary pumping can pump helium leaks up to 1 E-2 mbar.l/s. Additional pumps can be installed without breaking vacuum. • Air leaks up to 1 mbar.l/s can be tolerated. • The minimum time to repair a helium leak which requires opening of a 214 m vacuum sub-sector is 10.5 days. Replacement of an arc cryomagnet would take 25.4 days. Repair times for the QRL need to be determined. • RHIC exploits auxiliary pumping and executed repairs during annual shutdowns. • The objective to leak test individual components and subassemblies before their installation, and to fully leak test & repair the systems in the tunnel before cool-down, remains essential. Chamonix XIV Workshop, 17-21 January 2005

18. Cold/warm correlation for leaks Chamonix XIV Workshop, 17-21 January 2005

19. Leak testing techniques • Recall of methods with MSLD Chamonix XIV Workshop, 17-21 January 2005

20. Clamshell tools Chamonix XIV Workshop, 17-21 January 2005