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How to Deal with Leaks in the LHC Beam Vacuum

How to Deal with Leaks in the LHC Beam Vacuum. Vincent Baglin. CERN AT-VAC, Geneva. 1. Introduction 2. Air leaks in cryogenic temperature environment 3. He leaks in cryogenic temperature environment 4. Conclusions. Many thanks to :

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How to Deal with Leaks in the LHC Beam Vacuum

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  1. How to Deal with Leaks in the LHC Beam Vacuum Vincent Baglin CERN AT-VAC, Geneva 1. Introduction 2. Air leaks in cryogenic temperature environment 3. He leaks in cryogenic temperature environment 4. Conclusions Many thanks to : Paul CRUIKSHANK, Bernard JEANNERET, Berthold JENNINGER, Miguel JIMENEZ, Jean-Michel LAURENT, Rob Van WEELDEREN, Frank ZIMMERMANN For their contribution to this work And special thanks to : Noel HILLERET for his contribution to this talk and this work Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  2. 1. Introduction • Cryogenic elements • Unbaked vacuum chambers operating below 20 K in the arcs and in some components of the long straight sections • Air leak or He leaks could appears during operation : • The consequences are : risk of magnet quench, pressure bump • Magnet quench • At 7 TeV, a proton loss of 8 106 protons/m/s leads to a magnet quench (~ 9 W/m) • At 450 GeV, 7 108 protons/m/s • For comparison, the 100 h beam life time has an average loss rate of 3 104 protons/m/s/beam around the ring • Risk of magnet quench due to beam gas scattering onto the N2 or He molecules • The proton are lost in the cold mass by inelastic scattering close to the interaction point (B. Jeanneret et al., LHC PR 44) Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  3. 2. Air leak in cryogenic temperature environment • Description • The gas is condensed onto the cold surfaces in the neighborhood of the leak over ~ 0.4 m of the cold / warm transition • A leak rate of 10-6 Torr.l/s condense ~ 30 monolayers of gas over 0.4 m in a week • A leak rate of 10-8 Torr.l/s condense ~ 1 monolayers of gas over 0.4 m in a month • The gas accumulates and, in the electron cloud regime, when the beam is switch on, there is a vacuum transient due to the recycling by the electrons of the condensed N2 molecules into the gas phase. • The phenomenon stops when the equilibrium surface coverage is reached. • The continuous electron bombardment, flush the condensed N2 from the beam screen to the cold bore. Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  4. Example of vacuum transient • Effect of condensed N2 onto the BS over 0.4 m, 1 1015 N2/cm2, • heat load onto BS due to electron cloud : 1.5 W/m, 100 eV electron energy • Above the quench limit (2 10-7 Torr) during 10 minutes with 1.5 W/m • => risk of quench • Operate with reduced beam current above the background limit (3.2 1014 N2/m3) for 2 h with 0.1 W/m • => background Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  5. Leak rate with a risk of quench • With more than a monolayer, there is a risk of magnet quench at the start of a run (after a shutdown period) due to a vacuum transient induced by electrons bombardment • A leak rate above 10-8 Torr.l/s cannot be tolerated without a significant risk of magnet quench 22 days at cryogenic temperature without beam Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  6. Diagnostics • At each cold/warm transition, a cold cathode vacuum gauge can monitor the pressure. • It protects the experiment against background. • It protects the magnets against quench • At each arc extremity, the Beam loss Monitors (BLM) protects the machine against quench due to air leaks. Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  7. 3. He leak in cryogenic temperature environment • Description • A He pressure wave is developed with time along the beam vacuum chamber • The He wave can span over several tens of meter without being detected • Taking into account the scattering cross section, the average He gas density over 1 meter shall be below 1.7 1017 He/m3 to avoid quenching a magnet with a nominal proton beam at 7 TeV • This He pressure is equivalent to 3.3 10-8 Torr at 1.9 K and 4.2 10-7 Torr at 300 K • This gas density quench limit increase proportionally when the beam current decrease i.e. when operating with 1/3 of nominal beam, the He density limit is increased to 5.1 1017 He/m3 Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  8. String : case of a He leak 1 m Pressure at the level of the leak Quench limit : 4 10-7 Torr Pressure 73.5 m away from the leak 73.5 m, leak rate ~ 6 e-5 Torr.l/s E. Wallen, JVST A 15(6), Nov/Dec 1997 Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  9. He leak rate with risk of quench • A maximum Helium leak rate of : • 5 10-7 Torr.l/s with nominal beam current • 8 10-7 Torr.l/s with 1/3 of the nominal beam current (no electron cloud) • 2 10-6 Torr.l/s with 1/10 of the nominal beam current (1st year) • Can be tolerated without a significant risk of quench • For such leak rate : • The half-length of the helium wave will be ~ 75, 125 and 200 m • The speed of the He wave will be ~ 2, 4 and 6 cm/h • Lower leak rate : • Require a pumping of the beam tube on the yearly basis (cold bore >~4K) • Larger leak rate will provoke a magnet quench within : • 30 to 100 days beam operation for He leak rate of 10-6 Torr.l/s • A day of beam operation for He leak rate of 10-5 Torr.l/s 1 year of operation ~ 150 days Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  10. Diagnostics • Helium leak rate above 5 10-7 Torr.l/s shall be detected to avoid the risk of a quench • By design, a vacuum gauge is placed every 3 or 4 cells (320-428 m) i.e. 6 to 8 gauges per arc • BLM provide a measurement every 53 m at each arc quadrupole and inner triplet • Per cell, a power of ~ 1 W/m can be measured in cold masses • All quench due a He wave half-length larger than 53 m are detected by the BLM • At nominal, leaks rate below 7 10-7 Torr.l/s are detected during normal operation Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  11. Diagnostics • After a quench, the faulty magnet is identified by the triggered diode. • The cold bore is warmed up to more than 30-40 K • The He is flushed to the nearest unquenched magnet and condensed over ~ 10 m • Vacuum gauge or residual gas analyser can be locally installed, at the short straight section, in the tunnel for a leak detection (vacuum wish : spread the quench till the nearest SSS) • The minimum spacing between locally installed vacuum gauge is 53 m (if enough valves could be purchased) • Warm up of the cold bore to > 4 K and pump out of the He every month allows to operate with leak rates up to: • - 2 10-6 Torr.l/s and 1/3 of nominal • - 4 10-6 Torr.l/s and 1/10 of nominal • Time estimate ~ 1 day • Leak rates larger than 4 10-6 Torr.l/s requires an exchange of the magnet : • BE SURE that the OBSERVED QUENCH is due to a He LEAK at THEGIVEN MAGNET • (disconnecting capillaries might be considered) Repair Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

  12. 4. Conclusions • Air leaks at cryogenic temperature • Air leaks at cold/warm transition : • - Above 10-8 Torr.l/s risk of vacuum transient with quench • - Above 5 10-9 Torr.l/s risk of vacuum transient with background to the experiment • Diagnostics by cold cathode gauge • He leaks at cryogenic temperature • He leaks along the beam screen : above 5 10-7 Torr.l/s risk of quench • On line diagnostics, at nominal, by BLM up to 7 10-7 Torr.l/s • Measurement of the dissipated power in the cold mass of ~ 1 W/m • All beam losses are not due to leaks ! Leaks = He in the beam tube • Local diagnostic, after a quench, by vacuum gauges (gas composition) • Warm up of the cold bore to 4.2 K with He pump out is proposed for leaks below 4 10-6 Torr.l/s • The existence, the level and the position of the He leak shall be CLEARLY identified before intervention on to the capillaries or the exchange of the magnet Vincent Baglin LHC Project Workshop CHAMONIX XIV - 2005 - 17/21 January

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