procedures for detecting vacuum leaks in the LHC LTC open action from /36. LTC Frank Zimmermann, LHCCWG 23 October 2007

“Issues Effecting the Efficiency of Beam Commissioning (LTC Follow-Up) Chamonix XIV January 2006”, Steve Myers

revisiting the problem  He leak in cryogenic temperature environment leads to He pressure wave that may span several tens of meter without being detected; for large surface coverage He pressure reaches 17 Torr (saturated vapor pressure)!  average He gas density over 1 meter must be < He/m 3 to avoid quench with nominal p 7 TeV - equivalent to 33 nTorr at 1.9 K or 420 nTorr measured at 300 K (p~T 1/2 )  after quench: quenched magnet identified by triggered diode; cold bore warmed up to > K; He flushed to nearest unquenched magnet & condensed over ~ 10 m Vincent Baglin, Chamonix XIV

diagnostics vacuum gauges: placed every 3 or 4 cells ( m) i.e. 6 to 8 gauges per arc BLMs: every 53 m at each arc quadrupole and inner triplet power in cold masses: ~ 1 W/m over one cell can be measured (local quench level ~ 9 W/m at 7 TeV), needs ~6 h mobile vacuum gauges or residual gas analyser: can be installed locally, in SSS, every 53 m, for leak detection (→ vacuum wish : spread the quench till the nearest SSS) mobile radiation monitors: 32 can be installed per cell, sensitive down to “50 nTorr” (at which temperature?), needs ~¼ h [Thijs] warm up of cold bore to 4.2 K with He pump out for leaks < Torr.l/s existence, level and position of He leak shall be CLEARLY identified before intervention at capillaries or exchanging magnet Vincent Baglin, Chamonix XIV

sensitivity ranking radiation monitors (mobile, installation time ~1 day) cryogenic heat load BLMs vacuum gauges typically sensitivity decreases by factor 5-10 between two successive diagnostics tools listed above not discussed by Vincent: “snake” BLMs [Jeanneret] debunching and measuring e- current on biased BPMs (as in ISR & AA) [Poncet] notice: it seems to be more difficult to measure high leak rates than low ones?!

BLMs fixed BLMs: at each quad 3 monitors per beam mobile BLMs: 2 spare channels per card at each arc quad; more complicated in the LSS because of more elements Laurette Ponce, LHCCWG,

Alain Poncet’s proposal

Other ideas: use reflectometer to find local He bumps Fritz Caspers, today’s coffee

questions/actions can we indeed bias the BPMs? do we have the mobile radiation monitors? are the additional arc gauges and arc RGAs available? is it easy to access information on cryo heat load per cell? do we have snake BLMs? assess reflectometer sensitivity