Transcript What is an acceptable vacuum pressure in the LHC arcs?

What is an acceptable vacuum pressure in the LHC arcs?
B. Jeanneret, F. Zimmermann, CERN
Abstract
Beam-gas nuclear and Coulomb interactions introduce limitations on the tolerable vacuum pressure in the LHC and its
upgrade. We discuss local pressure limits set by the heat load on the cryogenics system or by loss-induced quenches, as
well as average pressure limits imposed by beam-lifetime degradation and emittance blow up. These pressure limits differ
for hydrogen and for heavier atoms which are present in the residual gas, like carbon, nitrogen, and oxygen.
The effect of Coulomb interactions also strongly depends on the beam energy.
tbeam& D/Dt for various gas species
issues:
• beam lifetime
nuclear elastic & inelastic
cross section
• emittance growth
multiple Coulomb scattering
elastic interactions
other heat loads & beam screen
cooling margin (at 2 K x5)
• heat load on beam screen
100-h beam lifetime
corresponds to heat load
of 0.05 W/m per aperture
a fraction of nuclear interactions
are elastic or quasi-elastic
for hydrogen
elastic pp scattering
d
 ae b ( t ,s ) t , where t  ( p ) 2
dt
• magnet quench
 rms 
limits local pressure
8x106
quench limit @ 7 TeV = loss rate
p/m/s
quench limit at 450 GeV = loss rate 7x108 p/m/s
for comparison: 100 hr beam lifetime ~ average
loss rate 6x104 p/m/s around the ring
  ;beam 
N
 8.8 10 6 rad
 x , y
2
at 450 GeV
single diffractive pp scattering,
similar cross section and b
emittance growth
2
d   1  13.6 MeV/  cnmgas

   
dt
2 
cp  X 0
magnet quench & local losses:
1
 5.9  10 4 rad
bp
b  14 GeV/c  for H
for heavier molecules
el. scattering with one of neff nucleons
& single diffraction off a nucleon weighted
with neff; b parameter similar to above
elastic proton-nucleus scattering neff  1.6 A1/ 3
single Coulomb scattering
 Mott  min    2 c 
2
Z2
2
Ebeam


1



cos


ln
1

cos

min
min 

1

cos

min


b pN  14.1A2 / 3 GeV/c   80 for O,C,N
2
 rms 
2.5  10 4 rad  30  ;beam
1

5
b pN p  1.6  10 rad  8  ;beam
some conclusions
1) cryogenics budget & pressure tolerances updated
2) ~1/2 of elastic nuclear interactions with heavier
atoms contribute to t at top energy
3) emittance growth rates at injection: 10-30 hours