Transcript Getters from Light Bulb to Accelerators

Reaching and
Measuring XHV
(10-12 Torr)
- Luminosity for accelerators
- Lifetime in storage rings
Reaching XHV is
commercially easier
than measuring it
A CERN modified Helmer gauge did
measure 10-14 Torr
XHV is not official - Pressure  10-7 Torr
are still called UHV
F. Le Pimpec
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How DID they make light bulb
work ?
♦ Since 1800 the electric arc ♦ The problem was in
light was known. However,
the life time of a filament
made of carbon was poor.
obtaining a high enough
vacuum in the bulb and
getting rid of the oxygen in
the residual gas
Use of the first
Evaporable
Getters
Bulb Installed in 1901 at
the Livermore’s (CA) Fire
station. C filament - 4 W
Phosphorus pumped
lamp tend to have a
red cast
F. Le Pimpec
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Why Measure Total Pressure ?
Extractor
Partial Pressure gives information
on the contents of the vacuum
Total pressure can be computed
from the partial P measurements
Operational in the same range (UHV)
The use of hot and cold gauge style
device need calibration for every
single species for accurate
readings – chemistry sensitivity
RGA’s electronics are sensitive to
the beam passage ! And are still
not cheap compared to gauges !
F. Le Pimpec
RGA
3
UHV - XHV Total Pressure
- Xray limitation due to the ehitting the grid : Ions are
desorbed from the Collector.
Remedies : Modulation
- ESD from the gauges
elements – Reducing emission
current : Wrong The grid will pump
then release molecules
- Installing a hot gauge in a small
tube – Transpiration effect
Modify Extractor gauge with hidden collector (U. Magdeburg)
Despite a higher pressure the
F
P
gauge will read lower. Solution:
A
nude gauges – but sensitivity to
stray ions from surroundingsF. Le Pimpec
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Ti vs Other Getters in Accelerator
Ba - Ca - Mg : High vapor pressure. Trouble if bake
out is requested
Zr - Nb - Ta : Evaporation temperature too high
1cm2
Typical sublimation rate 0.1 to 0.5 g/hr
0.5 g/hr ; MTi =48g/mol
0.5/(48*3600) = 2.9 10-6 mol.s-1
1ML~1015 atom.cm-2
2.9 10-6 mol.s-1. Avogadro =
1.74 1018 atom.s-1
1 ms is necessary to evaporate 1 ML
3 minutes = 180E3 ms hence ML for
1cm2
Ref. “Le Normand CERN vacuum note”
A) Pumping Speed calculated at RT
B) Wide variations due to film roughness
C) For H2, competition between desorption
and diffusion inside the deposited layers
Ref. “Sorption of Nitrogen by Titanium Films,” Harra and
Hayward, Proc. Int. Symp. On Residual Gases in Electron
Tubes, 1967
F. Le Pimpec
5
Also True for Thin Films TiZr and
TiZrV
V. Baglin et al.
F. Le Pimpec
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Ionodesorption by heavy energetic
ions on technical surfaces
1.5 109 Pb53+ ions (per shot) under 89.2° grazing incidence and 4.2 MeV/u
E. Mahner et al.
F. Le Pimpec
7
Ion-desorption on Al surfaces
Ion induced
desorption
yield
A.G.
Mathewson
M.H. Achard
M.H. Achard-R. Calder-A.G.
Mathewson
M.P. Lozano
1976
1976
1978
2001
15N +
2
Aluminium
as received
Aluminium after
24 hours baking
at 2000C (*)
at
2 keV
15N +
2
at
2 keV
K+ at
2 keV
K+ at 1.4 keV
Ar+ at 3 keV
H2
4.5 – 10
2.3
3.6 - 10
18
4-7
CH4
0.55 – 0.95
0.2
0.3 - 0.9
1
0.5 – 0.8
CO
7 – 10
2.5
3 - 10.5
7
0.9 – 1.5
CO2
1.8 – 3.2
0.5
1 - 3.7
1.2
1 – 2.5
H2
3.2 – 4
3.2
2.5
CH4
0.22 – 0.23
0.2
0.32
CO
2.8 – 2.9
2.2
1.5
CO2
0.75 - 1
0.18
0.35
(*) 300°C in the measurement of M.H. Achard
F. Le Pimpec
8
M. Pivi
Electron Cloud
fast head-tail instability
NLC Fast Head tail - straight
F. Le Pimpec
9
TiN/Substrate & Electron Cloud
Roughness is
an issue
a)
b)
c)
d)
e)
f)
F. Le Pimpec
COII - N - 2R
COIIIL
InjBELL1
Inj BEIR1
E3L
RFS2R
10
Getter & Electron Cloud
Low SEY : Choice for the NEG of
the activation T and t .
Conditioning (photons eions)
Contamination by gas exposure,
or by the vacuum residual
gas, increases the SEY;
even after conditioning.
TiZrV coating
TiZrV coating
Angles of
incidence, of
the PE, change
the shape of the
curve at higher
energy
2 h at 300C, CO injected at NEG T=60C
Hilleret et al.
F. Le Pimpec
11
TiN/Substrate & Electron Cloud
NLC: 130 eV e-beam
conditioning
TiN/SS
      e 1cos 
 =0.5
Roughness is
an issue
≤ 10-9 Torr Vacuum Recontamination
TiN/Al
F. Le Pimpec
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Alternative solution :
Playing with Roughness
Very rough surfaces emits
less SE, because SE can be
intercepted by surrounding
“walls”
Oxidized Cu disk
Al disk with triangular shape
F. Le Pimpec
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Alternative solution :
Playing with Roughness
Al disk with triangular shape
Reality
Simulation
Oxidized Cu disk
F. Le Pimpec
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Alternative solution :
Playing with Roughness
Al disk with triangular shape
Simulation
Reality
F. Le Pimpec
15
Dynamic UHV – NEG solution
In accelerator Cu, Al or SS are the technical
materials of choice, high conductivity
–
–
–
–
Cu and SS, can be baked at high temperature,
Al cannot (200°C)  special design, or ways, to
activate a NEG pumping solution
SS and NEG coating have a lower conductivity
compared to Cu or Al, wakefield issues  skin
depth & vacuum chamber size determination
A leak during an activation might lead to
scrapping the chamber (2m of Be chamber,
vertex detector, for LHC 106 CHF)
Cycles of venting/activation need to be
assessed for the lifetime of the machine
F. Le Pimpec
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