Transcript Beam-surface interaction

Beam-Surface Interaction
A Vacuum point of view
F. Le Pimpec
SLAC/NLC
Cornell May 2004
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Dynamic Vacuum
You want to address the terms of this formula
How to
measure the
Pressure ?
F. Le Pimpec - SLAC
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Outline
 Measuring and Reaching XHV
 XHV with Getters
 Beam Interaction with Technical
surfaces
- Desorption Induced by Electronic
Transition
- Electron Cloud
- Ion instabilities
 Summary and Conclusion
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Reaching and Measuring XHV
(10-12 Torr)
Luminosity
for accelerators
-14
10-13 10-12 10-11 10-10 10-9
Torr 10
10-8
10-7
10-6
10-5
10-4
10-3
Vacuum Gauges
Spinning Rotor gauge
Lifetime
Penning gauge
in storage rings
Reaching XHV
is commercially
easier than
measuring it
A CERN modified Helmer gauge
measured 10-14 Torr
XHV is not official
Pressure  10-7 Torr are
called UHV
Hot Cathod Ionization gauge, Bayard Alpert
Cold Cathod Discharge gauge
Extractor - Ionization gauge, Modified Bayard Alpert
Vacuum Pumps
Cryopump
Diffusion pump
Turbomolecular pump
Titanium Sublimation pump
Ion Sputter pump
Non Evaporable Getter pump & Cryogenic pump
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10-2
Why Measure Total Pressure ?
Total pressure can be computed
from the partial P measurements
BA –SVT305
Partial Pressure gives information
on the contents of the vacuum
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 - SLAC
RGA
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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 surroundings
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F. Le Pimpec - SLAC
UHV - XHV Partial Pressure
 The instrument of predilection is
the Quadrupole Mass Analyzer
 The Ion source is identical to that
of an ion gauge
- Same ESD problem as for a gauge. ESD ion
have higher energy than ionized gas
 Need to apply RF on the rod
- Resolution, and the price, is dependent on the
RF supply
Kr Trace
 Sensitivity (A/Torr) is non-linear
over few decades of pressure –
Ar Trace
space charge & collision at HV
 At XHV range, there is no
absolute calibration standard
10-7
Pressure (mbar)
10-4
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Reaching XHV in Static Vacuum
Reaching UHV from high vacuum is easy :
 Sputter/getter Ion pump
To reach XHV – Adding extra capture pumps
Diode
 Cryopump : lump or distributed pumping (LHC cold bore)
 Evaporable Getter : Ti sublimator (lump pumping)
 Non Evaporable Getter pump (distributed pumping)
Distributed Pumping
XHV is possible but is not
easy to reach because of
outgassing
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XHV Limit : Outgassing & Vapor Pressure
At which temperature is my
system going to be running ?
To minimize outgassing :
 Find a material with a low D coefficient
 Provide a diffusion barrier
 Installed a vacuum “cryostat”
 Degass the material …
F. Le Pimpec - SLAC
After Honig and Hook (1969)
Vapor Pressure :
True also for getters and
cryosystem
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Reaching Static XHV with NEG
C. Benvenutti
Lump pumping
LEP dipole chamber, getter St101 (ZrAl)
(1989-2000)
~24 km of NEG  P~10-12 Torr range
Inserted “linear” pump
The LEP : 1st
major success
of intensive use
of NEG pumps
Thin film getter
is the new
adopted way of
insuring UHV in
colliders or SR
light sources
DAFNE
ESRF
Inserted “total” pump
SOLEIL
DIAMOND
RHIC
TiZrV NEG Coating Setup at CERN (TiZrV) Surface pump
diffusion barrier
F. Le Pimpec -/ SLAC
LHC
NLC/GLC10??...
What are Getters ?
Getters are Capture Pumps
 Cryopumps and Sputter/getter-ion pumps
are also capture pumps.
 Differentiation is needed
– Physical getters (Zeolite)
– Work at LN2 temperature by trapping air gases
(including water vapor). Cheap primary dry pump.
– Recycling by warming up the zeolite
–
Chemical getters or simply : getters
– Includes Evaporable and Non Evaporable Getter
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How do Getters Work ?
Whatever the getter is, the
same principle applies :
Dissociation of residual
gases on a surface is not
systematic
The use of a clean
surface to form
chemicals bonds
 Covalent bond (sharing of the e-)
Tied bonds :
Chemisorption ≥ eV

Ionic bonds (1 e- is stolen by the
most electro- elements (Mg+O-))

Metallic bonds (valence electrons
shared)
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Titanium vs. Other Evaporable
Getters for Accelerator Use
Ba - Ca - Mg : High vapor pressure. Trouble if bake out is requested
Zr - Nb - Ta : Evaporation temperature too high
Photo courtesy of Thermionics Laboratory, Inc
Ref. “Sorption of Nitrogen by Titanium Films,” Harra and
Hayward, Proc. Int. Symp. On Residual Gases in Electron
Tubes, 1967
 Wide variations due to film roughness
 For H2, competition between desorption and
diffusion inside the deposited layers
 Peel off of the film ~50m
F. Le Pimpec - SLAC
Varian, Inc
Typical required sublimation rate
0.1 to 0.5 g/hr
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Non-Evaporable Getters
NEGs are pure metals or are alloys of several metals
: molecules.s-1.cm-2
  3.5 10 
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P
MT
CO, N2, CO2, O2
: sticking coefficient
P : Pressure (Torr)
H2
1ML : ~1015 molecules.cm-2
- Restoration is achieved by “activation” heating of the substrate on which the
getter is deposited. Joule or bake heating
- During activation, atoms migrate from
the surface into the bulk, except H2.
- Heating to “very high” temperature will
outgas the getter. This regenerates it but
also damages the crystal structure.
F. Le Pimpec - SLAC
NEG
CO, N2, CO2, O2
NEG
H2
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Non-Evaporable Getters : Uses
St 707 (ZrVFe)
Pump cartridge for Ion Pump or as lump pumps
Application of NEG are rather wide :
NEG is used in UHV (accelerators -tokamak)
Use of St 2002 pills to insure
a vacuum of 10-3 Torr
Used for purifying gases (noble gas)
Used for hydrogen storage, including isotopes
Lamps and vacuum tubes
…
Ref [7]
F. Le Pimpec - SLAC
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What Makes NEG So Attractive?

A
GREAT Material
– High distributed pumping speed
– Initial photo, electro-desorption coefficient
lower than most technical material (Al - Cu
- SS)
– Secondary Electron Yield (SEY) lower than
that of common technical materials

Drawbacks
– Needs activation by heating - Pyrophoricity
(200°C to 700°C)
– Does not pump CH4 at RT, nor noble gases
– Lifetime before replacement (thin film)
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Pumping Speed
0.6
H2
Ti32Zr16V52 (at.%)
0.005
Sticking probability
0.01
0
100
CERN/EST group
350
2 Hours Heating T (°C)
Pumping speed plots for getter are everywhere in the literature
• From sample to sample, pumping speed plots vary
• Many geometric cm2 are needed to see the pumping effects. Roughness (true
geometry)
•Temperature and/or time of activation is critical to achieve the pumping speed
required
•Capacity of absorption of the NEG is determined by its thickness
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Insuring Dynamic UHV
Beam Interaction
 Dynamic Outgassing should be studied for
every surfaces susceptible of being used
No existing coherent theory
 Source of gas are induced by photons
(SR), electrons and ions bombardment
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Photodesorption hCO at c = 194 eV
1.E-03
Sat
(13C18O) 13C18O
CO
1.E-04
ETA (molec/photon)
NEG St707 (Zr70V25Fe5)
NEG 0%
1.E-05
SS
1.E-06
Sat (13C18O) CO
1.E-07
1.E+18
1.E+19
1.E+20
1.E+21
NEG 100 %
1.E+22
1.E+23
Dose photons
An activated NEG desorbs less H2 CO CH4 CO2 than a 300°C baked SS
A saturated NEG desorbs more CO
a baked
F. Lethan
Pimpec
- SLAC Stainless Steel
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Also True For Thin films TiZr and
TiZrV
SS
Cu
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Electrodesorption hCO at Ee- = 300 eV
1.E-01
CO
NEG Sat (13C18O) 13C18O
1.E-02
Eta (molec/elect)
NEG St707
NEG Sat by CO
1.E-03
Cu
1.E-04
1.E-05
NEG 100 %
1.E-06
1.E+16
1.E+17
NEG Sat (13C18O) CO
1.E+18
1.E+19
1.E+20
1.E+21
Dose Electrons
An activated NEG desorbs less H2 CO CH4 CO2 than a 120°C baked OFE Cu
surface.
A saturated NEG desorbs less *C*O than a 120 °C baked OFE Cu surface
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Ion Desorption From 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
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Ion Desorption by Heavy Energetic
Ions on Technical Surfaces
1.5 109 Pb53+ ions (per shot) under 89.2° grazing incidence and 4.2 MeV/u
NEG Ti30Zr18V52
E. Mahner et al.
Measure at CERN for the LHC
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Other Beam Interactions
 Electron cloud & multipacting
 Free electron trapping in a p+
/ e+ bunch

Electron Cloud
Ion instabilities – link to the
pressure
- Pressure bump
- Fast beam-ion collective
instability
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SEY & Electron Cloud
NLC Fast Head tail
straight 1012
Electron cloud can exist in p+ / e+ beam
accelerator and arise from a resonant
condition (multipacting) between secondary
electrons coming from the wall and the kick
from the beam, (PEP II - KEK B - ISR - LHC).
3
Aluminium
Beryllium
Titane
2.5
M. Pivi
Secondary Electron yield
Copper OFHC
Stainless Steel
NEG St 707 (activated)
2
NEG TiZrV (activated 200°C- 2h)
1.5
1
0.5
0
200
400
600
800
1000
1200
1400
1600
1800
2000
Electron Beam Energy (eV)
SEY of technical surfaces baked at 350°C for 24hrs
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Thin Film & Electron Cloud
NLC: 130 eV e-beam
conditioning
Low SEY : Choice for the NEG of the
activation Temperature and time .
Conditioning (photons e- ions)
Contamination by gas exposure, or by the
vacuum residual gas, increases the
SEY; even after conditioning.
Angles of incidence, of the PE, change the
shape of the curve at higher energy
Roughness changes the SEY of a material
Variability
from sample
to sample
TiZrV coating
TiN/SS
Scheuerlein et al.
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Alternative Solution:
Playing with Roughness
Very rough surfaces emits
less SE, because SE can be
intercepted by surrounding
“walls”
Al disk with triangular shape
Experiment
1 mm
SEY Al flat grooved result
Real SEY Cu
 = 60°
Simulation
F. Le Pimpec - SLAC
G. Stupakov
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Ion Instability – Pressure Bumps
Ionized molecules are
accelerated toward the
wall by e+ /p beam
Ion impact energy as a function of beam current, LHC - Gröbner
Linked directly to hI
Dependant on surface
cleanliness
Dependant on the beam
pulse structure
Runaway condition is possible above a certain threshold
hi I critical 
e
i
Surface with a low h
S
Reduce the Pressure (S)
Use of clearing electrodes
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Fast Ion Instability
Fast ion instability can arise in e- beam accelerator
from ionization and trapping of the residual gas.
T. Raubenheimer
The amplitude of displacement yb
must be kept as small as possible due
to requested luminosity 
Diminishing the pressure
It is not, so far, a critical issue
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Conclusion
Reaching and measuring static XHV is possible and will become
necessary, as we push for higher luminosity
A NEG barrier diffusion solution provides pumping speed, low (hph hehi), low SEY and will insure dynamic UHV
 Ion instability – Pressure reduction
 Electron Cloud Issue
The vacuum solution has to be beam-dynamic friendly
 Wakefield (electrical conductivity) due to a film thickness
or surface roughness (or both)
 Lifetime of the solution (NEG) - % lifetime of the vacuum
device
 Heat Load in a cryogenic system (e-cloud)
…
F. Le Pimpec / SLAC-NLC
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Acknowledgement
SLAC :
R. Kirby, M. Pivi, T. Raubenheimer
CERN :
V. Baglin, JM. Laurent, O. Gröbner,
A. Mathewson
–
…
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References
1.
2.
3.
4.
5.
6.
7.
8.
9.
CAS Vacuum Technology: CERN 99-05
H. Brinkmann –Leybold Vacuum
R. Reid – Daresbury Vac group
CERN – Colleagues & web site
P. Danielson : Vacuum Lab
USPAS - June 2002
SAES getters
SLAC – colleagues
Web request for the beautiful pictures
–
…
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