Transcript Vacuum I - CERN Indico

Vacuum II
G. Franchetti
CAS - Bilbao
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Index
Creating Vacuum (continuation)
Measuring Vacuum
Partial Pressure Measurements
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Diffusion Ejector pump
operating pressure:
10-3 – 10-8 mbar
Laminar flow
Schematic of the
pump
Cold surface
Inlet
Outlet
Boiling oil
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Inlet
Pi
Laminar flow
Cold surface
Pump principle:
The vacuum gas diffuses into the jet
and gets kicked by the oil molecules
imprinting a downward momentum
Po
Outlet
The oil jets produces a skirt which
separate the inlet from the outlet
Boiling oil
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Inlet
Diffusion
Outlet
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Problems
Inlet
Pi
Back streaming
Cold surface
Back-Migration
Po
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Cures
Inlet
Baffle
Cold Cap
Cold surface
Pi
Cold surface
(reduces
the pumping
speed of 0.3)
Po
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with
The pumping speed Sm is proportional to the area
of the inlet port
100 mm diameter  Sm = ~ 250 l/s for N2
LEYBOLD (LEYBODIFF)
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Capture Vacuum Pumps
Getter Pumps
Principle
(evaporable, non-evaporable)
Capture vacuum pumps are based on the
process of capture of vacuum molecules
by surfaces
Sputter ion Pumps
Cryo Pumps
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Getter Pumps
Gas
Surface
Solid
Bulk
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Definition of NEG
Getters are materials capable of chemically adsorbing gas molecules. To do so their surface
must be clean. For Non-Evaporable Getters a clean surface is obtained by heating to a
temperature high enough to dissolve the native oxide layer into the bulk.
T = Ta
T = RT
T = RT
Native oxide layer
-> no pumping
Heating in vacuum
Oxide dissolution -> activation
Pumping
P. Chiggiato
NEGs pump most of the gas except rare gases and methane at room temperature
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Sorption Speed and Sorption Capacity
C.Benvenuti, CAS 2007
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Choice of the coating technique for thin film: sputtering
substrate to coat: vacuum chamber
target material: NEG (cathode)
driving force: electrostatic
energy carrier: noble gas ions
NEG composition
Ti
V
Zr
 The trend in vacuum technology consists in moving the pump progressively closer to the vacuum chamber
wall.
 The ultimate step of this process consists of transforming the vacuum chamber from a gas source into a
pump.
 One way to do this is by “ex-situ” coating the vacuum chamber with a NEG thin film that will be activated
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during the “in situ” bakeout of the vacuum system.
Dipole Coating Facility
Quadrupole Coating Facility
M.C. Bellachioma (GSI)
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Sputter ion pumps
Cathode
Anode
-
+
E
-
E
P2
B
Titanum
P1
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Sputtering process
+
trapped electrons
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The adsorbing material is sputtered around the pump
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A glimpse to the complexity
ion bombardment
with sputtering
Adsorbed ions
ionization
process
Trapped electron
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Example of Pumping speed
J.M. Laffterty, Vacuum Science, J. Wiley & Son, 1998
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Cryo Pumps
m
Stick to the
Wall !!
v1
Cold Wall
Dispersion forces between molecules and surface
are stronger then forces between molecules
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Schematic of a cryo pump
Vessel
nw
Pump
pw
= pressure warm
= pressure in the cold
= flux Vessel  Pump
= flux Pump  Vessel
n c pc
cold
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Pw
Pc
Iw
Ic
molecules stick to the cold wall
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By using the
state equation
Now
In the same way
If
no pumping although
Relation between pressures
Thermal Transpiration
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When the two pressures breaks the thermal transpiration condition a particle flow starts
We define
and
We find
Pc depends on the
capture process
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Cryocondensation: Pw is the vapor pressure of the gas at Tc
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Summary on Pumps
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N. Marquardt
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Gauges
Liquid Manometers
MacLeod Gauges
Viscosity Gauges
Thermal conductivity Gauges
Hot Cathode Gauges
Alpert-Bayard Gauges
Penning Gauges
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Liquid Manometers
P2
P1
Relation between the two pressure
h
issue: measure precisely “h” by eye +/- 0.1 mm
but with mercury surface tension depress liquid surface
High accuracy is reached by knowing the liquid density
Mercury should be handled with care: serious health hazard
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McLeod Gauge
First mode of use:
P2
The reservoir is raised till the mercury reaches the level
of the second branch (which is closed)
Closed
Δh
h0
Quadratic response to P2
Second mode of use: keep the distance Δh constant and measure the distance of the
two capillaries  linear response in Δh
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Viscosity Gauges
ω
φ
Gas
molecule
It is based on the principle that gas molecules
hitting the sphere surface take away rotational
momentum
R
The angular velocity of the sphere decreases
ρ = sphere density
α = coefficient of thermal dilatation
T = temperature
va = thermal velocity
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(K. Jousten, CAS 2007)
F.J. Redgrave, S.P. Downes, “Some comments on the stability
of Spinning Rotor Gauges”, Vacuum, Vol. 38, 839-842
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Thermal conductivity Gauges
A hot wire is cooled by the
energy transport operated by
the vacuum gas
hot wire
Accommodation factor
By measuring dE/dt we measure P
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Energy Balance
 Energy loss by gas molecules
Wc/2
WR
 Energy loss by Radiation
Wc/2
V
ε = emissivity
 Energy loss by heat conduction
When the energy loss by gas molecule is dominant
P can be predicted with contained systematic error
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Pirani Gauge
Example of power dissipated by
a Pirani Gauge vs Vacuum pressure
The Gauge tube is kept at
constant temperature and
the current is measured
So that
.
Through this value  E
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Ionization Gauges Principle
electrons
+
V
L
accelerating
gap
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E
new
electrons
i+ = current proportional
to the ionization rate
+
i-
V
positive
ions
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Electrons ionization rate
Sensitivity
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Hot Cathode Gauges
Electric field
Anode
Grid
30 V
180V
+
+
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Hot
Cathode
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Hot Cathode Gauges
In this region
the electrons
can ionize vacuum
particles
Anode
Grid
i+
30 V
180V
Hot
Cathode
+
A current between grid
and anode is proportional
to the vacuum pressure
+
i+
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Limit of use
Upper limit:
it is roughly the limit of the linear response
Lower limit:
X-ray limit
Anode
X-ray
new
electron
The new electron change the
ionization current
Pm
X-ray
limit
X-ray
Grid
P
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Alpert-Bayard Gauge
The X-ray limit is easily
suppressed by a factor
~100-1000
X-ray
Hot
Cathode
Anode
X-ray
the probability
that a new electron
hits the anode is now
very small
Grid
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Schematic of the original design (K. Jousten, CAS 2007)
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Penning Gauge (cold cathode gauges)
anode
cathode
E = electric field
B = magnetic
field
Electrons motion
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Under proper condition of (E,B) electrons get trapped in the Penning Gauge
-
+
+
One electron is trapped
+
+
+
+
+
+
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-
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-
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+
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-
+
+
One electron is trapped
+
+
+
+
+
+
-
-
-
-
-
-
one vacuum
neutral gas
enters into the
trap
-
-
-
-
-
-
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+
+
+
+
+
+
+
+
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Ionization process
Collision  Ionization
Before Collision
Neutral vacuum atom
-
Charged vacuum atom
-
Electron at ionization
speed
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Electron at
ionization
speed
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+
-
New
electron
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-
+
+
+
-
+
+
Ionization
-
Charged vacuum atom
-
-
-
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-
Electron at
ionization
speed
-
-
-
+
-
-
+
+
+
+
+
+
this ion has a too
large mass and relatively
slow
therefore+ velocity,
+
its motion is dominated
by the electric field and
not by the magnetic field
New
electron
+
+
+
+
-
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-
+
+
+
-
+
+
+
Charged
vacuum atom
Ionization
-
Motion of stripped ion
+
+
-
+
-
-
-
-
New
electron
-
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-
+
+
+
+
+
+
+
+
+
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More electrons are formed through the ionization of the vacuum gas and
remains inside the trap
-
+
+
- +
- +
+
+
- +
+
+
- +
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+
+
+
-
+
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-
Negative space charge formation
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When the discharge gets saturated each new ionization produce a current
- +
- +
+
+
- +
+
+
- +
-
+
+
+
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+
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--
-
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+
I
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The time necessary for the discharge formation depends upon the
level of the vacuum
lower pressure  longer time of formation
Sensitivity of a SIP (the same as
for the Penning Gauge).
J.M. Lafferty, Vacuum Science,p. 322
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Summary on Gauges
N. Marquardt
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Partial Pressure Measurements
These gauges allow the determination of the gas components
Partial pressure gauges are composed
Ion
Source
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Ion current
detection
System
Mass
Analyzer
G. Franchetti
data
output
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Ion Sources
Vacuum gas is ionized via
electron-impact

the rate of production of ions is
proportional to each ion species
Electron-impact ionization process
Before Impact
e
After impact
M
v
M+
inelastic scattering:
kinetic energy transfer
from the electron to
the molecule
e m
e
me
e
v
me
v
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At the appearance potential the production
rate is low
The minimum energy to ionize M
is called “appearance potential”
Electron
energy
(eV)
15
IP = 15 eV
M+ + e-
appearance
potential
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A. von Engel, Ionized Gases, AVS Classics Ser., p. 63. AIP Press, 1994
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Ion source
acceleration gap
Open source
Bayard-Alpert gauge
electrons
source
ion collector
ionization
region
ions
electrons
E
+
Vf
E
+
+
Vg
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Vg < Vc
Vc
53
A schematic
+
+
+
+
+
+
+
+
Mass analyzer
F = ion transmission factor
Ion Detection
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Ion Detection
 Faraday Cup
 Secondary electron Multiplier (SEM)
Gain
G=
# electrons output
# ion input
Idea: an ion enters into the tube, and due to the potential
is accelerated to the walls. At each collision new electrons are
produced in an avalanche process
V0 = applied voltage
d
V = initial energy of the electron
L
K = δ Vc
where
J.Adams, B.W. Manley IEEE Transactions on Nuclear Science, vol. 13, issue 3, 1966. p. 88
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δ = secondary emission coefficient
Vc = collision energy
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Mass Analyzers
Quadrupolar mass spectrometer
r0
y
s
Ion equation of motion
U, V constant
x
define:
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By rescaling of the coordinates the equation of motion becomes
Stability of motion
Mathieu
Equation
The ion motion can be stable or unstable
horizontal
vertical
q=0, a>0
unstable
stable
q=0, a<0
stable
unstable
The presence of the term q, changes the stability condition
Development of stable motion
Stable or unstable motion is referred to a channel which is infinitely long, but typically
a length correspondent to 100 linear oscillation is considered enough
Example:
 vs = 8301 m/s
For N2 at Ek = 10 eV
For a length of L = 100 mm
100 rf oscillation
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f = 8.3 MHz
typically
f ~ 2 MHz
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a
Stability Chart of Mathieu equation
10
10
a
5
5
Stable in y
0
5
10
5
10
15
15
5
5
Stable in x
10
10
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q
0
q
a
10
a
a
5
q0=0.706
a0=0.237
q
0
5
10
15
q
5
Stability region
in both planes
10
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a
Given a certain species
of mass M1 there are two
values U1, V1 so that
q=q0 and a=a0
(0.706; 0.237)
By varying V and keeping
the ratio V/U constant, the
tip of the stability is crossed
and a current is measured at V=V1
q
Stability region
in both planes
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V
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Magnetic Sector Analyzer
Example: A 900 magnetic sector mass spectrometer
B is varied and when a
current is detected then
Ez is the ions
kinetic energy
Resolving power
Wsource = source slit width
Wcollector = collector slit width
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Omegatron
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The revolution time is independent
from ion energy
collector
B
If the frequency of the RF is 1/τ
a resonant process takes place
and particle spiral out
E
Resolving power
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Conclusion
Creating and controlling vacuum will always be a relevant
part of any accelerator new development.
THANK YOU FOR YOUR ATTENTION
These two lectures provides an introduction to the
topic, which is very extensive: further reading material is
reported in the following bibliography
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Kinetic theory and entropy ,C.H. Collie, Longam Group, 1982
Thermal Physics, Charles Kittel, (John Wiley and Sons, 1969).
Basic Vacuum Technology (2nd Edn), A Chambers, R K Fitch, B S Halliday, IoP
Publishing, 1998, ISBN 0-7503-0495-2
Modern Vacuum Physics, A Chambers, Chapman & Hall/CRC, 2004, ISBN 0-84932438-6
The Physical Basis of Ultrahigh Vacuum, P. A. Redhead, J. P. Hobson, E. V.
Kornelsen, AIP, 1993, ISBN 1-56396-122-9
Foundation of Vacuum Science, J.M. Lafferty, Wiley & Sons, 1998
Vacuum in accelerators, CERN Accelerator School, CERN-2007-003 11 June 2007
Vacuum technology, CERN Accelerator School, CERN 99-05 1999
Acknowledgements: Maria Cristina Bellachioma
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