UHV Vacuum Techniques: Basic Concepts
Lecture Contents
Lecture 1: Basics
Aims:
●
●
What you ought to know
about using vacuum
What you might need
during your PhD
●
A quick starter for using
vacuum equipment
●
Cover entire range of
vacuum - not just UHV
►
Levels of vacuum and
subsequent applications
►
Modelling gas flow
►
Pressure measurement atmosphere to UHV
References
●
●
●
Two excellent books:
►
“Basic Vacuum Technology”, A. Chambers, R.K. Fitch and
B.S. Halliday.
►
“A Users Guide to Vacuum Technology”, O'Hanlon.
Vacuum equipment manufacturers:
►
Catalogues, websites
►
e.g. Leybold, Edwards, Varian, Pffeiffer, Alcatel etc.
Local knowledge (valuable, but finite resource!):
►
workshops
►
group technical support staff
Lecture 2: Achieving Vacuum
►
Vacuum pumps
►
Pump configurations
►
Types of chambers and
fittings
►
Complete systems
Why know about vacuum?
●
Nearly everyone here
needs to know a little about
vacuum:
●
Many people here will use
vacuum systems:
►
e.g. most research groups
use vacuum to some
extent
►
vacuum used very widely
in modern experimental
physics
►
►
mostly simple physical
principles involved
need to understand the
systems
►
might need to analyse
what is happening or
design modifications
►
what is achievable with
today's technology
►
the basics - so its easier to
go away and find out more
Historical Perspective
BC
1657
1900
Ancient Greeks
- Vacuum
“inconcievable”
Magdeburg
Hemispheres
Experiment
1950's
What is the equipment like?
Ultra High Vacuum
(~10-10 mbar)
2000+
Rough Vacuum
(~10-1 mbar)
Manufacture of
Lightbulbs
Mercury
Sealed
Pumps
Ultra High
Vacuum
Surface
treatment
research to
achieve XHV
Hydrocarbon
Sealed Pumps
Some Definitions
Assume the vacuum system is a sphere...
Units:
► Usually
► 1000
use mbar (although Pa are SI, sometimes Torr)
CHAMBER
(GAS)
5
mbar = 1 atm = 10 Pa = 760 Torr
PUMP
Levels of Vacuum:
► Low
Vacuum
► Medium
► High
► Ultra
Vacuum
Vacuum
-
1000 mbar to 1 mbar
-
1 mbar to 10-3 mbar
-
10-3 mbar to 10-8 mbar
High Vacuum -
► eXtreme
High Vac. -
-12
10
-8
mbar to 10 mbar
Kinetic Theory: reasonably valid
●
Maxwell Boltzman Distribution
v=

●
Mean free path
=
below 10-12 mbar
●

8kT
m
1
2 d2n
Impingement Rate
nv
J= 
4
For nitrogen at 295K:
n (litre-1)
λ
J (cm-2s-1)
1000
2.5x1022
66 nm
2.9x1023
1
2.5x1019
66 µm
2.9x1020
1x10-3
2.5x1016
66 mm
2.9x1017
1x10-6
2.5x1013
66 m
2.9x1014
1x10-10
2.5x109
660 km
2.9x1010
P (mbar)
Applications
Low Vacuum:
►
Ultra high vacuum (UHV):
mechanical handling
Medium vacuum:
►
industrial processes
►
vacuum drying/packaging
►
vacuum distillation
►
keeping surfaces clean for
hours (surface science,
epitaxial growth)
►
space simulation (~10-10
mbar at 1000 km)
►
Elementary Gas Transport
●
Throughput, Q, is the
volume of gas passing
through an area per second
at a specific temperature
and pressure:
Q= p
achieving ultra high
purities (e.g. for fusion)
►
●
Conductance is a
geometric property, which
represents the ability of a
gas to flow from a pressure
gradient:
dV
dt
C=
Q
P 2 P1
units, e.g. mbar.l/s
C
High vacuum (HV, λ>d):
►
►
Extreme high vacuum (XHV):
e-beams (welding, TV)
►
storage rings
►
ultra pure growth
vacuum evaporation or
coating
●
S=
Modes of Gas Flow
●
Fluid flow is described by the
Reynolds number and the
Knudsen Number (both
dimensionless)
u D


Kn=
D
R e=
For vacuum systems:
●
for round pipes
ρ = density
u = stream velocity
D = diameter
λ = m.f.p.
●
Flow is only turbulent at very
high pressures and pumping
speeds (e.g. during initial
evacuation, if unthrottled)
Viscous flow and the
transitional regime are
important at high pressures
(above about 10-3 mbar)
►
●
●
Turbulent flow if Re>2000,
laminar if Re<1200.
Viscous flow if Kn<0.01,
molecular flow if Kn>1.
●
Volumetric flow rate often
called the speed, S:
p1
dV Q
=
dt p
S
Molecular Flow
●
dominant for HV and UHV and well
understood
●
particles flow in all directions to reach
dynamic equilibrium
●
pumps 'wait and catch' gas particles high vacuum pumps do not 'suck'
PUMP
gas can be 'sucked out'
Molecular flow dominant
below about 10-3 mbar in
'normal' sized chambers.
►
gas flows through random
collisions with walls
p2
Q
Particles scattered from surface with
cosine distribution to surface normal
(Knudsen's cosine law)
Molecular Conductance of an Aperture
Net flow through an aperture
corresponds to the rate of impingement
from both sides:
●
P 1, n1
Q=k T  J 1J 2 A
=
C 0=


Conductance of Pipes
calculating conductance of
pipes give same results (see
O'Hanlon for details)
P2, n2
●
kT
p p A
2 m 1 2
A
J1
J2
►
C0 = 11.8A l/s (A in cm2) for nitrogen
(air) at room temp.
►
need to account for gas and temp.
●
For conductances in series,
add reciprocally:
►
●
1/CT = 1/C1 + 1/C2 + 1/C3
12.4 D / L
CS=
l/s
14D /3L
(D, L in cm) for nitrogen (air)
at room temperature.
C L=12.4 D 3 / L l/s
●
Accurate to about 10%
Effect of Conductance on Speed of a Pump
Conductance, C
For conductances in
parallel, add normally
►
C 0×C L
C 0C L
3
●
(D, L in cm) for nitrogen (air)
at room temperature.
Combining Conductances
For short pipes is convenient
to reciprocally add the
conductances for a long pipe
and equivalent aperture:
CS=

●
conductance depends on (T/m)1/2
For long pipes (L>>D):
3
D 2k T
C L=
6L
m
kT
A
2 m
►
●
Various methods for
CT = C1 + C2 + C3
C1
CHAMBER
C1
C2
C3
PUMP
PUMP
C2
C3
●
this assumes the volumes
are independent - need to
be careful
Effective pumping speed at
chamber is reduced by conductance
of any connecting pipe
The molecular conductance
of the entrance aperture
determines maximum speed of
any pump
Viscous and Transitional Flow
Molecular
●
Transitional
Conductances of Complex Shapes
Viscous
higher pressures in pipes to
mechanical pumps mean
transitional and viscous
flow may be important
●
conductance increases with
pressure
●
important in connections to
mechanical pumps - can
use smaller connections
●
accurate calculations
complex
●
results tabulated - often in
mfr's catalogues
●
Monte Carlo simulations
required in general
●
Refer to O'Hanlon
for simple 'standard'
shapes
●
Quite often, just need
approximate value
►
can get quite a long
way using simple
approximations
Approximate conductance is
the conductance of the two apertures,
added reciprocally...
From BOC Edwards Catalogue
A closer look at a generic system...
Outgassing: gradual loss
of particles adsorbed on
walls of the chamber water is the major problem.
●
CHAMBER
Backstreaming: many pumps
can lose fluids into the vacuum
system, causing contamination,
which can be difficult to remove.
Outgassing
Real Leak: a fine
passageway to the
air outside.
Quantitative Description of Pumping
Leak
PUMP
V
Process
Backstreaming
Evaporation
Virtual Leak: usually a
small trapped volume,
which acts like a real
leak, but will deplete with
time.
Evaporation: liquids (and greases)
will limit the vacuum until evaporated.
(Clean components and WEAR
GLOVES!)
QT = QP + QO + QL + QVL + QE + QB
●
dp
=Q T Sp
dt
Ultimate pressure (dp/dt=0)
pressure is simply
p = QT / S
►
●
Initial Air
Virtual Leak
Constant volume system
governed by
●
i.e. change in gas in chamber,
d(pV)/dt is load minus gas
removed by pump
Solving fully requires detailed
knowledge of QT - not
generally available
►
evaporation and
outgassing of range of
gases gives complex
behavior...
●
in the case of a leak,
QT~QL
For initial pumpdown, QL is
not important, so we obtain
p = p0 exp { -t / (v/S) }
►
easy to calculate initial
pumpdown times using the
pumping speed at the
chamber
Limit of Pressure - Outgassing
●
●
made up of general 'grot',
greases, water vapour etc.
log (pressure)
●
outgassing limits vacuum in a
clean, leaktight HV/UHV
chamber
for UHV, must accelerate
degassing of water by baking
to ~200°c for ~24 hours
►
●
►
rule of thumb: rate doubles
for every extra 10°c
►
can get to UHV in days
instead of years
what is the ultimate limit?
►
log(time)
desorption is activated
with Boltzmann factor
diffusion of H2 through
chamber walls
Measurement of Pressure
●
Require a measurable property which changes (linearly)
with pressure, preferably independent of gas type
●
No single principle is good between atm. and UHV.
Pressure (mbar)
●
In practise, integrated
systems now widely available
►
almost 'plug and play'
►
connect to PC for logging
►
can link to pump
controllers
combined pirani & ion
gauge
►
measure from atm. to UHV
●
Sense pressure by
mechanical deformation
●
Measure total pressure independent of gas type
●
Capsule gauge:
►
Still need care in operation
- e.g. responses different
for different gases
●
●
e.g. integrated ion gauge
and controller
10-3
10-6
10-9
10-12
Mechanical Total Pressure Gauges
Composite gauge heads can
measure over wide ranges
►
100
Capsule gauge
Diaphragm
Pirani (thermal cond.)
Capacitance manometer
Spinning rotor
Penning
Bayard-Alpert (Ionis.)
Inverted Magnetron
Extractor Ion
RGA Mass Spec.
Vacuum Gauges
●
103
►
simple mechanical lever from
expanding capsule to dial
►
good for 1-1000 mbar
Diaphragm gauges
►
sense by mechanical
deflection
►
sense by change in
capacitance, very accurate
and good to 10-5 mbar, but
expensive
Capsule Gauge
Annular
electrode
Disc
electrode
Diaphragm
pressure under
measurement
Chemical
Getter
Capacitance Manometer
Bayard Alpert Ion Gauge - HV and UHV
Pirani Gauge - Low to Medium Vacuum
●
●
●
RF
●
hot filament emits electrons,
which are attracted to grid and
spiral around
●
electron impact ionised
residual gas inside grid
●
positive gas ions reach
collector and are detected by
electrometer
RC
V
Heat Loss
●
measures thermal
conductivity from hot wire to
surroundings through vacuum
typically set up in bridge
arrangement with a
compensating filament, and
calibrated at HV
'standard' backing line gauge,
cost few £100
needs calibrating to atm. and
high vacuum
●
usable between about 10-3 and
10-11 mbar, cost ~£1000
●
limited by x-rays from grid
10-4
1
Pirani gauge and controller
P (mbar)
●
cold cathode ionisation gauge
- no filaments to blow
●
single ion initially created
spontaneously (e.g. cosmic
ray)
●
●
electrons attracted to anode,
but crossed B and E fields
cause long spiraling paths
electrons cause further
ionisation to maintain a stable
discharge
n
ion current given by i = kp
where k and n are constants,
and 1.1<n<1.2
B
●
●
►
emission current
ionisation probability of gas
Typical Correction Factor
0.16
1.0
1.4
2.4
5.8
Inverted Magnetron Gauge
+2kV
●
geometry of gauge
►
Penning Gauge
●
Gas
He
N2
CO2
Xe
C6H6
Response of gauge depends on:
►
180 V
30 V
response non-linear, so not
regarded as so accurate
some pumping effect
robust, widely used gauge
(industrially) for range 10-3 to
10-7 mbar
●
similar principle, but works
down to ~10-10 mbar
●
single initial ionisation event
few
gives electron
●
electrons perform cycloid
motion in crossed E and B
fields, ionising gas within
kV
●
produces stable discharge
similar to Penning gauge
●
some pumping effect
●
considerable sputtering at
high pressures (avoid >10-5
mbar for long periods deposits can cause leakage
currents)
B
●
provides alternative to ion
gauge
►
no filaments to blow
►
no light, no heating
►
but, slow to start at low
pressures and requires big
magnet
Precision Vacuum Gauges
●
●
most gauges are only
accurate to ~20% at best
(often worse)
●
wide variety of pumps used at all pressure levels
●
concentrate on main types of pumps used in research
a few 'precision' gauges are
available
►
►
●
Vacuum Pumps
All Vacuum Pumps
capacitance manometer
(limited to ~10-5 mbar)
Gas Transfer
spinning rotor gauge
(limited to ~10-7 mbar)
Positive
Displacement
precision gauges are
expensive, and limited range
but can be used to calibrate
ion gauges etc. when
pressure critical
Spinning Rotor Gauge
Kinetic
Entrapment
Moving components
displace and eject a
volume of gas.
Momentum imparted to
individual gas particles,
driving them to exhaust
e.g. reciprocating piston
or rotating vanes
e.g. rotor in a turbomolecular pump
Gas particles react
chemically and trapped,
or ionised, accelerated
and embedded in pump
walls.
Vacuum Pumps
Viscous Flow
Rotary Vane Pump
Molecular Flow
●
'standard' mechanical pump,
used to achieve rough
vacuum
●
sliding vanes rotate,
compressing and ejecting gas
to atmosphere
●
sealed with oil
Pressure (mbar)
103
100
10-3
10-6
10-9
Mechanical (Rotary) Pump
Diffusion Pump
Turbomolecular Pump
10-12
►
needs replacing periodically
►
need foreline trap to keep oil
from backstreaming into inlet
lines
►
special oils available for
pumping oxygen/aggresives else BANG!
Single stage rotary vane pump
Rotary Vane Pump
Rotary Vane Pump
Flexible pipeline
to chamber
●
Foreline trap
(catches oil in
adsorbant beads regen. periodically)
Inlet Connection
(to chamber)
Exhaust conn.
(always exhaust
to roof/outside SAFETY ISSUE)
Oil sight
glass
Motor
Pump housing
(oil casing)
2 stage pump gives better
ultimate pressure
►
●
difficult to expel condensables
►
e.g. water vapour
►
gas ballast helps (leak air in
between stages)
●
typically get down to 2x10-3
mbar with a good rotary pump
●
pumping speeds between
about 0.5 and 80 m3/hr
●
cost ~£1000 for few m3/hr
'Dry' Pumps
can avoid contamination
altogether with dry pumps,
if necessary
►
●
e.g. silicon wafer
processing
several mechanisms
available, e.g. diaphragm,
PTFE sealed pistons
●
Dry pumps generally:
►
lower pumping speed for
same size or cost
►
poorer ultimate pressures
►
more expensive
►
noisier
Two stage rotary vane pump
Other mechanical pumps
●
●
2 stages in series
●
Other mechanical pumps are
available, e.g.:
►
piston pumps
►
roots pump
15cm dia.
inlet flange
Roots
pump
Piston
pump
Usually designed for high
pumping speeds needed in
industry
Principle of
a roots pump
Large roots and piston pumps
used on a helium beam source
Getting to High Vacuum
●
●
●
Need different type of pump
to get below about 10-3
mbar
Diffusion Pump
Chamber
●
boils highly refined, high
molecular weight fluid
●
vapor jets impart downward
momentum to gas entering
pump
●
oil condenses on water cooled
body & recycled
●
Discharges to rough vacuum
Usually means using either:
►
diffusion pump
►
turbo pump
<10-6 mbar
HV
Pump
High vacuum pumps can't
discharge to atmospheric
pressure
►
►
~10-2 mbar
Backing
Pump
permanently need a rotary
pump as a 'support' or
'backing' pump
1000 mbar
Vent to
roof
●
reliable 'standard', gets down
to between ~10-7 and ~10-11
mbar depending on setup
Diffusion Pump
●
►
►
●
●
Diffusion Pump: Chevron Baffles
Ultimate pressure depends
on quality of fluid used
►
cheap fluid (e.g. Corning
DC704) ~10-6 mbar
Inlet
baffle
●
all diff pumps backstream
pump fluid when hot
●
cold, optically dense baffle
catches oil, but reduces the
pumping speed
good fluid (e.g. Edwards
L9) ~ 10-9 mbar
best fluid (Santovac 5)
~10-10 mbar (with baffle)
Different fluids have
different safety issues
Need chevron baffle to
catch backstreaming pump
fluid and achieve best
pressures
●
Thermal
Sensor
Water
cooling
Heater
'critical backing pressure'
~0.5 mbar (depends on
model)
Liquid N2
reservoir
To rest of
system
need liquid nitrogen temp.
to reduce vapour pressures
to UHV level
►
once nitrogen reservoirs
filled, need to be kept filled
►
internal water
condensation can refreeze in cracks and cause
leaks
To diffusion pump
Optically dense
chevron baffle,
connected to
cold reservoir
Diffusion Pump
Advantages:
Disadvantages:
●
fairly cheap (start ~£1000)
●
reliable - little to go wrong
►
►
heater is replaceable
►
►
cooling coils can be
descaled
►
●
Turbomolecular Pump
can take to bits and scrub
out inside
●
slow to start and stop
fast moving rotors and stators impart momentum to gas molecules
●
frequencies of up to ~1000 Hz
●
high precision greased/oiled or magnetic bearings
~½ hr to warm up
~1 hr to cool down before
venting
●
expensive to run large
pumps (£1000s per year)
●
require liquid nitrogen
baffles for true UHV
(require daily filling!)
●
Dependant on cooling
water – MUST BE
INTERLOCKED
often found 'lying around'
lab
●
Turbomolecular Pump
●
●
Advantages:
pumping speed varies with
gas and pressure
compression varies with
molecular mass
►
typically 109 for N2
►
typically ~104 for He
►
●
Turbomolecular Pump
3
typically ~10 for H2
difficult to remove lighter
gases
►
►
can add extra pump in series
to improve compression
add chemical pump in
parallel to pump reactive
species (e.g. H2)
Chamber
Getter
Pump
●
quick to start and stop
●
low electrical costs
●
clean – completely so for a
mag-lev - no baffles required
●
Turbo
Pump
●
Turbo
Pump
Backing
Pump
much more expensive to buy
and service (typically around
£10 000)
●
occasional catastrophic failure
►
something dropped in top
►
bearing seizes
water cooling much less
critical
►
magnetic controller fails
►
●
●
can mount in more
orientations
►
Vent to
roof
Disadvantages
pump should turn off if too
hot, usually after >1/2 hr
air cooling options available
mag-lev turbos good for
aggressive gases
●
need to be well secured
(SAFETY RISK)
►
e.g. Leybold T1600 has to
be secured to withstand
an impact torque of 20 000
Nm
Ion Pump
●
●
●
●
●
Getter Pump
●
type of entrapment pump,
operating like a series of
Penning gauges
Ion pump cell
●
electrons spiral in strong
magnetic field, ionising gas
species on impact
ions are accelerated to and
get buried in surface of anode
Choice of pump depends
on
►
►
level of vacuum
size of chamber
(outgassing)
►
gas throughput of process
►
pumpdown time
►
type of gas (nasty?)
►
level of cleanliness
required
►
size and positioning
►
finances available (!)
►
...
●
●
careful consideration and
risk assessment is vital
number of big mfrs.:
►
never pay list price for new
vacuum equipment
►
discounts of up to ~50%
are routine for academia
►
blocks of reactive, sintered
material (NEG)
limited capacity - has to be
used at very low pressures,
e.g. in a load-locked chamber
●
cannot pump inert/rare gases
Choice of Pumps
●
sublimated coatings (TSP
uses Ti/Mo alloy)
●
B
needs additional pump to get
down to about 10-4 mbar
before ion pump will start
►
particularly good at pumping
H2, unlike many other pumps
-kV
●
use strip getters along entire
length of particle accelerator
beamlines to achieve XHV
●
once activated, pumps without
power - portable!
various types of getter
●
+kV
clean pumping to UHV, but
requires large magnet
chemical 'getter' pump, reacts
with and contains reactive
elements
Low to High Vacuum Fittings
High vacuum fittings:
●
elastomer seals
●
usually use KF (Klein
Flange) fittings with o-rings
Ultra High Vacuum Fittings
metal flange
To achieve UHV:
●
●
not difficult, just need to do
the job properly and cleanly
●
require materials which are
not gassy
►
all metal fittings, plus
ceramics, glass(fibre), PTFE
►
no plastics, adhesives,
solder, brass
►
●
Need electrical, fluid and
mechanical connections to
experiment
►
commercial feedthroughs
ceramic
insulator
●
Electrical connections
►
ceramic to metal seals
►
delicate - avoid bending pins!
vacuum side
Electrical feedthrough
metal bellows to allow
motion in vacuum
http://outgassing.nasa.gov
usually use CF (Conflat) knife
edge flanges and copper
gaskets
►
Vacuum Feedthroughs
●
can seal with elastomer
gaskets temporarily (indicate
by using few bolts)
Knife Edge
Vacuum Valves
Diaphragm valve
(typically on backing lines)
Butterfly valve
(high conductance)
HV 90° valve
Mechanical connections
►
o-ring sealed for HV
►
differentially pumped or metal
bellows sealed for UHV
Push-pull linear feedthrough
Putting it all together
UHV 90° valve
Gate valve
(high conductance + UHV)
Standard Vacuum Symbols
Vacuum Pump,
General
Adsorption
Pump
Rotary Vane
Pump
Cryopump
Diffusion
Pump
Another Simple System
Manual
Valve
Variable
Leak Valve
UHV
Chamber
Getter
Pump
Electromag.
Valve
Roughing Line
UHV
Gate
Valve
Sputter-Ion
Pump
Turbo
Pump
Sorption
Trap
Turbomolecular
Pump
Getter
Pump
Vacuum
Gauge
Backing Line
●
Design system carefully
Then, if you have
problems...
●
●
●
●
Avoid contamination
►
degrease and clean all
components
►
assemble with clean
gloves
●
●
Try to identify source of
contamination
Cleaning How-To
●
●
Check system processes
Check system design and
construction
Locate and address leaks
►
Bake the system to achieve
UHV
►
a single unsuitable
component will limit the
pressure achieved
avoid gassy materials
Exhaust
to roof
Leak Test
Reaching the Ultimate Pressure
Normal procedure:
Exhaust
to roof
All Metal
Valve
Cold Trap
●
Not very exciting, but very
important to achieving good
high vacuum performance
Specialist cleaning possible
for certain materials
►
ceramics, glass
►
see textbooks
Some components difficult to
clean:
►
►
A generic procedure:
1. Start with 'mechanical
cleaning' (scrub with
detergent)
2. 'Buzz' in detergent in
ultrasonic cleaner for 5-15
minutes.
3. Rinse in clean tap water
4. Rinse in deionised water
clean as well as possible by
hand
5. Dry carefully
then use ultrasonic cleaner
6. Rinse in solvent
●
Need to work out the source
of the gas to fix the leak
►
seal off system, monitor
pressure
►
leak test
►
analyse gases in chamber
pressure
Fix real leaks:
►
re-seal leaking flanges
►
re-weld crack in welding
(avoid leak-sealant!)
►
leak-seal cracks in
feedthrough connections
virtual leak
pressure
pressure
Two common approaches
►
►
●
apply acetone + look for ANY
pressure change
Chamber
►
connect up and spray Helium
around potential leaks
►
versatile technique, used
widely outside 'vacuum field'
Difficult to 'degas' - can take a
long time
●
Avoid creating virtual leaks by
careful design in the first
place:
►
use internal welding
►
vent trapped volumes
directly
►
drill screw holes right
through
1. First try to detect the leak with
acetone
5. Use as little helium as
possible
2. Connect the leaktester to the
system (~4 helium leak
testers around Cavendish)
6. Ventilate the area
1. use a leak test port, or
use mass-spec helium
leaktester
Helium leak-tester is a simple
mass-spec designed to detect
helium
●
Helium Leaktest How-To
Helium
●
Almost VITAL to have some
leak testing facilities especially at UHV level
Trapped volumes of air (or
other gas / vapour) constitute
a virtual leak
both
Helium Leak Testing
●
●
real leak
time
●
Often present - is it important?
time
●
Virtual Leaks
time
Leaks
HV
Pump
Helium
Leak Tester
2. replace one of the backing
pumps
3. Apply a fine jet of helium from
a leak test cylinder (borrow
from liquifier facility)
Backing
Pump
4. Follow response of leaktester
over period of 10's of seconds
(depending on pumping path)
7. Work from the top down
8. Isolate sections of vacuum
system in plastic bags
9. If gas load is small, divert
entire backing load through
leaktester, to maximise
response.
10.Once leak identified, locate
its position precisely to help
fix it
Bakeout
●
Bake On
Generally necessary to
achieve p < 10-8 mbar without
waiting too long
●
Bake for as hot and as long
as possible!
●
Max. temperature determined
by components
►
Viton seals limit ~150°c
►
PTFE limit ~200°c
►
All metal systems are OK to
higher temperatures
Bakeout How-To
Bake Off
1. Check that the components in
the system are OK for baking
& decide on max. temp.
2. Disconnect all non bakeable
connections/fittings and store
3. Wrap sensitive components
(feedthroughs, windows) in
foil.
4. Position thermocouples at
various characteristic points of
the system (test them now!).
5. Either:
Baking allow UHV to be reached in
days, rather than months or years
1. Wrap chamber in heating
tape and several loose layer
of foil
RGA Mass Spectrometer
2. Enclose chamber in custom
made oven
6. Do final check on bake zone
7. Slowly increase temperature
to e.g. 200°c for 24 hours
(heat up over a period of a
few hours)
8. Monitor temperatures and
ensure an even heat - avoid
cold spots
9. Hold at temperature until
pressure sufficiently low
10.Cool down over a few hours
11.Degas filaments while
chamber still warm
RGA of a High Vacuum System
18 - water, chamber
needs baking
Residual
Gases
mass
filter
ioniser
ion
counter
4 (He), from process,
or diffusing through
elastomer gaskets
control
electronics
●
●
Provides lowest pressure
measurements - down to
pp. of ~10-15 mbar
●
allows analytical
measurements and
diagnosis
provides pressure
breakdown by mass
number
●
modern systems are run
from a PC
●
start at ~£5000
28, 32 - N2 (or CO)
and O2, suggest a
leak to atmosphere
only 2 (H2), 28 (CO)
and 44 (CO2) should
be present in
clean, leaktight UHV
Forest of peaks
around 40-70
suggests pump oil
contamination (fit
baffle or trap to
pump)
Cracking Patterns
●
●
Do:
different species give different mass patterns
►
●
Good Working Practice for HV / UHV
●
cracking, isotopic abundances etc.
allows individual components to be identified
►
understanding a process
►
diagnosing contaminants
●
cracking patterns tabulated by RGA mfrs. and in O'Hanlon
Species
Hydrogen
Helium
Methane
Water
Argon
Acetone
Mass
2
4
16
18
40
43
Abund.
100
100
100
100
100
100
Mass
1
2
15
17
20
58
Abund.
2.7
0.12
83
27
5.0
27
Mass
3
Abund.
0.31
14
16
36
27
15
3.1
0.36
8.0
keep a log of the behavior
of system - it can help
identify problems
record reference RGA
spectra of the chamber, if
possible
●
vent system to dry nitrogen
●
open smallest flanges
possible (keep under slight
overpressure of nitrogen?)
●
ensure regular
maintenance on pumps (if
possible!) - no one else will!
Don't:
●
vent HV / UHV systems
with air
●
vent system while
components are cold (e.g.
nitrogen traps, cryopumps)
●
handle any components
without gloves
●
get any fluid/oil on your skin
●
burn yourself on diff.
pumps(!)