Principles of Vacuum

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Principles of Vacuum
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Maxwells Distribution of Speed
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Maxwells Distribution of Speed
Argon
90K 200m/s
100K 220m/s
120K 240m/s
150K 280m/s
200K 300m/s
300K 360m/s
Speed(m/s)
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Maxwells Distribution of Speed
Helium
90K 620m/s
100K 660m/s
120K 720m/s
150K 800m/s
200K 920m/s
300K 1140m/s
Speed(m/s)
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Maxwells Distribution of Speed
90 K
130g/mol 110m/s
80g/mol 140m/s
40g/mol 200m/s
20g/mol 270m/s
4g/mol 610m/s
Speed(m/s)
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Collision Cross Section
Ideal elastic hard sphere collision
Where W is the collision cross-section
r2
r1
Where d is the collision distance
d
These equations negate potential interactions
between the two molecules (atoms), attractive and
repulsive, and assume spherical geometry.
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Mean Free Path (l)
collisions per unit time
Mean free path (length b/w collisions)
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Permeation
Outgassing
Real
Leaks
Virtual
Diffusion
Backstreaming
GAS LOAD (Q) IS EXPRESSED IN mbar liters per
second
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Outgassing
(s-1cm-2)
Adsorbed Air Molecules
(s)
a is a function of surface, adsorbate, and temp
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Gas Flow
2 regimes
Molecular Flow
Viscous Flow
100mTorr
Higher Pressure
Lower Pressure
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Pumping Speed
L/s, m3/s, ft3/min (cfm)
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The mass rate of flow through a vacuum system is
known as throughput (Q), torr L/s
The ability of an apparatus to transmit gas is
known as conductance (C), same units as pumping
speed.
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Conductance
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Conductance
In the viscous flow regime, the conductance is
geometry, pressure, and viscosity dependent.
(L/s)
tube
where D is tube diameter, L is length, P is in
Torr, and h is in micropoise.
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Conductance
In the viscous flow regime, the conductance is
geometry, pressure, and viscosity dependent.
(L/s)
where D is tube diameter, L is length, P is in
Torr, and h is in micropoise.
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Using the conductance equation, and knowing that
mass transfer is the same (Q) throughout the
system substitute the pressure values for the
pumping speed at aperture and pump.
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Pumpdown time
Pumpdown time is also useful info. This can
identify slow leaks, outgassing, etc.
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Pressure Gauges

• Four Major Types
• Mechanical Gauge
• Thermal-Conductivity
• Viscous-Drag
• Ionization Gauge

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Capacitance Manometer
Range 1000 Torr to 1x10-4 Torr Accuracy
0.01 Response 10ms
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Thermal-Conductivity
Heat loss to the surrounding gas
Comparison
Direct Measurement
Pirani Gauge
Thermocouple Gauge
Range 10 Torr to 1x10-3 Torr Accuracy
0.5 Response 10ms
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Viscous-Drag
Range 1x10-1 Torr to 1x10-6 Torr Accuracy
3 Response seconds
Measure time from 415-405Hz
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Ionization Gauge

• 2 types
• Hot Cathode (Ion Gauge)
• Cold Cathode (Penning Gauge)

e-

Collector
Range 1x10-3 to 1x10-10 Torr Response ms
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Other Gauges
Mass Spectrometry (RGA)
Range 1x10-5 to 1x10-14 Torr Accuracy
.01 Response ms
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Pumps

• 3 Types
• Mechanical
• Diffusion
• Entrainment

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Mechanical Pumps

• Rotory Vane Pumps
• Roots Blower
• Scroll Pump
• Diaphragm Pumps
• Molecular Drag Pump
• Turbomolecular Pump

Rough Pumps Atm-100mTorr
1 - 1x10-6 Torr
1x10-3 - 1x10-8 Torr
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Rough Pumps
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Turbo Pumps
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Diffusion
Vapor Jet
To Rough Pump
Cooling
Oil, Heater
Vapor Jet
50-50000 L/s
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Entrainment

• Sorption Pumps
• Getter Pumps
• Cryopumps
• Ion Pumps

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Getters and Ion
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Cryo Pumps
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Materials
Quartz and Pyrex UHV He Leak Ceramics
Alumina Good internal parts Brass Contains Zinc
Outgasses at 10-6 Copper and Oxygen
Free Oxygen free reduces outgassing for
heating Stainless Steel 300 series (304,
316) Weldable Aluminum 6000 series Porous and
Oxide surface outgasses better than
stainless Plastic Outgas at 10-7 Polyimide,
Delron, Kapton Good Be careful of wiring shield